Modified big muff problem - strange interaction with other pedals

[rats_eyes]

Hello all.

I haven't been on here for a while, but wonder if I could pick your collective brains for a moment as I'm stumped by a weird problem with a pedal. I built a modified green russian BMP for a guy. It works and sounds good on it's own, but behaves a bit strangely when used in combination with other pedals.

I made four modifications to the standard circuit...

1. a high / low gain switch - dpdt toggle, switching the first resistor between 39k and 390k (much like the black arts pharoah)
2. high end pot (much like the black arts pharoah)
3. a clean blend control - using this pcb... http://shop.pedalparts.co.uk/Blender_-_signal_mixer/p847124_8681943.aspx
4. a mid boost switch - switchable caps on the high pass filter

When i tested the pedal it worked fine. When my friend uses the pedal on it's own it works fine. However, when he uses the pedal with other dirt pedals in front of it (namely a Boss BD2 and Turbo Rat) the high / low gain switch behaves strangely. The low gain setting seems to work ok, but switch it to high and the gain seems to dissapear. This problem disappears as soon as you use the pedal first in the chain again. No circuitry appears to have been damaged.

I'm struggling to explain this fault, as I couldn't replicate it on my own setup. I'm not sure what kind of interaction between pedals is causing this problem. Could this be phasing? Something to do with the blend circuit? A property of the other pedals that are being added to the chain?

Have any of you experienced something like this? I'd be extremely grateful for any insight or discussion of this. It's got me stumped for now!  My plan (for now) is to increase the value of the resistor on the "high" side of the switch and see if that helps.

[GibsonGM]

Hmm, saw this on the blend site you linked to: 

"Please note: funny things can happen depending on the phase of the signals you're blending. If they're in-phase, you're ok. If they're out of phase you will get signal cancellation."

And what you reported is what it sounds like....that's where I'd start....with a phase reversal snippet, to be able to switch the phase and see if that changes things for the better....

[rats_eyes]

Thanks for responding so fast Gibson.

Could a phasing issue only affect the sound on high gain settings? I guess this would be a combination of inverted and non-inverted signals?

[GibsonGM]

YW, I was on same time as you....Yeah, that is odd, Rats....all I can say is "MAYBE"...depends on the 'foreign' mix thing you put in there.  If you can get the guy around, get around his rig - maybe you can bypass the clean blend, see how his stuff reacts??  You have to eliminate step by step.

Inverted and non inv - yes, that would definitely give off-phase issues.  Any time the 2 signals differ, there is SOME phase issue, but you often won't know it..I mean, every cap you run thru changes the phase a bit!    But if it's enough, it sounds like crap.

I suggest trying to bypass the blend, see what's.  Maybe all it needs is another 'block', to invert at the blend, on a switch....weird if it's only him, tho, so I'd find out just what his signal chain is....what is he blending?    Like it says - out of phase is just that....crud....

[rats_eyes]

He has had issues with a chain which has a BD-2 and a colorsound overdriver clone running into the unit I made, both of which invert the signal (to my knowledge). Think I need to gather some more info on exactly what is in the chain.

[GibsonGM]

He has had issues with a chain which has a BD-2 and a colorsound overdriver clone running into the unit I made, both of which invert the signal (to my knowledge). Think I need to gather some more info on exactly what is in the chain.

I'd say, first glance...don't rule out something HE's doing, and start blaming your pedal, til you figure out what he's doing to your nice things

If we can get "ID's" on the pedals, and schematics, you can see which would invert the signal...the vendor of the blend said that could be an issue - I wonder why they didn't design a correction into their blender?

[TejfolvonDanone]

If i understood correctly the blend control only blends the BMP to the input signal. So phasing can't be an issue becuase the phase of the BMP's output doesn't depend on the gain switch.

[duck_arse]

Hello all.
This problem disappears as soon as you use the pedal first in the chain again.

this makes it sound like a buffered/unbuffered loading type of problem thing. perhaps you could check for DC on his pedals' jacks.

[GibsonGM]

Good possibility, Duck...nice eye on that.  Some kind of bias shift?

[TejfolvonDanone]

There should be a decouple cap, shouldn't it?

[GibsonGM]

Depends on where it is, Tej...I think Duck is suggesting that a cap may be bad in one of the OTHER pedals, making it seem like THIS one has a problem...

[slacker]

What do you mean by "the gain seems to disappear"? Do you mean it sounds cleaner or something else?
Your mate could just be overloading the Big Muff if you hit them with too big a signal they can go all compressed sounding and lose volume.

[rats_eyes]

It seems to sound cleaner and more quiet. This change seems to happen gradually, starting normal but the gain dissappear (only on the high gain setting) after 5-10 minutes. It still works ok on the low gain setting. Apparently his pedals are not set to high gain. It's a bit of a mystery as I haven't been able to replicate the problem myself yet.

I wasn't aware that muffs can respond in that way to a powerful signal, I haven't experienced that myself

[slacker]

If it happens after gradually over time it's not the issue I was talking about. It could be a bad connection or leaky cap somewhere in the build but it's going to be difficult to figure out if you can't replicate the problem.

[GibsonGM]

Really sounds like HIS problem, and you'd need to isolate this to the one pedal that does this with yours.

You CAN measure for DC at then input and output of YOUR pedal, to determine that your caps are OK...

[rats_eyes]

Thanks for all of your input folks!

I got this pedal back to try and diagnose this and I have managed to replicate the problem on my own setup. I put a few other pedals in front of it (RAT and a pharoah). At first this worked fine. Eventually, after 10 minutes or so, the high gain setting reduced in gain until it sounds almost clean. The low gain setting remains the same as before. This issue remains for a while, the pedal function normally if it is unplugged for a while and then used again.

I then tried it on it's own and the problem occured again, with no other pedals in the signal path.

I don't know a lot about buffers and loading, so not sure how those issues may be affecting this. I've definitely got some learning to do there.

Could my gain switch be creating an issue. It is a break before make switch. Could the fact that it breaks contact before reaching the next pole be doing something to the circuit? Could the blend buffer circuit be interacting with the switch in some way?

[GibsonGM]

If it happens after gradually over time it's not the issue I was talking about. It could be a bad connection or leaky cap somewhere in the build but it's going to be difficult to figure out if you can't replicate the problem.

+1,000.     When something 'works up to a problem' like this, you suspect caps, things that can have a time constant associated with them.  OR something that gets warm (but how warm is something getting on 9V??).     

No clue why someone would want to put a Rat before a Muff, LOL...but to each their own, and this should be happening.

Really don't think your switch has anything to do with it.  Now, the path you are switching to VERY well may.   So - what's in that circuit branch for caps?    Are you using any 'blend' action here, or does this happen just Rat >> Muff?      You need to aggravate it, eliminate things.   Say it does this only if you are blending...then maybe a bad cap on the blend board?

Collect more data!!  Draw out how this is set up when you have the problem, with block diagrams, and post it...incl. buffer...a problem with a cap CAN 'interact' with a switch - it can be switched into a branch and cause problems....we'll look for you for more info, Rat! 

[rats_eyes]

I've been testing this thing all afternoon, it seems to fail on the high gain path eventually however it's used. I think this is accelerated by ramming a bigger signal into it, but I've managed to make the problem occur without stacking other pedals in front of it as well.

The only difference between the high and low gain signal path is one resistor. The first resistor in the circuit is usually a 39k. My switch toggles between this and a 390k (for low gain). My caps are a mix of electrolytics (63v and 25v) and mylars. Maybe one of these is leaking or faulty in some way. I've had the audio probe on this, so fra it's been hard to isolate the problem but I have more work to do here.

My BMP circuit is from this tagboardfx layout - http://tagboardeffects.blogspot.co.uk/2012/06/ehx-green-russian-big-muff.html

The only change I have made is I used a jumper in place of the first resistor, added a DPDT toggle switch before the input with a 39k and 390k resistor on each side.

I used this exact circuit previously, before adding the blend. It worked perfectly for a two hour rehearsal, so I think I can say that it was fine without the blend circuit. I feel comfortable interpreting the BMP circuit, but the blend buffer I'm not so good with.

I'm going to increase the resistance before the gain switch, so that both gain settings have some more resistance. Hopefully both sides might work this way.

[PRR]

> after 10 minutes or so, the high gain setting reduced

Check electrolytic cap polarity.

There aren't many things in a little pedal would cause a 10-minute wait.

That it only happens in one setting *may* be because the other setting is less sensitive to cap leakage.

Of course a 10-minute trouble *could* be overheating. I'd expect much shorter with small parts. But in a 9V system it is safe (may be loud!) to put your finger in there. If you have ditch-digger fingers like me, put your nose close: you may smell "cooking" but will also feel the rising heat on the less-calloused skin of your nostrils.

[GibsonGM]

Rats...the tagboard is the problem   I might get crap for this but - we talk in terms of schematics, not 'tagboard'.    If you made an error on there somewhere, you COULD cause an overheating issue.  Or, if you didn't wire the resistors in to the switch the right way - that's why I was asking for a drawing or schematic diagram that you're confident in, showing how you did it.
If the switch shorts something etc etc, that would explain it all.

Like PRR said, check the caps, check how the switch operates, be sure there's no overlooked problem due to the switch.  What you want to do is TOTALLY do-able...you shouldn't need to 're-adjust' the resistors to get this to work.  There's an error there, you just have to find it

Look REALLY CLOSELY at that jumper - I suspect there may be a problem there...maybe you should do the switch off-board...one position = straight thru to the 39k...other position ADDS IN a 330K in series...then you can keep the board as it is laid out...?