audio-talk

Nick wrote: ↑Mon Mar 13, 2023 3:49 pm Hang on, a x10 probe is a 1:10 attenuator, not a probe with gain. If the scope is set at 1v per div, and its not a scope that talks to the probe, then that is indeed 3.4v displayed on the scope, the applying the x10 probe, that means that the actual supply has 34v P/P ripple.

Okay, as I was.

Nick wrote: ↑Mon Mar 13, 2023 3:51 pm Its also hard to read any values from the scope. I would try turning the brightness down and try and play with the focus.

Some of that is the photo, not easy to get anything better with a phone. But that said that's as good as I can get it, it's just a broad line. I wondered if maybe there's some oscillation but I'm really just clutching at straws.

The right channel was almost identical up to the Aikido, the output cap and output phono changeable and looking like the following. Note the volts/square is now 100mV.
.
Sometimes two lines .
Sometimes one line .
Sometimes quite different .
The right channel is closer to the power supply than the left, but the RIAA filter and C3g are closer than the Aikido and I would have thought they'd be more likely to be affected?

Grateful for thoughts and suggestions.

My list of things to try/consider:

Try to reduce ripple on the raw DC without losing too much voltage

Increase the gridstopper on G1 of C3g. Currently it's 220R - I done have any carbon comps to hand but is it worth trying a 1k carbon film?

Separate the HT PS from the phono circuit to put some distance between them

Put some ferrites on the heater wires at the pcb end

Seem reasonable? Anything else worth trying?

Is that a choke in the center?

If yes, try moving it, or, at least rotating it, the gap on any choke will emanate a nasty hum field.

I would compare the trace to that with the input shorted to see if its a badly focused trace or something more before trying to fix it. Check the signal going into the riaa, if there is HF the riaa will attenuate it so checking before would show more if its there.

Andrew wrote: ↑Mon Mar 13, 2023 4:37 pm Is that a choke in the center?

If yes, try moving it, or, at least rotating it, the gap on any choke will emanate a nasty hum field.

HT transformer I think. From top to bottom: HT trafo, UF4007 bridge, 100V+100V cap, two resistors now shorted out (so only a single filter cap rather than two sections when I first wired it up), and heater trafo at the bottom.
.

Nick wrote: ↑Mon Mar 13, 2023 4:43 pm I would compare the trace to that with the input shorted to see if its a badly focused trace or something more before trying to fix it. Check the signal going into the riaa, if there is HF the riaa will attenuate it so checking before would show more if its there.

I haven't followed that. Do you mean the reged HT? All the other traces are with the input shorted.

The trace on the output side of the C3g coupling cap is at the input of the RIAA.

It might be worth mentioning that the "hash" for want of a better word seen at the input is very similar to the previous build. Which was perhaps why I wondered if there was some oscillation maybe. But the HT trafo is reasonably close to the audio circuit in that build too so perhaps it's proximity.

I can't help thinking just how good the phono would sound if I could track this down cos it's pretty special now I reckon.

Question for Andrew - how much scope is there for increasing the distance of the heater modules from the audio circuit? Could they go in a separate PS box with umbilical wires to the audio box? This might be preferable to passing AC through wires between boxes? Or would this not matter if the umbilical had only the 6no. heater wires? I imagine they could radiate EMF that could be picked up by the DC umbilical for the HT maybe?

All the other traces are with the input shorted.

No, sorry, I meant the input to the scope being shorted, that way you could see what the trace was like in the known absence of a signal.

Ah, got you - that makes sense!

Very good, you've done this before Nick . This is with the probe's ground connected to the tip, it's the same as at the phono inputs. Which suggests things are fine up to the Aikido?
. .
What does this mean for the probe/scope? I have two probes and they're both the same when I ground them.

Nick wrote: ↑Mon Mar 13, 2023 3:49 pm Hang on, a x10 probe is a 1:10 attenuator, not a probe with gain. If the scope is set at 1v per div, and its not a scope that talks to the probe, then that is indeed 3.4v displayed on the scope, the applying the x10 probe, that means that the actual supply has 34v P/P ripple.

D'Oh ! I didn't think that one through.

Which suggests things are fine up to the Aikido?

Maybe, is the circuit of the amp in this thread somewhere (will go and look)? It certainly makes me think that its not a issue with HF thats causing the wide trace. I think its just induced hum.