power supply buzz/hum

[daz]

This seemed odd to me. I have a tube screamer clone that i recently got and it's as quiet if not more than my old tube screamers. It is millennium bypass. With a battery dead quiet when bypassed. Then i tried 3 adapters, one a boss pedal adapter and all of them buzzed, some loudly. I thought for sure the boss would be filtered quite well. Then i tried a old i dunno what that i had gutted years ago and mounted in a rack i made for use with a tube screamer. Thee screamer was never buzzy so i tried it and it was dead quiet. Then i remembered i has a 1 spot by visual sound, a boutique pedal co. Surely this one must be quiet. Nope...buzzed also !

Now i DO realize millennium bypass shouldn't be affected by any noise but it was. Maybe getting in thru the ground i guess. But why would this be? The pedal worked fine with all of them, but they all buzzed when bypassed except that one. I even tried turning the 1 spot around because it doesn't have one prong larger than the other so i was able to try that. But still buzzing. I will just use that one, but it's just the tranny and circuit board and no case and i don't want to make some sort of box for it. The others, especially the 1 spot and boss should be quiet, no? My only idea is a ground loop between the signal cable and the PSU's, but then why is that one quiet?

[52 Bill]

Are the ones that buzz switching supplies? I'm pretty sure that the one spot is. Is the one that doesn't buzz an analog supply?

[daz]

Are the ones that buzz switching supplies? I'm pretty sure that the one spot is. Is the one that doesn't buzz an analog supply?

No idea. But tho i'm not even aware of what a switching supply is, i still can't understand why they would use that if it buzzes with pedals, which is what the boss and the 1 spot are made for.

[The Dude]

Polarity on all supplies is the same?

[daz]

Polarity on all supplies is the same?

Of course. As i sad they all worked, they just had that noise.

[52 Bill]

Switching supplies do not have a power transformer, analog ones do.

Most modern manufactured pedals have a certain amount of power supply filtering and decoupling parts added to the power supply input circuits. This is often just a diode or resistor in series with the input voltage, followed by a filter cap to get rid of any hum coming from the power supply.

Does your new pedal have any filtering in the power supply circuit? How do these power supplies work with other pedals?

[Mick Bailey]

Switching supplies do not have a power transformer, analog ones do.

Every SMPS I've ever seen uses a transformer, except one (a NiCad battery charger where the battery was enclosed). The switching frequency is so high that a tiny ferrite-cored transformer the size of a sugar cube will still provide a few amps at 9v. The output still needs isolation from the input, other than the EMI suppression cap coupled from the secondary to the primary.

[52 Bill]

Every SMPS I've ever seen uses a transformer, except one (a NiCad battery charger where the battery was enclosed). The switching frequency is so high that a tiny ferrite-cored transformer the size of a sugar cube will still provide a few amps at 9v. The output still needs isolation from the input, other than the EMI suppression cap coupled from the secondary to the primary.

Yes, my bad. I meant the large power transformer that we are all used to seeing. SMPS will have small transformers, chokes, etc.

[olddawg]

Yes, my bad. I meant the large power transformer that we are all used to seeing. SMPS will have small transformers, chokes, etc.

SMPSs have a chipper transistor that oscillates at a center frequency around 10khz usually. That's why they can use such a small transformer. But... You generally will not get a hum from a SMPS. They are filtering 10khz. You may get a very high pitch whine or the hash may mess with the clock on digital pedals. If you are getting a hum I would check if you are picking up something radiating into the ground like fluorescent lights.

[Jazz P Bass]

If the filtering inside the SMPS is not sufficient, that may allow double the line frequency through the supply.

[olddawg]

If the filtering inside the SMPS is not sufficient, that may allow double the line frequency through the supply.

Jazz.. I would find that unusual. Most SMPS I've seen have a simple half wave rectifier on the AC mains consisting of one big diode and little or no filtering at all going to the chipper transistor. The transformer secondary is completely isolated from the mains. Regulation is a achieved by varying the chipper frequency slightly around the Q of the transformer. I mean that's basically how they work. I suppose it could radiate into adjacent circuits from the primary side if not laid out correctly.

[R.G.]

I would suspect an oddity about the grounding first, then perhaps a marginal issue with internal oscillation. Actually, I would try to figure out whether the hum/buzz was power-line frequency, 2x power-line frequency, or unrelated to the power supply. This can be difficult to do without an oscilloscope, but sometimes good ears can do it. These will all have different causes.

There is another possible issue, and that is going to sound weird, but I've seen it: perhaps there's a problem with the grounding in your building. Switching power supplies can be so tiny because they work at very high frequencies. They switch above the top hearing limit (20kHz), sometimes a LOT above it. Some go at 1MHz. To keep them from radiating RF noise, the FCC and other national radio-regulators require certain grounding arrangements on the primary/input side. That works well, unless the "ground" on the wall plug is funny, and that includes the "neutral" side of the AC line, which is theoretically within a few volts of the safety ground. Sometimes that "grounded-ness" isn't all that good, and it causes noise emission issues from the power supplies even though they're working right and even though they're only two-prong. As a side note, we're going to be getting more of this kind of issue in the future, as laws requiring efficiency in wall transformers have the effect of outlawing the old-style iron transformers in all new manufacture, so - well, don't get me started.

I do know the insides of several SMPS devices, and full wave rectification is used above the very cheapest and smallest-power levels. The better ones are well filtered going into the chopper/primary side. These higher-end (! still cheap~ ) units also use PWM for regulation and have very wide regulation and good noise characteristics if they're fed a clean AC line.