Quote from: FleshOnGear on Today at 08:18:35 PMYou guys are rad. Rob, I used method 2 and the circuit is whisper quiet now. Thanks for the explanations. The transistors I'm using have hfe of about 509 for Q1 and 580 for Q2 if memory serves. The pre EQ is not quite the same. It sounds quite bassy now, and the distortion is a bit overblown in the lows. The pre EQ will need adjustment, and the tone control isn't quite right, either. Other than those nitpicks, the circuit sounds good. Thanks again.

That's good news.  I do have faith in the theory for this stuff!

For circuit #2, with those transistor gains:
- try raising the added base resistor from 130k to 180k.  That will reduce the bass.
- the cap on the base remains at 12n.
  However since circuit #2 isn't an exact match 10n will match the bass better.
  From that perspective even try 150k instead of 180k.

You guys are rad. Rob, I used method 2 and the circuit is whisper quiet now. Thanks for the explanations. The transistors I'm using have hfe of about 509 for Q1 and 580 for Q2 if memory serves. The pre EQ is not quite the same. It sounds quite bassy now, and the distortion is a bit overblown in the lows. The pre EQ will need adjustment, and the tone control isn't quite right, either. Other than those nitpicks, the circuit sounds good. Thanks again.

Mysterious vintage looking pedal with vintage looking parts.  The pedal could be build to deceive, or it could be someone's old DIY project - sometimes people put brand names on their DIY stuff (not even their brand, just something).    The mismatched sockets could be deliberate.

Quote from: FleshOnGear on Today at 08:53:24 AMMany thanks to you, antonis and Rob. This is all very helpful. Even after tinkering for years, I'm still learning about what BJTs can tolerate; they seem to be a lot touchier than tubes and FETs. This is the first time I've ever tried to do something like this with a BJT - attempting to get a higher than usual input impedance and sculpting the low end at the input.

With JFETs and Tubes the high value input resistors still cause noise.  However with BJTs the noise is much worse because there is a noise *current* at the base.  The base noise current causes an Inoise*Rbase noise voltage at the base.   As the resistance (actually impedance) at the base increases the noise increases.  You can change the operating current of the transistor to optimize the noise but that doesn't mean the noise ends up being low.  It's a big topic and not easy to understand or tap into.

https://hifisonix.com/wp-content/uploads/2019/08/Noise-in-Transistor-Circuits-P-J-Baxandall.pdf

QuoteLast night I tried shorting R4, and that definitely helped the noise. Then I tried removing R4 while keeping the 470p cap, because the bass was too bloated with R4 shorted. The noise came back when I did this. I was shocked that just having the lone 470p input cap would behave this way. Interestingly, the noise is more of a brown noise than white noise, as if the noise is mainly occurring at the frequencies that are blocked by the small input cap.

Yes it will have a brownish noise spectrum due to the 470pF cap.  As the frequency increases past 220Hz (f = 1/(2*pi*RC) = 1/(2*pi*1.5M*470pF) = 225Hz) the cap impedance becomes comparable to the resistance of R4.  The resistance on the base circuit decreases and that decreases the noise voltage.  At high frequencies the cap has a very low impedance and impedance seen at the base of Q1 is low and the noise from both R4 and R5 is removed.

QuoteSo, Rob, I will try your approach tonight. I was originally trying to keep a clear DC path between the buffer and the CE stage, because I didn't want any charge buildup on a coupling cap pinching the signal. But, I think the smaller values used in your examples should be just fine.

It's possible the two options with the parts moved don't sound like the original.  That's part of the redevelopment.  Circuit three is likely to have the least effect on the base cap charging.

https://www.diystompboxes.com/smfforum/index.php?topic=132305.0

Another issue is the circuit has some dependency on the gain of transistors Q1 and Q2.  I've assumed a hFE around 300 but if your parts are 400 the original circuit will behave slightly different that predicted in the simulations and the tweaked circuits won't be tweaked exactly right.

FWIW, in circuit 3 you might be able to tune the collector voltage to be exactly like your original circuit by tweaking R9 (I could have made it bias closer).    I've tried to make the frequency responses match quite closely but that meant using odd values (E24 series, 1%) for the parts - for the purpose of posting responses which match.  In practice you might be able to use rounder figure E12 values.

QuoteOnce a gain, thanks so much. Btw, what is the program you are using for your simulations?

No problem.  I used LTspice.  It's widely used these days and available on the web.

Quote from: FleshOnGear on Today at 08:53:24 AMBtw, what is the program you are using for your simulations?

LTSpice. It's free software and it's certainly "quirky" but given that it's free and really rather powerful, you can't really argue. It's a very useful tool if you can get over the interface weirdnesses! I use it a lot for looking at the frequency rsponse of different circuits - for that sort of tone control design, it's invaluable. But Rob has demonstrated another useful side of it here - noise analysis.

> Sus

What car collectors call "a tribute". Wrong frame, wrong engine, over-wrought paintjob. Some tributes are deliberately crafted, some just happen when you clean out the garage.

Quote from: duck_arse on Today at 11:24:06 AMscope the output pin of the 555. what frequency? can you scope the noise in the audio section as well?

About ~40khz. Ran out of time to scope the rest of the audio path, but it doesn't change when I can manipulate the oscillation frequency with knobs so I'm not sure its directly the issue (although that's not saying that it isn't getting modulated anywhere else in the circuit).

Quote from: R.G. on Today at 12:46:42 PMDA's right - first find out if the oscillator is emitting/leaking into the audio path.

More generally, the board seems to have the (very) high voltage oscillator section close to the J201. The J201 would be more than happy to pick up high frequency spikes.

Are you looking at the switching transistor for the PSU, not the FET? FET and the HV section are on opposite sides of the board, and there's not many traces from anywhere pre-tube that goes near HV from a quick squiz.

Quote from: PRR on Today at 01:23:34 PM

Quote from: drdn0 on Today at 01:20:52 AMat either max or min it is quiet.

The receiving antenna is probably the wire from the pot wiper.

That's what I was thinking re: the grid stopper routing being right up in the guts of the PSU, but moving it offboard didn't make a difference either. I didn't remove the R9 trace from being connected to the rest of the circuit so could still be causing grief I guess.

All good, PRR!

Just watched a demo and maybe this is operating as-designed. It's more subtle than I remembered.

Thanks everyone for chiming in. If incoming clock seems clean enough I guess I will move on to looking somewhere else!

Sorry, I mis-read.