No boost, just whistle

[Bluesician]

I built a KAT treble booster (with small mod) from a kit provided by Musikding, just to get back into the swing of things. It's just about the simplest pedal you can built, so what could possibly go wrong?

Well...

The engaged pedal produced nothing but a high pitched whistle (sinus wave somewhere in the 6kHz ballpark). I checked the soldering and the wiring a gazillion times, swapped out the transistor, but ended up stumped on the stomp. There's just not enough electronics on board to even produce a whistle.

So if all else fails, turn to google and I found this ancient thread.

https://www.diystompboxes.com/smfforum/index.php?topic=98488.0

Could it be that my cheap Chinese-hey-it's-just-for-testing-small-but-convenient power supply was the culprit?
Sure enough, after hooking up a 9V battery it works like a charm.

So now that I can do some better targeted googling, I learn that lack of amps can lead to whistle but I have two open questions.

1. Why?
2. How can this be the problem in my itty-bitty-booster-box that is not in any chain? It's just one 2N5088 drawing (what? 50-100 mA?) from a 1A-rated block.

[Bluesician]

Only now realize I used a Kat Kit.

[antonis]

Hi & Welcome..

First of all, we DO post schematics of what we refer on AND pictures of our build..

P.S.
2N5088 in that particular circuit barely draws 500μA.. (0.0005A)

[Bluesician]

Hi & Welcome..

First of all, we DO post schematics of what we refer on AND pictures of our build..

P.S.
2N5088 in that particular circuit barely draws 500μA.. (0.0005A)

Thanks for the welcome. I've actually been a member for 8 years, just extremely inactive 

Here are the pics.



(do I really have to post the build? This may cost me credits.)


 

[ElectricDruid]

If it's fine with a battery but noisy with the power supply, it sounds like the noise is coming from the power supply not the booster, right? Which isn't unlikely with a cheap no-name chinese power supply.

The only reason you've not had a problem with it before is because any other pedals you've used have better power supply smoothing and better power supply noise rejection. Simple transistor circuits like this are notorious for having basically no power supply noise rejection - any noise on the rails is going to come straight out of the output, more or less. Which never mattered when batteries were the default way of powering stuff like this, of course.

[antonis]

I'd try to throw some caps here and there..

[Bluesician]

If it's fine with a battery but noisy with the power supply, it sounds like the noise is coming from the power supply not the booster, right? Which isn't unlikely with a cheap no-name chinese power supply.

Right. It is definitely the power supply. By now I tried a few different ones and only the first one produces the whistle. But what I don't understand (with my miserable electronic skills) is how a power supply can cause such an issue and subsequently what I can do about it, other than using a different source.
From what I read it is not uncommon even with proven commercial pedals and branded bricks. There just seems to be a mismatch in some cases.

Someone somewhere who appears to be knowledgeable blames it on heterodyning, but I don't see it. What I forgot to mention is that the whistle occurs even without an input signal. So who's dyning who?

Getting serious about this whole pedal thing, I actually invested in tools and equipment including even a lab PS. Just need to find some time now to free the room and set it all up. I'm not so much looking for a solution as I am searching for knowledge and understanding. Man with a mission.

[Bluesician]

I'd try to throw some caps here and there..

Randomly? 

Can you elaborate a little on what you're doing here? Please?

[antonis]

470μF and  R1 form a LPF (of 3.4Hz corner frequency)..
100nF (preferably ceramic disk) bypasses high frequencies..
100pF feedback cap prevents high frequency oscillation (whining)..
(not essential here but it should make no harm either..)

Of course, none of the above might be effective for such a spaghetti wiring..

[Bluesician]

Of course, none of the above might be effective for such a spaghetti wiring..

What can I say? Italian blood. Don't make me regret posting my "craftsmanship".

[ElectricDruid]

What can I say? Italian blood. Don't make me regret posting my "craftsmanship".

LOL! Fantastic! 

[R.G.]

Every switching power supply has some ripple on its output. This is in general at the main frequency that the switching controller works. And in general this frequency is way above audio, usually 50kHz to 1MHz. But some circuit designs for the switching controls don't work well at very low loads.  For modestly complicated reasons, a given ripple peak may push the output voltage over the voltage needed for regulation, and the low load takes some time to pull the output back down into the regulation range; a heavier load would pull it back down in a short enough time to be ready for another pulse of power from the switching section. More elaborate switching power supplies build in some way to prevent this lower frequency ripple.

In sum, a very low load may cause the output voltage to peak, then ramp down much more slowly than normal operation. This can make the ripple be lower in frequency, back down in the audio range. And when that happens, a circuit without much power supply ripple rejection will let the ripple through and amplify it. Your setup is ticking all the boxes: cheap power supply, very low load, and a circuit that doesn't reject ripple well.

There are other possibilities, but with the info at hand this is my best guess. You could test this by putting a resistor in parallel with the effect. Something like a 100 ohm resistor should at least make it change frequency if my guess is right. Could be something else, but I'd guess this.

[amptramp]

There is a law that states all equipment must be FCC approved and the lower cutoff for generated frequencies is 9 KHz, so a converter that works at 6 KHz may be a deliberate design specification rather than just a side-effect of operating at too little load.  Indeed, there is someone on this board (I forget who) that makes a converter that operates below 9 KHz just to avoid having to go through expensive FCC testing.  Battery-powered equipment gets an automatic pass on conducted emissions because it has no way to conduct emissions to anything else.

[R.G.]

Hmmm. Good point. There is a thread of malicious design that might easily do that.

The EPA (and possibly other countries' equivalents) might have set this up, in a way, by mandating that the no-load power eaten from the AC mains must be below a certain level (which reduces over time) to avoid eating electricity as "vampire power". This had the consequence that switching power supplies started using controllers which went into cycle skipping - the practice of spacing out power pulses to be more infrequent. This cut the idling power, which is proportional to the number of times it switches per second. That has the effect of lowering the ripple frequency, sometimes down into the audio range.

That specifically effects things like musical/audio equipment. I was peripherally involved in an effort which obtained an exemption to the rule for power supplies intended specifically for musical equipment. I wonder if the power supply that started this would suddenly get quiet with more loading - to push it back into a power range that precludes cycle skipping.

[Bluesician]

I wonder if the power supply that started this would suddenly get quiet with more loading - to push it back into a power range that precludes cycle skipping.

I will put that to the test right after I figure out why the whole thing went dead after closing it up.
(Don't say spaghetti, Antonis!).

[antonis]

after I figure out why the whole thing went dead after closing it up.

I bet the whole thing will work just after opening it again..
(sometimes, it works with only loosening back cover screws..)