trying to build a simple Class D circuit

[Michael]

After thinking about the overheating of output transistors in a Class
AB scheme, I decided to try instead to build a Class D amplifier, with
about 1 W of output (powered by a 12V supply).

Yes, I know I could just buy a TPA3122D2 for about $3, but for some
odd reason I'm drawn to trying to build a small Class D system from
discrete components (or op-amps).

I am studying these:

http://www.eetimes.com/design/audio-design/4015723/Design-and-analysis-of-a-basic-class-D-amplifier
http://www.irf.com/product-info/audio/classdtutorial.pdf

Question:

Is a triangle wave or sawtooth wave preferred? In the first link the
"triangle" waves appear to actually be sawtooth waves.

Thanks,

Michael

 

[Rich Grise]

On Mon, 11 Oct 2010 21:42:55 -0700, Michael wrote:

After thinking about the overheating of output transistors in a Class AB
scheme, I decided to try instead to build a Class D amplifier, with about
1 W of output (powered by a 12V supply).

Yes, I know I could just buy a TPA3122D2 for about $3, but for some odd
reason I'm drawn to trying to build a small Class D system from discrete
components (or op-amps).

I am studying these:

http://www.eetimes.com/design/audio-design/4015723/Design-and-analysis-of-a-basic-class-D-amplifier
http://www.irf.com/product-info/audio/classdtutorial.pdf

Question:

Is a triangle wave or sawtooth wave preferred? In the first link the
"triangle" waves appear to actually be sawtooth waves.

It doesn't matter. The threshold just sets the duty cycle, and a triangle
or sawtooth will work fine to do that, as long as you don't have sync
problems or anything, but for audio frequencies, I don't think that'd be
a concern.

Have Fun!
Rich

 

[Dan Coby]

On 10/11/2010 9:42 PM, Michael wrote:

After thinking about the overheating of output transistors in a Class
AB scheme, I decided to try instead to build a Class D amplifier, with
about 1 W of output (powered by a 12V supply).

Yes, I know I could just buy a TPA3122D2 for about $3, but for some
odd reason I'm drawn to trying to build a small Class D system from
discrete components (or op-amps).

I am studying these:

http://www.eetimes.com/design/audio-design/4015723/Design-and-analysis-of-a-basic-class-D-amplifier
http://www.irf.com/product-info/audio/classdtutorial.pdf

Question:

Is a triangle wave or sawtooth wave preferred? In the first link the
"triangle" waves appear to actually be sawtooth waves.

In theory with either a perfect triangle wave or a perfect sawtooth wave
it does not matter which one you use.

With non-perfect waves there are some differences. If the slopes of the
wave are not straight lines then the output of the amplifier will be
distorted.

With a triangle wave, then both the rising and falling slopes need to
be as straight as possible.

With a sawtooth wave, the quickness of the transition at the end of the
wave reduces the effect of any non-linearity in this slope. Thus more
effort can placed on producing a straight ramp.


Dan

 

[whit3rd]

On Oct 11, 9:42 pm, Michael <mrdarr...@gmail.com> wrote:

After thinking about the overheating of output transistors in a Class
AB scheme, I decided to try instead to build a Class D amplifier, with
about 1 W of output (powered by a 12V supply).

Yes, I know I could just buy a TPA3122D2 for about $3, but for some
odd reason I'm drawn to trying to build a small Class D system from
discrete components (or op-amps).

Is a triangle wave or sawtooth wave preferred?  In the first link the
"triangle" waves appear to actually be sawtooth waves.

If you're using stereo, the sawtooth will result in switching
transients on both channels at the same time; I'd prefer the triangle.

More important, remember that the input currents are going
to be high frequency, and an order of magnitude (or more) higher
than the amplifier output currents; the power supply filtering
will be a large part of your design problem.

 

[Michael]

On Oct 12, 11:59 am, whit3rd <whit...@gmail.com> wrote:

On Oct 11, 9:42 pm, Michael <mrdarr...@gmail.com> wrote:

After thinking about the overheating of output transistors in a Class
AB scheme, I decided to try instead to build a Class D amplifier, with
about 1 W of output (powered by a 12V supply).

Yes, I know I could just buy a TPA3122D2 for about $3, but for some
odd reason I'm drawn to trying to build a small Class D system from
discrete components (or op-amps).
Is a triangle wave or sawtooth wave preferred?  In the first link the
"triangle" waves appear to actually be sawtooth waves.

If you're using stereo, the sawtooth will result in switching
transients on both channels at the same time; I'd prefer the triangle.

More important, remember that the input currents are going
to be high frequency, and an order of magnitude (or more) higher
than the amplifier output currents; the power supply filtering
will be a large part of your design problem.

Ok, from what I understand, I will need a sinusoidal audio input fed
to the (+) side of a high-speed comparator (no ordinary op-amp), and a
triangle generator (frequency at least 20x the max of the highest
audio frequency, or 20x 20 kHz) connected to the (-) side of the
comparator. The comparator output will be a square wave, varying from
+V to -V, requiring a dual supply.

Tough parts seem to be:

1. Feeding the resulting square wave into a half-bridge push-pull N-
channel and P-channel mosfet pair, without allowing shorting of the
power supply due to the mosfet requiring some time to turn off (OFF
delay).

2. Activating the N-channel mosfet's gate, since it wants a higher
voltage than +V.

Am I right so far?

Would (1) be partly solved by using XOR gates?

I'm looking at this:
http://geekcircuits.com/wp-content/uploads/2010/02/power-class-d-amp.png

and I'm wondering how the XOR gates can help here, since they are
before the MOSFET driver itself...

Michael

 

[Jasen Betts]

On 2010-10-13, Michael <mrdarrett@gmail.com> wrote:

On Oct 12, 11:59 am, whit3rd <whit...@gmail.com> wrote:
On Oct 11, 9:42 pm, Michael <mrdarr...@gmail.com> wrote:

After thinking about the overheating of output transistors in a Class
AB scheme, I decided to try instead to build a Class D amplifier, with
about 1 W of output (powered by a 12V supply).

Yes, I know I could just buy a TPA3122D2 for about $3, but for some
odd reason I'm drawn to trying to build a small Class D system from
discrete components (or op-amps).
Is a triangle wave or sawtooth wave preferred?  In the first link the
"triangle" waves appear to actually be sawtooth waves.

If you're using stereo, the sawtooth will result in switching
transients on both channels at the same time; I'd prefer the triangle.

More important, remember that the input currents are going
to be high frequency, and an order of magnitude (or more) higher
than the amplifier output currents; the power supply filtering
will be a large part of your design problem.




Ok, from what I understand, I will need a sinusoidal audio input fed
to the (+) side of a high-speed comparator (no ordinary op-amp), and a
triangle generator (frequency at least 20x the max of the highest
audio frequency, or 20x 20 kHz) connected to the (-) side of the
comparator.

the audio input should be analogue audio, needn't be sinusoidal

The comparator output will be a square wave, varying from
+V to -V, requiring a dual supply.

no, you could use a single supply and a H-bridge output stage or
series capacitor

Tough parts seem to be:

1. Feeding the resulting square wave into a half-bridge push-pull N-
channel and P-channel mosfet pair, without allowing shorting of the
power supply due to the mosfet requiring some time to turn off (OFF
delay).

you could put an inductor in series with the supply, ot just use a
mosfet driver chip....

2. Activating the N-channel mosfet's gate, since it wants a higher
voltage than +V.

Would (1) be partly solved by using XOR gates?

no.

I'm looking at this:
http://geekcircuits.com/wp-content/uploads/2010/02/power-class-d-amp.png

the 555 circuit looks wrong, I think trig and thrs should be connected
to the wiper of R3 not the end.

I suspect the business with the xor ates was inserted to compensate
for that.

and I'm wondering how the XOR gates can help here, since they are
before the MOSFET driver itself...

two of the XOR gatres act as buffers two as inverters.

r2-c2 is the delay for 'dead time' this will put both drivers in same
state for a short period.

I'd be interested in seeing a scope traces on the thrs pin of the 555.

--
ɹǝpun uʍop ɯoɹɟ sƃuıʇǝǝɹ⅁

 

[Michael]

On Oct 14, 2:06 am, Jasen Betts <ja...@xnet.co.nz> wrote:

On 2010-10-13, Michael <mrdarr...@gmail.com> wrote:



On Oct 12, 11:59 am, whit3rd <whit...@gmail.com> wrote:
On Oct 11, 9:42 pm, Michael <mrdarr...@gmail.com> wrote:

After thinking about the overheating of output transistors in a Class
AB scheme, I decided to try instead to build a Class D amplifier, with
about 1 W of output (powered by a 12V supply).

Yes, I know I could just buy a TPA3122D2 for about $3, but for some
odd reason I'm drawn to trying to build a small Class D system from
discrete components (or op-amps).
Is a triangle wave or sawtooth wave preferred?  In the first link the
"triangle" waves appear to actually be sawtooth waves.

If you're using stereo, the sawtooth will result in switching
transients on both channels at the same time; I'd prefer the triangle.

More important, remember that the input currents are going
to be high frequency, and an order of magnitude (or more) higher
than the amplifier output currents; the power supply filtering
will be a large part of your design problem.

Ok, from what I understand, I will need a sinusoidal audio input fed
to the (+) side of a high-speed comparator (no ordinary op-amp), and a
triangle generator (frequency at least 20x the max of the highest
audio frequency, or 20x 20 kHz) connected to the (-) side of the
comparator.

the audio input should be analogue audio, needn't be sinusoidal

Yep.

The comparator output will be a square wave, varying from
+V to -V, requiring a dual supply.

no, you could use a single supply and a H-bridge output stage or
series capacitor

Yes, an H-bridge output stage would be nice... I do not understand
though how a series capacitor will help. (Remember I am a beginner at
electronics!) The analog input will be positive and negative...
should I simply tie the Vs- end of the comparator to ground to get
pulses between Vsupply and 0?

Tough parts seem to be:

1.  Feeding the resulting square wave into a half-bridge push-pull N-
channel and P-channel mosfet pair, without allowing shorting of the
power supply due to the mosfet requiring some time to turn off (OFF
delay).

you could put an inductor in series with the supply, ot just use a
mosfet driver chip....

Good idea! This H-bridge mosfet driver looks interesting... 250+ kHz
switching frequencies...
http://www.intersil.com/data/an/an9405.pdf

2.  Activating the N-channel mosfet's gate, since it wants a higher
voltage than +V.
Would (1) be partly solved by using XOR gates?

no.

I'm looking at this:
http://geekcircuits.com/wp-content/uploads/2010/02/power-class-d-amp.png

the 555 circuit looks wrong, I think trig and thrs should be connected
to the wiper of R3 not the end.

I suspect the business with the xor ates was inserted to compensate
for that.

and I'm wondering how the XOR gates can help here, since they are
before the MOSFET driver itself...

two of the XOR gatres act as buffers two as inverters.

r2-c2 is the delay for 'dead time' this will put both drivers in same
state for a short period.

I'd be interested in seeing a scope traces on the thrs pin of the 555.

Ah ok. I had a feeling a 555 wasn't a good choice anyway... not such
a great comparator at high frequencies?

--
ɹǝpun uʍop ɯoɹɟ sƃuıʇǝǝɹ⅁

Greetings from the Northern Hemisphere

Michael

 

[Jasen Betts]

On 2010-10-15, Michael <mrdarrett@gmail.com> wrote:

Ah ok. I had a feeling a 555 wasn't a good choice anyway... not such
a great comparator at high frequencies?

It's good to around 1MHz, but I'm mystified about how it's supposed
to work in this circuit

 

[Michael Black]

On Fri, 15 Oct 2010, Jasen Betts wrote:

On 2010-10-15, Michael <mrdarrett@gmail.com> wrote:

Ah ok. I had a feeling a 555 wasn't a good choice anyway... not such
a great comparator at high frequencies?

It's good to around 1MHz, but I'm mystified about how it's supposed
to work in this circuit

It's elementary.

"Elementary Electronics" that is.

They ran an article in the mid-seventies about a Class-D amplifier using a
555, at a time when only a handful of Class-D amplifiers (or even
theoretical articles about them) had appeared in the hobby electronic
magazines.

I can't remember the layout.

But chances are good this derives from that, if it's not the same
schematic.

Michael

 

[Michael]

On Oct 15, 12:53 pm, Jasen Betts <ja...@xnet.co.nz> wrote:

On 2010-10-15, Michael <mrdarr...@gmail.com> wrote:

Ah ok.  I had a feeling a 555 wasn't a good choice anyway... not such
a great comparator at high frequencies?

It's good to around 1MHz,  but I'm mystified about how it's supposed
to work in this circuit

He's got a bunch of circuits here:

Thing is, they seem to be headphone amps. Not sure how that would
help me since I want to take the sound output from the computer and
attach that to some speakers via a homemade amplifier.

He seemed surprised that a basic transistor sounded better than his
second 555. Second circuit:
http://geekcircuits.com/2010/01/class-d-amp-made-easy-with-555-timer-ic/

 

[Michael]

On Oct 15, 2:16 pm, Michael <mrdarr...@gmail.com> wrote:

On Oct 15, 12:53 pm, Jasen Betts <ja...@xnet.co.nz> wrote:

On 2010-10-15, Michael <mrdarr...@gmail.com> wrote:

Ah ok.  I had a feeling a 555 wasn't a good choice anyway... not such
a great comparator at high frequencies?

It's good to around 1MHz,  but I'm mystified about how it's supposed
to work in this circuit

He's got a bunch of circuits here:

Thing is, they seem to be headphone amps.  Not sure how that would
help me since I want to take the sound output from the computer and
attach that to some speakers via a homemade amplifier.

He seemed surprised that a basic transistor sounded better than his
second 555.  Second circuit:http://geekcircuits.com/2010/01/class-d-amp-made-easy-with-555-timer-ic/

Bunch of circuits here: http://geekcircuits.com/category/class-d-amp/
still half asleep...

 

[Jasen Betts]

On 2010-10-15, Michael <mrdarrett@gmail.com> wrote:

On Oct 15, 2:16 pm, Michael <mrdarr...@gmail.com> wrote:
On Oct 15, 12:53 pm, Jasen Betts <ja...@xnet.co.nz> wrote:

On 2010-10-15, Michael <mrdarr...@gmail.com> wrote:

Ah ok.  I had a feeling a 555 wasn't a good choice anyway... not such
a great comparator at high frequencies?

It's good to around 1MHz,  but I'm mystified about how it's supposed
to work in this circuit

He's got a bunch of circuits here:

turns out the drawing is wrong. In his simulation he has 555 TRIG+THRS
connected to R3+C1 instead of to R3+D2 and that makes a lot more sense.

Thing is, they seem to be headphone amps.  Not sure how that would
help me since I want to take the sound output from the computer and
attach that to some speakers via a homemade amplifier.

the TC4451 used in this circuit should drive 4 ohms speakers at 20W or
so from a 12V supply that's a real 20W continuous sinewave not some
marketing hype 20W.

I can't vouch for the theoretical fidelity of this amplifier. The ramp
used by comparitor is not linear and the switching frequency is
modulated by the audio input, the non-linear ramp is likely to cause
distortion the FM could cause problems too.

He seemed surprised that a basic transistor sounded better than his
second 555.

for the purists there is the 'zen' mosfet amplifier (and space heater*).
said to be very good in sound quality.
(*gets hot, needs a really big heatsink)

http://www.tubecad.com/2009/10/blog0174.htm

 Second circuit: http://geekcircuits.com/2010/01/class-d-amp-made-easy-with-555-timer-ic/

ther second circuit deals with the PWM frequency problem
but not the linearity problem. this pone does that.

http://geekcircuits.com/2010/01/class-d-amp-using-555-timer/

substitute it for the 555 part of the power amplifier.

Still there's probably going to problems with noise from the powersupply
etc... these circuits are good as a learning exercise but in the real
world they have problems.

--
ɹǝpun uʍop ɯoɹɟ sƃuıʇǝǝɹ⅁

 

[Jasen Betts]

On 2010-10-15, Michael Black <et472@ncf.ca> wrote:

On Fri, 15 Oct 2010, Jasen Betts wrote:

On 2010-10-15, Michael <mrdarrett@gmail.com> wrote:

Ah ok. I had a feeling a 555 wasn't a good choice anyway... not such
a great comparator at high frequencies?

It's good to around 1MHz, but I'm mystified about how it's supposed
to work in this circuit


"Elementary Electronics" that is.

They ran an article in the mid-seventies about a Class-D amplifier using a
555, at a time when only a handful of Class-D amplifiers

But chances are good this derives from that, if it's not the same
schematic.

It turns sout his schematic was wrong.

--
ɹǝpun uʍop ɯoɹɟ sƃuıʇǝǝɹ⅁

 

[Michael]

On Oct 16, 2:03 am, Jasen Betts <ja...@xnet.co.nz> wrote:

On 2010-10-15, Michael <mrdarr...@gmail.com> wrote:

On Oct 15, 2:16 pm, Michael <mrdarr...@gmail.com> wrote:
On Oct 15, 12:53 pm, Jasen Betts <ja...@xnet.co.nz> wrote:

On 2010-10-15, Michael <mrdarr...@gmail.com> wrote:

Ah ok.  I had a feeling a 555 wasn't a good choice anyway... not such
a great comparator at high frequencies?

It's good to around 1MHz,  but I'm mystified about how it's supposed
to work in this circuit

He's got a bunch of circuits here:

turns out the drawing is wrong. In his simulation he has 555 TRIG+THRS
connected to R3+C1 instead of to R3+D2 and that makes a lot more sense.

Thing is, they seem to be headphone amps.  Not sure how that would
help me since I want to take the sound output from the computer and
attach that to some speakers via a homemade amplifier.

the TC4451 used in this circuit should drive 4 ohms speakers at 20W or
so from a 12V supply that's a real 20W continuous sinewave not some
marketing hype 20W.

TC4451? Which circuit is this in?

I can't vouch for the theoretical fidelity of this amplifier. The ramp
used by comparitor is not linear and the switching frequency is
modulated by the audio input, the non-linear ramp is likely to cause
distortion the FM could cause problems too.

He seemed surprised that a basic transistor sounded better than his
second 555.

for the purists there is the 'zen' mosfet amplifier (and space heater*).
said to be very good in sound quality.
(*gets hot, needs a really big heatsink)

http://www.tubecad.com/2009/10/blog0174.htm

Good outlook on life!

"I remember reading in Harpers magazine (at least I believe it was
Harpers) a list of the top problems facing high schools back in the
'50s. At the top of the list were stomach-wrenching problems that not
even the world's most capable community organizer could hope to solve,
such as chewing gum and being in the hallways without a pass. Today’s
list boasts much more tractable problems, as befits the hugely greater
investment we make in education today and the '50s lack of a United
States Department of Education (created in 1979), problems such as
murder, rape, arson, drugs, illiteracy… Ah, indeed, those days of old
when tubes glowed bold."

Class A as a space-heater... ah, now I get it! Perfect for Russia or
Norway I guess, but it gets HOT in my neck of the woods... was in the
90s all this week. Class D it is... should extend the life of
'lectrolytic caps too...

 Second circuit:http://geekcircuits.com/2010/01/class-d-amp-made-easy-with-555-timer-ic/

ther second circuit deals with the PWM frequency problem
but not the linearity problem. this pone does that.

http://geekcircuits.com/2010/01/class-d-amp-using-555-timer/

substitute it for the 555 part of the power amplifier.

Still there's probably going to problems with noise from the powersupply
etc...  these circuits are good as a learning exercise but in the real
world they have problems.

--
ɹǝpun uʍop ɯoɹɟ sƃuıʇǝǝɹ⅁

 

[Jasen Betts]

On 2010-10-16, Michael <mrdarrett@gmail.com> wrote:

Thing is, they seem to be headphone amps.  Not sure how that would
help me since I want to take the sound output from the computer and
attach that to some speakers via a homemade amplifier.

the TC4451 used in this circuit should drive 4 ohms speakers at 20W or
so from a 12V supply that's a real 20W continuous sinewave not some
marketing hype 20W.

TC4451? Which circuit is this in?

The one with the XOR gate. this is the corrected version:
http://geekcircuits.com/wp-content/uploads/2010/02/full-bridge-correction.png

The author's discussion here.
http://geekcircuits.com/2010/02/full-bridge-class-d-amp-using-555-timer/

ther second circuit deals with the PWM frequency problem
but not the linearity problem. this pone does that.

http://geekcircuits.com/2010/01/class-d-amp-using-555-timer/

substitute it for the 555 part of the power amplifier.

Still there's probably going to problems with noise from the powersupply
etc...  these circuits are good as a learning exercise but in the real
world they have problems.

--
ɹǝpun uʍop ɯoɹɟ sƃuıʇǝǝɹ⅁

 

[Michael]

On Oct 17, 2:03 am, Jasen Betts <ja...@xnet.co.nz> wrote:

On 2010-10-16, Michael <mrdarr...@gmail.com> wrote:

Thing is, they seem to be headphone amps.  Not sure how that would
help me since I want to take the sound output from the computer and
attach that to some speakers via a homemade amplifier.

the TC4451 used in this circuit should drive 4 ohms speakers at 20W or
so from a 12V supply that's a real 20W continuous sinewave not some
marketing hype 20W.

TC4451?  Which circuit is this in?

The one with the XOR gate. this is the corrected version:http://geekcircuits.com/wp-content/uploads/2010/02/full-bridge-correc...

The author's discussion here.http://geekcircuits.com/2010/02/full-bridge-class-d-amp-using-555-timer/

Ah! I see you posted to his blog and he corrected it the next day.

Thanks!

Michael

 

[Michael]

On Oct 17, 2:03 am, Jasen Betts <ja...@xnet.co.nz> wrote:

On 2010-10-16, Michael <mrdarr...@gmail.com> wrote:

Thing is, they seem to be headphone amps.  Not sure how that would
help me since I want to take the sound output from the computer and
attach that to some speakers via a homemade amplifier.

the TC4451 used in this circuit should drive 4 ohms speakers at 20W or
so from a 12V supply that's a real 20W continuous sinewave not some
marketing hype 20W.

TC4451?  Which circuit is this in?

The one with the XOR gate. this is the corrected version:http://geekcircuits.com/wp-content/uploads/2010/02/full-bridge-correc...

The author's discussion here.http://geekcircuits.com/2010/02/full-bridge-class-d-amp-using-555-timer/

Say... is it normal to run the TC4451 mosfet driver without any
mosfets to drive?

Here's a datasheet:
http://ww1.microchip.com/downloads/en/devicedoc/21987a.pdf

It appears to have the following specs:
Continuous output current = 2.6A (not sure why this is in the "Min"
column on page 3... typo?)
4.5V to 18V operating range

Just wondering where you got the 20W continuous power range from.

Thanks,

Michael

 

[Jasen Betts]

On 2010-10-18, Michael <mrdarrett@gmail.com> wrote:

On Oct 17, 2:03 am, Jasen Betts <ja...@xnet.co.nz> wrote:
On 2010-10-16, Michael <mrdarr...@gmail.com> wrote:

Thing is, they seem to be headphone amps.  Not sure how that would
help me since I want to take the sound output from the computer and
attach that to some speakers via a homemade amplifier.

the TC4451 used in this circuit should drive 4 ohms speakers at 20W or
so from a 12V supply that's a real 20W continuous sinewave not some
marketing hype 20W.

TC4451?  Which circuit is this in?

The one with the XOR gate. this is the corrected version:http://geekcircuits.com/wp-content/uploads/2010/02/full-bridge-correc...

The author's discussion here.http://geekcircuits.com/2010/02/full-bridge-class-d-amp-using-555-timer/

Say... is it normal to run the TC4451 mosfet driver without any
mosfets to drive?

About as normal as using a 555 for an audio amplifier. (in other
words, "not really") Whilst TC4451s are not designed to drive
loudspeakers they can do it.

Just wondering where you got the 20W continuous power range from.

2.6A at 12V (the voltage on the schematic) is about 31W, but
a sine wave that peaks at 2.6A has a lower RMS current
(a factor of 0.71) and there's also losses in the 4451a and
the output filters.

my estimate of 20W might actually be a bit on the high side,

Thanks,

Michael

--
ɹǝpun uʍop ɯoɹɟ sƃuıʇǝǝɹ⅁

 

[Michael]

On Oct 18, 1:07 am, Jasen Betts <ja...@xnet.co.nz> wrote:

On 2010-10-18, Michael <mrdarr...@gmail.com> wrote:



On Oct 17, 2:03 am, Jasen Betts <ja...@xnet.co.nz> wrote:
On 2010-10-16, Michael <mrdarr...@gmail.com> wrote:

Thing is, they seem to be headphone amps.  Not sure how that would
help me since I want to take the sound output from the computer and
attach that to some speakers via a homemade amplifier.

the TC4451 used in this circuit should drive 4 ohms speakers at 20W or
so from a 12V supply that's a real 20W continuous sinewave not some
marketing hype 20W.

TC4451?  Which circuit is this in?

The one with the XOR gate. this is the corrected version:http://geekcircuits.com/wp-content/uploads/2010/02/full-bridge-correc...

The author's discussion here.http://geekcircuits.com/2010/02/full-bridge-class-d-amp-using-555-timer/
Say... is it normal to run the TC4451 mosfet driver without any
mosfets to drive?

About as normal as using a 555 for an audio amplifier. (in other
words, "not really")  Whilst TC4451s are not designed to drive
loudspeakers they can do it.

Just wondering where you got the 20W continuous power range from.

2.6A at 12V (the voltage on the schematic) is about 31W, but
a sine wave that peaks at 2.6A has a lower RMS current
(a factor of 0.71) and there's also losses in the 4451a and
the output filters.

my estimate of 20W might actually be a bit on the high side,



Thanks,

Michael

--
ɹǝpun uʍop ɯoɹɟ sƃuıʇǝǝɹ⅁

Ok I understand now. 2.6 A x 12V / sqrt(2) ~ 20A.

Thanks for all your help!

Michael

 

[transmogrifox]

On Oct 15, 2:16=A0pm, Michael <mrdarr...@gmail.com> wrote:
On Oct 15, 12:53=A0pm, Jasen Betts <ja...@xnet.co.nz> wrote:

On 2010-10-15, Michael <mrdarr...@gmail.com> wrote:

Ah ok. =A0I had a feeling a 555 wasn't a good choice anyway... no
su=
ch
a great comparator at high frequencies?

It's good to around 1MHz, =A0but I'm mystified about how it'
supposed
to work in this circuit

He's got a bunch of circuits here:

Thing is, they seem to be headphone amps. =A0Not sure how that would
help me since I want to take the sound output from the computer and
attach that to some speakers via a homemade amplifier.

He seemed surprised that a basic transistor sounded better than his
second 555. =A0Secon
circuit:http://geekcircuits.com/2010/01/class-d-amp=
-made-easy-with-555-timer-ic/


Bunch of circuits here: http://geekcircuits.com/category/class-d-amp/
still half asleep...

For something simple, I found a work-around to the shoot-through problem:
You can use 4 N-channel MOSFETs in an H-bridge. The low side MOSFETS ar
switched at a high frequency. The High Side MOSFETs are switched accordin
to the input signal polarity. This gives you a push-pull amplifie
topology.

On positive signal polarity, quadrants 1 and 3 are off, quadrant 2 is ful
on, and quadrant 4 is switching at a high frequency. The opposite is tru
on a signal negative polarity. Now the speed of the high-side switchin
transition can be tuned to the amount of crossover distortion you ca
tolerate. You may even be able to reduce the shoot-through trouble b
placing an inductor in the source of the high-side MOSFETs, thus greatl
reducing your crossover margin. That way it will only source averag
currents through the load, but will not respond to the fast pulse when th
signal is near zero, or silence.

The nice thing is this relaxes design requirements for high frequenc
switching of the high side, and reduces power handling requirements o
transistors driving those gates (though for a 1W audio amp, you don't eve
need gate drivers). Everything I can think about this method works i
favor of the designer: You don't have as much power loss due to powe
dissipated in the gate driver circuits, shoot-through is minimal and can b
tuned away, at low signal levels this can be tuned to a slight DC offset t
eliminate switching current...

Just an idea. I wish I had a schematic up as that would make it mor
clear. I haven't been able to find this mode of operation on the web..
and probably for good reasons. The concept is so old and outdated tha
people have forgotten that hobbyists may be interested in trying this. Th
thing that is popular today are the switching schemes that effectivel
double the switching frequency (like Crown's "Class I"), or other thing
you have been looking at.

A really simple ramp generator can be made with a comparator, capacitor
diodes and a few other parts. It's a classic relaxation oscillator. Th
key to making a linear ramp is to use a transistor as a constant curren
source charging the capacitor where most schematics only show a resistor.
I have made a triangle oscillator by triggering a constant current sink t
discharge the capacitor instead of simply dumping it all at once...it seem
it was only good at audio frequencies though. Depending on the comparator
this can go to several MHz as a ramp.



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