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stellavox 19th December 2009 12:59 PM

Looking for (Generic) Preamp Troubleshooting Advice

I've got a discreet "op amp" type preamplifier similar to:

Except the input FET's are in a single package. And it doesn't work - doesn't pass a signal and the output voltage is up against one rail. Because the design is all direct coupled, I don't know how/where to start looking for a problem - other than removing and testing every transistor.

Can anyone suggest (or point me to) a "systematic / generic" way to troubleshoot this?

Thank You


djoffe 19th December 2009 05:59 PM

the secret is getting the dc bias conditions right...

First, ground both inputs...the grab a DC meter.

J1 is a constant current diode...I don't know the current it's set for, let's assume 2 mA, under this assumption,

Then there should be 1 Volt across R34. If there isn't, you've found the problem. Work your way from left to right, checking all the DC voltages, across R55, R20, R7...they should be quite can compare it to your spice simulation, if you have one...(does spice think this works?)

Hopefully this will more info as you have it...

scott wurcer 19th December 2009 06:13 PM

Sorry about that mistake, you could go back and check the "Yet another discrete op-amp thread" for the fix and further discussion. If problems continue you can PM me.

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