Mixing 4 audio channels to 3?



I've yet to have a problem doing it.
NT
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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
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snipped-for-privacy@spam.com (Don Pearce) writes:

I don't see component references, either. However, assuming R1-R4 are the resistors from the inputs to ground, they may be necessary to provide a proper impedance to the source driving them.
To reduce crosstalk, I'd replace the two resistors from the 220nF cap junctions to the pot with four resistors from each of the four inputs to the pot. Presumably, the output impedance of the amps is low compared to the resistors reducing crosstalk.
For mixing signals there is a method using amps and feedback where the connection between multiple sources is at a virtual ground, eliminating crosstalk.
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Sorry. They didn't make onto the jpg file. Added here:
<
http://i40.tinypic.com/5ets9w.jpg

OK, can do that.
Suggested values for resistors?
Thanks.
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If I'm on the right track, what are the suggested values for resistors?
Thahks.
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10 kohms for all fixed resistor should work...
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Tomi Engdahl (http://www.iki.fi/then /)
Take a look at my electronics web links and documents at
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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
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--
Why not go active and get rid of the caps altogether?


.LA>-+---[10k]-+-[10k]-+-------------> -(LA+LB)
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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
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--
I used an LT1007 in the simulation.

Did you run it?
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Is the type of resistor important (wire wound, etc.)?
Thanks.
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Don't use wirewound.

5% 1/4 watt carbon film is fine.
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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.
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You can buy them directly from Linear:

http://www.linear.com/purchase/LT1007
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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.
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They're free:

http://www.linear.com/designtools/software /
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Should all pots be log taper?
Thanks.
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The input pots, OK. The feedback pot, probably not.

Plot the change in output voltage as a function of wiper position to
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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
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DaveC <newsgroups> wrote:

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.

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 < snipped-for-privacy@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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