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.
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
Hi Tomi. Thank you for your advice. I have enjoyed your creative designs for
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...)
Only a single supply (in the amplified speakers) is available to power this
circuit. I can tap this supply for my circuit:
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?
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 < firstname.lastname@example.org> AE6EO
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