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

[Harry Bissell]

ho-kay here goes...

----- Original Message -----
From: Justin Owen <juzowen at gmail.com>
To: SDIY List <synth-diy at dropmix.xs4all.nl>
Sent: Tue, 12 Jul 2011 12:03:48 -0400 (EDT)
Subject: [sdiy] The annual I/O impedance/protection thread! (bumped)

Now everybody's back - I hope nobody minds me giving this a bump...

I'd really appreciate it if you could spare a moment to have quick look over these PDFs to see if you can answer any of the questions below. Most of them are dead basic - but fiddly enough that they need an answer from someone who knows (or at least someone who has an opinion...).

Hopefully your answers will be helpful to more people than just me.

1) Input: http://www.sdiy.org/juz/euro_input_01.pdf

Q: My understanding is the 100R at the socket protects the Zeners from a short when plugging a jack into the socket - is 100R big enough and if not - should I also adjust the impedance setting resistor down accordingly?

The 100R is to protect the zeners. You choose the highest voltage that you will apply (and figure out how much current that input might be... you might have some current limit in the stage driving it as well...) and see what the power will be in the zeners, and in the series resistor. If you expect
120VAC input, no... 100 ohm,s isn't enough. Another factor... how long do you need to stand an overvoltage ? I see no additional way of removing the
voltage, so the answer is forever (in your case). If you know the overvoltage was only a short pulse, the zener and resistor wattage can be lower.

Another way to back into the answer... what is the wattage of the zeners and resistor ?  You can figure out what overvoltage causes you to exceed the power rating.

ex. if its a 1/4W resistor, you could apply 5V and that would be 1/4W. That resistor will get HOT at that point... you should shoot for no more than 1/2 of the resistor power rating to be safe. If this answer isn't good, make the resistor larger.... (The zeners are at .5W at this point btw,,,)

Q: Input impedance is set to 100K on the non-inverting config via a 100K to ground. This should be to ground right? Not in series with the input?

Ground, right.

Q: On the non-inverting input - is the 100R/100K combo redundant? Can I just use a 100K at the input to look after the zeners and set the input impedance?...

uhhh maybe and no. this would put all the zener parasitics on the input (the clamping will be poor at such a low power level, as well as the zener
capacitance forms a lowpass filter with the series 100K... Your drawing is really the correct way unless you can really stand the zeners on the other side of the 100K.. If you can. it will work as well...

Q:...and if Yes to the above - were would the AC coupling cap go? Between Input and 100K (formerly 100R) or between Zeners and inverting input?

ugly any way you slice it. The cap should go between the inout and the zeners imho, with ANOTHER resistor to ground right at the input, so both sides of the capacitor have a bleed-off path to ground (no pops when you jack in) AND the non-inverting input can get bias current (or the opamp runs to the rail and stops working.

more in another e-mail need to re-open your next file...

H^) harry



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

Q: The 100R on the output is to protect the Zeners, again - is 100R big enough?

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

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



3) Summing: http://www.sdiy.org/juz/euro_summing_01.pdf

Q: Is there a case for a buffer before the attenuation Pot? If I'm protecting my I/O from unknowns - is it odd to assume that suddenly that input will be prepped to 1K impedance no matter what?



Thanks heaps.

Justin

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-- 
Harry Bissell & Nora Abdullah 4eva