Very quick question about 3PDT

[gcme93]

Hi all,

It seems we invariably use a 3pdt with two of the switches for the signal (necessary for true bypass and no high end losses), and the other to turn on an LED.

Why is it that we don't have the LED unswitched (i.e always on when power is on) and use this extra switch to turn power on and off like a conventional "power on"? Is it something to do with popping, power surges, power up delays...? I can't quite imagine the disadvantage of this, and it would seem to maximise battery efficiency if you only ever have power running through the pedal when its in use, not just plugged in? (the number of times I've left a pedal plugged in after use and found it dead a couple of days later...)

Thanks for any/all replies

[R O Tiree]

Yes, you're right - it's all to do with power surges. It takes a while for the various voltages in a circuit to stabilise. As you switch on, the sudden step change in voltage at one end of each cap passes a current through the cap - that's bad for the next circuit in the chain, your amp and, potentially, for the pedal before it. Almost every piece of audio gear I've ever owned popped as I switched it on. That would be a PITA if it happened every time you engaged one of your pedals. Remember, the current through a cap is proportional to the rate of change of voltage across it. "Pop!" Additionally, as voltages build up, you can get distortion of the signal until it's all stable.

So, you can hopefully see why it is conventional, not to mention that it prevents potential damage to your amp and speakers, to power each pedal up first, then your amp and then design systems to switch input and output simultaneously as silently as possible during the course of play.

Arguably, pure TB (every pedal in the chain, that is) does nothing to improve high-end losses and, indeed, worsens them. Gazillions of buffers in the chain can also be detrimental. Electronically-switched pedals can be made to do so absolutely silently. People often have problems trying to make TB pedals "pop-free", especially high-gain circuits or circuits that have high-gain circuits after them in the chain. For me, the secret is some buffered, some TB - best of both worlds. Pete Cornish, who makes bespoke pedal boards for a great many professional musicians has written a very good article on this subject - The Case Against True Bypass

[gcme93]

Fantastic explanation! Thanks a lot.

I think I'm with you on the buffering vs. TB. It seems very easy for people to "take sides" as purist TB or buffered when a mix is the healthiest solution. Especially with us builders - we can choose exactly where and how we buffer our chains which is surely the ultimate luxury

Thanks again!

[tommycataus]

Sorry in advance for hijacking this thread, but that "essay" is a really informative angle on the whole true bypass debate. What Pete is talking about... is that some sort of buffer box that would go first in the pedal chain from the guitar? What he is talking about sounds like some sort of patchable daisy chain situation:

"My system, which I devised in the early 70's, is to feed the guitar into a fixed high impedance load, which is identical to the amp input, and then distribute the signal to the various effects and amps by low impedance buffered feeds."

I'm quite interested to try this... but being an amateur, I must ask the inevitable question - can anyone explain this concept in layman's terms?

Cheers!

[gcme93]

Last paragraph:
"All my current effects pedals (excluding Vintage Series) which are derived from our large systems have, as the main input stage, a fiendishly clever pre amp that has the same characteristics as the input of a tube amp (1 Megohm/20pF), a highly efficient filter to eliminate the possibility of radio breakthrough and a low output impedance so that any following pedals/ cables etc. will not impose a load on the guitar signal. This pre amp is fitted to all our large stage systems and has always met with huge approval; not only from the guitarist but also the PA operator who is so happy to have constant level and tone presented to his mixing board."

So basically each of his pedals has a buffer before it so that the "high input impedance" ideal is satisfied at EVERY pedal, not just at the amp (which would be the case if your long chain of pedals were all in true bypass mode). The schematic of these buffers are available with a quick search so you can add them in if you'd like, e.g at the start of your effects chain, or perhaps the start and end of your effects chain...

Hope this helps? (p.s no worries about hijacking! The best threads are the ones that develop into lots of ideas)

[Keppy]

Why is it that we don't have the LED unswitched (i.e always on when power is on) and use this extra switch to turn power on and off like a conventional "power on"?

In many/most cases, the LED draws MORE current than the audio circuit. For distortions/boosts, this would kill the battery FASTER. Plus the reasons stated above.

[tommycataus]

You lot are a wonderful bunch! And yes, it was late, I assumed that there would be little chance I would find anything via Google... seems like I have you to thank once more

Tom