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Technology of the Tube Screamer Answered

Some time a circuit Screams wrong as this foot switch flipflop
I can see the arrangement if the design wanted independent control
but that would allow a lack of signal.



8 years ago

Not quite clear on what you're saying, iceng.

The drawing replaces JFETs with "conceptual switches," but two JFETs would still be needed to simulate a SPDT switch in that circuit, if you really need to break each connection when the other is made.

Even though the TS has an input buffer, not breaking the "bypassed" or the "on" signal (obviously not true bypass) must have some detrimental effect. But the buffer eliminates the need for DPDT switching, at least.

It's possible, but you'd be hard-pressed to find technical mistakes in R.G. Kean's writings. He's probably the most well-known technical expert in the field of guitar FXs. He's the wizard behind Visual Sound, and unlike guys like me, an actual EE. ;-). RG also gives of his time and expertise quite freely on websites like Stompboxes.com (and many others), as well as his own GeoFex site... He's got a page dedicated to switching methods as well.

It's possible, of course, to replace the switching logic with a "true bypass" DPDT footswitch. But switching might be noisier than the circuit as designed. From reading RG's comments in the past, I know he's not a fan of "true bypass" switching, and prefers buffered like the TS. But he didn't design the TS, so the geofex page just explains the original TS switching schem... 

I've never built one 'cause I bought a TS clone awhile back for $10, used ;-).


Reply 8 years ago

Thank you for your input and eloquent references.  I myself am an EE also.
I am no audio type, but do understand that contact bounce on a normal
SPDT switch is undesirable.

Consider mercury whetted switches mechanical or an Hg reed switch
activated by a stomp magnet or a tilt Hg switch also operated by a simple
stomp mechanism.

Yes,  a pair of Jfets on a flip-flop does the first layout but it takes an
added low voltage smooth power supply.
If you do add a power supply,  then go all the way  into isolation  and
have the flip-flop drive a pair of opto-mosfets ( see last pic ).



Reply 8 years ago

iceng, it is really nice to have an EE posting here...

Re: mercury switches--they wouldn't be practical in this application. They don't call them "stompboxes" for nothing. If you're not kicking your own FX box by accident, you're dragging your whole effect chain across the stage when you get a little too excited, or your singer or bass player is tripping over your cords. Or the stage is jumping 1/8" because the drummer is kicking his double bass drums in unison. ;-)

That, and try to find anything that's not RoHS in any modern music gear. You won't even find lead solder in Chinese-made gear anymore. No country will let you import mercury switches; not anywhere, not anymore...

With a few exceptions, stompboxes are now almost universally 9V, so the power supply isn't a problem. The audio section needs a quiet supply too.

One of the biggest reasons is cost--a few transistors, some discrete components and a tactile switch cost (far) less than a hi-quality DPDT or 3PDT stomp switch. I doubt you could source a quality footswitch for under a dollar even in quantity. A few transistors and such is probably fifteen or twenty cents...

RG mentions the opto solution in the switching link. Nothin' wrong with that idea. But one nice thing about transistors is there's always a substitute part (or 20 different ones) with the same footprint.

RG has also written about the reliability of typical mechanical footswitches in the past--they are the most likely thing to fail in a pedal. And one of the most expensive to replace.

Frankly, I think it also comes down to the engineering culture in some companies--some, like Boss or Ibanez always seem to use silicon switching. It's probably considered an "elegant solution" to those engineers...

Home builders like me hate SS switching--more complexity and for a one-off they're somewhat more expensive. And a tiny tactile switch requires a carefully-engineered plastic case (which is also cheaper than a metal box). For first-time builders that's more solder joints to ruin, too. There's usually a little more current draw from the SS switches, too.

icengLithium Rain

Reply 8 years ago

Ha ha, Don't you trifle with us so far from your shore,
gmoon is teaching me neat new stuff in the sub USEE dialect :-D