Mixing 4 audio channels to 3?

[DaveC]

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

 

[DaveC]

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.

 

[DaveC]

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

Should all pots be log taper?

Thanks.

 

[DaveC]

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.

 

[DaveC]

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

Thanks.

 

[DaveC]

Is it possible to use single-supply amps throughout?

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

 

[DaveC]

Latest rev:

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.

 

[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

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:

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

Suggestions?

Thanks.

 

[Don Pearce]

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

 

[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]

First you can replace C1-[4?] 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]

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

 

[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.

 

[DaveC]

I was going to suggest that, since the mixer is going to be external

to the amp, he use a couple of wall-warts to get the dual supplies,
but I like your solution a lot better.

Enough to suggest a nice circuit? ;-)

Thanks,
Dave

 

[DaveC]

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.
...
Dave P.

I'm learnin'! Thanks for the explanation. I'll use standard aluminum 'lytics
here, connected as noted.

Dave C.

 

[[email protected]]

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

Hint: Don't use the chassis ground symbol as a "Vref" symbol. It's confusing
and someone along the line might get hurt. Grounds should be.

 

[DaveC]

Hint: Don't use the chassis ground symbol as a "Vref" symbol. It's
confusing

and someone along the line might get hurt. Grounds should be.

Yeah, I wasn't sure about that. How should I have indicated 2 separate
grounds?

Thanks.