AW: Re: external input (again)

[Don Tillman]

   Date: Mon, 27 May 1996 21:41:06 -0800
   From: haines at apc.net (Matt Haines)

   >Folks in my band with advanced degrees in electrical engineering
   >seem incapable of getting the grounding right.  If your input signal
   >finds itself riding on on top of a 115v 60Hz sine wave it's a good
   >idea to have reverse-biased diodes to the power +/- supplies as well
   >as in input resistor.

   That's a very good point. Unlike in the 'safe' enviroment of the modular
   synth, bringing in outside signal sources could be frought with peril. So
   on the power supplies of the op amp, I should have:


   +15     ----------------+-------------------- +v op-amp in
			   |
			  ___
			  / \
			   |
   GND     ----------------+-------------------- GND
			   |
			  ___
			  / \
			   |
   -15     ----------------+-------------------- -v op-amp in


   Right? So would a couple of 1w 16v zeners do?

Ummm, no, that's not what I meant.  What you've drawn is a circuit 
intended to protect the opamp from overvoltage and reverse polarity
power supplies, but it won't really work.

You never mentioned how you were going to power this thing; I assumed
it was to be hard wired to the power supply of your synth, but I guess
I have no idea what you're doing in this respect.  If your unit is
going to be connected to random sources of power at random times, then
yes, power supply protection is important.  Otherwise it's not.

I meant input proection like this: Input jack to 1.0k resistor, other
side of that resistor goes to the anode of a diode to +15v, and to the
cathode of a diode to -15v, and to the actual input (I guess that
would be the pot in this case).

   >With opamp circuits it's a good idea to roll of the high end in the
   >feedback network to limit the gain in the region where the opamp is
   >less able to handle it.

   Like a 30pF cap in parallel with the feedback resistor?

Yes, depending on the specific feedback resistor.  Make it roll off at
a time constant of 40KHz or so.

   >   As far as getting a book on op-amps, yes it's something I need
   >   to do. I'm much more likely to get answers specific to my
   >   problem here on DIY though. 

   >It's important... at the very least it will give you the warm fuzzies
   >about how these things work.

   Hey, I've got the Radio Shack Engineer's Mini-Notebook, the op-amps volume!
   That's a start. :)

Maybe good, maybe not.  I don't know this publication, so I can't say.
I suggested Walt Jung because he has a reputation for doing things
correctly. 

  -- Don