[sdiy] Audio mixers

David dingebre at 3dphysics.net
Fri Jun 17 09:38:42 CEST 2011


Like I really know anything...

I've been reading Douglas Self's book, "Small Signal Audio Design". He has
three rules for gain structures:

1. Don't amplify then attenuate.
2. Don't attenuate then amplify.
3. The signal should be raised to the nominal internal level as soon as
possible to minimize contamination with circuit noise.

He has a whole chapter on mixer architectures. I've liked his writing style
so far. I would recommend a look in this book for some excellent ideas on
designing a good, low noise mixer.

David

-- -----Original Message-----
-- From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
-- bounces at dropmix.xs4all.nl] On Behalf Of Tom Wiltshire
-- Sent: Friday, June 17, 2011 12:51 AM
-- To: stew
-- Cc: synth-diy at dropmix.xs4all.nl
-- Subject: Re: [sdiy] Audio mixers
-- 
-- I like the way that Stewart's plan works. But it puts more active
components in the audio
-- path, which is generally something you're trying to reduce for hi-fi,
since distortion will
-- build up. So there's a trade-off here, as ever. Do you use a buffer, in
which case you can
-- lower the summing node resistors and the mixer noise, but add any
distortion from the
-- buffer, or go for the simpler mixer with larger resistors and take the
noise hit instead?
-- 
-- In practice modern op-amps are so quiet, it's probably 'measurable
differences' rather than
-- 'perceptible differences'.
-- 
-- But it's very instructive to look at the schematics of mixing desks. They
typically use a lot
-- more op-amps than I was expecting to see in the audio path, though
there's very little you
-- don't recognize ("ooh, buffer!" "mixer!" "tone control!" etc).
-- There's some good ones at:
-- 
-- http://www.ddaconsoles.com/
-- 
-- T.
-- 
-- 
-- On 17 Jun 2011, at 03:25, stew wrote:
-- 
-- > If you want a low noise mixer you'd be best off using op amp buffers
for each input with
-- a 100k input resistor to ground to set the input impedance then follow
these by 5k or 10k
-- voltage divider pots and 10k summing resistors. The lower value resistors
will reduce the
-- noise. Also use low noise op amps like the LM833 or NE5532, and the
associated coupling
-- capacitors required to combat the offset voltage caused by the bipolar
input stages of the
-- op amps.
-- >
-- > Regards,
-- > Stewart.
-- >
-- >
-- >> Chris Muir <cbm at well.com> wrote:
-- >>
-- >>
-- >> I believe that he is talking about removing the summing resistors from
-- >> the sum node (aka mix point). I agree that in an ideal world that
might
-- >> give the lowest noise, but it is none too practical in many cases. You
-- >> would have to run your sum node around on potentially longish wires to
-- >> hit all the switches.
-- >>
-- >> Here's a thought experiment using a typical line mixer without a
master
-- >> gain like he shows in his figure 4. the way you get a system like this
-- >> the quietest is generally to turn all the channels down. What does
that
-- >> do in most circuits? It grounds the resistor going to the sum node
-- >> because the bottom of the pot is at ground. The only thing grounding
the
-- >> input with a switch does is make both sides of the level pot sit at
-- >> ground.
-- >>
-- >> I think this article
-- >> On Jun 16, 2011, at 2:30 PM, Richie Burnett wrote:
-- >>
-- >>>> Another question I should have put in my first email.  If I am using
-- >> a
-- >>>> switch to remove a particular input from the mix, route it
-- >>>> somewhere else
-- >>>> etc, what is the proper way to handle the open ended wire
-- >>>> when the switch
-- >>>> is "off"?  Just leave it open?  Connect it to ground?  Thanks again!
-- >>>
-- >>> If you are using the virtual-earth active mixer then every resistor
-- >> you connect to the mixing bus actually increases the output noise of
the
-- >> op-amp. Whilst the gain from any single input to the output is not
-- >> affected by adding more inputs, the noise gain of the op-amp goes up
as
-- >> you load the summing node with lots of resistors, even if each input
is
-- >> shorted firmly to ground!  So in this case it's best to leave un-used
-- >> inputs floating rather than connect them to ground if you want the
-- >> lowest noise figure possible.
-- >>>
-- >>> Rod Elliot gives a more detailed explanation...
-- >>>
-- >>> "Virtual earth mixers have an interesting characteristic that will
-- >> seem strange at first. Even though the gain for a signal from each
-- >> individual channel may be unity (a common approach), the circuit has a
-- >> far greater gain for noise. This "noise gain" is created because all
of
-- >> the input (mixing) resistors are effectively in parallel. So while the
-- >> signal gain for one channel may be unity, the noise gain is ...
-- >>>
-- >>> An = Rfb / ( Rmix / N )
-- >>> ... where An is noise gain, Rfb is the value of the feedback
-- >> resistors, Rmix is the value of the mixing resistors and N is the
number
-- >> of channels.
-- >>> For the 3 channel mixer shown, the noise gain is therefore 3, and
this
-- >> applies whenever the inputs are connected to a source. Noise gain is
-- >> minimised by disconnecting all mixing resistors that are not being
used.
-- >> The signal gain is not affected when channels are connected or
-- >> disconnected because of the virtual earth mixing scheme, and there are
-- >> no clicks or pops provided there is no DC in any of the channels."
-- >>>
-- >>> quoted from...
-- >>>
-- >>> http://sound.westhost.com/articles/audio-mixing.htm
-- >>>
-- >>> -Richie,
-- >>>
-- >>> _______________________________________________
-- >>> Synth-diy mailing list
-- >>> Synth-diy at dropmix.xs4all.nl
-- >>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
-- >>
-- >> Chris Muir                     | "Space is big. You just won't believe
-- >> how vastly, hugely,
-- >> cbm at well.com              |  mind-bogglingly big it is. I mean, you
may
-- >> think it's a long
-- >>                                    |  way down the road to the
-- >> chemist's, but that's just peanuts to space."
-- >> http://www.xfade.com  |            - Douglas Addams
-- >>
-- >>
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