[sdiy] mixer

[harry]

Hi Scott... (et al)

Scott Bernardi wrote:

> The Johnson noise from all source resistors (kTqR) always add as an RMS sum
> regardless of whether they are in parallel or not. In addition, the current noise
> from the opamp input flows through the input resistance and produces another noise
> source - that's why low input resistance is desirable. Most "low noise" opamps
> (OP-27, NE5532, etc) are optimized for low voltage noise, not current noise. All
> these input referred noise sources are multiplied by the closed loop gain of the
> opamp.
> Also, tricks used for error cancellation in precision circuits, like having a
> resistor in series with the + input that is the same as the equivalent resistance
> seen by the - input (eg, Rf || Rin) is actually detrimental, because there is no
> phase cancellation of noise between the + and - inputs - everything adds as RMS
> (the square root of the sum of the squares of all the individual noise sources).

(I agree... isn't this what i said ;^)?

So if you can stand it... do not try to adjust for the bias current with the non
inverting input... AC couple the output instead...

H^) harry  (size DOES matter...  smaller is better)

>
>
> Glen wrote:
>
> > At 11:11 AM 11/2/01 , harry wrote:
> >
> > >The self-noise of the resistor is proportional to resistance, so the bigger
> > >we make the resistor, the worse the noise gets.
> > >
> > >The trade is to use the lowest input resistor we can stand, that does not
> > >interfere with the driving stage.
> > >
> > >H^) harry
> >
> > Thanks Harry. So I was basically correct?
> >
> > If one is only mixing typical, unbalanced line-level equipment, the noise can
> > be reduced by using a lower input resistance on the mixer. Instead of 47K, I
> > would probably use 5K, which I admit sounds extreme, but I'm very fussy about
> > noise. Remember, this assumes that the input will always be feed with an
> > unbalanced, line-level signal. If you need to feed this same input with things
> > like electric guitars, or low-impedance balanced mics, then you will need an
> > appropriate preamp to insert between the signal source and the mixing input.
> > The original post mentioned nothing about guitars or microphones, but only
> > line-level devices like synths and effects units. My suggestion should work
> > well for those devices.
> >
> > Later,
> > Glen
>
> --
> Scott Bernardi
> sbernardi at home.net

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