Mixing 4 audio channels to 3?

[Dave Platt]

In article <0001HW.CADEF1F900A742BFB02069DF@news.eternal-september.org>,
DaveC <newsgroups> wrote:

Only a single supply (in the amplified speakers) is available to power this
circuit. I can tap this supply for my circuit:

One cautionary note with regards to this circuit, as drawn: be careful
when you wire up R15. If you use a standard three-terminal
potentiometer, make sure that you wire both the wiper, and one of the
two ends to U3's inverting input. Don't just wire up the wiper!

The reason: pots occasionally go "open" due to dirt or wear. If you
have only the wiper connected, and it goes open, you'll have no
feedback path around U3, and it'll immediately and enthusiastically
slam its output against one of the rails (or both in rapid succession
if there's a signal present). This will let out a really unholy
THWOMP from your subwoofer, and may pop the cone out of the cabinet or
at last shove the voice coil out of the gap. Expensive damage.

With a three-terminal wire-up, the resistance in this part of the
feedback loop will never be more than the bulk value of the pot (i.e.
open wiper == wiper all the way at one end) and this will limit the
maximum subwoofer volume. You can choose the maximum loudness by
setting the value of the pot.

--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
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[Stuart]

In article <0001HW.CADEF1F900A742BFB02069DF@news.eternal-september.org>,
DaveC <invalid@invalid.net> wrote:

How do I go about getting a 1/2 Vcc ground reference? (See my non-EE
attempt.) What values to use for the divider resistors?

The easiest way is to take two equal value resistors connected in series
across your supply (as you have done R16/17). The centre point is then
connected to the + input of another op-amp which has its output strapped
back to its input - terminal. this forms a voltage follower and the output
is your Vcc/2 reference. This provides a low impedance source and the
resistors can be almost any value you like - anything between 10k and 1M
say.

--
Stuart Winsor

Only plain text for emails
http://www.asciiribbon.org

 

[DaveC]

Latest rev:

<http://i40.tinypic.com/35m026h.jpg>

What needs to be reference to the new "ground"? Everything between the
input
caps and output caps?

Pretty much... each of the op amps' noninverting inputs, and the
"bottom ends" of the potentiometers, as you have drawn them. *NOT*
the V- input to the op amp(s), of course.

You do mean each of the op amps' *inverting* inputs, yes?

You might want to add "pop preventer" resistors at the inputs and
outputs... say, 100k to DC ground, from the "outside" end of each of
the DC-blocking capacitors.

Is this what you mean (see link)?

Thanks.

 

[John Fields]

On Tue, 8 Nov 2011 21:07:44 -0800, DaveC <invalid@invalid.net> wrote:

Latest rev:

http://i40.tinypic.com/35m026h.jpg

What needs to be reference to the new "ground"? Everything between the
input
caps and output caps?

Pretty much... each of the op amps' noninverting inputs, and the
"bottom ends" of the potentiometers, as you have drawn them. *NOT*
the V- input to the op amp(s), of course.

You do mean each of the op amps' *inverting* inputs, yes?

---
No.

Your drawing is wrong; signal goes to the inverting (-) inputs and the
Vcc/2 reference goes to the non-inverting (+) inputs.

--
JF

 

[DaveC]

Your drawing is wrong; signal goes to the inverting (-) inputs and the
Vcc/2 reference goes to the non-inverting (+) inputs.
JF

Thanks guys. Fixed:

<http://i44.tinypic.com/r1k8qa.jpg>

All else looks good?

Are cap values reasonable? I added C8 & C9 out of habit of seeing in other
designs. Values for these?

Thanks.

 

[DaveC]

You still have the signal go to the non-inverting input.

The current version of the drawing has signal going to the inverting input

Are cap values reasonable? I added C8 & C9 out of habit of seeing in other
designs. Values for these?

I worry that the RC time constant would have the reference be not at the
1/2 way point while C8 charges on powerup.

Suggestions?

I don't see a C9.

That means you're not looking at the right version of the drawing. Copy &
paste this into a browser:

<http://i44.tinypic.com/r1k8qa.jpg>

Might do something not so good to the subwoofer.
[M. Moroney]

Suggestions?

Thanks.

 

[Don Pearce]

On Wed, 9 Nov 2011 09:07:15 -0800, DaveC <invalid@invalid.net> wrote:

Your drawing is wrong; signal goes to the inverting (-) inputs and the
Vcc/2 reference goes to the non-inverting (+) inputs.
JF

Thanks guys. Fixed:

http://i44.tinypic.com/r1k8qa.jpg

All else looks good?

Are cap values reasonable? I added C8 & C9 out of habit of seeing in other
designs. Values for these?

Thanks.

What does U4 do?

d

 

[Dave Platt]

In article <0001HW.CADFF34300E39070B038C9DF@news.eternal-september.org>,
DaveC <newsgroups> wrote:

Your drawing is wrong; signal goes to the inverting (-) inputs and the
Vcc/2 reference goes to the non-inverting (+) inputs.

Agreed.

Thanks guys. Fixed:

http://i44.tinypic.com/r1k8qa.jpg

All else looks good?

Are cap values reasonable? I added C8 & C9 out of habit of seeing in other
designs. Values for these?

I'd eliminate C9. Some op amps aren't able to drive capacitive loads
without exhibiting instability.

If you do want some noise reduction on your reference, I'd add a
small decoupling resistor (say, 47R) between U4 and C9, and perhaps
use another .1 uF for C9. If you're using a good low-noise op amp,
you can probably just omit the filtering here and feed U4's output
directly to your "common".

I'd also recommend decoupling your 16-volt power supply, with a .1 uF
located as close as practical to the V+/V- pins of each op amp.

Remember to get the polarities of C1-C7 correct when you install them
(+ to the op-amp side, - to the outside world).

--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

 

[Michael Moroney]

DaveC <invalid@invalid.net> writes:

Your drawing is wrong; signal goes to the inverting (-) inputs and the
Vcc/2 reference goes to the non-inverting (+) inputs.
JF

Thanks guys. Fixed:

http://i44.tinypic.com/r1k8qa.jpg

All else looks good?

You still have the signal go to the non-inverting input. The way the
schematic is, U1-U3 will throw their output hard to a rail or oscillate
with positive feedback from R13-R15.

Are cap values reasonable? I added C8 & C9 out of habit of seeing in other
designs. Values for these?

I worry that the RC time constant would have the reference be not at the
1/2 way point while C8 charges on powerup. I don't see a C9. Might do
something not so good to the subwoofer.

 

[DaveC]

I'd eliminate C9. Some op amps aren't able to drive capacitive loads
without exhibiting instability.

OK, done.

If you're using a good low-noise op amp,
you can probably just omit the filtering here and feed U4's output
directly to your "common".

I'd also recommend decoupling your 16-volt power supply, with a .1 uF
located as close as practical to the V+/V- pins of each op amp.

Sound like basic good advice.

Remember to get the polarities of C1-C7 correct when you install them
(+ to the op-amp side, - to the outside world).

I presumed that such coupling caps should be non-polar. No?

Thanks.

 

[Bob E.]

What does U4 do?

Provides a Vcc/2 local "ground" so I can use these op amps with a single
supply voltage.

 

[DaveC]

I'd also recommend decoupling your 16-volt power supply, with a .1 uF
located as close as practical to the V+/V- pins of each op amp.

Since the V- pin is already PS ground, I need decouple caps only on the V+
pins, yes?

 

[DaveC]

with 1uF each. Replace C8 with 220uF, and
omit U4 & C9 entirely.

Leave C5-7 as is?

You dont want to use a 50k pot followed by a 10k load (R5-12).

Teach this man to fish: why don't I want to use 50K pot & 10K load
combination?

I'd go
with 10k pots and 100k for R5-12, adjusting the nfb Rs accordingly.
NT

"adjusting" means replace those with 100K's also?

Thanks.

 

[Don Pearce]

On Wed, 9 Nov 2011 12:10:19 -0800, Bob E. <bespoke@invalid.tv> wrote:

What does U4 do?

Provides a Vcc/2 local "ground" so I can use these op amps with a single
supply voltage.

More useful to let it oscillate as a square wave generator at 100kHz
or so, and rectify the output into a negative 15V rail. That way you
can run the op amps the way they are meant to be run.

d

 

[NT]

On Nov 9, 5:07 pm, DaveC <inva...@invalid.net> wrote:

Your drawing is wrong; signal goes to the inverting (-) inputs and the
Vcc/2 reference goes to the non-inverting (+) inputs.
JF

Thanks guys. Fixed:

http://i44.tinypic.com/r1k8qa.jpg

All else looks good?

Are cap values reasonable? I added C8 & C9 out of habit of seeing in other
designs. Values for these?

Thanks.

First you can replace C1-5 with 1uF each. Replace C8 with 220uF, and
omit U4 & C9 entirely.
You dont want to use a 50k pot followed by a 10k load (R5-12). I'd go
with 10k pots and 100k for R5-12, adjusting the nfb Rs accordingly.


NT

 

[DaveC]

More useful to let it oscillate as a square wave generator at 100kHz
or so, and rectify the output into a negative 15V rail. That way you
can run the op amps the way they are meant to be run.

d

Suggest a circuit...?

Thanks.

 

[Dave Platt]

In article <0001HW.CAE01DBF00081152B01849DF@news.eternal-september.org>,
DaveC <newsgroups> wrote:

Remember to get the polarities of C1-C7 correct when you install them
(+ to the op-amp side, - to the outside world).

I presumed that such coupling caps should be non-polar. No?

No need for that. You're going to have an 8-volt bias sitting on each
cap (half of your supply voltage), and the audio signals that they see
will only be a volt or two, peak-to-peak, so the caps will always be
polarized in the direction I indicated.

It's entirely usual and standard practice to use polar electrolytics
in this sort of situation. If you want to get fancy I'm sure you
could find an exotic 'lytic (like one of the new solid-electrolyte
types), but I see no need for that in this application.

You *could* use nonpolar 'litics if you have them around, but as
they're usually more expensive I don't see the point.

--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

 

[Dave Platt]

I'd also recommend decoupling your 16-volt power supply, with a .1 uF
located as close as practical to the V+/V- pins of each op amp.

Since the V- pin is already PS ground, I need decouple caps only on the V+
pins, yes?

Good practics is to put the bypass caps as close to the IC leads as is
practical, and run short traces (or wires) to the IC pins.

I wasn't suggesting one bypass cap from V+ to ground and another from
V- to ground... since you're using a single-sided supply and V- is DC
ground, that would be redundant (as you have noted).

--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

 

[Stuart]

In article <4ebae303.100219082@news.eternal-september.org>,
Don Pearce <spam@spam.com> wrote:

Provides a Vcc/2 local "ground" so I can use these op amps with a single
supply voltage.

More useful to let it oscillate as a square wave generator at 100kHz
or so, and rectify the output into a negative 15V rail. That way you
can run the op amps the way they are meant to be run.

KISS

--
Stuart Winsor

Only plain text for emails
http://www.asciiribbon.org

 

[Stuart]

In article <3quqo8-5e3.ln1@radagast.org>,
Dave Platt <dplatt@radagast.org> wrote:

I'd eliminate C9. Some op amps aren't able to drive capacitive loads
without exhibiting instability.

It's largely redundant anyway.

--
Stuart Winsor

Only plain text for emails
http://www.asciiribbon.org