TS circuits - dry signal leaking through with hot input signal?

[grolschie]

Hi there,

I have an issue where with my Bad Monkey I notice when I play with high output humbuckers, and strike the strings hard, I can hear the clean signal loudly through the dirty signal. If I wind back the volume knob on my guitar this effect seems lessened. I am sure I heard similar things with my friends TS-9.

Is there a part of the TS design that would cause very hot input signal to leak through?

Thanks in advance.
grol

[BAARON]

It wouldn't be a fault with the Tubescreamer circuit, but rather a fault with the electronic bypass system used in those pedals.  You'll notice a similar problem on the Boss SD-1 Super Overdrive: if you turn the gain up high and then bypass the effect, you'll be able to hear a faint amount of the dirty signal bleeding through into your bypassed signal.  The SD-1 has a workaround/fix, but I don't know about the Bad Monkey.

[grolschie]

Thanks for the reply BAARON.

Can anyone please help me understand the properties of transistor bypass circuits that would cause this effect to happen? And what can be done, besides converting to true-bypass? I noticed a small amount of the wet signal bleeding through on my Arion SCH1 Chorus when on bypass mode once some time ago. Thanks in advance.

[aron]

Wait a minute, this happens when you are in distortion mode, not bypassed right? This is normal. If you search back through the archives, there are posts on this and in fact, if you look a the schematics - Shaka Braddah (original), I make a reference to this. Also, some of my tube screamers do this too. There IS clean signal mixed with the distorted signal in a TS circuit (I believe).

[aron]

Try a search for screamer and clean.

[ayayay!]

Aron is correct.  This is definitely most noticeable w/ high output humbuckers.  So that result is always there (but not as noticeable with other pickup) which is one of the reasons I think a TS sounds great with a Tele, and a turd with a Les Paul. 

[grolschie]

Dag nabbit! Thanks guys. 

Yep it happens in distortion mode. So there is nothing that can be done? Perhaps some LED or diode clippers to dirt like in a Rat, but doubled up enough to only clip the very peaks?

[Processaurus]

That's interesting, it totally makes sense, because using the non inverting opamp configuration, the math is that you get a gain of 1 + whatever happens in the feedback loop (clipping from diodes and gain from the pot) to what the opamp sees on its non inverting input.

Sounds like you have an idea of what to do, you can clip that gain of 1 signal (the clean signal on the noninverting input that pokes through), with a shunt diode arrangement (back to back diodes to ground) after the opamp, like the Rat, remembering a small resistor in series with the opamp output so it doesn't try to stuff a bunch of current down the diodes.

LEDs might let the regular distortion through and only clip big clean transients like your guitar does.

Not that wikipedia's the ultimate source for electronics knowledge, ha ha, but here's a bit on the mechanics at work:
http://en.wikipedia.org/wiki/Operational_amplifier_applications#Non-inverting_amplifier

[grolschie]

Thanks for that. What clipping threshold voltage would it need to be with diodes after the first opamp stage? I am thinking that I don't want to clip single coils with the diodes to ground, just a real hot signal from the high-ouput humbuckers. So maybe I should double some diodes up.

[grolschie]

Sounds like you have an idea of what to do, you can clip that gain of 1 signal (the clean signal on the noninverting input that pokes through), with a shunt diode arrangement (back to back diodes to ground) after the opamp, like the Rat, remembering a small resistor in series with the opamp output so it doesn't try to stuff a bunch of current down the diodes.

Interesting. I added red LEDs to dirt between the op-amp stages, after the 5k resistor. One of the LED's is on all the time, the other doesn't light up, and the signal is massively reduced. I would've thought the LED only lights up when clipping. It seems like it's shunting a heap more than just the peaks. Should I be doubling up LEDs in series? Should they be between the op-amp stages, or near the output?

[Processaurus]

Oh, that must mean that there is a DC bias on the signal (the rat must decouple the signal with a cap before the diodes).  You can put a cap (.47uf would work) in between the connection from the back to back diodes (red LEDs for you) and ground.  That will block DC bias (you likely need to keep the bias to get things downstream to work right) and only let AC signals get shunted to ground by the diodes. 

EDIT: You can use a electrolytic cap to block the DC, might work out better to use a bigger value (how about 4.7uF), to be sure to clip low frequencies just like the mids and highs.  Just put the minus side to ground since there'll be ~3v coming through the diodes on the plus side.

[grolschie]

Thanks for that. Yep, the Rat has a cap before the clipping diodes. Does it matter if the cap is between the signal path and the LEDs, vs between the LEDs and ground?

[slacker]

Instead of using a cap you can just connect the LEDs to the 4.5 volt vref instead of ground.

[Processaurus]

Instead of using a cap you can just connect the LEDs to the 4.5 volt vref instead of ground.

That'll work, and save a cap.

If you do use a cap it would need to be between the leds and ground, not inline with the audio signal to the rest of the circuit, as that will remove the dc bias the next opamp needs to see (unless it gets re-biased somewhere further along).

[grolschie]

Thanks guys. Will try this. :-)

[grolschie]

Tried it with red LEDs. Could still hear the clean sound. Tried germanium diodes instead. Really dirty, and couldn't hear the clean sound (I think). So somewhere in between could work.

[John Lyons]

Do you have a schematic of what you have built?
With LED as in the Landgraff dynamic OD, son of screamer, YATS (yet another tube screamer),
they all don't have any clean coming though. It may be a matter of some other part values, not
just the diodes.

[grolschie]

Interestingly, I noticed the same thing a while back with my friends TS-9. Only with high output humbuckers when hit hard. I didn't notice anything with single coils.

[BAARON]

...using the non inverting opamp configuration, the math is that you get a gain of 1 + whatever happens in the feedback loop (clipping from diodes and gain from the pot) to what the opamp sees on its non inverting input.

...you can clip that gain of 1 signal...

For the record, I'm pretty (i.e., very) certain that when the output of a non-inverting op-amp is 1+r1/r2, it doesn't mean "a unity signal at the output of the opamp, and a seperate amplified signal in the feedback loop at the same time."  It means that if r1=400 and r2=200, then the overall gain is 1 + (400/200), which works out to 1 + (2) = 3.  The gain would be 3... not 1, with another signal with gain of 2 happening in a different place.



I've never noticed a clean signal leaking through any true bypass screamer-style circuits, whether with low output single coils or hot hot humbuckers (or any kind of clipping diode or lack thereof).  I don't know how it could actually be possible, except possibly by interference from tone-goblins.
However!  Something like an Ibanez TS-9 or a Digitech Bad Monkey is not true bypass.  It's electronic bypass, so the effect and the bypass part of the circuit are never physically separated: rather, the signal is "electronically persuaded" to choose one path or the other, you might say.  And I know from experience that sometimes electronic bypasses leak in one direction or another: as I mentioned before, the dirty signal from a Boss SD-1 mixes/leaks into the bypassed signal if you turn up the effect's volume and gain knobs, so it's quite conceivable that the clean/bypass signal on a TS-9 or a Bad Monkey might mix into the dirty signal if you're using high gain pickups, which is exactly what you're describing.  I really don't think the culprit is the overdrive part of the circuit: it must be the bypass circuitry, because I've never heard it happen with any true-bypass version of the Screamer style circuit.

[Processaurus]

...using the non inverting opamp configuration, the math is that you get a gain of 1 + whatever happens in the feedback loop (clipping from diodes and gain from the pot) to what the opamp sees on its non inverting input.

...you can clip that gain of 1 signal...

For the record, I'm pretty (i.e., very) certain that when the output of a non-inverting op-amp is 1+r1/r2, it doesn't mean "a unity signal at the output of the opamp, and a seperate amplified signal in the feedback loop at the same time."  It means that if r1=400 and r2=200, then the overall gain is 1 + (400/200), which works out to 1 + (2) = 3.  The gain would be 3... not 1, with another signal with gain of 2 happening in a different place.

Thanks, I realized my sentence is kind of garbled, what I meant is that a non-inverting amplifier circuit's output will have an exact copy of what it has on the non-inverting input added to whatever kind of gain you are making happen with the feedback loop to the inverting input.

Opamps use their output to try and keep their inputs at the same voltage, so with a circuit using feedback to the inverting input, any voltage seen on the noninverting input will be amplified at least by a gain of 1, to try and make it back to the (-) input.  Put something in the way, like back to back diodes, and the output instantly rises to a diodes turn on voltage, and once there, will continue to rise if the (+) input is higher than the diode turn on voltage.  That's why small signals (P-P less than the turn on voltages of the diodes) don't show up sounding clean on the TS's clipping stage output, there is a ton of clipped gain from the output trying to produce something that after being shoved through the TS's complex feedback circuit, will look like a unity gain signal to the (-) input.  But a big signal, like a big strummed guitar with atomic humbuckers, bigger than the turn on voltage of the diodes, will make the output of the opamp turn them on and continue to go bigger to make a matching signal on the (-) input as it sees on the (+) input.

Maybe then the most scientific solution to clamping the output is not clipping diodes on the output of the opamp, but clipping diodes (same as the ones in the feedback loop, Si probably) on the non inverting input, to Vref (or through a cap to ground).  That way the opamp's (+) input never sees a signal bigger than the clipped signal the opamp makes with the diodes in the feedback loop.  But the original poster's diodes after the output will work fine too.