Forum-Vibe AC hum issue

[JimSalabim]

Hi! When turning on my Forum-Vibe, I get this AC hum (see MP3). I am powering the pedal with a 18V AC wall wart adapter.
Any ideas how to fix this? Would removing the rectifier and powering it with 18V DC help?
Here's the MP3: http://www.matthiasstockert.de/forum_vibe_hum.mp3

[GibsonGM]

Hi, sounds pretty good, really, Jim!  But yes, I can hear a little bit of 60Hz hum in there.  There are many reasons this can happen.  Yes, using 18VDC would remove that if your source of DC is well-regulated!   But....

I haven't seen the schematic, can you post it?  A pic of the guts would help, too, showing where power comes in & is rectified.  Often you can get rid of this sorta thing with a resistor and cap, just some more power supply filtering.

Another way the hum may get in there is to be coupled into other components...power wires can act like small transmitters and couple this hum into nearby signal wires.  The way that's cured is generally by examining the layout, and being sure your power wires are routed well away from signal lines, usually along the wall of the chassis.  Using shielded input wires with one end grounded is very helpful.  If the hum gets in before the 1st amplification stage, it will also be amplified - it's less likely to make an appearance 'down the line', as your nice signal has been amplified above it...unless it's coupling to those lines too!  Layout can be critical... 

[JimSalabim]

Thank you very much! Here is the layout and pictures of the pedal.

Note that I don't use exactly the same wiring as shown in the wiring layout: I don't use the Cancel-Bulb switch, I use a simple power switch instead (connected to the PWR points on the circuit board).
I have also done two simple mods (one for unity gain and the other one with a trimpot, but I don't really remember how the mod was called – anyway, the hum was there before the mods already)

[GibsonGM]

Ok Jim - nice work on the pedal - it looks pretty good    From where I'm sitting, I think I can see signal wires crossing power, and power wires would ideally be twisted together for hum cancellation.   Not that there is at ton of room in the enclosure!   

Looks like maybe some power wires are also passing very close to a jack at the bottom of the pic?  And signal wires passing over the rectifier...

You can try dressing the power wires better and using shielded wire for your input & output, or go with your initial idea of dumping the bridge and going to DC.  The power is well-filtered on the board, so it's not coming from there, I think, other than maybe coupling to the blue wire passing over.

I'd plug it in and use a wooden chopstick to 'mess around' with the wires, see if the hum decreases if you move something around, which would tell you you're in the right area!   

FYI, If a signal wire absolutely must cross a power wire, it should do so at right angles...but you really should not have AC anywhere near signal if humanly possible...and twisting them is a great idea.  Look at the tube heater wiring in this pic for an idea:  http://homepage.univie.ac.at/markus.breier/php/amp_ac/building-of-the-amp/wiring/

Let us know how it works out!

[JimSalabim]

Thank you! I was just a bit too fast now, because I was too curious if dumping the bridge rectifier and using DC would work. First I have used thicker power wires now and routed them along the wall of the chassis and away from the signal lines (at least as far as possible), but this alone didn't help. I haven't tried twisting them together unfortunately, but I will keep this in mind for any future pedals.
Then I removed the bridge rectifier and took the DC power from my Fuel Tank Junior with a voltage doubler cable. And ... the hum is all gone now and everything works fine
I'd be interested in knowing if twisting the wires together would also have helped, but now, as everything is fine and I don't even need an external AC adapter anymore, I'll leave it as it is ;-)
Thanks so much for your help!

[GibsonGM]

YW Jim, glad it worked out!  Yes, the DC is fine, I just didn't want you to "waste" the effort you'd put in.   Yup, twisting the wires helps a lot (different phase relationship, the noise cancels..).   This is from building tube amps - common practice. 

You don't have to use thick wire for low voltage like that!  Might be easier to fit it all if the gauge were smaller; only a concern for line power.  And lastly, the routing of the signal wires - that'll be important to you, so note it on your next build    Plus the shielded input wire, can't hurt to include it.  Esp on high-gain pedals (distortions...).

Enjoy the vibe!

[R.G.]

One recurring issue that comes up with vibe clones is the AC power jack. You simply MUST make this jack be isolated from the metal enclosure. If either side of the incoming AC is attached to the chassis by the jack, the connection of the signal ground to the chassis with the signal jacks makes the "ground" side of the incoming AC jump up and down by one diode drop every AC half cycle, and this in turn makes for intractable hum.

There are other ways to get hum, but this one is both in-your-face big hum and very subtle to figure out.

I can't tell from your pictures if you have this problem, but it's a so common that whenever anyone says "...-vibe" and "hum" it springs to mind.

[JimSalabim]

Thanks R.G.! In my case it was not this problem, but I have experienced exactly what you say, because on my search for the hum I have connected one side from the AC power jack to the signal ground only for a few seconds accidentally and that gave really gruesome hum – far far worse than the little hum that I had.