Fender rhodes peterson vibrato preamp

  
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There is considerable hiss brought on by the circuit. Cutting the treble knob helps greatly (but no fun with treble cut!). When the vibrato circuit is active, the his is worse. Following these discoveries, I re-capped the board given the schematic values as well as those shown on this picture: At this point I`ve only changed out the electrolytics.
Fender rhodes peterson vibrato preamp - schematic

Swapping them had little effect on overall hiss, but the swap caused the vibrato to thump in the attack portion of waveform. There was no thumping before the re-cap. When I received this preamp I noticed that someone had altered the cap values. If you follow that picture, the 4x 47uf caps and 1x 16uf cap were all installed as 100uf. I know there is a quick-fix for thumping vibrato by adding some RC networks to the oscillators, but I want to avoid this if possible. This changes the characteristics of the vibrato wave to a triangle and I think it will negatively impact the sound. Hiss is not generally caused by filter caps. They fail and cause hum or unwanted DC leakage. Usually in a low voltage circuit like this my first suspects for hiss would be semiconductors. Changing the e-caps is probably a good idea though. If the treble control drops the hiss, then the source is before that control, so the input stage. It only has two transistors. SCope it to see, or just replace them. Drawing says 2N3392, but most any nice low noise signal transistor should work fine. The circuit only runs on +25v, so most transistors cover that. Low noise NPN signal transistors My own go-to choice would probably be an MPS8097 or maybe 2N5088. And if there are any left in my drawer some MPSA18. I forget now what I used them in a lot, but I have a bunch of 2N5210 too. Those are old school numbers, there may be some more recent favorites, but I have...



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