CMOS vs Relay vs FET vs ??? switching for a NPN Fuzz Face

[drolo]

I wanted to build a FF for a friend who needs the switch to be as quiet as possible, so 3PDT's can't be used.

Over the last week i have been reading a lot about CMOS, Relay and FET switching.

I guess FET wont work out too well with a Fuzz Face as it will eliminate *the magic loading* of the pickups ...

Relays seem like they could work in that sense, but how about CMOS switching?


While i was thinking about switches, I wondered if LDR's could be used for that.
They would certainly provide an absolute popless gradual switch, but i'm not certain the on/off resistance is small/big enough for use in a switch ...

Has any one ever tried switching with LDR's ?

[merlinb]

A relay is a switch, so it would click just as much as a 3PDT. CMOS also switches fast, in general, so will introduce click unless you switch during a quiet passage.

JFETs are the way to go. No need to worry about the 'magic loading'; the on-resistance of a JFET is only tens of ohms, which is neglibile compared with the pickups.

LDRs would work too, but it may be difficult to get decent offness and on-ness, unless you use series/shunt switching.

[earthtonesaudio]

I agree with Merlin, sounds like you'd do well with a buffer-less JFET switch.

[drolo]

I agree with Merlin, sounds like you'd do well with a buffer-less JFET switch.

hmm i though JFET switches would always present a buffer, hence the fear the high impedance would mess with the Fuzz Face loading.

Would you have an example of a JFET bufferless switch schematic ?

[merlinb]

hmm i though JFET switches would always present a buffer, hence the fear the high impedance would mess with the Fuzz Face loading.

The FET is only ever fully off, or fully on, so it does not have any active amplifying (buffering) properties under these conditions.

Would you have an example of a JFET bufferless switch schematic ?

Second diagram in this document:
http://www.geofex.com/Article_Folders/bosstech.pdf

i.e., this, but don't forget the input and output caps to block DC:

[R.G.]

Yep, Merlin's right. For truly noiseless switching with cheap, easy to get analog parts, the JFET is the only good choice.

JFETs (and bipolars, and MOSFETs, and tubes, and...) can be used as either analog/amplifier parts or as switches. They have different degrees of on-ness (that is, near-zero resistance when passing signal) and off-ness (acting like an open circuit)  as well as quirks - offset voltages, clicks, pops, and downright difficult-ness to drive.

JFETs are probably the simplest to drive well, and have the fewest quirks. Analog signal switching is one place where the wide variation in gain an other properties don't hurt you much at all.


JFET switches are quiet because (1) they have very little internal bleedthrough of the control signal on their gate, (2) the control signal can be quite high impedance and (3) the control signal can be made very slow without changing the variable-pass-through nature of the JFET.

This last is what makes them really quiet, because the control signal can be slowed down by a simple R-C-Diode filter til the  change from open/closed happens over several milliseconds. This is slow enough that even the small amount of bleedthrough of the control signal is such a low frequency that the coupling caps filter it out and you might not be able to hear it anyway.

Yeah, go JFET. The critical parameter in a JFET data sheet is rdson. This is 100-1K ohms for normal JFETs, often 10-50 ohms for JFETs specially made for switching. The other important factor is Vgsoff. You have to have enough control signal to turn the JFET fully off. In 9V ciruits, this usually means a Vgsoff of 4.5V or less.

The impedance of a pickup, generally 4-18K in series with 2-4H of inductance, is far larger than an "on" JFET, so the JFET causes no perceptible issues.

[drolo]

Great I'm sold :-)

R.G., out of curiosity, with all this FET goodness, what made you go the CMOS way for the Visual Sound pedals (maybe not all versions ..)? Cost, Space, ease of automation ?

[R.G.]

R.G., out of curiosity, with all this FET goodness, what made you go the CMOS way for the Visual Sound pedals (maybe not all versions ..)? Cost, Space, ease of automation ?

I was solving a different problem. Well, different set of problems, or different set of compromises, depending on how you look at it.

CMOS analog switches require much more care to make them click-less, but give you more flexibility in using combinations of logic to make them do slightly different things at different times, and offer a better "systems" approach for a whole series of signal switching, if you want to go that way. I rarely design just one pedal. Generally I do five to seven designs at the same time as part of a product family. Once the basic designs are set, I spend time between "families" working on incremental improvements, and getting more consistent quality and reliability from manufactured units.

The good thing about having things manufactured in quantity is that if you can get the design right, they all come out very consistent. The bad thing about that process is the same as the good thing - mess up the design and you live with paying for several thousand mediocre - or worse - copies. I have the manufacturing engineer's recurring nightmare - that I missed some small thing in a design that turns out to make it unusable **after** many customers have bought it. That's an ugly scenario, and an expensive one.

But I'm wandering again.

Different set of problems. Horses for courses.

[drolo]

What would account for the reputation these Boss and Ibanez pedals have about their "tone sucking" in bypass mode? Is it the use of a simple transistor with lowish impedance as buffer? Or are some of them just badly designed?

- mess up the design and you live with paying for several thousand mediocre - or worse - copies. I have the manufacturing engineer's recurring nightmare - that I missed some small thing in a design that turns out to make it unusable **after** many customers have bought it.

That reminds me to not forget that it was a real person with feelings who messed up the EHX 16 second delay reissue by including a 4 bar count-in, that cannot be turned off, making the looper unusable in a live setup ... i will stop hating him as of today :-)