Pacific RIAA phono pre: failure!

[Osvaldo de Banfield]

Which is the link between audio distortion at high frequency and gate stoppers?

[kevinkr]

Oscillation... Good practice to isolate gate from external stray inductance and capacitance, more critical with mosfets than fets, but I have had situations with fairly high transconductance fets that oscillated without gate stoppers. May or may not fix the issue at hand..

[Osvaldo de Banfield]

OK, I agree, but the guy explicit that audio distortion is at high frequencies, I believe that oscillation must affect all audio spectrum.

[Malefoda]

Originally Posted by kevinkr

Oscillation... Good practice to isolate gate from external stray inductance and capacitance, more critical with mosfets than fets, but I have had situations with fairly high transconductance fets that oscillated without gate stoppers. May or may not fix the issue at hand..

It did fix the issue!
Many thanks, a big hug to ya all

Here is the 100Ω and the tiny 51Ω (it's far smaller than it looks, flash and zoom worsen its appearance, but minimelf and micromelf are a soldering challenge!), and now all flows smooth and dynamic!

[The Space Egg Corp]

BOO YAH !

[The Space Egg Corp]

BOO YAH !

[Malefoda]

In fact I'm not sure it means " great " but if it's the case: sadly not :'(
Overall volume is louder on left channel. I've cleaned the mess, solder flux, tracks, close matched the 30n with 2x15, it's almost perfect to the eye. But no use...
Before I go any further I need to say that I find weird that with only the right IC connected to the turntable I get no sound on left (!) and a very slight level of music on right, then left only connected, no sound on right but a terrific loud bassy mess on left... wtf!
May that be some loop at work here? My IC are all connected both wires both sides.

So the question is, if there is nothing odd there, how can I fine adjust gain? That's lying on no solid tests but it sounds like a +2/3dB on left most felt at kHz... (I've said it's "à vue de nez" (translate by "seen-by-the-nose") but that's the best I can tell about what I hear, not TT related as inverting left/right does nothing for this).
I'm glad I did not started a bigger project! That's so annoying...

But I still have to go, time to shift to side B

[merlin el mago]

Check the signal with the scope, comparing both channels. Can be a cold joint.

[Malefoda]

Hello Catalonian man
I may be able to check with scope but I don't know when sadly :'(
Solder is still warm, a cold joint is unlikely to me, few parts but I'll take a look when I'll manage to scope output signal.

[kevinkr]

Check the tone arm wiring, and does the problem swap to the other channel when you swap the left and right inputs?

Left and right inputs both return to ground somewhere on your nicely built boards? (No opens)

Measure dc voltages on fet drains and make sure channel to channel that they match to say within 5 - 10%, if they do not, try to match some fets as variations in transconductance, pinch off voltage, etc. may effect the channel to channel gain match. I would not expect the differences to be that large unless the fets were grossly different.

Even better would be to measure the signal levels directly in each stage, verifying that levels match after the first stage, after the passive eq, and finally the second stage.

Post a reasonably sized picture of each side of the PCB and let us have a look, perhaps something different between the two channels will stand out.

Finally have someone else take a look at your handiwork.