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

[Harry Bissell]

Yes, the order you mention at the output is what I had in mind. From another post (not posted yet) you probablky don't
need the zeners unless you are going to plug in some external output (guitar amp maybe) to the output.  Not a real good idea
anyway...

H^) harry

----- Original Message -----
From: Justin Owen <juzowen at gmail.com>
To: Harry Bissell <harrybissell at wowway.com>
Cc: SDIY List <synth-diy at dropmix.xs4all.nl>
Sent: Tue, 12 Jul 2011 16:08:20 -0400 (EDT)
Subject: Re: [sdiy] The annual I/O impedance/protection thread! (bumped)

-----Original Message-----
From: Harry Bissell [harrybissell at wowway.com]

2) Output: http://www.sdiy.org/juz/euro_output_01.pdf

>>Q: Again, on the inverting config can the 100R/100K combo be combined into a single 100K at the input?
>>

>here its 1K and 100 ohm right ?
>

Yes - my bad.

>One resistor is protecting the zeners from an externally applied voltage at the output, the other is protecting against
the opamp trying to drive above 10V output. I'd make them both 500 ohms if this is what you intended. There are better ways to limit an output to 10V if that is what you want. Zeners in the feedback of the opamp is one way. That could use just one resistor in series with the opamp output to protect
the opamp output. If you tried to drive a voltage into the output, the opamp would drive to keep the voltage within the 10V range set by the zeners.
>

Yes, the idea was to protect the op amp from a voltage going in to an output. i.e. what I do regularly.

Would a 500R, Zeners, 500R Output Socket config sort that and leave me with 1K output impedance?

>>Q: Again, if yes, where does this put the AC coupling cap?
>>

>I'd put the cap after the series resistor at the opamp output (so the opamp does not see the capacitive load directly) and add a 1M resistor
to ground at the jack end, to prevent thumps as you plug in.
>

So, from left to right...

Op Amp Output, 500R Series Resistor (formerly 1K), Cap, Zeners to Ground, 500R series resistor (to make up 1K impedance), 1M to Ground, Output Socket?

You can see how interpreting this from even the best advice of the archives can get ambiguous.

Thanks again.


-- 
Harry Bissell & Nora Abdullah 4eva