Need Help! Unwanted Noise!!!

[J0K3RX]

I guess it's no secret that I love high gainers around here! But, I am having a little bit of a problem with most or all of these designs I have gathered from the Russian forums... I can get them all to work and they sound absolutely excellent except for one damn thing and it's buggin the livin hell outa me!!! The problem is that when I turn the volume back on the guitar I get increased background noise/buzz/hum or whatever you want to call it? I can't understand why it increases/surges as the volume is rolled back but it actually gets louder, as loud as the guitar signal itself which sound like total sh!t! So, if I want to roll back the volume on my guitar to get it to clean up a bit I just get buzzzzzz over the top of the guitar signal??? Doesn't happen when the volume on the guitar is at full throttle only when I turn it down. If I turn it all the way down I hear this faint rustling/static/popping noise also? Sounds like wind blowing over a live mic or somebody touching a mic and rubbing their finger on it? Should just be silence, no? Am I making any sense??

I use a 1Spot 9v DC adapter, no ground loops... tried them all with just 9v batteries, still does the same thing? All my other pedals don't do this - Dr.Boogie, Retail AMT P1, Retail AMT E1, AMT SS-30 etc...? Just these pedals. I have tried just about everything I can think of but still the buzzzz lives on?

Now, here's the funny part! If I stick a tube screamer for example in front of the pedal the noise disappears, no noise at all! I can roll my volume back on the guitar and it's perfect! If I roll the volume all the way off no static noise either??? If I hit the bypass on the tube screamer there's the noise again.. Turn it back on and perfect again?     Any ideas on how to solve this short of building a tube screamer circuit into the front of these things? Which doesn't sound bad at all by the way but the pedals have plenty of gain so that's just a good thing to tighten up the bottom end a bit! I just want to get these things working 100% and this is the only problem setting them back! When I get them working I will share all of the layouts etc and it will be almost a full line of high gain pedals for everybody to build!   I would almost be willing to pay a reward for the solution!    

Shoot me some ideas and I will tell you if I have tried them already or not... I have tried too may to list in one sitting!

Here is my little collection, there are a few others not in here but they are all relatively similar if you look at them. I am working with J201's and 2N5458's in place of the KP303's btw...
https://www.dropbox.com/sh/7sovzyd44dbck7x/2DBRj8mM5a/Preamps

[samhay]

Is your tube screamer true bypass?

[drolo]

I had a very similar thing with a Jazzmaster i put together recently.

In my case it was happening when i was going through my pedal board. Connected directly to the amp, nothing happens.

I read some theories about similar issues. All seem to point towards an impedance phenomenon.

I still need to take some time and try different configurations in order to determine whether if it's the guitar, the cable, a pedal, or maybe a ground loop causing the issue ...

The stock Tubescreamer impedance is rather lowish if my understanding is correct. That could account for the reduction of higher frequency noise.

Sorry, have no definitive solution to offer, but maybe some leads ;-)

[J0K3RX]

I had a very similar thing with a Jazzmaster i put together recently.

In my case it was happening when i was going through my pedal board. Connected directly to the amp, nothing happens.

I read some theories about similar issues. All seem to point towards an impedance phenomenon.

I still need to take some time and try different configurations in order to determine whether if it's the guitar, the cable, a pedal, or maybe a ground loop causing the issue ...

The stock Tubescreamer impedance is rather lowish if my understanding is correct. That could account for the reduction of higher frequency noise.

Sorry, have no definitive solution to offer, but maybe some leads ;-)

I think you are right about the impedance.. How can I raise or lower the input impedance on the pedal? I am mathematically challenged by the way so keep it semi-stupid please  

[jhferguson]

Happened to me on my recent Telecaster build. In my case the volume pot was grounded to the shielding that I had put inside the cavity on the guitar. Could also explain why it is happening with every pedal.

[drolo]

No worries, I am mathematically disabled myself ... ;-)

If i understood correctly, the pedals you are talking about are based on FET stages, so in theory they all have high impedance (which is a good thing usually, i gather) but it also allows more high frequencies through.

The fact the the TS buffer killed those is because its lower impedance, it worked in your favor in this case.

I am really not an expert but before messing with the pedals, try if you can get a good quality cable, and see if that makes any difference. Also try different guitars if you have some available.

You can try to lower the impedance of one of your pedals (maybe a smallish resistor accross the imput, must try some to find a value that works) but it might also change the character of your pedal. I am absolutely not sure about the quality of the advice i am providing here, so use at your own risk :-D

[samhay]

I asked about the true bypass as I was thinking about impedance too. If you load down your guitar (with a relatively low impedance input stage), you will roll of the highs. However, if I recall correctly, the stock tube screamer is buffered bypass and therefore the impedance into it doesn't change when it is 'off'.

[Jdansti]

For the hell of it, try a different guitar. You might want to try a different cable too. These would be a couple of quick checks you could do with what you have lying around.

[R.G.]

Welcome to systems engineering. 

Guitar electronics is a long way from where it started, and it's no longer simple. Worse yet, we have some legacy things that mean we have to just keep pasting over the underlying problems.

Most electric guitars need a high impedance input to work right. So we put high impedance inputs on all pedals, not just the ones that the guitar plugs into. We have to, because guitarists simply will stick every possible plug into every possible jack, even when they're not chemically enhanced. It's what we do to honor Nigel Tufnel, I guess.

High impedance inputs are OK, within reason, but then we want monster gain per pedal, and then we stack many pedals in series, series-parallel, and some people even hook up feedback paths from the outputs to the inputs of pedal chains just to see what happens. And we use an -um, eclectic  setup for powering the clot of pedals and grounding the audio signal, and mix all that together. Sometimes there are issues. Many times there are issues.

Your issue is that your pedal(s) have enough random feedback paths from the wiring, power and grounding, as well as simply electric field and magnetic field around them that they oscillate with some input loading/supply impedances and not with others. It's not necessarily the low or high impedance, it's that you can get lots of feedback paths, and some of them will scream. High gain makes it worse.

The buzz you're getting is probably an RF oscillation inside the pedals that the pedals are reconverting down into the audio range, much like some pedals pick up 1-2MHz RF and convert it down into hearing radio stations. There is a child's garden of nearly every possible noise that can come out of this process.

The cure is to make the pedals stop oscillating internally, and then to see what sensitivities you have left with input impedances. Some pedals only scream when their inputs are open, so the stock true bypass circuit makes them scream. This is what the true bypass variant that grounds the input fixes, but that's not the problem you describe.

The Russian circuits - do they have internal power supply decoupling and filtering? Is there any attention paid to grounding and how to not mix power ground with signal ground on the wires? I would start with that.

[J0K3RX]

Welcome to systems engineering.  


The cure is to make the pedals stop oscillating internally, and then to see what sensitivities you have left with input impedances. Some pedals only scream when their inputs are open, so the stock true bypass circuit makes them scream. This is what the true bypass variant that grounds the input fixes, but that's not the problem you describe.

The Russian circuits - do they have internal power supply decoupling and filtering? Is there any attention paid to grounding and how to not mix power ground with signal ground on the wires? I would start with that.

First off... Thanks all you guys for the replies!

Tried 4 different guitars, one with EMG actives and the other 3 passive - Still does it
I have really good cables and the main one I use is an American Recorder Technologies George Lynch guitar cable, the others are short patch cables - Elixir
So, I hope it's not the cables since they cost about as much as my friggin guitar But like I said, my other pedals are silent so I doubt it's the cables...

My tube screamer is one that I built so it has true bypass...

drolo (David) - I tried resistors across the input but still noise when I roll the vol pot back.. Even if I lower the gain on the pedal way down it still does it. Still investigating this impedance thang!

R.G. - To be honest, I was really hoping you would respond! I read most of your replies in other post even if I am not particularly interested in the topic... You my friend are a gold mine of knowledge, totally respect your opinions!
Ok - yes, there is power supply decoupling and filtering.. Is it enough? I have no idea?  
Next question - Is there any attention paid to grounding and how to NOT mix power ground with signal ground on the wires? I think you have something there! Yes, the power ground and signal ground are all connected. How would you suggest I isolate those from each other? This may be the problem with the BF as well?

Thanks,
Jim

[R.G.]

Can you do a sketch of the setup - guitar to pedal1 to pedal2.... to pedalN to amplifier, and how the power is applied to them?

Do any of them use input jack power switching?

Does a single pedal cause this? Any two pedals? Any three... any N? Does leaving one special pedal out change things? Does the first pedal in the line change things?

There may be one especially-prone pedal, or one that has the problems worst when it's in front of others.

[J0K3RX]

Can you do a sketch of the setup - guitar to pedal1 to pedal2.... to pedalN to amplifier, and how the power is applied to them?

Do any of them use input jack power switching?

Does a single pedal cause this? Any two pedals? Any three... any N? Does leaving one special pedal out change things? Does the first pedal in the line change things?

There may be one especially-prone pedal, or one that has the problems worst when it's in front of others.

I am only running this one pedal into a USB recording interface to a PC. No other pedals except for when I stick the tube screamer in front of the preamp pedal which corrects the problem (when switched on). If I just run my (guitar -> pedal -> USB interface) then I get the noise. Or, if I run just my (guitar -> pedal -> poweramp) I get the same volume pot roll back noise, either way. And there is that strange rustling/popping/static noise with the volume all the way off on the guitar?

No input jack power switching.

Here is the pedal I am working with and a few mods I did, mainly just replaced the Russian KP303 transistors, and a trimmer on the J201/Q1 that's about it..

[R.G.]

I am only running this one pedal into a USB recording interface to a PC. No other pedals except for when I stick the tube screamer in front of the preamp pedal which corrects the problem (when switched on). If I just run my (guitar -> pedal -> USB interface) then I get the noise. Or, if I run just my (guitar -> pedal -> poweramp) I get the same volume pot roll back noise, either way. And there is that strange rustling/popping/static noise with the volume all the way off on the guitar?

That helps. I don't seen anything obvious, but here's what I'd do first:
1. Ensure that R1, R3, and C3 meet at a single point, no more than a couple of mm of wire/trace between them.
2. Install two 1K resistors in series with the input to the first JFET. From their intersection, take a 1nF **ceramic** capacitor to that common junction of R1, R2, and C3. C1 can be left in, but is parenthetical.

If that doesn't fix it:
3. Install a 0.01 to 0.1uF **ceramic** capacitor from the power supply lead of R2 (right at the component, no more than a couple of mm away) to the common junction of R1, R3, C3, and that new 1nF.

If that doesn't fix it:
4. Install a 0.1uF ceramic cap from the +4.5 bias voltage end of R6 to the ground end of R10, shortest lead length possible.

If that doesn't fix it:
5. Repeat (4) for R13->R17 and R18->R21.

If that doesn't fix it:
6. repeat (3) for the power-to-ground for R16->R17 and R20-21.

If that doesn't fix it:
7. Insert a 1K resistor between the drain side of R20 and the gate of the following JFET, as close to the JFET gate lead as you can get it. Also, insert a 100R resistor between the source of the JFET and the junction of R25/C19.

If that doesn't fix it:
8. Insert a 1K between the wiper of the volume control and the gate of the following JFET, placing the resistor as close to the gate of the JFET as you can physically get it.

If that doesn't fix it:
9. Insert a 470R resistor between the source of the last JFET and C22.

For all of these, lead length matters. The FETS you have in there are quite capable of gains in the hundreds of megahertz. A few mm of lead can make a difference.

If that doesn't fix it, we'll have to look harder.   

[J0K3RX]

Ok, I got as far as #5 without much difference.. Got tired and went to bed. I am gonna try the rest tonight.

I did happen to dig out a retail tube screamer "TS-7" one of those ugly grey ones that I picked up at a pawn shop for $10.. Not a bad TS by the way, just as good as all the rest in my opinion.. I did do the TS-808 mod on it a long time ago but anyway it does have the buffered bypass so, I hooked it up in front of the engl 3 preamp and left it in bypass. Totally makes the engl work perfectly, that buffer does the trick! Would it be stupid to just build a buffer circuit and stick it in front of the preamp permanently/always on with the preamp and then just have true bypass when the preamp is bypassed? It just works so good with it on..! Maybe there is a better suited buffer for this option? Suggestions?

[R.G.]

Ok, I got as far as #5 without much difference.. Got tired and went to bed. I am gonna try the rest tonight.

The more RF experienced will recognize that set of stuff as what one might to do quell internal oscillations. It's a bit of a pain, but it's good practice.

I did happen to dig out a retail tube screamer "TS-7" one of those ugly grey ones that I picked up at a pawn shop for $10.. Not a bad TS by the way, just as good as all the rest in my opinion.. I did do the TS-808 mod on it a long time ago but anyway it does have the buffered bypass so, I hooked it up in front of the engl 3 preamp and left it in bypass. Totally makes the engl work perfectly, that buffer does the trick! Would it be stupid to just build a buffer circuit and stick it in front of the preamp permanently/always on with the preamp and then just have true bypass when the preamp is bypassed? It just works so good with it on..! Maybe there is a better suited buffer for this option? Suggestions?

That adds a bit more data. What appears to be going on is that the circuit oscillates if there is a certain critical source impedance on its input. One of the two important features about a buffer is that it has a low output impedance. Putting a buffer in front effectively "shorts" the input with the low source impedance, which can drag down any unwanted feedback and prevent the oscillation.

Works either way. Either make the circuit less prone to oscillation, or put a buffer in front to quell it.