Help me with my Wah hum

[mr_sooty]

I have a Vox Wah V847, the old USA made non-adaptor version. I am currently powering all my pedals with a Godlyke PA9 Powerall, similar to a 1spot. It has 1700 ma capacity, which is more than enough by far even for my fairly sizeable board.

The Powerall comes with a battery clip adaptor, an extension that goes from the daisy chain output to the battery clip inside the wah. I used to power my Wah in a similar fashion, but from a powered pedal board. It worked fine, with no issues.

But with the Powerall, the Wah emmits this low, growly hum. It's OK clean, but quite problematic when exaggerated by overdive.

Does anybody know if there's a way to fix this? I don't have any problems with my other pedals off the Powerall, and I have 10 pedals plus an AB box and a Tuner running off it.

I also have Tru-bypassed it - a mod I'm considering undoing because of the 'pop' I get now. But I had this issue before the tru-bypass mod.

Cheers.

[ChrisK]

If by "low, growly hum" you mean at or twice the mains frequency, the pedal may suffer from inadequate Power Supply Rejection Ratio (PSRR).

This is excessive sensitivity to noise/signal on the power input.

If you know the current draw for the pedal, and know what the lowest operating supply voltage is, you can make a power supply filter.

For instance, if it draws 50 mA and can work with a 7.5 VDC input voltage, and the supply is outputting 9 VDC (boy, those digital multi-meters sure come in handy when working on electronical things), the difference of 1.5 VDC divided by 50 mA (0.05 A) indicates a series resistor of 30 Ohms. At the pedal power input end, add a 1,000 uF 15 VDC electrolytic cap for additional filtering.

Also an isolation diode in series with the power to the effect can be tried.

Or, a 1,000 to 3,300 uF 15 VDC electrolytic cap can be connected to a free end of one of the power supply cables to bulk filter the entire DC bus.


Otherwise, it could be ground loop issues.

[mr_sooty]

Ok, I tried plugging the adaptor straight in with no daisy chain, and the hum was worse, so I imagine it is to do with the overloading you suggested, because if definitely sounds like a mains hum to me (not that I'd really know).

The current draw is apparently 0.54 ma. Not much huh. I have no idea what the minimum voltage is though, and I'm not sure how I can find out.

[ashcatlt]

He didn't say overloading, he said PSRR. It's not that there's too much voltage, but rather that the power supply itself is not giving a steady 9V DC. It's actually some AC signal which swings (hopefully not too far) around 9V.

The discussion of lowest operating voltage is to help determine the maximum amount of filtering you can apply without severely degrading performance. Some voltage will be dropped across the filter, you see.

To determine lowest operating voltage you could wire a pot across the power supply on the way to the pedal. One lug to supply +, one to supply and pedal -, and wiper to pedal +. Turn it down until the pedal fails to operate to your requirements, then measure the voltage between wiper and the - lug.

This is going to be a subjective thing, more like "how much voltage can you live without". I think this circuit it will start to suck long before it stops passing signal.

[ChrisK]

Ok, I tried plugging the adapter straight in with no daisy chain, and the hum was worse, so I imagine it is to do with the overloading you suggested, because if definitely sounds like a mains hum to me (not that I'd really know).

In this case it would be UNDER LOADING.

I believe that your mains frequency is 50 Hz. Full wave rectified AC has a 100 Hz base frequency (2x mains frequency).

Assuming that the low E string on a guitar is aboot 80 HZ, 100 Hz is smack dab between G and G#.

For 60 Hz mains, 120 Hz is smack dab between A# and B.

Now, I said UNDER LOADED since the noise should be LESS if it were a loading issue and all other loads were removed. It may be that the power input filter caps on the other effects are helping a weak or under-sized filter in the adapter. It's just a guess.

It may also be that the new power supply is a switching regulator. This may have some high frequency issues (20 to 100 Khz) on the power output and causing some non-harmonically related sound artifacts especially with overdrive/distortion, again as a function of PSRR. Your old battery-only pedal never considered these things since they weren't used then.


Either way, I'd add some additional bulk filtering capacitance.

For this effect alone, since the input current is under 1 mA, I'd try a 510 to 1K Ohm series resistor from the adapter output to a 1,000 uF 15 VDC bulk capacitor across the power input to this effect.

You could also try connecting the 1,000 uF of so cap directly across the adapter's power output. Mind the polarity please.


Do you have a separate adapter compatible with this effect, and does it hum when powered by it alone?

Is there mains AC wiring (extension cord, amp cord, etc) run nearby and parallel with ANY signal cabling?

[mr_sooty]

Aug 29, 2008 12:12:26 GMT -5 ChrisK said:

Do you have a separate adapter compatible with this effect, and does it hum when powered by it alone?

Unfortunately not, I gave away my last one.

Is there mains AC wiring (extension cord, amp cord, etc) run nearby and parallel with ANY signal cabling?

There is obviously an amp plugged in nearby, and the adaptor is often plugged into and extention cord, but I don't know they're 'in parallel with any signal cabling', because I don't really get what you mean.

For this effect alone, since the input current is under 1 mA, I'd try a 510 to 1K Ohm series resistor from the adapter output to a 1,000 uF 15 VDC bulk capacitor across the power input to this effect.

OK, well I can get those part easily enough - but I'm not sure exactly where to put them. So would I hook the resistor up to the positive side of the extention that goes to the bettery clip? Only I'm not sure which one's the poisitive because they're both black, it's one of this thin double wires you get with adaptors.

And the cap across the input, where abouts do you actually wire that? Sorry guys, my electronics understanding is pretty limited, so I basically need dummies level instruction.

[JohnH]

I think Chris means to check if there is any power cables running across the floor physically near and parallel to any signal cables - if so, move them away.

Just a check - if you dont run this wah on the adapter, and instead run it off a battery, does the hum go away? If it hums the same with battery power then its not the power supply which has a problem.

But back to the cap/resistor idea. Ive done that too and it works well enough to turn an unstabilized unsmoothed supply, into a smoothed (but still unstabilized) one without significant hum. But Im suprised if your good power supply, that works well with other pedals needs this - hence wondering if its another problem.

To use the cap/resistor, the cap goes directly across the power inputs to the wah, ie, if it only has battery terminals, across these or some points connected to them inside. The cap will have a + and - polarity that your must follow, as marked on the can. Then the resistor goes from the + power input on the wah to the + power supply from the adapter. The - power input and - supply are connected together as usual.

If you cant tell which is + and -, you can use a meter, or work it out from the shape of the battery clips, comparing those on a real battery, noting that the clips on the wah will of course, be the opposite way round to those on a battery.

John

[ChrisK]

Thanks John, good interpretation.

It might not be a bad idea to try the wah with a battery again, maybe something has failed since the last time you used one with it (like its internal bulk filter capacitor).

[mr_sooty]

I really appreciate your help guys, and I'm trying to get my head around this as best I can.

I am running it with a battery currently - no hum. This problem only arose when I changed the way the wah was powered. As I say, I used to have a pedal board that had a 600ma 9v adaptor that pugged into it, and 6 mini jack plugs that leads plugged into and ran to the pedals. There were capacitors of some kind between each mini jack plug. When I ran a battery clip extention lead from one of these plugs I didn't have any problems (apart from a ploarity issue, which I fixed that by choping the lead and switching the wires around).

I've moved on from that board because it was getting too small and the little mini jacks kept breaking and needing replacing. Now I have this Godlyke thing, a 1700ma switching adaptor with a daisy chain or two. It's only when running off this that I have the problem with the wah, no other issues apart from the odd radio frequency!

Here's a photo of the guts:



Obviously the grey lead connects to the daisy chain. The tape on the lead is just some protection for the lead at the point it exits the casing.

So how would I connect the resistor to the two positives without butchering the lead? Do I just solder it to the battery clip terminals and not clip the positive ones together - is that the idea? Then clip the negative terminal together only?

As you can see, the battery clip goes into one of those little white plasitc thingees, so there aren't really any points to solder the connect the capasitor - again, would I have to cut the wires to do this? I guess that's be the easiest way. Just concerned about butchering them in case it doesn't work.

[mr_sooty]

If I was just to go ahead and chop the wires, is this how I would do it?:

[ChrisK]

If I was just to go ahead and chop the wires, is this how I would do it?:

Precisely!

Tape the solder joints to ensure a complete lack of shorting.


The small blue cylindrical cap on the PCB (C2 methinks), is fairly small. It appears to be the only electrolytic cap. I doubt that there ever was significant bulk filtering capacitance since this effect was intended to only work from a battery.

[ashcatlt]

How's about making a filtered battery clip adapter to go in the middle? 2 dead batteries (CAREFULLY deconstructed) OR 2 battery clips snipped from elsewhere, 2 wires, a resistor and a cap.

Now you've modified neither the pedal nor the adapter.

It's interesting that this Godlyke adapter advertizes that it has no transformer and therefore no noise.

[JohnH]

Aug 29, 2008 22:56:56 GMT -5 ashcatlt said:

How's about making a filtered battery clip adapter to go in the middle? 2 dead batteries (CAREFULLY deconstructed) OR 2 battery clips snipped from elsewhere, 2 wires, a resistor and a cap.

Now you've modified neither the pedal nor the adapter.

I think that is a good idea. It would look like sootys diagram above, with an extra battery clip on the right leading to the wah. This second clip, assuming it was a bought one with red and black wires attached, would be wired with colours reversed, ie red top and black bottom, to get the correct male and female plug orientation.

John

[newey]

+1 to Ash for the filtered battery clip idea! Nice, no permanent alterations necessary.

If you can diagram this, please post it for others' future reference.

And, it occurs to me that perhaps we should have a sub-board under "effects pedals" for posting of completed schematics/diagrams of pedal mods, thus separating these from the general effects pedal discussion herein. Any thoughts?

[ashcatlt]

I could diagram it, but like JohnH said...

Here's a link to instructions on making battery clips out of dead batteries.

[mr_sooty]

Cool, yeah that's a good idea, making an extra inbetween extention. I think the local electronics shop sells battery clips with wires pretty cheaply, so I'll definitely give this a go! Cheers guys, really appreciate your help as always. I'll let you know how it goes!

[ChrisK]

It's interesting that this Godlyke adapter advertises that it has no transformer and therefore no noise.

Sounds like marketing.

If it has no transformer, it has no (SAFETY) isolation from the mains. Some galvanic isolation means must be in use.

Since this is an off-line switching power supply, it likely uses a high frequency converter that utilizes a ferrite coil inductor with a secondary winding(s)...also known as a transformer.

What they (didn't necessarily) mean to say is that it has no mains (50 - 60 Hz) transformer, and therefore no coupled mains noise.

It is odd that you are getting noise. The adapter likely works at the 40 Khz and up range (perhaps even 100 - 200 Khz) which should be well out of the audio range.

Maybe something else is afoot.

[mr_sooty]

Another point of note is that I have heard of other people getting this hum with this model wah when using a 1 Spot, which is the same technology as the Godlyke Powerall. So it isn't unique to my system. It only happens when the Wah is engaged.

[ashcatlt]

Here's a good link re: the Technology of the Wah Pedal. Examines a "typical" Vox-style wah circuit. Maybe not exactly what you've got, but it's not likely too far off.

Does that help any?

[ChrisK]

I'm beginning to think that it's more than just PSRR. Perhaps there are some EMI issues with the tuned wah circuit.

Does the noise change as the wah is frequency swept?

When you add the electrolytic cap to your interposing battery contact adapter, also solder a 0.1 or 0.047 uF cap across the electrolytic. That will ensure some high frequency bypassing as well.


And, is the negative terminal of the battery actually connected to circuit ground?

[mr_sooty]

Nah, the noise doesn't change, just a constant growl when the pedal is engaged. So this extra cap - just solder it in parralel to the elctro one?

The negative from the battery clip goes into that white plastic thing, and I don't really know where it's connected. But remember that this hum is not there with batteries, and it had been powered off a powered pedal board with no hum.

[mr_sooty]

Aug 30, 2008 23:13:40 GMT -5 ChrisK said:

When you add the electrolytic cap to your interposing battery contact adapter, also solder a 0.1 or 0.047 uF cap across the electrolytic. That will ensure some high frequency bypassing as well.

Wait a sec, wouldn't this short it? Given that the cap would be going between the postive and the negative? I assumed the electrolytic would be OK seeing as how it has a positive and a negative end - not that I know why, I just figure you guys know what you're talking about. But is it OK to wire a non-electro cap across there too?

[ChrisK]

Wait a sec, wouldn't this short it?

No.

Given that the cap would be going between the positive and the negative? I assumed the electrolytic would be OK seeing as how it has a positive and a negative end - not that I know why, I just figure you guys know what you're talking about.

Polarised caps have a positive and negative terminal because they require that the correct polarity voltages be applied on each terminal.

But is it OK to wire a non-electro cap across there too?

Non-polarised caps do not require such polarity concerns.

[ashcatlt]

Capacitors block DC current. The present less and less resistance (terminology correction pending, wait for it...) to higher and higher frequencies of AC.

It will be close enough to a short to higher frequencies, which is what we're looking for here. You would like for anything of greater frequency that 0 Hz bypass your pedal altogether and stop working its way into your signal. These AC currents will not be anywhere near big enough to cause any damage.

[mr_sooty]

Seriously, you guys are absolute freakin' genius's. This worked first go. My local store didn't have those battery clips unfortunately, so after ripping the house apart to find two relatively dead 9v batteries, I made the clips as per Ash's link.

The Electro Cap is 1000 uf 16 V, couldn't find a 15 v, figured this would have to do. The Resistor is 750 ohms (right in the middle of the 500 - 1K suggested), and the green cap is 0.1 uf.

Of course it's all insulated now and tucked away in the Wah casing, but I took this photo before that:



So, the deal is this. The low growl is gone - totally. There is quite a bit of a hiss in the treble half of the sweep, but that is also there with a battery, without the 'Interposing Battery Contact Adapter', so I don't think it's power related. Also, I'm relatively sure that hiss has always been there, but anyway, if you know how to fix that one, suggestions are welcome.

Also, I re-connected the buffer - at least I think that's what I did, that is if this pedal actually had a buffer in the first place - I think it did. When I did the true bypass mod, I clipped the brown wire as shown in this diagram:

homepage.ntlworld.com/s.castledine/greenfuz/effects/tb3.png

So today I just replaced that wire with the black one you can see at the top of my photo. It runs from the top space in that white plastic thing to the input jack at the same point it originally went to - which is the same point the other brown wire is in the diagram. So I think I reconnected the buffer (?). It does seem to have fixed the 'pop' I was getting when engaging the effect.

One interesting thing I noticed while doing this: I wanted to see if reconnecting that particular wire would cause 'tone suck' (I have no idea why it would - but remember, I have no idea about any of this stuff, I just do what you guys tell me!). So after I connected it, I tried my signal plugged straight into the amp, and plugged in through the bypassed wah. There was a definite noticeable loss of brightness - tone suck - through the wah. The interesting thing is, when I disconnected this 'buffer' (?) wire, going back to the 'true bypass' specs, it still sucked the tone just as much!

So I have left this 'buffer'(?) wire in, because it seems to make the 'pop' go away without making any difference to the bypass tone - which is compromised either way. Any comments on this?

[JohnH]

Sooty – first, congrats on your de-humm-idifier device. I’m glad that worked.

I have a comment on pops, since I’ve also been chasing some of these down in my pedal chain recently.

I started building pedals each with a buffered output, because I think at least one buffer at the pedal board or in the guitar is a very good thing for keeping tone from degrading due to cable capacitance. But I’ve been adding more pedals recently, and with my rehearsal setup comprising, guitar, tuner, Bad Monkey, Delay, and JohnH-designed overdrive, booster and cab-sim DI box to go to the PA, I had a total of 7 buffer stages, even with no effects. Actually, it was not too bad practically, only a very little extra hiss when not playing. But I wanted to cut out some unnecessary stages.

It seems the main culprits for stomp-box switching pops are the input and output caps that decouple the circuitry from the jacks, and the right design is to have the outer ends of these caps with a high value resistor to ground. If this is not done, the voltages float and then give a pop when connected by a switch to something at a different voltage. I’ve been adding 3.3M to tie down input caps and 1M on output caps.

But I got stuck for a long time, fixing up a box with this setup, and still getting loud pops when switching it. What I’ve only just realized is that these tie down resistors need to be applied to the whole chain not just one box, and having done that its now working better. I have one more box to fix, and now my pops are reduced to a small crack from what was a huge thump. Hopefully my shop-bought boxes are correctly designed, and so I’m hoping for a nice quiet chain.

John

[ashcatlt]

Yay team!

Glad we figured that out.

According to this schematic, it's not a buffered bypass. In bypass mode, the switch connects input jack to output jack, leaving the input of the effect also connected, but the output of the effect just hangs there. Some people say that this causes tone suck. One would expect the signal to follow the ~0 resistance through the switch, and ingore the rest of the circuit, but it doesn't.

Your true bypass switch disconnects the input jack from the effect input, connecting it instead directly to the out jack. It also leaves the output of the effect disconnected, and seems to short the effect input. That part seems a bit superflous to me, but...

Since both of these arrangements are causing loss of high frequnecies, I'm suspecting it has very little to do with the effect itself. My hypothesis is that you've simply (approximately) doubled the cable length between guitar and amplifier, increasing cable capacitance and lowering the cutoff frequency of the pickup/cable/amp filter.

The hiss, I think, comes from your guitar and is amplified fairly significantly when the resonant peak of the wah filter is swept up into the higher range. I'd be interested to hear it though. Does this hissing seem to change "color" along with the guitar signal as the pedal is swept?

[D2o]

I don't know what you just said, Ash, but +1 from me too.

You are always there with an ever-increasing range of knowledge and advice to offer to those in need.

DD

P.S. - now that enough time has passed to exalt the other members of your circuit posse, +1 to ChrisK and JohnH as well.

Well done, boys!

And of course to mr_sooty, the brave! +1

[mr_sooty]

Sept 2, 2008 13:59:55 GMT -5 ashcatlt said:

My hypothesis is that you've simply (approximately) doubled the cable length between guitar and amplifier, increasing cable capacitance and lowering the cutoff frequency of the pickup/cable/amp filter.

Yeah, that's what I thought, so I tried a cable that was about double the length of the ones I was using - without the effect - and no such tone loss that I could notice.

I wonder if it's actually the lack of a buffer that causes the tone loss? And the fact that your signal is going through those skinny little hookup wires with no buffer? The other interesting thing is that I seem to get no tone suck when I put my signal through my whole pedal board (10 pedals - including the wah - plus a tuner and AB box). I wonder if that's because a few of those pedals have buffers? Maybe they make up for the lack of a buffer on the wah?

So, does it make any sense that the pop from engaging the wah seems to be gone when I rehook up this 'clipped' wire to the input jack?

The hiss, I think, comes from your guitar and is amplified fairly significantly when the resonant peak of the wah filter is swept up into the higher range. I'd be interested to hear it though. Does this hissing seem to change "color" along with the guitar signal as the pedal is swept?

Well it doesn't change colour as such, it just goes away. No hiss at the bottom of the sweep, just a gradual increase which is quite obvious at the top of the sweep.