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

[Harry Bissell]

Neil:

I just read the draft of your paper. Did you know that this configuration (minus
the opamp) is common in electric guitar to avoid the trouble of having the volume control
of one pickup load (and maybe even short out) another pickup? In this case the impedance of the
coil is the first resistor, the pot is (uhhh...) the pot, and the 'mix bus' is the amplifier
input.

backward thinking. brilliant !

H^) harry

----- Original Message -----
From: Neil Johnson <neil.johnson97 at ntlworld.com>
To: Justin Owen <juzowen at gmail.com>
Cc: synth-diy at dropmix.xs4all.nl
Sent: Thu, 14 Jul 2011 09:05:19 -0400 (EDT)
Subject: Re: [sdiy] The annual I/O impedance/protection thread! (revised)

Hi,

>>Outputs
>> http://www.sdiy.org/juz/euro_output_03.pdf
>
>>And for the non-inverting circuits add anti-parallel
>>Schottky diodes across the op-amp inputs and a resistor to the +ve input.
>
> I've added Schottky diodes based on advice from you and Harry B - it's also
> been suggested that I could use 1N4148s.
> Can you clarify what you mean about the resistor to the +ve input? Where
> to/from? Why? Value based on?

In the non-inverting case, definitely on the path to the +ve input to
the op-amp before the diodes to limit any fault current that might
flow from the signal source either into the op-amp or through the
diodes.  Better still a matching resistor on the connection to the -ve
pin.  10k would be enough.

> You'd ditch the zeners and their associated protection resistor from the
> output as well?

Well, since you seem unable to provide any *specific* requirements for
what you are hoping to protect the outputs from, I'm assuming that the
worst case is an input of -15V and the op-amp trying to push +15V.
All you need to do then is put some resistance, say 1k0, in the path
to limit the fault current.

>>Summing
>> http://www.sdiy.org/juz/euro_summing_03.pdf
>
>>2/ R1 will appreciably load the input pot to give a non-linear control
>>curve.  This might not be an issue for you (designers choice) but it
>>irks me.
>
> So how would you lay out an attenuated input for an inverting summing config
> or what would you suggest revising about that input with regards to
> component choice, value, order ,etc?

Its something I'm working on:

http://www.milton.arachsys.com/nj71/index.php?menu=2&submenu=2&subsubmenu=4

>>5/ Depending on what the internal load looks like that 1N4001 could
>>behave in a rather non-linear fashion.  And assuming enough current
>>passes through it to turn it on, it will drop about 1V from the output
>>of the op-amp.
>
> Yes, agreed, but considering it will probably be feeding something along the
> lines of my beloved single transistor VCA - it does the job just fine. When
> I need a precision solution I'll use a 'proper' rectifier.

Well, that's your call.  And its the first time you've given any
information about what these circuits are supposed to be driving.

>>6/ When the 5V1 zener turns on its going to provide a nice low-impedance
>>path to ground to pull lots of current out of the op-amp, which will
>>overheat and possibly melt.  Not quite desirable.
>
> I've used this config plenty of times and not even noticed any undue warmth
> - and I do check. Certainly never had one melt.

What was the op-amp output pin sitting at when you did the overvoltage test?

> Your impedance point does concern me though. Considering that the aim of the
> 1N4001/5V1 Zener combo is to allow an AC or negative V input to be used as a
> CV source, then attenuated, biased, blocked and capped so you only get e.g.
> 0-5V - what would you suggest for capping the upper DC limit?

A zener in the op-amp feedback path would be a better starting point.

Neil
-- 
http://www.njohnson.co.uk
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-- 
Harry Bissell & Nora Abdullah 4eva