Thread: PLZ pre-preamp
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Old 23rd February 2019, 07:11 PM
bikerhifinut bikerhifinut is offline
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Default Re: PLZ pre-preamp

Originally Posted by bob orbell View Post
Just took mine apart, there is no regulator although the circuit has 3 volts to part of it and about 2.5 to other sections adjustable, the PCB is copper solder wiped, not silver and the transistors look like FET's, but I may be wrong, all 6 capacitors are electrolytic, 4 off 10 v 470uF, and 2 off 10v 10uF . BOB
So the mystery potentiometer could be to trim the supply voltage?
The other thing that crosses my mind is are the 470uF supply bypass/decoupling capacitors or are they emitter bypass jobs? Did you trace the path to see how they are connected Bob?
The 10uF coupling caps could be usefully swapped for polyprops, as the voltage is 3V or less 6.3V jobs or better will be good and that gets the physical size down if using film caps. If Stuarts feeding a higher Z input as is usual on Valve amps then you can get away with a 4,7 uF poly or even 1uF into 50k will still be plenty, and they can be very small physically. I'm just a little bit phobic about electrolytics as coupling caps but I imagine using something of the quality of Nichicon FG or Sanyo Oscon etc with low ESR etc wont have a detrimental effect on sound quality.
There are small polywotsit films but I think at that capacitance they'll be too large to fit the PCB so its electrolytics. They seem to have lead pitches around 22mm which is a bit bigger than a 10uF electrolytic.
I'd love a photo with an idea of sizes, there must be some service info out there somewhere.
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