Mixing 4 audio channels to 3?
 

I want to connect the audio out (headphone jack) of 2 computer sound cards to
a desktop & woofer speaker-amp combination. (The original input was via USB
only and I'm modifying this for analog audio input.)

This is my guess at the necessary resistors to mix down these 4 outputs to
the 3 inputs in the amplifier (L, R, sub):

http://i41.tinypic.com/97mpud.jpg

The data sheet for the Philips TDA8510J amp IC shows an application for 2
channel input:

http://i40.tinypic.com/5ets9w.jpg

Suggestions welcome.

Thanks.

Mixing 4 audio channels to 3?
 

If I'm on the right track, what are the suggested values for resistors?

Thahks.

Mixing 4 audio channels to 3?
 

On Oct 26, 7:58*am, DaveC wrote:
I want to connect the audio out (headphone jack) of 2 computer sound cards to
a desktop & woofer speaker-amp combination. (The original input was via USB
only and I'm modifying this for analog audio input.)

This is my guess at the necessary resistors to mix down these 4 outputs to
the 3 inputs in the amplifier (L, R, sub):

http://i41.tinypic.com/97mpud.jpg

Lose Rs 1-4, keep 5-8, lose R11, and lose C1,2,3, then itll be good.


NT

Mixing 4 audio channels to 3?
 

On Wed, 26 Oct 2011 08:50:55 -0700 (PDT), NT
wrote:

On Oct 26, 7:58*am, DaveC wrote:
I want to connect the audio out (headphone jack) of 2 computer sound cards to
a desktop & woofer speaker-amp combination. (The original input was via USB
only and I'm modifying this for analog audio input.)

This is my guess at the necessary resistors to mix down these 4 outputs to
the 3 inputs in the amplifier (L, R, sub):

http://i41.tinypic.com/97mpud.jpg

Lose Rs 1-4, keep 5-8, lose R11, and lose C1,2,3, then itll be good.


NT

Is there something wrong with my browser? I can't see any of these
component references. Actually I see what you mean. One problem here
is that R9 and R10 are going to cause crosstalk. Without some active
electronics there is no way around that apart from taking the sub from
one channel only. This is actually quite common.

d

Mixing 4 audio channels to 3?
 

Is there something wrong with my browser? I can't see any of these
component references.

Sorry. They didn't make onto the jpg file. Added he

http://i40.tinypic.com/5ets9w.jpg

Actually I see what you mean. One problem here
is that R9 and R10 are going to cause crosstalk. Without some active
electronics there is no way around that apart from taking the sub from
one channel only. This is actually quite common.
d

OK, can do that.

Suggested values for resistors?

Thanks.

Mixing 4 audio channels to 3?
 

Lose Rs 1-4, keep 5-8, lose R11, and lose C1,2,3, then itll be good.
NT

Callouts added:

http://i40.tinypic.com/5ets9w.jpg

R11 provides some adjustment for sub. Otherwise no way to have any difference
between L & R channel volume and sub volume.

The caps are suggested on the data sheet example:

http://i40.tinypic.com/5ets9w.jpg

Why would you do away with them?

What values for the resistors do you suggest?

Thanks.

Mixing 4 audio channels to 3?
 

On Wed, 26 Oct 2011 10:10:12 -0700, DaveC wrote:

Lose Rs 1-4, keep 5-8, lose R11, and lose C1,2,3, then itll be good.
NT

Callouts added:

http://i40.tinypic.com/5ets9w.jpg

R11 provides some adjustment for sub. Otherwise no way to have any difference
between L & R channel volume and sub volume.

The caps are suggested on the data sheet example:

http://i40.tinypic.com/5ets9w.jpg

Why would you do away with them?

What values for the resistors do you suggest?

Thanks.

For that particular circuit with that chip, the caps are necessary
because those inputs are not at ground potential. If you are feeding
normal hi fi unit inputs the caps aren't needed. Resistor values
around 5 to 10 k would be what you need.

d

Mixing 4 audio channels to 3?
 

For that particular circuit with that chip, the caps are necessary
because those inputs are not at ground potential. If you are feeding
normal hi fi unit inputs the caps aren't needed. Resistor values
around 5 to 10 k would be what you need.

And the value for the potentiometer?

Thanks.

Mixing 4 audio channels to 3?
 

On Wed, 26 Oct 2011 11:18:39 -0700, DaveC wrote:

For that particular circuit with that chip, the caps are necessary
because those inputs are not at ground potential. If you are feeding
normal hi fi unit inputs the caps aren't needed. Resistor values
around 5 to 10 k would be what you need.

And the value for the potentiometer?

That depends entirely on the input impedance of the subwoofer. You
could try a 100k pot and connect it as a normal volume control - one
end to the incoming signal, the other end to ground and the slider to
the subwoofer.

d

Mixing 4 audio channels to 3?
 

That depends entirely on the input impedance of the subwoofer. You
could try a 100k pot and connect it as a normal volume control - one
end to the incoming signal, the other end to ground and the slider to
the subwoofer.

d

The data sheet says that the 2 single-ended inputs (R & L) are 50K impedance.
The bridge-tied load ("BTL") input -- used for the sub -- is 25K impedance.

Does this suggest any change to your recommendation of a 100K pot tied to
ground?

Thanks.

Mixing 4 audio channels to 3?
 

On Wed, 26 Oct 2011 11:52:49 -0700, DaveC wrote:

That depends entirely on the input impedance of the subwoofer. You
could try a 100k pot and connect it as a normal volume control - one
end to the incoming signal, the other end to ground and the slider to
the subwoofer.

d

The data sheet says that the 2 single-ended inputs (R & L) are 50K impedance.
The bridge-tied load ("BTL") input -- used for the sub -- is 25K impedance.

Does this suggest any change to your recommendation of a 100K pot tied to
ground?

Thanks.

No, that would be about right.

d

Mixing 4 audio channels to 3?
 

Mixing 4 audio channels to 3?
 

On Oct 26, 5:09*pm, (Don Pearce) wrote:
On Wed, 26 Oct 2011 08:50:55 -0700 (PDT), NT
wrote:

On Oct 26, 7:58*am, DaveC wrote:
I want to connect the audio out (headphone jack) of 2 computer sound cards to
a desktop & woofer speaker-amp combination. (The original input was via USB
only and I'm modifying this for analog audio input.)

This is my guess at the necessary resistors to mix down these 4 outputs to
the 3 inputs in the amplifier (L, R, sub):

http://i41.tinypic.com/97mpud.jpg

Lose Rs 1-4, keep 5-8, lose R11, and lose C1,2,3, then itll be good.

NT

Is there something wrong with my browser? I can't see any of these
component references. Actually I see what you mean. One problem here
is that R9 and R10 are going to cause crosstalk. Without some active
electronics there is no way around that apart from taking the sub from
one channel only. This is actually quite common.

d

I was hinting that the OP might decide to add numbers to the
components, otherwise its sufficiently tedious to talk about them that
people often just wont bother.

Yes the caps are necessary, but for the amp IC, not for the mixer.
House them together and its the same difference, house them apart and
the distiction matters.


NT

Mixing 4 audio channels to 3?
 

Yes the caps are necessary, but for the amp IC, not for the mixer.
House them together and its the same difference, house them apart and
the distiction matters.
NT

This is unclear to me.

Do you mean that if the mixer resistor matrix is housed close to the amp IC
that the capacitors won't be needed? And if they are separated and connected
by shielded audio cables that the caps will be needed?

Thanks.

Mixing 4 audio channels to 3?
 

Latest rev:

http://i40.tinypic.com/300feaq.jpg

All resistors 5K-10K. Pot 100K.

Comments welcome.

Thanks.

Mixing 4 audio channels to 3?
 

On Oct 27, 5:17*am, DaveC wrote:
Yes the caps are necessary, but for the amp IC, not for the mixer.
House them together and its the same difference, house them apart and
the distiction matters.
NT

This is unclear to me.

Do you mean that if the mixer resistor matrix is housed close to the amp IC
that the capacitors won't be needed? And if they are separated and connected
by shielded audio cables that the caps will be needed?

Thanks.

Nothing like that, no. The amp IC requires the caps, so theyre needed,
no question about it. But theyre needed for the amp, not for mixing
purposes, so if the 2 circuits can be disconnected by the end user,
then the caps need to be with the IC amp part, not the mixer part.


NT

Mixing 4 audio channels to 3?
 

DaveC writes:

If I'm on the right track, what are the suggested values for resistors?

10 kohms for all fixed resistor should work...


--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at
http://www.epanorama.net/

Mixing 4 audio channels to 3?
 

On Wed, 26 Oct 2011 21:17:16 -0700, DaveC wrote:

Yes the caps are necessary, but for the amp IC, not for the mixer.
House them together and its the same difference, house them apart and
the distiction matters.
NT

This is unclear to me.

Do you mean that if the mixer resistor matrix is housed close to the amp IC
that the capacitors won't be needed? And if they are separated and connected
by shielded audio cables that the caps will be needed?

Thanks.

---
Since the chip is being powered by single positive DC supply, The
only way to get signals which swing below ground into and out of the
chip is to use capacitive coupling.

--
JF

Mixing 4 audio channels to 3?
 

"John Fields" wrote in message
...
On Wed, 26 Oct 2011 21:17:16 -0700, DaveC wrote:

Yes the caps are necessary, but for the amp IC, not for the mixer.
House them together and its the same difference, house them apart and
the distiction matters.
NT

This is unclear to me.

Do you mean that if the mixer resistor matrix is housed close to the amp
IC
that the capacitors won't be needed? And if they are separated and
connected
by shielded audio cables that the caps will be needed?

Thanks.

---
Since the chip is being powered by single positive DC supply, The
only way to get signals which swing below ground into and out of the
chip is to use capacitive coupling.

Or, as is not uncommonly done these days in portable digital players,
establish a floating ground for the purpose of providing and signal for the
(floating) load, so that the difference between the output and the floating
ground can become negative.

Mixing 4 audio channels to 3?
 

Or, as is not uncommonly done these days in portable digital players,
establish a floating ground for the purpose of providing and signal for the
(floating) load, so that the difference between the output and the floating
ground can become negative.

Which raises the question I was pondering recently: is it OK to connect 2
computers' sound cards' grounds together?

The PS for the powered speakers is via "wall wart" (not grounded).

Thanks.

Mixing 4 audio channels to 3?
 

DaveC wrote:
Or, as is not uncommonly done these days in portable digital players,
establish a floating ground for the purpose of providing and signal for the
(floating) load, so that the difference between the output and the floating
ground can become negative.

Which raises the question I was pondering recently: is it OK to connect 2
computers' sound cards' grounds together?


It should be. If you are concerned, check with a meter first.

The PS for the powered speakers is via "wall wart" (not grounded).

Thanks.


--
Les Cargill

Mixing 4 audio channels to 3?
 

On Thu, 27 Oct 2011 09:11:17 -0700, DaveC wrote:

Or, as is not uncommonly done these days in portable digital players,
establish a floating ground for the purpose of providing and signal for the
(floating) load, so that the difference between the output and the floating
ground can become negative.

Which raises the question I was pondering recently: is it OK to connect 2
computers' sound cards' grounds together?

No. You will certainly create a ground loop which will result in mains
hum.

d

Mixing 4 audio channels to 3?
 

On Thu, 27 Oct 2011 17:59:59 GMT, (Don Pearce) wrote:

On Thu, 27 Oct 2011 09:11:17 -0700, DaveC wrote:

Or, as is not uncommonly done these days in portable digital players,
establish a floating ground for the purpose of providing and signal for the
(floating) load, so that the difference between the output and the floating
ground can become negative.

Which raises the question I was pondering recently: is it OK to connect 2
computers' sound cards' grounds together?

No. You will certainly create a ground loop which will result in mains
hum.

This is certainly likely but not a foregone conclusion. In any case, it's
"OK" (i.e. not dangerous). As long as the computers are plugged into the same
circuit, it's quite possible that he can get away with it, assuming line
levels. If hum is a problem, coupling capacitors or cheap audio transformers
will usually solve it.

Mixing 4 audio channels to 3?
 

is it OK to connect 2 computers' sound cards' grounds together?

No. You will certainly create a ground loop which will result in mains
hum.
d

This kinda kills the project, doesn't it? (At least, the straightforward
solution.)

The alternative is...?

Thanks.

Mixing 4 audio channels to 3?
 

On Thu, 27 Oct 2011 11:12:00 -0700, DaveC wrote:

is it OK to connect 2 computers' sound cards' grounds together?

No. You will certainly create a ground loop which will result in mains
hum.
d

This kinda kills the project, doesn't it? (At least, the straightforward
solution.)

The alternative is...?


Transformers. You can buy decent ones in Maplin for use in car audio.
Capacitors have been suggested, but they don't help. If they are big
enough to pass bass, the ground loop is still there. It doesn't
require a DC connection to function.

d

Mixing 4 audio channels to 3?
 

On Oct 27, 5:11*pm, DaveC wrote:
Or, as is not uncommonly done these days in portable digital players,
establish a floating ground for the purpose of providing and signal for the
(floating) load, so that the difference between the output and the floating
ground can become negative.

Which raises the question I was pondering recently: is it OK to connect 2
computers' sound cards' grounds together?

The PS for the powered speakers is via "wall wart" (not grounded).

Thanks.

I've yet to have a problem doing it.


NT

Mixing 4 audio channels to 3?
 

10 kohms for all fixed resistor should work...

Hi Tomi. Thank you for your advice. I have enjoyed your creative designs for
many years.

Do you have any comments on my revisions of the circuit (ie, are caps
recommended? should I be concerned about cross talk if I take resistors from
all 4 channels to drive the sub woofer? etc...)

Cheers,
Dave

Mixing 4 audio channels to 3?
 

On Sat, 29 Oct 2011 00:49:32 -0700, DaveC wrote:

10 kohms for all fixed resistor should work...

Hi Tomi. Thank you for your advice. I have enjoyed your creative designs for
many years.

Do you have any comments on my revisions of the circuit (ie, are caps
recommended? should I be concerned about cross talk if I take resistors from
all 4 channels to drive the sub woofer? etc...)

Cheers,
Dave

---
Why not go active and get rid of the caps altogether?


..LA-+---[10k]-+-[10k]-+------------- -(LA+LB)
.. | | |
.. | | +V |
.. | | | |
..LB-|-+-[10k]-+--|-\ |
.. | | | -+ 10K
.. | | +--|+/ +-[POT]-+
.. | | | | | |
.. | | GND -V | +5 |
.. | | | | |
.. | +------[10K]---+---+-|-\ |
.. | | | --+-- -(LA+LB+RA+RB)
.. +--------[10K]---+ +-|+/
.. | |
.. +--------[10K]---+ GND
.. | |
.. | +------[10K]---+
.. | |
.. | |
.. | |
..RA-+-+-[10k]-+-[10k]-+
.. | | |
.. | | +V |
.. | | | |
..RB-+---[10k]-+--|-\ |
.. | -+------------- -(RA+RB)
.. +--|+/
.. | |
.. GND -V

Seems to work pretty well...

Version 4
SHEET 1 880 968
WIRE 112 -544 48 -544
WIRE 224 -544 192 -544
WIRE -96 -448 -528 -448
WIRE 32 -448 -16 -448
WIRE 48 -448 48 -544
WIRE 48 -448 32 -448
WIRE 112 -448 48 -448
WIRE 224 -432 224 -544
WIRE 224 -432 176 -432
WIRE 256 -432 224 -432
WIRE 112 -416 80 -416
WIRE -96 -352 -432 -352
WIRE 32 -352 32 -448
WIRE 32 -352 -16 -352
WIRE 80 -352 80 -416
WIRE -528 -240 -528 -448
WIRE -416 -240 -528 -240
WIRE 64 -240 -336 -240
WIRE 112 -240 64 -240
WIRE 240 -240 192 -240
WIRE -432 -192 -432 -352
WIRE -352 -192 -432 -192
WIRE 64 -192 64 -240
WIRE 64 -192 -272 -192
WIRE -272 -144 -336 -144
WIRE 64 -144 64 -192
WIRE 64 -144 -192 -144
WIRE -176 -96 -240 -96
WIRE 64 -96 64 -144
WIRE 64 -96 -96 -96
WIRE 128 -96 64 -96
WIRE 240 -80 240 -240
WIRE 240 -80 192 -80
WIRE 272 -80 240 -80
WIRE 128 -64 96 -64
WIRE 96 0 96 -64
WIRE 112 112 48 112
WIRE 224 112 192 112
WIRE -336 208 -336 -144
WIRE -96 208 -336 208
WIRE 32 208 -16 208
WIRE 48 208 48 112
WIRE 48 208 32 208
WIRE 112 208 48 208
WIRE 224 224 224 112
WIRE 224 224 176 224
WIRE 256 224 224 224
WIRE 112 240 80 240
WIRE -240 304 -240 -96
WIRE -96 304 -240 304
WIRE 32 304 32 208
WIRE 32 304 -16 304
WIRE 80 304 80 240
WIRE -528 688 -528 -240
WIRE -432 688 -432 -192
WIRE -336 688 -336 208
WIRE -240 688 -240 304
WIRE -144 688 -144 656
WIRE -48 688 -48 656
WIRE -528 832 -528 768
WIRE -432 832 -432 768
WIRE -432 832 -528 832
WIRE -336 832 -336 768
WIRE -336 832 -432 832
WIRE -240 832 -240 768
WIRE -240 832 -336 832
WIRE -144 832 -144 768
WIRE -144 832 -240 832
WIRE -48 832 -48 768
WIRE -48 832 -144 832
WIRE -528 928 -528 832
FLAG 144 192 +5
FLAG 144 256 -5
FLAG 80 304 0
FLAG 144 -464 +5
FLAG 144 -400 -5
FLAG 80 -352 0
FLAG -528 928 0
FLAG -144 656 +5
FLAG -48 656 -5
FLAG 160 -112 +5
FLAG 160 -48 -5
FLAG 96 0 0
SYMBOL Opamps\\LT1007 144 160 R0
SYMATTR InstName U1
SYMBOL voltage -240 672 R0
WINDOW 3 24 96 Invisible 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V3
SYMATTR Value SINE(0 1 1000)
SYMBOL res 0 192 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value 10k
SYMBOL res 0 288 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 10k
SYMBOL res 208 96 R90
WINDOW 0 -43 58 VBottom 2
WINDOW 3 -37 60 VTop 2
SYMATTR InstName R3
SYMATTR Value 10k
SYMBOL Opamps\\LT1007 144 -496 R0
SYMATTR InstName U2
SYMBOL res 0 -464 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R4
SYMATTR Value 10k
SYMBOL res 0 -368 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R5
SYMATTR Value 10k
SYMBOL res 208 -560 R90
WINDOW 0 -43 58 VBottom 2
WINDOW 3 -37 60 VTop 2
SYMATTR InstName R6
SYMATTR Value 10k
SYMBOL voltage -48 784 R180
WINDOW 0 -50 102 Left 2
WINDOW 3 24 16 Left 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 5
SYMBOL voltage -144 672 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value 5
SYMBOL res -80 -112 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R7
SYMATTR Value 10k
SYMBOL res -176 -160 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R8
SYMATTR Value 10k
SYMBOL res -256 -208 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R9
SYMATTR Value 10k
SYMBOL res -320 -256 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R10
SYMATTR Value 10k
SYMBOL Opamps\\LT1007 160 -144 R0
SYMATTR InstName U3
SYMBOL res 208 -256 R90
WINDOW 0 -43 58 VBottom 2
WINDOW 3 -37 60 VTop 2
SYMATTR InstName R12
SYMATTR Value 5k
SYMBOL voltage -336 672 R0
WINDOW 3 24 96 Invisible 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V4
SYMATTR Value SINE(0 1 1700)
SYMBOL voltage -432 672 R0
WINDOW 3 24 96 Invisible 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V5
SYMATTR Value SINE(0 1 2300)
SYMBOL voltage -528 672 R0
WINDOW 3 24 96 Invisible 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V6
SYMATTR Value SINE(0 1 4260)
TEXT -562 952 Left 2 !.tran .005


--
JF

Mixing 4 audio channels to 3?
 

Why not go active and get rid of the caps altogether?

Thanks John. Good simple design.

Can you suggest a good op amp for such an audio application?

Is it possible to use single-supply amps throughout?

Thanks,
Dave

Mixing 4 audio channels to 3?
 

On Sun, 30 Oct 2011 02:26:06 -0700, DaveC wrote:

Why not go active and get rid of the caps altogether?

Thanks John. Good simple design.

Can you suggest a good op amp for such an audio application?

---
I used an LT1007 in the simulation.

Did you run it?
---

Is it possible to use single-supply amps throughout?

---
Yes, but you'd have to use caps on the inputs and outputs and
reference the opamp + inputs to Vcc/2.

--
JF

Mixing 4 audio channels to 3?
 

Can you suggest a good op amp for such an audio application?

I used an LT1007 in the simulation.

Did you run it?

No tools.

I made the L & R op amps fixed gain and added input level control pots:

http://i43.tinypic.com/2hh3y12.jpg

Comments?

Thanks.

Mixing 4 audio channels to 3?
 

http://i43.tinypic.com/2hh3y12.jpg

Should all pots be log taper?

Thanks.

Mixing 4 audio channels to 3?
 

I used an LT1007 in the simulation.

Can't find them on Mouser.

Suggest another one that's good for audio?

How about LM833? ::

http://www.national.com/ds/LM/LM833.pdf

The SR and GBW beat the 1007... ;-)

Thanks.

Mixing 4 audio channels to 3?
 

Is the type of resistor important (wire wound, etc.)?

Thanks.

Mixing 4 audio channels to 3?
 

On Sun, 30 Oct 2011 21:44:33 -0700, DaveC wrote:

Can you suggest a good op amp for such an audio application?

I used an LT1007 in the simulation.

Did you run it?

No tools.
---
They're free:

http://www.linear.com/designtools/software/
---


I made the L & R op amps fixed gain and added input level control pots:

http://i43.tinypic.com/2hh3y12.jpg

Comments?

---
Looks OK.


--
JF

Mixing 4 audio channels to 3?
 

On Sun, 30 Oct 2011 21:47:34 -0700, DaveC wrote:

http://i43.tinypic.com/2hh3y12.jpg

Should all pots be log taper?

---
The input pots, OK. The feedback pot, probably not.

Plot the change in output voltage as a function of wiper position to
get a better handle on it.

--
JF

Mixing 4 audio channels to 3?
 

On Mon, 31 Oct 2011 00:16:15 -0700, DaveC wrote:

I used an LT1007 in the simulation.

Can't find them on Mouser.

---
You can buy them directly from Linear:

http://www.linear.com/purchase/LT1007
---

Suggest another one that's good for audio?

How about LM833? ::

http://www.national.com/ds/LM/LM833.pdf

The SR and GBW beat the 1007... ;-)

---
Go for it; they're a lot cheaper!

--
JF

Mixing 4 audio channels to 3?
 

On Mon, 31 Oct 2011 01:11:57 -0700, DaveC wrote:

Is the type of resistor important (wire wound, etc.)?

---
Don't use wirewound.

5% 1/4 watt carbon film is fine.

--
JF

Mixing 4 audio channels to 3?
 

Is it possible to use single-supply amps throughout?

---
Yes, but you'd have to use caps on the inputs and outputs and
reference the opamp + inputs to Vcc/2.
[J. Fields]

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

http://i41.tinypic.com/2vlo2t2.jpg

I've added input & output caps. Are these values sound? ;-)

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?

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

Thanks,
Dave

Mixing 4 audio channels to 3?
 

In article ,
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:

http://i41.tinypic.com/2vlo2t2.jpg

I've added input & output caps. Are these values sound? ;-)

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?

If you want to do it with just resistors, you could make R16 and R17
somewhere around 4k7 each, and add a few uF of bypass capacitance
around one or both resistors. 1k would give a stiffer reference if
you don't mind the additional power consumption.

If you can spare an op-amp section, you can get a better (stiffer) ground
reference than you'll get with just resistors, with lower power
dissipation (I think). Use R16 and R17, and a small cap from the
junction point, to create a Vcc/2 reference, but do not "ground" this
directly to your internal reference point (the rectangular-looking
ground symbol). Instead, feed this to the noninverting input of an
op-amp section, feed the op-amp output back to the inverting input
(i.e. create a unity-gain follower), and use the op amp's output as
your ground reference. In this arrangement R16 and R17 can be
high-value (100k?) as their junction point will be looking into a
high-impedance op amp input.

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 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.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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