Pumpkin preamp - ordered by Steen , official making thread

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Cookbook mentions nothing about capacitors.

Hi Andy,

the cookbook mentions the two caps........



I am so sorry that you had no success with the new PSU....:mad:

I see only the possibility you build one (or two) channels meanwhile point to point in your B1 case, to compensate for the endless frustration.......
I would give up at this point...:joker:

until you get from Zen Mod new stuff..... and when your PTP are running well ........ you nail them on the wall:D:D:D
 

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When I built the Pumpkins they oscillated with the 5pf silver mica compensation caps C1 and C2. The oscillation was about what you are describing. I installed some twisted wire and started trimming. If I remember correctly the 5pf cap caused the frequency response to peak at about 300MHz.

BDP
 
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When I built the Pumpkins they oscillated with the 5pf silver mica compensation caps C1 and C2. The oscillation was about what you are describing. I installed some twisted wire and started trimming. If I remember correctly the 5pf cap caused the frequency response to peak at about 300MHz.

BDP

My stocks of low pF caps are nil. Consequently I'm having to buy nnew values every time.

where did you put the twisted pair (PSU wiring perhaps). What value did you substitute for C1/C2 ?

One of the Pumpkins now has 27K for R5 and R6 with no change to the 5pF caps. The oscillation is the same.

Can anyone suggest a brutal approach to see if this the area of the problem.

(I'm a computer systems engineer - not an audio designer)
 
I need a few suggestions here.

I have a few ideas but not the knowhow how to implement them.

Firstly reduce gain to Less than 1, then it CAN'T oscillate.

Secondly reduce bandwidth to peanuts (say 10kHz), then it CAN'T oscillate.

ONLY once I've got rid of the oscillation can I start to take some meaningful measurements.

ALL the matched components were supplied by ZenMod - that's not to say that either a component failure or a mismatch isn't causing the problem.
ALL components are correctly inserted, and their values checked correct.

Amp (2) has R5 and R6 at 27K and C1 and C2 at 33pF. The only difference now is that Amp (1) (Original vlaues) oscillates at 20MHz and Amp (2) oscillates at 9MHz ????????
 

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Take C1 and C2 OUT. You have 100k feedback resistors - these things are not doing anything to help stop oscillation.

R2 and R4 should be left at 10k.

You need to drastically reduce the loop gain of this pre amp. I proposed yesterday that you changed WR1 to 500 Ohms. If you do not have a 500 Ohm pot, try inserting a 470 ohm resistor either side of the 47 ohm pot.