Odp: [sdiy] 1/8th inch questions

[Grant Richter]

Not to contradict you, but...

According to Bernie Hutchins in an article about input and output
structures. The 1K value was chosen to make passive mixing of outputs
possible.

Also the 1K output impedance is taken from Analogue computer where the
coefficient potentiometers are 1K in value.

The 1K/100K combination gives an error of 1% (1V/Octave voltage sources
don't have 1K resistor). The 100/100K reduces the error to 0.1% and could be
useful in some configurations.

> From: John Blacet <john at blacet.com>
> Date: Tue, 22 Oct 2002 15:54:28 -0700
> To: synth-diy at dropmix.xs4all.nl
> Subject: Re: Odp: [sdiy] 1/8th inch questions
> 
> I believe that this refers to the practice of including the output
> resistor (1K) in the feedback loop of the op amp. For example, if you
> have a 100K feedback R and a 1K output R, the actual output is taken
> from the junction of the two, not the output pin of the amp. Of course,
> this changes the actual gain, so you have to figure the Rs for that. I'm
> not sure if this is something you can do with a buffer configuration.
> 
> In practice, we have opted to use a 100 ohm resistor for the output. The
> 074X series for example, is rated for indefinite short circuits to the
> supplies and ground. The 100 ohm is just extra current limiting, but
> gives us 10X the "fanout" of the 1K typically used.
> 
> --
> Regards,
> --/////--
> John Blacet
> Blacet Research
> http://www.blacet.com
> 
> 
>