Fuzz circuit onboard guitar bypass question

[guitylerham]

Hey everyone,

I'm in the middle of a Jazzmaster build and I plan to insert a simple fuzz circuit in place of the rhythm circuit on this style of guitar. This works out well as there is the slider switch for on/off and the two roller knobs for vol/tone. My question is regarding the switch and bypassing the circuit. My ideal design would be: first, to use a stereo output jack so that the battery negative is grounded only when the guitar is plugged in and second, use a 3pdt slider switch to break the battery + from the circuit while in bypass mode to effectively keep the circuit from draining battery when I'm not using the fuzz. I am well aware that this simply isn't done in stompbox design because of pops and switch pole availability (using LED's on the third pole) and dependability, but in this case, is there any way to make this work? Could I insert a capacitor of the correct size to slowly charge once the switch is turned on to try and avoid a loud pop? I bet I could live with a slight hesitation before I'd play my first fuzz note.

What say you guys? Worse case, I'll just use the stereo jack as my only power switching but I wanted to see if I could make this situation even better for my needs. Thanks!

[MaxPower]

I believe a large resistor (1 or 2 Mega Ohms) is typically used to eliminate/prevent loud pops. The pops are apparently due to capacitors (leakage).

This may be somewhat helpful:
http://www.muzique.com/news/pulldown-resistors/

Someone usually comes along and corrects me if I'm wrong, which is usually the case....

[R.G.]

It is only possible to do this if the fuzz is designed in a special way so that its output does not change fast enough to be heard when its power is turned on. This is very difficult to do for effects powered by a single-ended power supply, like a battery.

The "outside" of an effect circuit must have a DC voltage of 0V for a large number of reasons. If the effect circuit is powered by a single-sided battery voltage, the idling "no-signal" DC level INSIDE the circuit has to be somewhere in the middle of the battery voltage. Input and output caps are there to keep this internal DC voltage from getting out to the outside.

When the circuit is turned on, the "inside" DC voltage must be charged onto the inside of the input and output caps, while their "outside" voltage has to be held at 0V. Holding the outside voltages to 0V is one purpose of pulldown resistors, but the resistors are sized for the tiny currents involved in capacitor leakages, not for the sudden, large jumps in voltage and current in the caps at power on.

So the pulldown resistors cannot handle this large voltage change, from fully discharged to something like half the power supply voltage, at power-on without letting through thumps and pops. The change is too big for them to handle gracefully.

The simple thing to do is to use a low-current fuzz circuit and turn on the power when a jack is inserted. Most simple fuzz circuits don't pull all that much current; the biggest current eater is often the indicator LED. Make the battery be a rechargeable LiIon type, and plug the charger into the hidden battery charge jack you have so thoughtfully provided for it on the guitar.

There are other ways, but they are complicated, and relatively untried. You want onboard stuff to be simple.

[anotherjim]

If you're worried about battery life, I suggest that you can't win with that one. A battery will fail sometime and we don't know how good they really are until we use them. If it lasts a very long time, the more chance you'll be caught out when it does fail.

How long will you leave the guitar plugged in? You can switch the battery with a TRS jack socket just like in a stompbox and you don't need an indicator LED if the switch position is easily seen.

As RG suggested, you only have a chance of pop free powering with a balanced or bi-polar powered circuit. If it only uses op-amps, then a conventional circuit should work if the Vref (half supply bias point) is tied to Ground instead of the battery negative. That would work for Tube Screamer or Rat type circuits where any transistor stages are replaced by an op-amp.

For something that must use discrete transistors, like a  fuzz face, you could have a timed mute circuit on the output - but I think that would involve some circuitry that is always powered. Maybe somebody has done that (nothing new under the sun). Search for power-on mute? It has been done with delay effects to mute the garbage in the delay line when power comes on.  It might look something like Boss FET Bypass I think, but with a delay before changeover while powering the FX on or off.

                 

[PRR]

> a chance of pop free powering with a balanced or bi-polar

Not a panacea.

The output will go to zero when the chip is fully powered, the rails are +/-9V, sure.

With some care in finding low-volt chips, maybe +/-2V.

But the rails are coming up from zero. There will always be some time when the rails are too low, the chip is "dead", or at least not fully alive and able to hold its output at zero.

Very typically the output "clings" to one rail until it wakes-up.

So something like a zero to 1V to zero thump.

1V thump is "big" in guitar work.

The universal hack in "real" stuff is muting relays. Which may eat more power than you are trying to save.

The hack in popular-price stuff (car-audio) is to set the ramp-up time long enough that the thump is sub-sonic (the cones jump but you don't hear it).

Way I see it-- we have good (or workable) tricks for many common problems. The ring-finger battery ground so the pedal goes off when un-plugged. The 1Meg bleeders for signal switching pops. There -isn't- a good trick to leave the FX powered off until needed, despite 50 years of crying need.

[R.G.]

I hate it when I think of things belatedly.

It is possible that this could be done with a P-channel JFET set up in parallel with the output pulldown resistor, and with its gate connected to a capacitor to ground and a resistor to V+.

A P-JFET is on unless it's gate is pulled positive, the opposite of an N-channel. A P-channel would hold the output to ground with a couple of hundred ohms channel resistance until its gate was pulled up by a volt or two. It is possible that the R-C value of the gate circuit could be juggled to be slower to turn the JFET off than the fuzz circuit and output capacitor was to turn on.

That would solve the turn-on pop. The problem would then be how to stop the turn-off pop, because this will potentially be as bad as the turn-on pop. A germanium or Schottky diode across the gate timing resistor *might* be tinkered to do this. Maybe.

It's still a question of which what is faster/slower than the other what. But maybe.

Best advice is still just to leave the fuzz power on all the time.

[guitylerham]

Wow, thanks for schooling me on this idea, guys. I think what you're saying is an overly complex solution is possible for the simple problem of having to replace/recharge the onboard battery slightly more often.

The finessing of the values to establish the correct time response so that the output is muted as the pop would occur might just be too much work. I had no idea what to even search for as far as keywords for this topic but I'm glad I asked and received such detailed responses. Good information to have in the archives.

Ok, so I'll concede to just using the stereo jack as my switching. I'm sure the other issue I'll contend with is minimizing noise/interference when having such a high gain fuzz circuit INCHES from the pickups!! The circuit I plan on using is a Mosrite Fuzzrite clone. I really liked the clips of a Catalinbread Merkin Fuzz and so that's what I'm going for. Wish me luck! Thanks again everyone.

[ghostsauce]

Back in the 70s, dad solved this problem on his tele by putting the battery on its own switch. Turn it on when you pick up the guitar, off when it gets put down. No prob!

[guitylerham]

Yeah, that would be a decent option actually. You'd have to remember to turn it on before you take your solo! I like it. I'll see if I can find an inconspicuous place for a switch. Thanks for the suggestion!

Back in the 70s, dad solved this problem on his tele by putting the battery on its own switch. Turn it on when you pick up the guitar, off when it gets put down. No prob!

[thehallofshields]

I'm just gonna throw this out there; someday I will own a Jazzmaster, and when I do I'll make the Rhythm Circuit put both pickups in series. Higher Output, Hum Cancelling, and probably less Treble and more Mids. No Battery required.

[guitylerham]

Do what I'm gonna do on mine and put a 4-way selector switch in place of the Jazzmaster toggle switch, just like my tele. You get your series setting with the 4th switch position. Now you still have the rhythm circuit to keep stock or do something cool too!