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Volume Pedal Noise Mystery

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Volume Pedal Noise Mystery

Postby thelbert » Thu Apr 15, 2021 11:46 pm

Bought a Dunlop DVP4 volume pedal and experienced noise issue (described below). Returned it and received a NEW identical pedal only to have identical problem. Tested two OTHER volume pedals (Boss & Vox - both also passive) with same results. More troubleshooting ensued - I'll bullet point all known information below:

The Noise:
- Definite crackle - not 60Hz hum, nor radio talk or frequency sweep
- Only noisy 0 - 99% - full volume is quiet.
- Noise is persistent (Doesn't only occur during foot pedal transitions)

Electrical:
- Power source is Furman power conditioner.
- Noisy even w/ most basic chain (GTR -> PEDAL -> AMP)
- All tested VP pedals were passive so only one power source to consider.

The Amp:
- NO PROBLEMS using OTHER amps. (The "problem" amp is a 2x12 Fender Deville)
- However, I have no problems when the Deville runs any pedal that is not a volume pedal.

Summary: It seems my amp has some strange reaction to only volume pedals. wtf could be going on? Any ideas are much appreciated. I'll burn the house down if it will fix the noise issue. Thanks
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Re: Volume Pedal Noise Mystery

Postby forumuser931182 » Fri Apr 16, 2021 6:08 am

Very mysterious indeed.
Is there noise with just the pedal plugged into the amp and no guitar connected?
Assuming I’m looking at a suitable amp circuit diagram the fender has a 1meg ohm resistor to ground then a 68k resistor feeding into a 12AX7A valve.
Perhaps the amp would prefer a higher impedance pedal - the Dunlop uses a 50k pot.
Do you always have the volume pedal as the last thing in the chain before the amp - try putting another pedal after the vol pedal e.g a boost or buffer pedal would be good.
Your guitar doesn’t have active pickups does it?
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Re: Volume Pedal Noise Mystery

Postby Wonks » Fri Apr 16, 2021 6:14 am

Have you tried more than one guitar, and if so were the results the same?

Have you tried different guitar leads, changing one at a time?
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Re: Volume Pedal Noise Mystery

Postby Wonks » Fri Apr 16, 2021 6:39 am

forumuser931182 wrote:the Dunlop uses a 50k pot.

According to the manual it has a 250k pot for the volume control and an additional 10k pot for use in an expression pedal mode. https://www.jimdunlop.com/content/manuals/DVP4.pdf

You are plugging the guitar into the ‘input’ on the pedal and the pedal’s output ti the amp?

Is the switch for the aux output in ‘tuner’ mode? if not, I’d put it in that mode, just in case, even if the aux output is unconnected.

As the problem happens with more than one volume pedal, it’s probably not the pedal so the leads are my first suspect. Have you got a multimeter? If so, I’d check the leads for low resistance from tip to tip and sleeve to sleeve (should be less than or very close to 1 ohm) and between tip and sleeve for high resistance (should read open circuit). Anything else and the lead needs replacing or repairing.

With the volume pedal set at max, do you get the same noise if you turn the volume control down on the guitar?
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Re: Volume Pedal Noise Mystery

Postby ef37a » Fri Apr 16, 2021 6:46 am

I suspect there is a tiny amount of DC on the Fender's input. It is standard practice not to put a coupling capacitor on the first stage valve of guitar amplifiers because in theory and mostly in practice the grid is tied to earth/chassis/ground but there is the possibility that it is 'floating' for some reason or, you might have an input valve (almost certainly an ECC83/12AX7) that has a bit of 'grid current and thus a bit of DC voltage on the grid. Swap it around for another.

Or, put another pedal between VC and amp.

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Re: Volume Pedal Noise Mystery

Postby Wonks » Fri Apr 16, 2021 6:53 am

ef37a wrote:I suspect there is a tiny amount of DC on the Fender's input. It is standard practice not to put a coupling capacitor on the first stage valve of guitar amplifiers because in theory and mostly in practice the grid is tied to earth/chassis/ground but there is the possibility that it is 'floating' for some reason or, you might have an input valve (almost certainly an ECC83/12AX7) that has a bit of 'grid current and thus a bit of DC voltage on the grid. Swap it around for another.

Or, put another pedal between VC and amp.

Dave.

If so, the problem should also show itself with just the guitar volume control and the guitar plugged straight into the amp.
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Re: Volume Pedal Noise Mystery

Postby ef37a » Fri Apr 16, 2021 9:28 am

Wonks wrote:
ef37a wrote:I suspect there is a tiny amount of DC on the Fender's input. It is standard practice not to put a coupling capacitor on the first stage valve of guitar amplifiers because in theory and mostly in practice the grid is tied to earth/chassis/ground but there is the possibility that it is 'floating' for some reason or, you might have an input valve (almost certainly an ECC83/12AX7) that has a bit of 'grid current and thus a bit of DC voltage on the grid. Swap it around for another.

Or, put another pedal between VC and amp.

Dave.

If so, the problem should also show itself with just the guitar volume control and the guitar plugged straight into the amp.

Possibly not Wonks, guitar VCs are often loaded by the relatively low resistance of the pickups whereas I would guess the pedal is a standard 1 meg or a half that? The grid current will be nAamps so need a very high resistance to develop a voltage.

But, we is all guessing!


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Re: Volume Pedal Noise Mystery

Postby Wonks » Fri Apr 16, 2021 9:43 am

No, we are not guessing about the volume pedal. It's a 250k pot (in this Dunlop pedal at least, don't know about the other pedals used). It is wired as a potential divider with the input from the guitar at one end of the track, ground at the other and the output to the amp from the wiper, so it should be exactly the same as the volume pot in the guitar, and basically wired in parallel with it, though separated by the length of the first guitar lead.

If the guitar has passive humbuckers and 500k volume pots, then with both the guitar and pedal volumes fully up, you'll certainly get a resultant 166k equivalent resistance which will load the pickups and knock off some treble.

If the guitar has passive single coils and 250k volume pots, then you'll get a resultant 125k equivalent resistance which will also load the pickups and knock off some treble.

I'd always recommend putting an 'always on' clean boost pedal before the volume pedal, set at or just above unity gain, to avoid the pickup loading. I've got a Donner volume pedal which has 100k pot, and the treble loss without a clean boost pedal in front of it is very noticeable indeed.
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Re: Volume Pedal Noise Mystery

Postby ef37a » Fri Apr 16, 2021 9:48 am

I meant we are guessing as to the source of the noise Wonks. I would still swap that front end bottle as a first check.

Treble loss? DON'T get me started on "true bypass"!

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Re: Volume Pedal Noise Mystery

Postby Wonks » Fri Apr 16, 2021 10:11 am

I know bad or missing ground connections within guitars can cause noise issues with reduced volumes, which is why I want to know if it happens with more than one guitar. But that dodgy ground can equally apply to the guitar leads.

Those Planet Waves (now badged D'Addario) leads with the leaf spring additions to the outer jack sleeve always gave dodgy connections on some of my pedals so I threw them away. The idea seems sound, but something in the implementation goes amiss with certain types of jack sockets.

I suppose you could always put a lead in the amp and use a multimeter between tip and sleeve on the jack on the other end to check for any DC voltages present. They'd be in the low mV range if they were, I'd assume?
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Re: Volume Pedal Noise Mystery

Postby jjlonbass » Fri Apr 16, 2021 10:40 am

I'd suspect that the first valve may be imposing some DC voltage on the input too.

This can easily be checked with a digital multimeter with an input resistance of 10M ohms or greater - turn the volume controls right down then plug a lead into the amp input and measure the DC voltage between tip and sleeve of the plug at the free end of the lead. Anything more than a few millivolts here points to a faulty valve exhibiting either grid emission or gas current.

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Re: Volume Pedal Noise Mystery

Postby Hugh Robjohns » Fri Apr 16, 2021 11:01 am

I'll third the query about DC from the first valve. Tiz the most common cause of crackles on volume pots.

Stick an FX pedal with an electronic (not hard wired) bypass between the volume pedal and amp and see if that cures it.
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Re: Volume Pedal Noise Mystery

Postby Martin Walker » Fri Apr 16, 2021 6:04 pm

Hugh Robjohns wrote:I'll third the query about DC from the first valve.

Damn - that leaves me to fourth this suggestion, since it was the first thing that occurred to me as well ;)


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Re: Volume Pedal Noise Mystery

Postby thelbert » Wed Apr 21, 2021 2:36 pm

*UPDATE*: Buffered pedal placed after VP solved noise problem!

Thank you to everyone who gave their thoughts! For the sake of my own understanding and any future reader with a similar problem, could someone explain the electrical principles behind what was happening with my rig?

I just got a multimeter and would happily test the input jack of the amp if that is needed to be more certain of the cause.
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Re: Volume Pedal Noise Mystery

Postby jjlonbass » Wed Apr 21, 2021 4:02 pm

The crackling was due to the fact that ordinary carbon track potentiometers as fitted to your volume pedal tend to be noisy when a DC voltage is applied to them.

The DC voltage in question is coming from the input stage of your Fender amp - the grid of the first valve has a DC voltage on it that was finding its way into your volume pedal.

The valve shouldn't have a DC voltage on its grid of any appreciable magnitude and steps are taken in the design to prevent this. If there is an appreciable DC voltage on the grid, the valve is passing grid current, is faulty and should be replaced - it could cause more damage if it is left as it is.

Here http://www.r-type.org/articles/art-171.htm is a technical description of grid current and its causes.

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