[sdiy] mixer

[Scott Bernardi]

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).

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