Signal amplifier works

[Uncle Steve]

.... unless there is no signal at the input.

I started with some information at the following two URLS:

http://www.mysticmarvels.com/amplifier.html
http://www.talkingelectronics.com/projects/TheTransistorAmplifier/TheTransistorAmplifier-P1.html

The first URL goes through the process of building a high-gain
amplifier, and it works O.K. with the exception that there is no
power. The second URL goes into more detail and shows how to deliver
some power to the speaker as in figure 16. I have different parts on
hand, and I am working with a 12V supply instead of 9V as in the first
URL. Since I am looking to deliver big chunky volts to my speaker,
I went ahead and modified Figure 16 and the example in the first URL
to obtain the following circuit:


12V +---+----------+----------+ R1 = 510K
| | | | R2 = 5.9K
| > R2 > R3 | R3 = 220
R1 > < < | C1 = 1uF
< | | | Q1, Q2 = 2N2222
| +-- | | Q3 = 2N3055
input | | \ b |c |
o---||-+-(Q1) -----(Q2) -----(Q3)
C1 | |e / |
1uf | --- SPKR
| |
| |
o----------+---------------------+

SPKR is a ribbon tweeter with a 2 Ohm, 10 Watt resistor in series.

R2 ought to be 6K, but 5.9 is what I have on hand.


Driving the circuit with a 50KHz signal produces 10V across the
speaker, which decreases to about 8V as the power transistor heats up.
I had a problem with thermal runaway until I got a sufficiently beefy
heatsink on Q3 and the 7812 supplying the 12V rail. Q3 runs quite
hot, but I think I can fix that by substituting a TO-3 package for
the TO-220 currently in the circuit, or by reducing the current.
Not sure which is the proper solution, but the speaker handles
the existing power.

220 Ohms for R3 was derived by guessing, and noting that 1K
attenuates the signal too much, but I don't really know how I should
be calculating the proper value. The speaker is usually paired with a
5.1 Ohm, 5W resistor, but I found it got too warm in this circuit, so
I used the 10W part instead. I have no idea what the speaker
impedance really is in operation.

The horrible fact is that this circuit works as long as there is a
signal present at the input. I am using a simple signal generator
from my DSO, but it doesn't allow the amplitude to be changed, so I
put a 100K trim pot on the positive side of C1. Before I modified the
circuit by adding Q2, the trim pot worked as expected. Now,
attenuating the signal even a little causes the output across the
speaker to fall off very quickly, with serious distortion occurring as
it falls (it seems the duty cycle goes from 50 to 90%, but that is
occurring somewhere in-circuit.)

If there is no signal present at the input, the circuit goes nuts very
quickly and ends up producing a much-distorted 9KHz signal at full
power at the speaker. This seems to be occurring at Q3, but I don't
really understand what is going on. My thinking is that with Q1c-Q2b
at ~6V, the DC load through the speaker is too much for Q3, but when
a signal is present, it 'works'.

What I'd like to do:

In the original form of the circuit, the speaker is driven through a
capacitor connected at Q2c (3300uF, 50V), but of course there is no
power available, and the voltage drops from 11V to 1 or 2 volts at the
positive terminal of the speaker. With the above circuit, it is not
obvious to me how I should rearrange things to decouple the speaker
from the DC signal.

I would like to figure out why the circuit only works at 'full on',
and blows up otherwise. The last problem is the power-on state, which
launches a huge spike through the speaker driver, and indeed, changing
the input signal frequency on the DSO causes a similar discontinuity
that is being amplified quite a bit as well. Suggestions as to how to
reduce this spike would be appreciated. Off the top of my head, I
suppose I could install an SCR in line with the speaker driver and
have a switch to turn on the power and another to enable the speaker
output via the SCR. I would test this, but I don't have an SCR
available ATM.

The entire objective with this little project is to develop a simple
amplifier that I can use to drive this speaker I have, where fidelity
is not as important as stability. Plus, it would be nice to be able
to use the parts I have on-hand.



Regards,

Uncle Steve

--
More than a century has passed since science laid down sound
propositions as to the origins of the universe, but how many have
mastered them or possess the really scientific spirit of criticism? A
few thousands at the outside, who are lost in the midst of hundreds of
millions still steeped in prejudices and superstitions worthy of
savages, who are consequently ever ready to serve as puppets for
religious impostors. -- Peter Kropotkin

 

[Michael A. Terrell]

Uncle Steve wrote:

... unless there is no signal at the input.

I started with some information at the following two URLS:

http://www.mysticmarvels.com/amplifier.html
http://www.talkingelectronics.com/projects/TheTransistorAmplifier/TheTransistorAmplifier-P1.html

The first URL goes through the process of building a high-gain
amplifier, and it works O.K. with the exception that there is no
power. The second URL goes into more detail and shows how to deliver
some power to the speaker as in figure 16. I have different parts on
hand, and I am working with a 12V supply instead of 9V as in the first
URL. Since I am looking to deliver big chunky volts to my speaker,
I went ahead and modified Figure 16 and the example in the first URL
to obtain the following circuit:

12V +---+----------+----------+ R1 = 510K
| | | | R2 = 5.9K
| > R2 > R3 | R3 = 220
R1 > < < | C1 = 1uF
| | | Q1, Q2 = 2N2222
| +-- | | Q3 = 2N3055
input | | \ b |c |
o---||-+-(Q1) -----(Q2) -----(Q3)
C1 | |e / |
1uf | --- SPKR
| |
| |
o----------+---------------------+

SPKR is a ribbon tweeter with a 2 Ohm, 10 Watt resistor in series.

R2 ought to be 6K, but 5.9 is what I have on hand.

Driving the circuit with a 50KHz signal produces 10V across the
speaker, which decreases to about 8V as the power transistor heats up.
I had a problem with thermal runaway until I got a sufficiently beefy
heatsink on Q3 and the 7812 supplying the 12V rail. Q3 runs quite
hot, but I think I can fix that by substituting a TO-3 package for
the TO-220 currently in the circuit, or by reducing the current.
Not sure which is the proper solution, but the speaker handles
the existing power.

220 Ohms for R3 was derived by guessing, and noting that 1K
attenuates the signal too much, but I don't really know how I should
be calculating the proper value. The speaker is usually paired with a
5.1 Ohm, 5W resistor, but I found it got too warm in this circuit, so
I used the 10W part instead. I have no idea what the speaker
impedance really is in operation.

The horrible fact is that this circuit works as long as there is a
signal present at the input. I am using a simple signal generator
from my DSO, but it doesn't allow the amplitude to be changed, so I
put a 100K trim pot on the positive side of C1. Before I modified the
circuit by adding Q2, the trim pot worked as expected. Now,
attenuating the signal even a little causes the output across the
speaker to fall off very quickly, with serious distortion occurring as
it falls (it seems the duty cycle goes from 50 to 90%, but that is
occurring somewhere in-circuit.)

If there is no signal present at the input, the circuit goes nuts very
quickly and ends up producing a much-distorted 9KHz signal at full
power at the speaker. This seems to be occurring at Q3, but I don't
really understand what is going on. My thinking is that with Q1c-Q2b
at ~6V, the DC load through the speaker is too much for Q3, but when
a signal is present, it 'works'.

What I'd like to do:

In the original form of the circuit, the speaker is driven through a
capacitor connected at Q2c (3300uF, 50V), but of course there is no
power available, and the voltage drops from 11V to 1 or 2 volts at the
positive terminal of the speaker. With the above circuit, it is not
obvious to me how I should rearrange things to decouple the speaker
from the DC signal.

I would like to figure out why the circuit only works at 'full on',
and blows up otherwise. The last problem is the power-on state, which
launches a huge spike through the speaker driver, and indeed, changing
the input signal frequency on the DSO causes a similar discontinuity
that is being amplified quite a bit as well. Suggestions as to how to
reduce this spike would be appreciated. Off the top of my head, I
suppose I could install an SCR in line with the speaker driver and
have a switch to turn on the power and another to enable the speaker
output via the SCR. I would test this, but I don't have an SCR
available ATM.

The entire objective with this little project is to develop a simple
amplifier that I can use to drive this speaker I have, where fidelity
is not as important as stability. Plus, it would be nice to be able
to use the parts I have on-hand.

Where did you get a speaker that works at 50 KHz?

 

[Uncle Steve]

On Fri, Feb 08, 2013 at 02:59:33PM -0500, Michael A. Terrell wrote:

Uncle Steve wrote:

... unless there is no signal at the input.

I started with some information at the following two URLS:

http://www.mysticmarvels.com/amplifier.html
http://www.talkingelectronics.com/projects/TheTransistorAmplifier/TheTransistorAmplifier-P1.html

The first URL goes through the process of building a high-gain
amplifier, and it works O.K. with the exception that there is no
power. The second URL goes into more detail and shows how to deliver
some power to the speaker as in figure 16. I have different parts on
hand, and I am working with a 12V supply instead of 9V as in the first
URL. Since I am looking to deliver big chunky volts to my speaker,
I went ahead and modified Figure 16 and the example in the first URL
to obtain the following circuit:

12V +---+----------+----------+ R1 = 510K
| | | | R2 = 5.9K
| > R2 > R3 | R3 = 220
R1 > < < | C1 = 1uF
| | | Q1, Q2 = 2N2222
| +-- | | Q3 = 2N3055
input | | \ b |c |
o---||-+-(Q1) -----(Q2) -----(Q3)
C1 | |e / |
1uf | --- SPKR
| |
| |
o----------+---------------------+

SPKR is a ribbon tweeter with a 2 Ohm, 10 Watt resistor in series.

R2 ought to be 6K, but 5.9 is what I have on hand.

Driving the circuit with a 50KHz signal produces 10V across the
speaker, which decreases to about 8V as the power transistor heats up.
I had a problem with thermal runaway until I got a sufficiently beefy
heatsink on Q3 and the 7812 supplying the 12V rail. Q3 runs quite
hot, but I think I can fix that by substituting a TO-3 package for
the TO-220 currently in the circuit, or by reducing the current.
Not sure which is the proper solution, but the speaker handles
the existing power.

220 Ohms for R3 was derived by guessing, and noting that 1K
attenuates the signal too much, but I don't really know how I should
be calculating the proper value. The speaker is usually paired with a
5.1 Ohm, 5W resistor, but I found it got too warm in this circuit, so
I used the 10W part instead. I have no idea what the speaker
impedance really is in operation.

The horrible fact is that this circuit works as long as there is a
signal present at the input. I am using a simple signal generator
from my DSO, but it doesn't allow the amplitude to be changed, so I
put a 100K trim pot on the positive side of C1. Before I modified the
circuit by adding Q2, the trim pot worked as expected. Now,
attenuating the signal even a little causes the output across the
speaker to fall off very quickly, with serious distortion occurring as
it falls (it seems the duty cycle goes from 50 to 90%, but that is
occurring somewhere in-circuit.)

If there is no signal present at the input, the circuit goes nuts very
quickly and ends up producing a much-distorted 9KHz signal at full
power at the speaker. This seems to be occurring at Q3, but I don't
really understand what is going on. My thinking is that with Q1c-Q2b
at ~6V, the DC load through the speaker is too much for Q3, but when
a signal is present, it 'works'.

What I'd like to do:

In the original form of the circuit, the speaker is driven through a
capacitor connected at Q2c (3300uF, 50V), but of course there is no
power available, and the voltage drops from 11V to 1 or 2 volts at the
positive terminal of the speaker. With the above circuit, it is not
obvious to me how I should rearrange things to decouple the speaker
from the DC signal.

I would like to figure out why the circuit only works at 'full on',
and blows up otherwise. The last problem is the power-on state, which
launches a huge spike through the speaker driver, and indeed, changing
the input signal frequency on the DSO causes a similar discontinuity
that is being amplified quite a bit as well. Suggestions as to how to
reduce this spike would be appreciated. Off the top of my head, I
suppose I could install an SCR in line with the speaker driver and
have a switch to turn on the power and another to enable the speaker
output via the SCR. I would test this, but I don't have an SCR
available ATM.

The entire objective with this little project is to develop a simple
amplifier that I can use to drive this speaker I have, where fidelity
is not as important as stability. Plus, it would be nice to be able
to use the parts I have on-hand.


Where did you get a speaker that works at 50 KHz?

It's a ribbon tweeter, ostensibly rated to 65KHz. It is difficult to
say what it's actual output is at those frequencies, but I can put two
of them face-to-face, drive one of them, and get signal from the other
with a scope. With the above circuit operating as described, the
second driver shows 54mV at the terminals -- using ribbons which are
sub-optimal. So I guess it works.


Regards,

Uncle Steve

--
More than a century has passed since science laid down sound
propositions as to the origins of the universe, but how many have
mastered them or possess the really scientific spirit of criticism? A
few thousands at the outside, who are lost in the midst of hundreds of
millions still steeped in prejudices and superstitions worthy of
savages, who are consequently ever ready to serve as puppets for
religious impostors. -- Peter Kropotkin

 

[Uncle Steve]

On Fri, Feb 08, 2013 at 03:36:30PM -0500, Uncle Steve wrote:

On Fri, Feb 08, 2013 at 02:59:33PM -0500, Michael A. Terrell wrote:

Where did you get a speaker that works at 50 KHz?

It's a ribbon tweeter, ostensibly rated to 65KHz. It is difficult to
say what it's actual output is at those frequencies, but I can put two
of them face-to-face, drive one of them, and get signal from the other
with a scope. With the above circuit operating as described, the
second driver shows 54mV at the terminals -- using ribbons which are
sub-optimal. So I guess it works.

Actually, here's someting wierd. At 4MHz, I measure 200mV, almost
nothing at 6MHz, and 60mV or so at 8MHz. 4MHz may be a resonant
frequency of the ribbon, given the speed of sound in copper, but
that's just a guess.


Regards,

Uncle Steve

--
More than a century has passed since science laid down sound
propositions as to the origins of the universe, but how many have
mastered them or possess the really scientific spirit of criticism? A
few thousands at the outside, who are lost in the midst of hundreds of
millions still steeped in prejudices and superstitions worthy of
savages, who are consequently ever ready to serve as puppets for
religious impostors. -- Peter Kropotkin

 

[Jon Kirwan]

What's the specific ribbon tweeter? What will be used to
drive the amplifier when in practical use? (It's output
impedance, Vpp.) What's the application? (Pestering bats?)
How much power output do you expect? What's the range of
frequencies to be emitted? A pure tone of 50kHz?

The first stage looks all wrong to me, anyway. Are you saying
it worked? That's hard to imagine. All I see is that Q1 is
hard-saturated with its collector very near ground and the
Darlington emitter follower not doing much of anything. What
in heck were you driving that thing with via C1??? Do you
have your design notes that you might share?

Jon

 

[Jim Thompson]

On Fri, 08 Feb 2013 14:59:33 -0500, "Michael A. Terrell"
<mike.terrell@earthlink.net> wrote:

Uncle Steve wrote:

... unless there is no signal at the input.

I started with some information at the following two URLS:

http://www.mysticmarvels.com/amplifier.html
http://www.talkingelectronics.com/projects/TheTransistorAmplifier/TheTransistorAmplifier-P1.html

The first URL goes through the process of building a high-gain
amplifier, and it works O.K. with the exception that there is no
power. The second URL goes into more detail and shows how to deliver
some power to the speaker as in figure 16. I have different parts on
hand, and I am working with a 12V supply instead of 9V as in the first
URL. Since I am looking to deliver big chunky volts to my speaker,
I went ahead and modified Figure 16 and the example in the first URL
to obtain the following circuit:

12V +---+----------+----------+ R1 = 510K
| | | | R2 = 5.9K
| > R2 > R3 | R3 = 220
R1 > < < | C1 = 1uF
| | | Q1, Q2 = 2N2222
| +-- | | Q3 = 2N3055
input | | \ b |c |
o---||-+-(Q1) -----(Q2) -----(Q3)
C1 | |e / |
1uf | --- SPKR
| |
| |
o----------+---------------------+

SPKR is a ribbon tweeter with a 2 Ohm, 10 Watt resistor in series.

R2 ought to be 6K, but 5.9 is what I have on hand.

Driving the circuit with a 50KHz signal produces 10V across the
speaker, which decreases to about 8V as the power transistor heats up.
I had a problem with thermal runaway until I got a sufficiently beefy
heatsink on Q3 and the 7812 supplying the 12V rail. Q3 runs quite
hot, but I think I can fix that by substituting a TO-3 package for
the TO-220 currently in the circuit, or by reducing the current.
Not sure which is the proper solution, but the speaker handles
the existing power.

220 Ohms for R3 was derived by guessing, and noting that 1K
attenuates the signal too much, but I don't really know how I should
be calculating the proper value. The speaker is usually paired with a
5.1 Ohm, 5W resistor, but I found it got too warm in this circuit, so
I used the 10W part instead. I have no idea what the speaker
impedance really is in operation.

The horrible fact is that this circuit works as long as there is a
signal present at the input. I am using a simple signal generator
from my DSO, but it doesn't allow the amplitude to be changed, so I
put a 100K trim pot on the positive side of C1. Before I modified the
circuit by adding Q2, the trim pot worked as expected. Now,
attenuating the signal even a little causes the output across the
speaker to fall off very quickly, with serious distortion occurring as
it falls (it seems the duty cycle goes from 50 to 90%, but that is
occurring somewhere in-circuit.)

If there is no signal present at the input, the circuit goes nuts very
quickly and ends up producing a much-distorted 9KHz signal at full
power at the speaker. This seems to be occurring at Q3, but I don't
really understand what is going on. My thinking is that with Q1c-Q2b
at ~6V, the DC load through the speaker is too much for Q3, but when
a signal is present, it 'works'.

What I'd like to do:

In the original form of the circuit, the speaker is driven through a
capacitor connected at Q2c (3300uF, 50V), but of course there is no
power available, and the voltage drops from 11V to 1 or 2 volts at the
positive terminal of the speaker. With the above circuit, it is not
obvious to me how I should rearrange things to decouple the speaker
from the DC signal.

I would like to figure out why the circuit only works at 'full on',
and blows up otherwise. The last problem is the power-on state, which
launches a huge spike through the speaker driver, and indeed, changing
the input signal frequency on the DSO causes a similar discontinuity
that is being amplified quite a bit as well. Suggestions as to how to
reduce this spike would be appreciated. Off the top of my head, I
suppose I could install an SCR in line with the speaker driver and
have a switch to turn on the power and another to enable the speaker
output via the SCR. I would test this, but I don't have an SCR
available ATM.

The entire objective with this little project is to develop a simple
amplifier that I can use to drive this speaker I have, where fidelity
is not as important as stability. Plus, it would be nice to be able
to use the parts I have on-hand.


Where did you get a speaker that works at 50 KHz?

The amplifier clearly needs a bootstrap >:-}

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Uncle Steve]

On Fri, Feb 08, 2013 at 12:56:20PM -0800, Jon Kirwan wrote:

What's the specific ribbon tweeter? What will be used to
drive the amplifier when in practical use? (It's output
impedance, Vpp.) What's the application? (Pestering bats?)
How much power output do you expect? What's the range of
frequencies to be emitted? A pure tone of 50kHz?

The driver is a proprietary design; the label sez "Quadral / 923 112 /
6 ohms", but it is normally attached to a crossover with a 5.1 ohm
5W resistor. Practical use is in a speaker cabinet powered by a
conventional stereo system, but as I'm trying to replace the ribbons,
I need a way to test the thing without installing it in the cabinet.

Now that you mention it, there is a vampire infestation in Toronto so
I may use one as a perimeter defense weapon, but that's after I prove
A replacement ribbon. In practice the device is intended to produce
audible audio frequencies, but for testing purposes I'm happy to use
ultrasonics to keep the workbench noise level down.

The first stage looks all wrong to me, anyway. Are you saying
it worked? That's hard to imagine. All I see is that Q1 is
hard-saturated with its collector very near ground and the
Darlington emitter follower not doing much of anything. What
in heck were you driving that thing with via C1??? Do you
have your design notes that you might share?

You're correct to be surprised it worked, and I still don't know
exactly what happened. I wired up a TO-3 version of Q3 and
substituted it into the circuit and found out that the old Q3 had let
some of it's Magic Smoke out, but was still working as I described. I
have no idea how that happened. Ah, it seems I miswired Q2; emitter
to GND, collector to the base of Q3, and to +12 through R3.

Again, wired correctly, and without Q3, I have 10.6V square wave at
the collector of Q1, and 3.6V at the emitter of Q2, open circuit. Is
it safe to apply the output of Q1 to the base of Q3?

Oh, and I'm driving it with the signal generator output of a DSO Quad,
which is a handheld 4-channel scope which retails about $220.


Regards,

Uncle Steve

--
More than a century has passed since science laid down sound
propositions as to the origins of the universe, but how many have
mastered them or possess the really scientific spirit of criticism? A
few thousands at the outside, who are lost in the midst of hundreds of
millions still steeped in prejudices and superstitions worthy of
savages, who are consequently ever ready to serve as puppets for
religious impostors. -- Peter Kropotkin

 

[Michael A. Terrell]

Jim Thompson wrote:

Michael A. Terrell wrote:

Where did you get a speaker that works at 50 KHz?

The amplifier clearly needs a bootstrap >:-}

If it can hit 50 KHz, it's too late for a jock strap. ;-)

 

[Uncle Steve]

On Fri, Feb 08, 2013 at 03:13:36PM -0800, George Herold wrote:

On Feb 8, 3:56 pm, Jon Kirwan <j...@infinitefactors.org> wrote:
What's the specific ribbon tweeter? What will be used to
drive the amplifier when in practical use? (It's output
impedance, Vpp.) What's the application? (Pestering bats?)
How much power output do you expect? What's the range of
frequencies to be emitted? A pure tone of 50kHz?

The first stage looks all wrong to me, anyway. Are you saying
it worked? That's hard to imagine. All I see is that Q1 is
hard-saturated with its collector very near ground and the
Darlington emitter follower not doing much of anything. What
in heck were you driving that thing with via C1??? Do you
have your design notes that you might share?

Jon

It looked crazy to me too.

I suppose so. It just doesn't seem to have enough discreet
components. The first stage is simple enough. The bias resistor, R1
and load resistor R2 specify that the voltage at Q1 collector is about
6V with no input signal, and allows the output to swing about 10V.
The second transistor is used to boost the current, and then in theory
Q3 does the heavy lifting. I'm not really clear on how Q2 does its
thing, but that's what the experimentation is for.

It may help to know that initially, I started with C1, R1, R2, Q3, and
a capacitor coupling the collector with the speaker + terminal.
That's when I discovered there was no power available to drive the
speaker. I substituted a 2N2222, and then made the circuit above. It
didn't work properly because I wired it wrong (see previous message)
but as I described it did output something like the the input signal
once the 2N3055 halfway burned out.

Uncle Steve, you need to find some better websites.

Maybe. There's no shortage of crap out there.

You could try an opamp with a push pull output stage.
Or just a power opamp.
Hey Jan had a nice audio amp...

Hey, I have all these transistors lying around and since I hear
they're good for amplifying signals, I thought I might as well use
them.

If you want to do a class A thing, then google the Zen amplfier.

Googling... Interesting, but I don't care about linearity all that
much. This is just something to run one frequency through the driver
at any given time, and therefore does not have to be special or
complex.


Regards,

Uncle Steve

--
More than a century has passed since science laid down sound
propositions as to the origins of the universe, but how many have
mastered them or possess the really scientific spirit of criticism? A
few thousands at the outside, who are lost in the midst of hundreds of
millions still steeped in prejudices and superstitions worthy of
savages, who are consequently ever ready to serve as puppets for
religious impostors. -- Peter Kropotkin

 

[George Herold]

On Feb 8, 3:56 pm, Jon Kirwan <j...@infinitefactors.org> wrote:

What's the specific ribbon tweeter? What will be used to
drive the amplifier when in practical use? (It's output
impedance, Vpp.) What's the application? (Pestering bats?)
How much power output do you expect? What's the range of
frequencies to be emitted? A pure tone of 50kHz?

The first stage looks all wrong to me, anyway. Are you saying
it worked? That's hard to imagine. All I see is that Q1 is
hard-saturated with its collector very near ground and the
Darlington emitter follower not doing much of anything. What
in heck were you driving that thing with via C1??? Do you
have your design notes that you might share?

Jon

It looked crazy to me too.

Uncle Steve, you need to find some better websites.
You could try an opamp with a push pull output stage.
Or just a power opamp.
Hey Jan had a nice audio amp...
If you want to do a class A thing, then google the Zen amplfier.

George H.

 

[Jon Kirwan]

On Fri, 08 Feb 2013 19:09:51 -0500, Uncle Steve
<stevet810@gmail.com> wrote:

On Fri, Feb 08, 2013 at 03:13:36PM -0800, George Herold wrote:
On Feb 8, 3:56 pm, Jon Kirwan <j...@infinitefactors.org> wrote:
What's the specific ribbon tweeter? What will be used to
drive the amplifier when in practical use? (It's output
impedance, Vpp.) What's the application? (Pestering bats?)
How much power output do you expect? What's the range of
frequencies to be emitted? A pure tone of 50kHz?

The first stage looks all wrong to me, anyway. Are you saying
it worked? That's hard to imagine. All I see is that Q1 is
hard-saturated with its collector very near ground and the
Darlington emitter follower not doing much of anything. What
in heck were you driving that thing with via C1??? Do you
have your design notes that you might share?

Jon

It looked crazy to me too.

I suppose so. It just doesn't seem to have enough discreet
components.

Um. That's not the way I'd say it. But that would be one of
the symptoms.

The first stage is simple enough.

I think the word is "simplistic." Not simple.

The bias resistor, R1
and load resistor R2 specify that the voltage at Q1 collector is about
6V with no input signal,

Not in this universe.

and allows the output to swing about 10V.

No, not really.

The second transistor is used to boost the current, and then in theory
Q3 does the heavy lifting. I'm not really clear on how Q2 does its
thing, but that's what the experimentation is for.

Okay. So this is when a little theory goes a long way
because, it turns out, there is good theory for a circuit
like this. Experiments are for when theory fails you because
it is incomplete or too complex to completely solve. This
circuit isn't too complex and theory here would be complete
enough for the job.

It may help to know that initially, I started with C1, R1, R2, Q3, and
a capacitor coupling the collector with the speaker + terminal.
That's when I discovered there was no power available to drive the
speaker. I substituted a 2N2222, and then made the circuit above. It
didn't work properly because I wired it wrong (see previous message)
but as I described it did output something like the the input signal
once the 2N3055 halfway burned out.

Uncle Steve, you need to find some better websites.

Maybe. There's no shortage of crap out there.

You could try an opamp with a push pull output stage.
Or just a power opamp.
Hey Jan had a nice audio amp...

Hey, I have all these transistors lying around and since I hear
they're good for amplifying signals, I thought I might as well use
them.

If you want to do a class A thing, then google the Zen amplfier.

Googling... Interesting, but I don't care about linearity all that
much. This is just something to run one frequency through the driver
at any given time, and therefore does not have to be special or
complex.

Okay. So let's look at the circuit you built. Here it is:

,--------+--------+--------+-------,
| | | | |
| | | | |
| | \ | |
| | / R3 | |
| \ \ 220 | ---
| / R2 / | - V1
| \ 5.9k | | --- 12
\ / | | -
/ R1 | | | |
\ 510 | | | |
/ | | | |
| | |/c Q2 | gnd
| +------| 2N2222 |
| | |>e |
| | | |
C1 | | | |
|| 1u | |/c Q1 | |/c Q3
IN---||----+------| 2N2222 '------| 2N3055
|| |>e |>e
| |
---, | |
| | \
| | / SPEAKER
gnd gnd \ 2 Ohm
/
|
|
gnd

Ignore everything from the base of Q2 and beyond (to the
right.) Ignore C1 for a moment (assume it isn't connected
up.) Just look at R1, R2, Q1, and the +12V supply rail:

+12 +12
| |
| |
| \
| / R2
| \ 5.9k
\ /
/ R1 |
\ 510 |
/ |
| +----Q1 collector ???
| |
| |
| |/c Q1
'------| 2N2222
|>e
|
|
|
gnd

No signal input (obviously.) So you think Q1's collector will
be at 6V? Really?

Let's see. Assume Q1 Vbe=0.7V (within 0.1V, probably) for
now. This means Ib=(12-0.7)/510=22.2mA. Ic, even in the worst
possible case where Q1 is completely saturated and its Vce=0,
would be Ic=12/5.9k=2mA. So the base current is about ten
times the maximum possible collector current. Now THAT is
overdriven, with beta=0.1. You don't see that very often. But
the gist is that Q1's Vce will be driven so close to 0 as to
make no difference at all.

It won't be 6V. The collector is hard-clamped against ground.

Now this doesn't mean you can't drive the input with
something that would move it. But this circuit is going to be
sitting with Q1's collector clamped hard to ground for half
the input cycle, at least. If you drive it hard enough, at
50kHz (Xc of a 1uF is only about 3 ohms), you will decidedly
be able to pull down hard enough during part of the cycle to
actually get the collector to move off of ground, I suppose.
But this is a mess.

And the quiescent Q1 collector will not sit at 6V. Measure
it.

Jon

 

[Jim Thompson]

On Fri, 08 Feb 2013 19:43:23 -0800, Jon Kirwan
<jonk@infinitefactors.org> wrote:

On Fri, 08 Feb 2013 19:09:51 -0500, Uncle Steve
stevet810@gmail.com> wrote:

On Fri, Feb 08, 2013 at 03:13:36PM -0800, George Herold wrote:
On Feb 8, 3:56 pm, Jon Kirwan <j...@infinitefactors.org> wrote:
What's the specific ribbon tweeter? What will be used to
drive the amplifier when in practical use? (It's output
impedance, Vpp.) What's the application? (Pestering bats?)
How much power output do you expect? What's the range of
frequencies to be emitted? A pure tone of 50kHz?

The first stage looks all wrong to me, anyway. Are you saying
it worked? That's hard to imagine. All I see is that Q1 is
hard-saturated with its collector very near ground and the
Darlington emitter follower not doing much of anything. What
in heck were you driving that thing with via C1??? Do you
have your design notes that you might share?

Jon

It looked crazy to me too.

I suppose so. It just doesn't seem to have enough discreet
components.

Um. That's not the way I'd say it. But that would be one of
the symptoms.

The first stage is simple enough.

I think the word is "simplistic." Not simple.

The bias resistor, R1
and load resistor R2 specify that the voltage at Q1 collector is about
6V with no input signal,

Not in this universe.

and allows the output to swing about 10V.

No, not really.

The second transistor is used to boost the current, and then in theory
Q3 does the heavy lifting. I'm not really clear on how Q2 does its
thing, but that's what the experimentation is for.

Okay. So this is when a little theory goes a long way
because, it turns out, there is good theory for a circuit
like this. Experiments are for when theory fails you because
it is incomplete or too complex to completely solve. This
circuit isn't too complex and theory here would be complete
enough for the job.

It may help to know that initially, I started with C1, R1, R2, Q3, and
a capacitor coupling the collector with the speaker + terminal.
That's when I discovered there was no power available to drive the
speaker. I substituted a 2N2222, and then made the circuit above. It
didn't work properly because I wired it wrong (see previous message)
but as I described it did output something like the the input signal
once the 2N3055 halfway burned out.

Uncle Steve, you need to find some better websites.

Maybe. There's no shortage of crap out there.

You could try an opamp with a push pull output stage.
Or just a power opamp.
Hey Jan had a nice audio amp...

Hey, I have all these transistors lying around and since I hear
they're good for amplifying signals, I thought I might as well use
them.

If you want to do a class A thing, then google the Zen amplfier.

Googling... Interesting, but I don't care about linearity all that
much. This is just something to run one frequency through the driver
at any given time, and therefore does not have to be special or
complex.

Okay. So let's look at the circuit you built. Here it is:

,--------+--------+--------+-------,
| | | | |
| | | | |
| | \ | |
| | / R3 | |
| \ \ 220 | ---
| / R2 / | - V1
| \ 5.9k | | --- 12
\ / | | -
/ R1 | | | |
\ 510 | | | |
/ | | | |
| | |/c Q2 | gnd
| +------| 2N2222 |
| | |>e |
| | | |
C1 | | | |
|| 1u | |/c Q1 | |/c Q3
IN---||----+------| 2N2222 '------| 2N3055
|| |>e |>e
| |
---, | |
| | \
| | / SPEAKER
gnd gnd \ 2 Ohm
/
|
|
gnd

Ignore everything from the base of Q2 and beyond (to the
right.) Ignore C1 for a moment (assume it isn't connected
up.) Just look at R1, R2, Q1, and the +12V supply rail:

+12 +12
| |
| |
| \
| / R2
| \ 5.9k
\ /
/ R1 |
\ 510 |
/ |
| +----Q1 collector ???
| |
| |
| |/c Q1
'------| 2N2222
|>e
|
|
|
gnd

No signal input (obviously.) So you think Q1's collector will
be at 6V? Really?

Let's see. Assume Q1 Vbe=0.7V (within 0.1V, probably) for
now. This means Ib=(12-0.7)/510=22.2mA. Ic, even in the worst
possible case where Q1 is completely saturated and its Vce=0,
would be Ic=12/5.9k=2mA. So the base current is about ten
times the maximum possible collector current. Now THAT is
overdriven, with beta=0.1. You don't see that very often. But
the gist is that Q1's Vce will be driven so close to 0 as to
make no difference at all.

It won't be 6V. The collector is hard-clamped against ground.

Now this doesn't mean you can't drive the input with
something that would move it. But this circuit is going to be
sitting with Q1's collector clamped hard to ground for half
the input cycle, at least. If you drive it hard enough, at
50kHz (Xc of a 1uF is only about 3 ohms), you will decidedly
be able to pull down hard enough during part of the cycle to
actually get the collector to move off of ground, I suppose.
But this is a mess.

And the quiescent Q1 collector will not sit at 6V. Measure
it.

Jon

You forgot the bootstrap effect >:-}

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Jon Kirwan]

On Fri, 08 Feb 2013 21:18:13 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

big snip of everything

You forgot the bootstrap effect >:-}

...Jim Thompson

I didn't miss out on the "Negative Feedback in Audio
Amplifiers" thread. But since this is posted in .basics it
seems appropriate to put on the kid gloves. There's still
plenty of time to add in a bootstrap, too!

Jon

 

[Jon Kirwan]

Here's a schematic of something close to what you want. It is
set for around 50kHz (I'm assuming you wanted that) and uses
your three transistor types. If your IN drive is low
impedance, then you don't want it to go more than about
40-50mV p-p. I set the quiescent Vc=7.5V or so. Gain is
40*Iq*R2, but since Iq is about 400uA that's about Av=160. It
will actually be a little lower than that. But it's still
likely to be somewhere around 120 to 140. With about +/-3V
around the 7.5V center, roughly speaking, that would mean no
more than 6vp-p/140 or about 43mVp-p. Which is why I said
40-50mv p-p input at C1.

I've even included the ever-popular bootstrap (R7 and C4):

: ,---------------+---------------+------------+----------,
: | | | | |
: | | | | |
: \ \ | | ---
: / R4 / R2 | | - V1
: \ 150k \ 10k | | --- 12
: / / | | -
: | C1 | |/c Q3 | |
: | || .047u +-------------| 2N2222 | |
: IN------------||---, | |>e | |
: | || | | | | gnd
: | | | | |
: | | | | |/c Q2
: | R7 | |/c Q1 '-+--------| 2N3055
: +---/\/\---+--| 2N2222 | |>e
: | 220k |>e | |
: | C4 | \ |
: | ||.047u | / R6 | C3
: +-------||------+ \ 2.2k | || 3.3u
: | || +-------, / +----||-----,
: | | | | | || |
: \ \ | | | |
: / R3 / R1 --- C2 gnd \ |
: \ 33k \ 2.7k --- .47u / R5 )| /
: / / | \ 1k )|/ speaker
: | | | / )|\ 2 ohm
: | | | | )| \
: gnd gnd gnd | |
: gnd |
: gnd

Since I've no idea what is driving the thing, except that it
appears to be some kind of low impedance driver if it expects
a 5.1 ohm resistor, then I feel fine stopping here. You may
need to supply that DC path, though, as a resistor divider
and then drive C1 from that.

Jon

 

[Jon Kirwan]

On Fri, 08 Feb 2013 19:43:23 -0800, I wrote:

snip

Ignore everything from the base of Q2 and beyond (to the
right.) Ignore C1 for a moment (assume it isn't connected
up.) Just look at R1, R2, Q1, and the +12V supply rail:

+12 +12
| |
| |
| \
| / R2
| \ 5.9k
\ /
/ R1 |
\ 510 |
/ |
| +----Q1 collector ???
| |
| |
| |/c Q1
'------| 2N2222
|>e
|
|
|
gnd

No signal input (obviously.) So you think Q1's collector will
be at 6V? Really?

Let's see. Assume Q1 Vbe=0.7V (within 0.1V, probably) for
now. This means Ib=(12-0.7)/510=22.2mA. Ic, even in the worst
possible case where Q1 is completely saturated and its Vce=0,
would be Ic=12/5.9k=2mA. So the base current is about ten
times the maximum possible collector current. Now THAT is
overdriven, with beta=0.1. You don't see that very often. But
the gist is that Q1's Vce will be driven so close to 0 as to
make no difference at all.

It won't be 6V. The collector is hard-clamped against ground.
snip

I made a mistake reading your values. You are using a 510k,
not 510 for R1. This reduces Ib by a factor of 1000, to
22.2uA. With a beta of 200 or so for the 2n2222, this
suggests a possible Ic=4.44mA. That times a 5.9k ohm
collector resistor, this is a drop of 26.2V across R2. Not
possible at a supply of 12V. Instead, you will see a drop of
about 11.8V with Ic=2mA.

Not as "hard-clampled" as I wrote earlier. But it is still in
saturation and will clip half-cycles. Not so good.

Jon

 

[Jim Thompson]

On Sat, 09 Feb 2013 00:56:15 -0800, Jon Kirwan
<jonk@infinitefactors.org> wrote:

Here's a schematic of something close to what you want. It is
set for around 50kHz (I'm assuming you wanted that) and uses
your three transistor types. If your IN drive is low
impedance, then you don't want it to go more than about
40-50mV p-p. I set the quiescent Vc=7.5V or so. Gain is
40*Iq*R2, but since Iq is about 400uA that's about Av=160. It
will actually be a little lower than that. But it's still
likely to be somewhere around 120 to 140. With about +/-3V
around the 7.5V center, roughly speaking, that would mean no
more than 6vp-p/140 or about 43mVp-p. Which is why I said
40-50mv p-p input at C1.

I've even included the ever-popular bootstrap (R7 and C4):

: ,---------------+---------------+------------+----------,
: | | | | |
: | | | | |
: \ \ | | ---
: / R4 / R2 | | - V1
: \ 150k \ 10k | | --- 12
: / / | | -
: | C1 | |/c Q3 | |
: | || .047u +-------------| 2N2222 | |
: IN------------||---, | |>e | |
: | || | | | | gnd
: | | | | |
: | | | | |/c Q2
: | R7 | |/c Q1 '-+--------| 2N3055
: +---/\/\---+--| 2N2222 | |>e
: | 220k |>e | |
: | C4 | \ |
: | ||.047u | / R6 | C3
: +-------||------+ \ 2.2k | || 3.3u
: | || +-------, / +----||-----,
: | | | | | || |
: \ \ | | | |
: / R3 / R1 --- C2 gnd \ |
: \ 33k \ 2.7k --- .47u / R5 )| /
: / / | \ 1k )|/ speaker
: | | | / )|\ 2 ohm
: | | | | )| \
: gnd gnd gnd | |
: gnd |
: gnd

Since I've no idea what is driving the thing, except that it
appears to be some kind of low impedance driver if it expects
a 5.1 ohm resistor, then I feel fine stopping here. You may
need to supply that DC path, though, as a resistor divider
and then drive C1 from that.

Jon

How much power can that deliver to the 2 Ohm load ?:-}

The OP seemed to want 10V P-P across the load. The OP was somewhat
vague on the load, 2 Ohm in series with a "ribbon tweeter". Wonder if
that was a piezoelectric or voice-coil type?

I'd suggest a class-B structure with some feedback (and some
engineering thought

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Jon Kirwan]

On Sat, 09 Feb 2013 10:28:34 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

On Sat, 09 Feb 2013 00:56:15 -0800, Jon Kirwan
jonk@infinitefactors.org> wrote:

Here's a schematic of something close to what you want. It is
set for around 50kHz (I'm assuming you wanted that) and uses
your three transistor types. If your IN drive is low
impedance, then you don't want it to go more than about
40-50mV p-p. I set the quiescent Vc=7.5V or so. Gain is
40*Iq*R2, but since Iq is about 400uA that's about Av=160. It
will actually be a little lower than that. But it's still
likely to be somewhere around 120 to 140. With about +/-3V
around the 7.5V center, roughly speaking, that would mean no
more than 6vp-p/140 or about 43mVp-p. Which is why I said
40-50mv p-p input at C1.

I've even included the ever-popular bootstrap (R7 and C4):

: ,---------------+---------------+------------+----------,
: | | | | |
: | | | | |
: \ \ | | ---
: / R4 / R2 | | - V1
: \ 150k \ 10k | | --- 12
: / / | | -
: | C1 | |/c Q3 | |
: | || .047u +-------------| 2N2222 | |
: IN------------||---, | |>e | |
: | || | | | | gnd
: | | | | |
: | | | | |/c Q2
: | R7 | |/c Q1 '-+--------| 2N3055
: +---/\/\---+--| 2N2222 | |>e
: | 220k |>e | |
: | C4 | \ |
: | ||.047u | / R6 | C3
: +-------||------+ \ 2.2k | || 3.3u
: | || +-------, / +----||-----,
: | | | | | || |
: \ \ | | | |
: / R3 / R1 --- C2 gnd \ |
: \ 33k \ 2.7k --- .47u / R5 )| /
: / / | \ 1k )|/ speaker
: | | | / )|\ 2 ohm
: | | | | )| \
: gnd gnd gnd | |
: gnd |
: gnd

Since I've no idea what is driving the thing, except that it
appears to be some kind of low impedance driver if it expects
a 5.1 ohm resistor, then I feel fine stopping here. You may
need to supply that DC path, though, as a resistor divider
and then drive C1 from that.

Jon

How much power can that deliver to the 2 Ohm load ?:-}

The OP seemed to want 10V P-P across the load.

No power supply bypassing, either. Anyway, half that at best.
And with plenty of distortion.

The OP was somewhat
vague on the load, 2 Ohm in series with a "ribbon tweeter". Wonder if
that was a piezoelectric or voice-coil type?

I'd suggest a class-B structure with some feedback (and some
engineering thought

Ah, come on. It's got the three transistors in the original
configuration and it has a bootstrap, too!

Now _you_ want a differential pair, current mirrors, a VAS
and a Vbe multiplier, two to perhaps four output BJTs, loop
feedback, speaker reactance compensation, a bipolar power
supply or perhaps a bridge-tied speaker, and THD of 0.1% with
bandwidth to 50kHz? With crafted design thinking to boot?

Yeah, right. It took less than 10 minutes for me to hack up
the above with only a few calculations and then use my custom
program to convert the schematic into ASCII text. I'm sitting
pat at that level of effort to help.

Jon

 

[Jim Thompson]

On Sat, 09 Feb 2013 11:27:27 -0800, Jon Kirwan
<jonk@infinitefactors.org> wrote:

On Sat, 09 Feb 2013 10:28:34 -0700, Jim Thompson
To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

On Sat, 09 Feb 2013 00:56:15 -0800, Jon Kirwan
jonk@infinitefactors.org> wrote:

Here's a schematic of something close to what you want. It is
set for around 50kHz (I'm assuming you wanted that) and uses
your three transistor types. If your IN drive is low
impedance, then you don't want it to go more than about
40-50mV p-p. I set the quiescent Vc=7.5V or so. Gain is
40*Iq*R2, but since Iq is about 400uA that's about Av=160. It
will actually be a little lower than that. But it's still
likely to be somewhere around 120 to 140. With about +/-3V
around the 7.5V center, roughly speaking, that would mean no
more than 6vp-p/140 or about 43mVp-p. Which is why I said
40-50mv p-p input at C1.

I've even included the ever-popular bootstrap (R7 and C4):

: ,---------------+---------------+------------+----------,
: | | | | |
: | | | | |
: \ \ | | ---
: / R4 / R2 | | - V1
: \ 150k \ 10k | | --- 12
: / / | | -
: | C1 | |/c Q3 | |
: | || .047u +-------------| 2N2222 | |
: IN------------||---, | |>e | |
: | || | | | | gnd
: | | | | |
: | | | | |/c Q2
: | R7 | |/c Q1 '-+--------| 2N3055
: +---/\/\---+--| 2N2222 | |>e
: | 220k |>e | |
: | C4 | \ |
: | ||.047u | / R6 | C3
: +-------||------+ \ 2.2k | || 3.3u
: | || +-------, / +----||-----,
: | | | | | || |
: \ \ | | | |
: / R3 / R1 --- C2 gnd \ |
: \ 33k \ 2.7k --- .47u / R5 )| /
: / / | \ 1k )|/ speaker
: | | | / )|\ 2 ohm
: | | | | )| \
: gnd gnd gnd | |
: gnd |
: gnd

Since I've no idea what is driving the thing, except that it
appears to be some kind of low impedance driver if it expects
a 5.1 ohm resistor, then I feel fine stopping here. You may
need to supply that DC path, though, as a resistor divider
and then drive C1 from that.

Jon

How much power can that deliver to the 2 Ohm load ?:-}

The OP seemed to want 10V P-P across the load.

No power supply bypassing, either. Anyway, half that at best.
And with plenty of distortion.

The OP was somewhat
vague on the load, 2 Ohm in series with a "ribbon tweeter". Wonder if
that was a piezoelectric or voice-coil type?

I'd suggest a class-B structure with some feedback (and some
engineering thought

Ah, come on. It's got the three transistors in the original
configuration and it has a bootstrap, too!

Now _you_ want a differential pair, current mirrors, a VAS
and a Vbe multiplier, two to perhaps four output BJTs, loop
feedback, speaker reactance compensation, a bipolar power
supply or perhaps a bridge-tied speaker, and THD of 0.1% with
bandwidth to 50kHz? With crafted design thinking to boot?

Yeah, right. It took less than 10 minutes for me to hack up
the above with only a few calculations and then use my custom
program to convert the schematic into ASCII text. I'm sitting
pat at that level of effort to help.

Jon

OK. How much signal does it put into 2 Ohms ?

My guess is that it just peak charges the 3.3uF (C3) which then decays
slowly thru R5 (1K).

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Jon Kirwan]

On Sat, 09 Feb 2013 12:34:56 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

On Sat, 09 Feb 2013 11:27:27 -0800, Jon Kirwan
jonk@infinitefactors.org> wrote:

On Sat, 09 Feb 2013 10:28:34 -0700, Jim Thompson
To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

On Sat, 09 Feb 2013 00:56:15 -0800, Jon Kirwan
jonk@infinitefactors.org> wrote:

Here's a schematic of something close to what you want. It is
set for around 50kHz (I'm assuming you wanted that) and uses
your three transistor types. If your IN drive is low
impedance, then you don't want it to go more than about
40-50mV p-p. I set the quiescent Vc=7.5V or so. Gain is
40*Iq*R2, but since Iq is about 400uA that's about Av=160. It
will actually be a little lower than that. But it's still
likely to be somewhere around 120 to 140. With about +/-3V
around the 7.5V center, roughly speaking, that would mean no
more than 6vp-p/140 or about 43mVp-p. Which is why I said
40-50mv p-p input at C1.

I've even included the ever-popular bootstrap (R7 and C4):

: ,---------------+---------------+------------+----------,
: | | | | |
: | | | | |
: \ \ | | ---
: / R4 / R2 | | - V1
: \ 150k \ 10k | | --- 12
: / / | | -
: | C1 | |/c Q3 | |
: | || .047u +-------------| 2N2222 | |
: IN------------||---, | |>e | |
: | || | | | | gnd
: | | | | |
: | | | | |/c Q2
: | R7 | |/c Q1 '-+--------| 2N3055
: +---/\/\---+--| 2N2222 | |>e
: | 220k |>e | |
: | C4 | \ |
: | ||.047u | / R6 | C3
: +-------||------+ \ 2.2k | || 3.3u
: | || +-------, / +----||-----,
: | | | | | || |
: \ \ | | | |
: / R3 / R1 --- C2 gnd \ |
: \ 33k \ 2.7k --- .47u / R5 )| /
: / / | \ 1k )|/ speaker
: | | | / )|\ 2 ohm
: | | | | )| \
: gnd gnd gnd | |
: gnd |
: gnd

Since I've no idea what is driving the thing, except that it
appears to be some kind of low impedance driver if it expects
a 5.1 ohm resistor, then I feel fine stopping here. You may
need to supply that DC path, though, as a resistor divider
and then drive C1 from that.

Jon

How much power can that deliver to the 2 Ohm load ?:-}

The OP seemed to want 10V P-P across the load.

No power supply bypassing, either. Anyway, half that at best.
And with plenty of distortion.

The OP was somewhat
vague on the load, 2 Ohm in series with a "ribbon tweeter". Wonder if
that was a piezoelectric or voice-coil type?

I'd suggest a class-B structure with some feedback (and some
engineering thought

Ah, come on. It's got the three transistors in the original
configuration and it has a bootstrap, too!

Now _you_ want a differential pair, current mirrors, a VAS
and a Vbe multiplier, two to perhaps four output BJTs, loop
feedback, speaker reactance compensation, a bipolar power
supply or perhaps a bridge-tied speaker, and THD of 0.1% with
bandwidth to 50kHz? With crafted design thinking to boot?

Yeah, right. It took less than 10 minutes for me to hack up
the above with only a few calculations and then use my custom
program to convert the schematic into ASCII text. I'm sitting
pat at that level of effort to help.

Jon

OK. How much signal does it put into 2 Ohms ?

My guess is that it just peak charges the 3.3uF (C3) which then decays
slowly thru R5 (1K).

Yeah, I see that clearly now. Active one side only. Well, I'm
just an ignorant hobbyist. So I'll chalk up last night to
that, plus a kick in the head. Thanks. Well, at least it was
only a few minutes wasted time last night.

Jon

 

[Uncle Steve]

On Fri, Feb 08, 2013 at 07:43:23PM -0800, Jon Kirwan wrote:

On Fri, 08 Feb 2013 19:09:51 -0500, Uncle Steve
stevet810@gmail.com> wrote:

On Fri, Feb 08, 2013 at 03:13:36PM -0800, George Herold wrote:
On Feb 8, 3:56 pm, Jon Kirwan <j...@infinitefactors.org> wrote:
What's the specific ribbon tweeter? What will be used to
drive the amplifier when in practical use? (It's output
impedance, Vpp.) What's the application? (Pestering bats?)
How much power output do you expect? What's the range of
frequencies to be emitted? A pure tone of 50kHz?

The first stage looks all wrong to me, anyway. Are you saying
it worked? That's hard to imagine. All I see is that Q1 is
hard-saturated with its collector very near ground and the
Darlington emitter follower not doing much of anything. What
in heck were you driving that thing with via C1??? Do you
have your design notes that you might share?

Jon

It looked crazy to me too.

I suppose so. It just doesn't seem to have enough discreet
components.

Um. That's not the way I'd say it. But that would be one of
the symptoms.

The first stage is simple enough.

I think the word is "simplistic." Not simple.

The bias resistor, R1
and load resistor R2 specify that the voltage at Q1 collector is about
6V with no input signal,

Not in this universe.

Sorry, I suppose it is more correct to say that it is about 6V when
the input signal is crossing zero.

and allows the output to swing about 10V.

No, not really.

The second transistor is used to boost the current, and then in theory
Q3 does the heavy lifting. I'm not really clear on how Q2 does its
thing, but that's what the experimentation is for.

Okay. So this is when a little theory goes a long way
because, it turns out, there is good theory for a circuit
like this. Experiments are for when theory fails you because
it is incomplete or too complex to completely solve. This
circuit isn't too complex and theory here would be complete
enough for the job.

Hey, I'm just a noob so the theory is not something I have a really
good handle on yet. The experimentation allows me to fool around with
my scope and see what happens with different values on input and with
different component values.

It may help to know that initially, I started with C1, R1, R2, Q3, and
a capacitor coupling the collector with the speaker + terminal.
That's when I discovered there was no power available to drive the
speaker. I substituted a 2N2222, and then made the circuit above. It
didn't work properly because I wired it wrong (see previous message)
but as I described it did output something like the the input signal
once the 2N3055 halfway burned out.

Uncle Steve, you need to find some better websites.

Maybe. There's no shortage of crap out there.

You could try an opamp with a push pull output stage.
Or just a power opamp.
Hey Jan had a nice audio amp...

Hey, I have all these transistors lying around and since I hear
they're good for amplifying signals, I thought I might as well use
them.

If you want to do a class A thing, then google the Zen amplfier.

Googling... Interesting, but I don't care about linearity all that
much. This is just something to run one frequency through the driver
at any given time, and therefore does not have to be special or
complex.

Okay. So let's look at the circuit you built. Here it is:

,--------+--------+--------+-------,
| | | | |
| | | | |
| | \ | |
| | / R3 | |
| \ \ 220 | ---
| / R2 / | - V1
| \ 5.9k | | --- 12
\ / | | -
/ R1 | | | |
\ 510 | | | |
/ | | | |
| | |/c Q2 | gnd
| +------| 2N2222 |
| | |>e |
| | | |
C1 | | | |
|| 1u | |/c Q1 | |/c Q3
IN---||----+------| 2N2222 '------| 2N3055
|| |>e |>e
| |
---, | |
| | \
| | / SPEAKER
gnd gnd \ 2 Ohm
/
|
|
gnd

R1 is 510K ohms, so I'll just amend the following diagram to suit.

Ignore everything from the base of Q2 and beyond (to the
right.) Ignore C1 for a moment (assume it isn't connected
up.) Just look at R1, R2, Q1, and the +12V supply rail:

+12 +12
| |
| |
| \
| / R2
| \ 5.9k
\ /
/ R1 |
\ 510K |
/ |
| +----Q1 collector ???
| |
| |
| |/c Q1
'------| 2N2222
|>e
|
|
|
gnd

No signal input (obviously.) So you think Q1's collector will
be at 6V? Really?

Let's see. Assume Q1 Vbe=0.7V (within 0.1V, probably) for
now. This means Ib=(12-0.7)/510=22.2mA. Ic, even in the worst
possible case where Q1 is completely saturated and its Vce=0,
would be Ic=12/5.9k=2mA. So the base current is about ten
times the maximum possible collector current. Now THAT is
overdriven, with beta=0.1. You don't see that very often. But
the gist is that Q1's Vce will be driven so close to 0 as to
make no difference at all.

From my notes:

Ic/Hfe = Ib

..01A / 50 = .0002A

I chose 50 somewhat arbitrarily. Initially I was making this circuit
with the 2N3055, for which the data sheet says that the DC current
gain is 20min and 100max, so 50 seemed like a good starting point.

Vcc - Vbe = Vb

12V - 1.8V = 10.2V

Again the data sheet says that the Base-Emitter on voltage is 1.8V
max. Perhaps I should have used 1V, but there you go.

Vb / Ib = Rbias

10.2V / .0002A = 51K ohms

(1/2 Vcc) / Ic = Rc

6V / .01A = 600 ohms

Next, I decided I could live with an order of magnitude less quiescent
current, and so chose 510K and 6K ohms respectively for R1 and R2.

I suppose it doesn't help that after moving on, I changed Q1 to a
2N2222. So, if I fix the calculations based on the assumption of .7V
Q12 Vbe, the calculations are as follows:

Ic = .001A / 50 = .00002A (quiescent current)

12V - .7V = 11.3V

11.3V / .00002A = 565K ohms

6V / .001A = 6K ohms

I happen to have a 560K ohm resistor, so I could use that if it was
necessary.

It won't be 6V. The collector is hard-clamped against ground.

Not exactly.

Now this doesn't mean you can't drive the input with
something that would move it. But this circuit is going to be
sitting with Q1's collector clamped hard to ground for half
the input cycle, at least. If you drive it hard enough, at
50kHz (Xc of a 1uF is only about 3 ohms), you will decidedly
be able to pull down hard enough during part of the cycle to
actually get the collector to move off of ground, I suppose.
But this is a mess.

I agree, but not quite for the reason you say.

And the quiescent Q1 collector will not sit at 6V. Measure
it.

Sorry, I meant to say that with a driver coupled to the collector with
a capacitor, the (zero-crossing) voltage should be about 6V. In
theory. After poking around a bit it is now clear that the behavior
of this circuit is wierd.

Firstly, the signal generator output has an amplitude of about 1V. I
put a 100K trim-pot in front of the capacitor to vary the input from
the signal generator. I'd been using a square wave previously, in
part because the sine-wave caps at 20KHz with current DSO firmware.

With a 560K bias resistor, and an open output, the sine-wave output at
Q1c is thin (duty cycle of 34%, whereas input is 50%) and the output
saturates at 11V. Just for fun, I changed R1 to 2M ohm and found that
the output waveform is more accurate, although requiring more input
attenuation to achieve. In this case the output signal attains a 3.x
DC bias and the peak-to-peak output is 7.xV. I assume this is a
result of interaction of the input signal and C1, so I suppose I have
to look into the proper way to float the input signal. I'm guessing
that the impedance of the signal generator comes into play here.

Earlier today, I replaced the broken 2N3055 at Q3 and found that the
output of Q2 was now about 10.6V, whereas I measured it's open-circuit
voltage as peaking at 3.something. Unfortunately, I had set the scope
input to AC, so that was a case of GIGO. Open-circuit, the Q2e output
shows 4Vpp with a DC offset of 5V. The waveform is distorted, and I
suppose this is no surprise since Q2b is getting driven with too much
voltage. Adding in Q3 and a load produces just so much random noise.
Driving the speaker directly from Q2e doesn't do anything useful.

So clearly this is the wrong way to do it, and figure 16 on
http://www.talkingelectronics.com/projects/TheTransistorAmplifier/TheTransistorAmplifier-P1.html
is bogus.

Evidently the original miswired circuit only worked because Q3
malfunctioned in a particular way.

Back to the drawing board.


Regards,

Uncle Steve

--
More than a century has passed since science laid down sound
propositions as to the origins of the universe, but how many have
mastered them or possess the really scientific spirit of criticism? A
few thousands at the outside, who are lost in the midst of hundreds of
millions still steeped in prejudices and superstitions worthy of
savages, who are consequently ever ready to serve as puppets for
religious impostors. -- Peter Kropotkin