[sdiy] The annual I/O impedance/protection thread! (revised)

[Harry Bissell]

Hi Justin

Inputs:

A, C: does not need resistor to ground at input jack

Outputs:

A, C: does not need resistor to ground at output jack

Summing:

Does not need resistor to ground at input jack

more inline
 

----- Original Message -----
From: Justin Owen <juzowen at gmail.com>
To: SDIY List <synth-diy at dropmix.xs4all.nl>
Sent: Wed, 13 Jul 2011 13:17:42 -0400 (EDT)
Subject: [sdiy] The annual I/O impedance/protection thread! (revised)

Based on the advice Harry B gave yesterday (thanks again Harry...), I've revised and uploaded new schems for my Eurorack Input, Output and CV Summing.

http://www.sdiy.org/juz/euro_input_02.pdf
http://www.sdiy.org/juz/euro_output_02.pdf
http://www.sdiy.org/juz/euro_summing_02.pdf

Go take a look - go pull 'em apart.

I made the following changes:
1) Added 1M to ground at socket on all I/O to stop pops and thumps

see above

2) Moved the AC Coupling cap to become the first component in series with the socket - placed after the 1M de-thump resistor
3) Set value of Zener protection resistor according to the relationship between expected input overvoltage/current, power rating of the Zeners the resistor is meant to protect and the required input impedance
4) Added optional 1M Resistor to Ground to taylor the curve of the two CV Pots on the Summing config

The resistor has to be much less than 100K to affect the curve of the pot. This will also change the input impedance quite a bit with
pot rotation. If you can't stand that, a buffer is in order before the pot...

I hope I got that all right...

I still have a couple of questions:

1) Non Inverting Input
Does the 1M at the input affect the Impedance?

Yes, its in parallel with the other input impedance (100K). Being 1M it will make only a small change in the overall impedance and can
practically be ignored.

2) Inverting Input
If the Impedance of this input is the sum of the 100R Zener protection resistor and the 100K resistor at the inverting input of the op amp? At some point, if I raise the 100R high enough to provide better protection for the Zeners - I would have to drop the 100K to compensate and to keep the impedance close to 100K?

Yes, if you want real precision the sum of those resistors in series needs to be 100K total.  Once you exceed (or get near) the zener voltage, the
impedance drops to the protection value. It will happen over a small range of voltage (called the "knee" of the zener, where small voltage changes
make big changes in zener current)

H^) harry

I'd still be up for any input on this...

Thanks - much appreciated,

Justin

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-- 
Harry Bissell & Nora Abdullah 4eva