Thread: PLZ pre-preamp
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Old 1st March 2019, 11:33 PM
bikerhifinut bikerhifinut is offline
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Default Re: PLZ pre-preamp

I think I may have stumbled on the circuit.
I had asked a friend who is very knowledgeable on amplification and apparently the large power devices are used because of their base spreading resistance properties.
I was given a name of a designer, W Marshall Leach and I looked it up on the web.
I have attached the link.
All is revealed I think its a common base amplifier to this design. An examination of the PCB suggests this with the emitters commoned and the preset pot is configured as a variable resistance which could be in place of R2 in the published circuit to adjust the very low bias current quoted here as 125uA (0.125 mA).
I'd be loth to fiddle about with the 470uF electrolytics unless you are absolutely sure they havent upset the biasing. The 100 uF as you can see from the circuit are simply DC blocking coupling caps to the output.
Interesting comment too about certain designers grinding off the transistor markings as if their type was some sort of industrial secret when its been lifted from a design in the public domain!
OK stuart I think you've got something to go on there.
Try it with the existing caps in, you won't blow anything up with 3V across it. You've already prodded about with your DVM so its had a few volts bunged in places already.

http://leachlegacy.ece.gatech.edu/headamp/

Andy.

Last edited by bikerhifinut; 1st March 2019 at 11:57 PM.
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