[sdiy] mixer

[mikko.a.helin at nokia.com]

I think the 10 k is close to optimum when noise is regarded (for NE5532 at
least, I think someone in this group analyzed the noise figure for it years
ago).  For most circuits this is just fine, but maybe you could use input
stage FET follower (what's a low-noise FET, don't know, maybe BF245 is OK?)
for let's say two channels for high-impedance sources (guitar, bass etc.).
Use a 10 k pot as voltage divider for channel gains, this can also be the
source resistor for the FET, though you may need a cap there to avoid DC
offsets. If you have four 10 V p-p sources and if they are in phase you'll
get 40 V p-p output unless you have negative gain for the (inverting)
summing stage, also you may want that the output is in phase with the input,
so I recommend using 10 k input transistors with 4.7k feedback resistor
followed by another inverting stage with 4k7 input resistor and 1k + 10k log
pot feedback resistor for setting the output level. That way you can avoid
the overloading of the summing stage and avoid the phase inversion. 

-Mikko


> -----Original Message-----
> From: ext harry bissell [mailto:paia2720 at yahoo.com]
> Sent: 02. November 2001 0:56
> To: Glen; Theo; matti; synth-diy
> Subject: Re: [sdiy] mixer
> 
> 
> I'd vote (like anyone asked me...) for the
> 47K... its not too high and not too low.
> 
> I assume that the source might be driving more
> than one load... 5K in parallel with other loads
> gets real low, real fast.
> 
> If noise is that much of a problem. I'd be using
> a low noise preamp / buffer at the lowest impedance
> I could stand... then mix with... 47K ???   ;^P
> 
> H^) harry
> 
> 
> 
> --- Glen <mclilith at ezwv.com> wrote:
> > At 01:14 PM 11/1/01 , Theo wrote:
> > >For a line mixer just a summing amp is ok, use 47k
> > for the summing
> > >resistors.
> > 
> > Isn't that excessively high? Remember, the higher
> > the input resistance value,
> > the higher the background noise will be. All the
> > line-level equipment that I
> > use will handle loads of 5K with no problem.
> > 
> > Later,
> > Glen
> > 
> > 
> 
> 
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