Small Stone Noise Issue

[morgan]

I recently purchased a vintage Small Stone V2 that I'm getting some noise when the color switch is engaged. It makes a sound similar to surf/waves on a beach every other pass. It does not make the noise in regular mode. Sounds quite nice actually.

I've replaced all the caps and freshened up all the solder joints and it's still present. Any other tips at things I could check on this bird to get it up an running?

[Mark Hammer]

This is a common enough issue with some phasers.  The Color switch does three things at once.  It changes the LFO waveshape and speed range, and alters the amount of feedback/regeneration.  In one position the waveshape is more triangular, the speed faster, and the feedback reduced.  In the other, the waveshape is hypertriangular, the speed slower, and the feedback increased.

The problem introduced by increasing feedback is that any residual noise from the devices that make up the phase-shift stages is effectively increased.  What  op-amp based phasers do is stick a small-value cap in the op-amp feedback loop to trim back high-frequency hiss and keep it from getting exaggerated.  Since the SS uses OTA chips, the solution has to be a little different, by trimming hiss in the overall feedback path from the circuit output back tothe mixing node near the input.  I could recommend a solution, but first I'd need to know the particulars of the circuit to identify a particular spot to intervene. 

"Vintage" covers a lot of time and different issues, for a pedal that is 40 years in production.  Couldyou offer a little more identifying info?  E.g., five EH-1048 chips or six?   Any 14-pin or 16-pin chips in there?

[morgan]

"Vintage" covers a lot of time and different issues, for a pedal that is 40 years in production.  Couldyou offer a little more identifying info?  E.g., five EH-1048 chips or six?   Any 14-pin or 16-pin chips in there?

Mark, thank you for your interest in helping. Mine is the six chip version. 5 EH-1048 and 1 EH-10368 which I believe to be the same chip as the EH-1048 possibly thrown in to confuse rival competitors? The closest schematic I could find to mine is this one below with the 500uf taken off the power rail to ground (appears to be from your own collection!  )



Edit: I'll add that there's a few caps in there I am missing and not having traced the whole board so there may be some resistors mine does not have. For instance I'm not seeing that .05uf or that 50uf cap. I do have a 10uf that the positive side goes to all the pin 3's and the negative goes to all the pin 4's of the four phase OTA's. Also all the 8 & 7 pins are tied to ground on all six chips. So if the EH-1048's are CA3094's they must be flipped around? Again maybe to confuse rival competitors? 

[Mark Hammer]

Can't speak to the markings on the chips, but assuming that this schematic is close, I would suggest the following.

The 27k resistor seen just above the output provides the pathfor the feedback signal back to the input.  If you were to replace it with a 15k and 12k resistor in series, that would yield the same resistance.  If you were to run a 1500pf cap from their junction to ground (the 15k coming "first"), that would roll off content above 5900hz in the feedback.  In principle, that would prevent any hiss and undesirable  harmonic content from getting recirculated and made more noticeable, without necessarily detractingfrom what you like in that Color switch position.  If the effect is too much, drop the 1500pf down to 1200.

[morgan]

Can't speak to the markings on the chips, but assuming that this schematic is close, I would suggest the following.

The 27k resistor seen just above the output provides the pathfor the feedback signal back to the input.  If you were to replace it with a 15k and 12k resistor in series, that would yield the same resistance.  If you were to run a 1500pf cap from their junction to ground (the 15k coming "first"), that would roll off content above 5900hz in the feedback.  In principle, that would prevent any hiss and undesirable  harmonic content from getting recirculated and made more noticeable, without necessarily detractingfrom what you like in that Color switch position.  If the effect is too much, drop the 1500pf down to 1200.

Thanks Mark! I added a 1500pf cap in there, it's showed some improvement but there's still some hiss in there. Should I try upping that to say 1600pf -1800pf?

[Mark Hammer]

Sure.  Can't harm anything, apart from reducing the perceived "intensity" of that color switch setting.  You could even move up to 2200pf.  Worse comes to worse, you replace the 15k resistor with another 12k.

I don't know how high up the notches get swept.  Note that you will still have the full signal bandwidth coming from the clean/dry path, and whatever the "full" bandwidth is from the phase shift path.  What you're changing is the bandwidth of what gets fed back.  And there you're really only interested  in the content immediately surrounding the notches and adjacent peaks.  Anything outside of that is largely moot.

[morgan]

Sure.  Can't harm anything, apart from reducing the perceived "intensity" of that color switch setting.  You could even move up to 2200pf.  Worse comes to worse, you replace the 15k resistor with another 12k.

I don't know how high up the notches get swept.  Note that you will still have the full signal bandwidth coming from the clean/dry path, and whatever the "full" bandwidth is from the phase shift path.  What you're changing is the bandwidth of what gets fed back.  And there you're really only interested  in the content immediately surrounding the notches and adjacent peaks.  Anything outside of that is largely moot.

I tried the 2200pf because that's all I had on hand and the hiss is totally gone with that value but it makes the effect a little darker. I'll have to make a run up to the local electronics store and see about a 1600pf or 1800pf, I'm betting the 1800pf will be the best of both worlds in this case. Thanks for all your help Mark!

[Mark Hammer]

And my thanks, in turn, for the helpful feedback on the feedback. 
So often (though thankfully not always) someone will provide a solution to a problem and we don't hear back.  Your feedback will be usedul for others in future.