I vaguely recall discussion of audio mixer circuitry where the reason to
omit the resistor was that it adds noise. If that's right, I guess here you
make the tradeoff that less offset is more important than less noise. It
looks like for example the audio input mixer on the 440 filter does not have
a resistor on the + input.
What would compensate for it... I would guess the fact that the oscillator
core is basically integrating the noise and acts like an LPF?
...Craig
-----Original Message-----
From:
motm@yahoogroups.com [mailto:
motm@yahoogroups.com] On Behalf Of Paul
Schreiber
Sent: Saturday, August 01, 2009 8:39 PM
To: Richard Brewster
Cc:
motm@yahoogroups.comSubject: Re: [motm] Missing resistor
> According to the Musical Engineer's Handbook (Bernie Hutchins, 1975), 3a
> (10), "The purpose of the resistor is to compensate for the offset due to
> the current that actually flows into the other input." It is important to
> use in precision DC summing circuits.
This is sort of a "hand waving" answer you write on a test when you forgot
the real answer :)
> The MEH also tells how to calculate the value.
Well that part is correct.
> I've wondered how important this resistor is in an inverting summer.
Application dependent.
I don't know the answers to question #2.
>
It's a toughie :)
Paul S.
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