Can a squealing/feedbacking guitar pedal harm an audio interface?

[garrettendi]

I have a pedal that emits a loud squeal when switched on sometimes but not playing. It is tolerable on my amp if I turn the level on the pedal down, but I'm loathe to try it into my audio interface in case I overload something.

This isn't for recording purposes at the moment, just playing around, so the fact its squealing when I'm not playing is not an issue, and its true bypass so no squeals when disengaged. I can fix it properly further down the line, and I can use a VST noise gate in Reaper to help.

Will this loud feedback-y type sound damage the preamps/interface/headphones if its too loud?

[CS70]

No, in the interface etc, an excessively hot signal will simply be clipped (which will be perceived as distortion), so you can't really damage audio kit with any audio signal.

A squeal is likely something that's not excessively loud but it just sounds awful (and can damage your ears if you listen to it at too high playback volume).

[garrettendi]

Thank you kindly CS70!

[ef37a]

Assuming the pedal is 9V powered the absolute maximum output it could deliver is about 3V rms or 9V peak to peak (with a bang up battery) . Any AI with phantom power must stand the pulse as that juice is connected and removed so no, you won't damage it.

I would also suggest that the whole reason that pedal squeals is BECAUSE it is "True Bypass"!
The input 'load', which is indeterminate, is finnaglled round to the output and there is always the possibility that daft T**Tish designer did not switch a load onto the input when the former was removed.

With a PROPERLY designed bypass circuit you have a buffer and an output circuit and all states remain the same and the switching is done by turning a couple of FETs (sometimes bipolar transistors) on and off.

Rant over!

Dave.

[garrettendi]

Thanks!

Yup it's 9V powered from a PSU, the pedal is a Behringer HM300.

The AI is the (also) Behringer UMC204HD so has phantom power, although I am of course not using it on the instrument input!

[garrettendi]

Also just to clarify: the squeal is constant, except when I play the guitar or it is switched off...

[ef37a]

Also just to clarify: the squeal is constant, except when I play the guitar or it is switched off...

Is the noise independent of control settings?

Dave.

[garrettendi]

Turning down the level makes it much more tolerable, I haven't tried the other knobs.

[Hugh Robjohns]

The pedal inherently has a very high gain -- and by your description it sounds like there is an AGC function in there -- so will always be prone to feedback and self oscillation.

You may find that moving the input and output cables away from each other helps to reduce the tendency for self oscillation. Or it could be related to a ground-loop issue with the mains PSU. More likely is that the PSU isn't properly regulating and isolating the supply outputs, so there's either cross-contamination between pedals, or the Behringer pedal is particularly prone to a floppy power supply rail!

It would be worth trying it on an internal battery as an experiment to see...

But there is no need to be conerned about it causing damage to the interface. Your speakers/headphones/ears, on the other hand, is a different story!

[garrettendi]

Yes running on battery doesn't cause any sqeualing so I figured it was the PSU. I will upgrade PSU eventually for sure.

But there is no need to be conerned about it causing damage to the interface. Your speakers/headphones/ears, on the other hand, is a different story!

How can I ensure my headphones/ears are safe with adjusting levels etc?

[Hugh Robjohns]

Keep the volume turned down!

Interesting that it's quiet on a battery supply. So somehow the audio output is getting back to the input through the supply. You may be able to resolve that by hacking the ground connection in one of the audio cables in the signal chain... But it could just as easily be a problem caused by a contaminated power connection on another pedal, or just a floppy, poorly decoupled power rail.

Does this mains PSU feed other pedals?

[CS70]

Yes running on battery doesn't cause any sqeualing so I figured it was the PSU. I will upgrade PSU eventually for sure.

How can I ensure my headphones/ears are safe with adjusting levels etc?

Always start with all levels down and increase them carefully and with attention.
Have a fast mute option readily available (pulling the plug works in extreme cases).

Basically be aware that every time you turn on a system you may have unwanted noises and be careful accordingly-

[garrettendi]

Yep, I had aleady planned to let the pedal squeal, and adjust all volumes accordingly form the lowest setting, to ensure that Reaper's Master bus stays green. Then go from there.

Does that sound like the right thing?

Yes the PSU is powering a total of 6 pedals, 5 analogue and one digital (the Boss TU-3 tuner). I have no issues with any of the other pedals, and 3 of the other are also gain pedals, two of them quite high gain.

EDIT: I was also going to mute my headphones until the levels are green in Reaper.

[Hugh Robjohns]

I suspect what's happening is that one or more of these pedals is allowing the audio signal to modulate the power rail, due to a poor internal design (not unusual), and the power supply unit isn't isolating the power outputs from each other very well (also not unusual).

The result is that the Behringer pedal is receiving a power rail that is being modulated slightly by the audio -- either its own, or from a pedal further downstream, and that audio is then getting back into the high-gain amp's input resulting in feedback and howlround.

A better power supply with properly regulated and isolated feeds would undoubtedly solve the problem.

You might also find that changing the order of the pedals helps, or that removing one particular pedal cures it...

As I recall, there was a discussion here a while back about power supplies and why the good ones are expensive...

[garrettendi]

Yep I will get a better PSU eventually for sure. But for now it's not an option sadly, due to funds.

Will my plans for the setting of level at least protect my headphones and ear? I'll also put a noise gate on the Reaper channel.

[ef37a]

Ah, did not get the power supply point at first reading.

So, do NOT desert us G but the guys at 'thefretboard.co.uk. have had lots of experience with less than well designed pedal supplies.

That said, such problems can be just as much the fault of the pedal design in not keeping signal and supply lines well 'apart'.

Dave.

[Hugh Robjohns]

Yes to the protection -- in so far as you'll know what the maximum level can be.

A noise gate won't help since that just mutes the channel when the sound level falls below a certain threshold... but in this case the problem is that the pedal generates full level output in the absence of an input signal.

In your situation, I think I'd run the Behringer pedal on a rechargable battery as -- to me at least -- the cost of that would be easier to tolerate than nasty howlrounds!

If you were interested in some DIY electronics then it might be practical to improve the stiffness of the power supply by fitting some additional smoothing /decoupling capacitors in the supply outputs... but that would require a careful analysis of the existing circuitry before working out how to improve it and then modifying it in a safe way.

[garrettendi]

Yes to the protection -- in so far as you'll know what the maximum level can be.

Ok great I'll go ahead with that for now knowing I'm safe equipment/hearing-wise

A noise gate won't help since that just mutes the channel when the sound level falls below a certain threshold... but in this case the problem is that the pedal generates full level output in the absence of an input signal.

Damn. Ah well, at least now I know it won't help.

In your situation, I think I'd run the Behringer pedal on a rechargable battery as -- to me at least -- the cost of that would be easier to tolerate than nasty howlrounds!

I could get one of those 9V battery cable things that plug into the power input? It would power from a battery that way with the benefit of my easily being able to disconnect it when not in use. I could then leave the guitar cables connected without running the battery down. It could as you say be rechargable too so I don't keep using up normal batteries.

If you were interested in some DIY electronics then it might be practical to improve the stiffness of the power supply by fitting some additional smoothing /decoupling capacitors in the supply outputs... but that would require a careful analysis of the existing circuitry before workign out how to improve it...

That's out of my ability I'm afraid!

Anyway, at least I know I can use this pedal safely in the meanwhiles.

[Hugh Robjohns]

I could get one of those 9V battery cable things that plug into the power input? It would power from a battery that way with the benefit of my easily being able to disconnect it when not in use. I could then leave the guitar cables connected without running the battery down. It could as you say be rechargable too so I don't keep using up normal batteries.

That sounds like a very practical compromise solution to me.

[garrettendi]

Thanks Hugh. Will order the cable tonight.

[ef37a]

Maplin used to do a range of 'battery boxes' Get one for 6 AAs and you have 9V with MASSIVELY more capacity than a PP3.

If a search does not throw one up PM me as I am sure I have a spare one kicking around I can Jiffy to you. .. Nuffin but you can buy your own AAs!

Ah, beware a battery clip to co ax power cable? I bet most are wired for the logical centre +ve? Most pedals are not.

Dave.

[garrettendi]

I’ve found the Voodoo Labs PPBAT-R which I’ve bought. I checked and it’s centre negative!

[Sam Spoons]

Simply plugging an unterminated DC plug into the socket will usually disconnect the battery, and is much cheaper than a battery power pack.

[ManFromGlass]

Brilliant - tip of the day! Thanks.

[garrettendi]

I've only just realised the Voodoo Labs PPBAT-R is for wiring the battery input from a vintage pedal to a PSU. Not powering a pedal via a battery.

Well that was a waste of £6. I've already ordered it.

I have some reversed polarity cables from the PSU I have. Could I connect the PPBAT-R to the pedal using one of those so I use the PPBAT-R in reverse?

What if I get the positive/negative the wrong way round when using the pedal??