1980 EHX Small Stone - Power Filter Issue?

[Helmholtz]

Leave the cap where it works best.

Would have to see the complete schematic to decide where I would connect the cap.
Maybe we don't agree what after the resistor means?

Anyway the power supply doesn't seem to be good "audio" quality. Is it a SMPS?

BTW, did you check the 33µ "bias" cap regarding capacitance and ESR?

[clarisso11]

I wouldn't have described the power supply as low quality, but I'm not sure. From what I've read it has a "2-stage switch mode technology", so SMPS? I also used a separate power supply during testing though (True Tone 1 Spot), it produced the same noise.

I had checked the 33µ cap, it measured close to what it should be, and ESR was low.

[mozz]

Both of those supplies are switch mode, lots of noise. Find a old power supply with a transformer, you'll be good then.

[Helmholtz]

I wouldn't have described the power supply as low quality, but I'm not sure. From what I've read it has a "2-stage switch mode technology", so SMPS? I also used a separate power supply during testing though (True Tone 1 Spot), it produced the same noise.

Yes, switch mode means SMPS.
Many cheap SMPS supplies around with poor filtering.
You wouldn't have had a problem if the power supplies would provide sufficient output filtering, i.e. if they would contain the 100µ output cap.
In the early 80s effect power supplies were linear type.

That said, many newer effect pedals do provide additional filtering for external power supply.

[Mick Bailey]

I've done a few pedal board builds (at least 6) using customer-supplied Cioks DC7s and they haven't suffered any noise problems. One of these was a large analog board for a pro player, with dual supplies for both recent and vintage chorus, delays, many distortions and a compressor. I've never used this particular PSU for a small-stone, though. This brand/model PSU is pretty well-regarded and has a good reputation, so it would be interesting to know for sure if it's a PSU issue that's causing the problem.

[mozz]

Even those one spots, i have heard they often get noisy after a few years. I have 1 here that's noisy but never put it on a scope to see what's going on.

[Mark Hammer]

Noise from/with switching supplies is a widely misunderstood phenomenon, that needs to be appreciated in historical context.

If you're old enough, you'll remember with no fondness whatsoever, that when dad turned on a power tool, or mom used the sewing machine, or the fridge compressor turned on, all audio in the house went to hell in a handbasket. The TV sounded like barf, and the radio and stereo got very noisy. Why? Because every appliance in the house shared power with everything else plugged into the wall, and nothing manufactured took any steps to prevent motor-created spikes from being spread out across the power line. As time went on, more and more electrical appliances had protections built in so that a) they wouldn't send out any spikes on the power lines, and/or b) be sensitive to any such spikes coming in on the power.

T'was a similar progression with digital pedals and switching supplies. Both relay on clocks, and initially both could generate spikes on the power line, and in the case of digital FX, they would be sonically and audibly corrupted by power-line spikes, via heterodyning. Heterodyning functions a bit like ring modulation, producing sideband products. If I have two digital devices operating at 2mhz...nominally, with no incoming or outgoing protection on a shared power line, I will certainly not hear the sum of their clocks, but device A might clock at 1.999mhz and device B might clock at 2.001mhz. Pretty tight spec, if you ask me, but their difference is 2khz, and THAT will be audible as an annoying whine. Given how digital devices often divide down a master clock to accomplish various processing steps, there can be a great many different spike frequencies on the power line. So it should come as no surprise that, 20 years ago, when digital FX were new to many, a person who had one digital pedal that was clean, quiet, and well-behaved, would go to the store, try out another digital pedal that impressed them enough to plunk down their credit card, and when they got home and hooked it to the same daisy-chain power supply shared with the first one, they would be greeted with the proverbial "hornet's nest" of noise, leading them to return the new pedal they thought was the culprit, and "defective". The problem was that the two digital pedals, and the power source they shared, lacked the protections against outgoing and incoming power-line spikes.

In much the same way that nearly all of our AC-operated appliances are regulated enough these days that we do not live in fear of mom, dad, or any siblings or kids, turning on anything with a motor and messing up our TV viewing or music listening, musical-equipment manufacturers have taken note of the heterodyning noise risk and begun designing digital pedals and power supplies to avoid and prevent such problems. How well such problems are avoided can depend on what generation of musical device one is using (the newer the pedal, the better the protection). I consider RG Keen, who designed the One-Spot, as well as power supplies for his previous very large employer, a longtime friend (we began corresponding on newsgroups in 1991), and I know he has been vigilant about problems that have cropped up in its use, and done his utmost to protect against them as well as assure a long life for the unit. If noise on a daisy-chained pedal-board does crop up, a "declining" One-Spot would be just about the LAST place I would suspect trouble. And if there IS noise, the clock frequency of a switching power supply will likely be well above what any rock musician could hear. The noise is more likely to be the interaction of the clock in the One-Spot, and a clock in some device, or multiple devices, it is powering, especially if they are older pedals. And remember, clocks are not unique to digital pedals. Analog delays, chorus pedals, and flangers all have clocks.

[mozz]

You can say what you will about the one-spot, but i do know for a fact as they age sometimes they cause problems. Something is going bad in them and making noise. I had 1 here and have also read of other people who told me the same thing, after a few years they get noisy. This is a 1 pedal setup i tried it on. I think i pried/cracked it open and swapped out a few caps with no improvement. Probably tossed it in the garbage. Having worked for a major electronics firm myself, long life is not on the drawing board for 99% of anything you buy today. That was taken away from any engineering dept a long time ago and replaced with lowest cost.