Hum from DC supply on MXR DynaComp?

[Mikhael]

My question concerns trying to use a DC power supply on the bugger. Being screwed into my pedalboard, it's a pain to unscrew and remove it every time the battery goes dead. I've drilled a hole in the side, and tried several different wall-wart power supplies on it. I get the same result no matter what DC supply I use, or if the supply's plugged into another circuit or not: a horrendously noticeable HUM that you can even hear when I play, and (of course, because of the compression) is extremely loud when I'm not playing, especially if using any distortion (which I don't use a lot of).

It's driving me nuts, because I want to use it, but I don't wish to run off batteries, since a dead one can nail you in the middle of a song onstage. I've tried using caps across the DC input, to reduce noise on the DC supply, from .1uF to 22uF. No change at all was noted in the hum (which I expect is 60Hz). Why this little bugger hums so loudly on a 9VDC supply, yet not on a battery, mystifies me. I know these wall-wart supplies aren't the best on the planet, but I checked them out on an oscilloscope to see how much AC noise is on them, and it varied greatly from one to the other. However, the amount of hum didn't change between supplies; it was always loud.

One other data point: when the guitar input jack was unplugged, there was no hum, even though the LED remained on (I thought maybe there was a switch on the input jack, but the LED shouldn't be on if this was so). But whether the input was connected to a guitar, another pedal, wireless receiver, or even a cable plugged into nothing, made no difference; it hummed as long as anything was plugged into the input jack.

Is it some sort of ground loop, since this unit was not originally designed to run off a DC supply instead of a battery? I don't know - I don't wish to destroy it, so I haven't taken it completely apart. I've done enough to it by drilling a hole in the side, and replacing the original black insulating foam (which rotted away) with orange insulating foam (which will not rot)...

So, does anyone have any ideas?

[Enzo]

22uf? Sorry, but 1000uf is much more like it. Got a 470uf handy? clip that on and see if it starts to help. I have a drawer full of 2200uf 15v I might try. 25v better yet.

And watch the grounding. is one side grounded in the battery connection?

[Mark Hammer]

Adaptors (wall-warts) can vary widely in the degree of regulation they provide. What they aim for is to be as rock-steady as a battery, but the circuit needs to be more complex to get there. Unfortunately, because we generally only get to see them as little sealed black boxes, you can't really tell what is inside. To some extent, weight can be associated loosely with extent of regulation (assuming that more parts = more weight) but that's still no guarantee, because switching supplies can offer substantial regulation, provide loads of current, and weight next to nothing.

If the adaptor itself offers good regulation/smoothing of AC power, then having insufficient smoothing in the pedal itself poses no problem. If the pedal provides good "secondary regulation", then poorer regulation in the adaptor poses less of a problem. The problem occurs when you plug a cheapo adaptor into a pedal that takes only minimal steps to smooth external power.

I frequently recommend that people make themselves a power distribution box. It's a simple thing, really. A little plastic box, a couple of jacks, and a big cap between hot and ground. You plug the wallwart into one of the jacks, and the other jacks can be used to feed several paralleled outputs with the current that is further smoothed out by the big capacitor in the box (1000uf is a good value to use).

[olddawg]

Have you tried a switching power supply like a Onespot or a Godlyke? You will not get a 60 hz hum from a switching supply since they are usually running above 10Khz. I run my whole pedal board off of one $30 Godlyke. They are small, inexpensive, well regulated, and provide a suprizing amount of current for their size. Your standard cheap wall wort is just a transformer, bridge, and pi filter. An invitation to Hum City.

[Enzo]

Well yeah, the average wall wart has minimal filtration. That is why a Boss pedal hums like crazy using a generic wart, but not with the Boss wart. The pedal expects the superior filtration of the boss.

[Mark Hammer]

When it comes to power filtering/smoothing and the sharing of responsibilities between adaptor and supply, it can be a bit like "I thought YOU packed the kids' bathing suits!" when you're 100 miles from home. Each side might mistakenly assume the other is taking care of things.

Wouldn't it be nice if there were some universal standard and accompanying code for adaptors that would let you look on the back of the adaptor and see something like "RL-3" to let you know it incorporated some specific degree of regulation? That way you would know in advance that it was the "right" kind of adaptor for your pedals.

It is the absence of that sort of info, plus the requirement to explain plug polarity, current and voltage requirements, and the difference between AC and DC, that leads companies like Roland/Boss to simply "command" you to buy their adaptors. They probably make no money on the adaptors, but it saves them the grief of handling customer complaints, product failures, and service calls.

[Mikhael]

Well, I found the problem. You'll notice I was mentioning the LED in the original post; well, I found the schematics online, and discovered this bugger never actually HAD an LED! Someone installed a current-limiting resistor and an LED in the box!

Clipping the wire to the LED resulted in a much lower amount of noise, that can be drained with a cap to ground. Now, of course, I can't tell when I have it on without playing, but it works. Interesting, though, that the battery was that much quieter.

I got a schematic from a site called "GEO" that has a "millinium" indicator for pedals, using a bunch of inverters to drive the LED. I'll eventually put one of those together, and stick in there.

Yes, those wall-warts are noisy buggers, and not very well-regulated. My problem is that the units on my pedalboard take 9VDC, 12VDC, 10VAC, and 13VDC. Go figure...

[Mark Hammer]

Batteries are almost always quieter, or should I say never noisier, because there is nothing to rectify and smooth out.

Be careful about the Millenium bypass. It won't hurt anything, but my understanding is that, depending on some of the characteristics of the output stage, some versions can work more reliably or trouble-free than others. That's why you'll see so many different versions of it. So, you may find yourself spending more time than necessary to get it working right.

Ideally, simply adding a resistor and LED should not introduce noise other than possibly an audible click when switching.

[vincediamond]

The wall warts might just be pulsating DC thats why its humming

if the DC is pulsating at 60hz thats why u get hum

AC ripple wouldn't hum , just causes high frequency noises in the power supply

60hz Hum is mostly half-wave pulsating DC
120hz hum is full-wave pulsating DC

[Mark Hammer]

Um, I don't mean to be picky, but the only reason why DC might "pulsate" at 60hz is because it IS improperly regulated AC. It only becomes DC once all that pulsation is removed. If there were something in the regulation circuit that caused a fluctuation over time such that DC rose steadily, then fell sharply, or something like that, you might call it DC pulsation. But if the rate is the rate at source - the wall - then it is the remnants of AC.

[vincediamond]

yes its not AC regulated because most guitar pedals don't the right capacitor value for a wall wart

If you use a wall wart to battery snap adapters you will get alot of HUM

The hum will either look like pulating DC or large AC ripply , its not a steady straigh line voltage

you can put a 1000uf across the battery snaps + and - to filter the wall wart better

circuit that caused a fluctuation over time such that DC rose steadily

Regulators don't cause the fluctuation unless its defective, its the filter capacitors value that determines the discharge time

A short discharge time will cause fluctuation in pulsating DC
you will get a fall time slope from each peak to peak of a pulsating DC waveform which is not good if you want clean power supply and no hum

[bjolly1]

Anyone know where to get the replacement foam for an MXR Dyna Comp? I got a 77 and the foam is a mess.

Thanks!

[Mark Hammer]

I don't think there is any "official" foam. I use "floating" boards like MXR did, and find that I can generally rely on things like the pink foam that non-CMOS chips are often packed in at the retail counter. Other useful substitutes are things like cheap carpet underlay, "hobby/fun" foam that can be purchased in 12"x12" squares at hobby stores.

[EffectsGuru]

I would say that you should velcro your pedals to the board and not screw them down, batteries sound better than power supplies, dollar store batteries. Thats my solution.

[R.G.]

Originally Posted by EffectsGuru

... batteries sound better than power supplies, dollar store batteries...

Define "better".