Vox V847 power supply mod noise

[fiveightandten]

I've added this power supply to a Vox V847 wah. It's dead quiet when running off a power supply that takes a 3 prong plug, but has a lot of hum when running off 3 different 2 prong wall warts. I tried running 9V straight from the jack and it's even louder that way.

Any ideas on what I can do to quiet things down?

[MrStab]

hi five (!),

i'm finding it a bit difficult to understand your diagram. does "power from harness..." come from the power source, and "GRN" go off to the wah circuit?
if so, the 1k5 resistor should be BEFORE the 100uF cap for the power filtering to work. if you have enough current for the wah after the 1k5 resistor (which seems quite high), that RC filter should be pretty strong in killing off mains hum.

don't worry about the safety earth/ground pin on the wall warts, i don't think that's an issue here. Your 3-prong supply is probably quieter for other reasons.

[fiveightandten]

hi five (!),

i'm finding it a bit difficult to understand your diagram. does "power from harness..." come from the power source, and "GRN" go off to the wah circuit?
if so, the 1k5 resistor should be BEFORE the 100uF cap for the power filtering to work. if you have enough current for the wah after the 1k5 resistor (which seems quite high), that RC filter should be pretty strong in killing off mains hum.

don't worry about the safety earth/ground pin on the wall warts, i don't think that's an issue here. Your 3-prong supply is probably quieter for other reasons.

Thanks for the reply. I pulled this diagram from a google search. It was in an old discussion on a different forum and people were using it for their wahs.

Red ("power from harness") is 9V out to the wah circuit.
Green is 9V in from the jack.

It's actually dropping down to about 8.3V loaded, so maybe that 1K5 needs to be adjusted. The hum is noticeable with the wah in toe down position. In heel down, the frequency response of the effect does away with it, so it's more of a high frequency thing.

[MrStab]

ah! okay. 1k5 is pretty high, but if you're getting 8.3V then i wouldn't worry about it. 8.3V is more than fine and inductor wahs aren't generally power-hungry.

so is it more of a whine than a hum? that would likely be caused by switch-mode power supplies. if your filter on the positive rail doesn't remove the noise, i wonder if some is coming through the negative rail.

do the same wall warts affect different pedals in the same way? ones that they might not be designed for?

[fiveightandten]

ah! okay. 1k5 is pretty high, but if you're getting 8.3V then i wouldn't worry about it. 8.3V is more than fine and inductor wahs aren't generally power-hungry.

so is it more of a whine than a hum? that would likely be caused by switch-mode power supplies. if your filter on the positive rail doesn't remove the noise, i wonder if some is coming through the negative rail.

do the same wall warts affect different pedals in the same way? ones that they might not be designed for?

Here's a video if it helps:

https://youtu.be/Xeo5F4tq2fc

I'm pretty ignorant when it comes to power supply design. Is there a way to filter the negative rail?

I tried it with 5 different power supplies
-One spot (hums)
-Boss wall wart (hums)
-Bluecoil wall wart (hums)
-Onespot CS12 (no hum)
-Friedman Power Grid 10 (no hum)

The one Spot, for instance, is 1500mA and proper polarity. It should have no issue powering the circuit. The big CS12 and Friedman are silent.

[willienillie]

The simple solution is obviously to use one of the quiet power supplies, or a battery.  Since wah pedals accentuate higher frequencies by nature, they will be very picky about switching power supplies.  You may improve your filtering somewhat by adding a 100n ceramic cap in parallel with the electrolytic and diode.  Not sure it will remove enough noise, but some.

The 1.5K could be reduced to 47 or 100 ohms and still get the job done, but that's not contributing to your noise, just a little wasteful.

[anotherjim]

Just the other week I had a Jen Super Cry Baby in with power problems. Those were exactly like a standard Jen wah but with an ordinary centre positive 1/8" mono jack socket for the AC adapter and no extra supply protection/smoothing whatsoever. Anyway, the owner wanted to use it with standard 2.1mm stomp power cables so the necessary 2.1mm jack in place of the 1/8" jack that had worn out switch springing preventing battery power from working. Not surprising after 40 years or so. Amazingly, the stomp switch and wah pot look original and still work well!

I used the same power filter wiring scheme copied from a search result (that I cannot find again!). Although the cap was 220uF. The 1k5 resistor isn't excessive and if only about 8v gets through that is fine. Remember those pedals worked ok when designed with ordinary zinc-carbon batteries which usually got down below 7v before anyone thought they were spent.
However, I also added a 100nF ceramic cap across the 220uF as ceramics are better to bypass fast noise spikes.

A tougher problem to solve is the wah pedal with metal audio jacks relies on the screwed up connections for ground continuity via the shell. This makes them prone to ground hum issues. Anyway, I found it to work well with my power supplies, including a switch mode type, so off it went back to the owner. However, in use with an Engl amp, it worked perfectly until the high gain lead channel was engaged when it created one hell of a racket! I solved the problem by telling the owner to put it in the amp's FX loop so the high gain was before the wah, and he's happy with that. But a better solution is to sort out the grounding scheme. Note that modern Wha's have insulated audio jacks so a proper hard wired grounding scheme can be employed.

What I probably should have added, and it's cheap and easy, is to fit a low-value resistor in series with the DC jack negative pin. 10 ohm might be plenty (But even 100R wouldn't stop it working) and just might persuade audio ground currents to avoid the power supply ground. So I think that's worth a shot.

[MrStab]

You may improve your filtering somewhat by adding a 100n ceramic cap in parallel with the electrolytic and diode.

good call, didn't spot that. ceramics are used specifically for higher-frequency noise (and discharge quicker, but not so much a concern here).