Switching Questions

[niggez]

I was looking at the different kinds of bypasses at geofex, so far ive always used dpdt true bypass without an indicator.
I was wondering if it was possible to use some kind of diode to prevent signal loss in a non-true bypass configuration. Using a dpdt switch, i could use one side for a status led, and the other to switch guitar signal to effect input, or to effect bypass. At the output of the effect, i would place the diode...so that current would only pass from inside the effect, but not from the bypass back into the effect...would this keep any signal from entering the circuit "from behind"?

[R.G.]

Diodes are one-way valves for electricity. Signal is alternating current, flows one way for a while, then the other way for a while.

So yes, it would keep the signal from entering the circuit backwards - in one direction only. It would also keep signal from leaving the effect in one direction only, and subtract its 0.5 to 0.7V diode voltage from what signal it let pass.

Good, creative thinking, but not practical.

[niggez]

Okeydoke nevermind then 
How about this idea here: Im using one half of the dpdt to switch the outputs, the other half to switch between 2 diode/photocell configurations, with a status led lighting up together with either one of them. Could this work?
Does it make a difference weather the actual switch or the ldr-switch is located at input or output?
Are there photocells that have opposite from normal effect, that way it could all be done with one led and 2 photocells, one resistance when on, one when off, and the other way around...

[R.G.]

It could work - except that LDRs are not all that wide-range. An LDR does not vary between 0 ohms and infinite ohms. It varies from a low of a few hundred ohms to a few thousand ohms, up to a maximum of a few hundred Kohm to a few megohms.  That is, the ratio of off-resistance to on-resistance is not large. And that's the killer with LDRs.

If you have an LDR that goes from, say, 1K when its LED is on to 2M when its LED is off, and it's feeding a 1M amp input, then the voltage at the amp when the LED is on is essentially the full signal (1M/(1K +1M) times the signal). And when the LED is off? It's 1M/(1M +2M) or 1/3 of the normal signal - it does not turn fully off, and you still hear the "off" signal bleeding through.

The on to off resistance ratio is one measure of the "Goodness" of any switch element, and the metal-contact switch or relay is the best on that particular measurement; these go from milliohms to thousands of megohms. Nearly as good are JFETs, which can go from hundreds of ohms to hundreds of megohms, and MOSFETs, ohms or tens of ohms to thousands of megohms.

The LED-photoFET modules H11F1 through F3 would probably work in the setup you show. They are JFET elements set up to be turned on and off by LED light. See my discussion of this in the "Technology of Bypassing" article at GEOFEX, where I show the Clinton Bypass, named by its similarity to William Jefferson Clinton saying under oath that he didn't think the questioner could ... prove... it wasn't true.

If you're doing this as a mental exercise, good for you. But go read GEOFEX and Kevin O'Connor's The Ultimate Tone on switching first. It will save you a whole lot of needless searching and running up against things that have already been done or discarded.

There is one switching setup which uses diodes to switch analog signals, the diode-ring switch. But it can only handle signals under about 25mV without heavily distorting the signal.

[niggez]

The article is very informative. Nevertheless, I will experiment some more before copying one of those circuits exactly! Thanks for the hint with the photophet hint, i will try this out next.

[niggez]

is there any way i could measure how much of the signal is actually "lost" in different non-truebypass switching methods?
Right now my fairest guess would be running a constant signal into the circuit and testing "by ear" if theres a change...
Could i measure the signal? With / without true bypass? How?

[R.G.]

AC RMS voltmeters are designed for exactly that.

However, be aware that there has been at least fifty years of people messing with this ahead of you. Do yourself a favor: go read what they did and stand on their shoulders. Studiously ignoring past practice is interesting and aesthetically satisfying no doubt, and sometimes you do - how did that go? - discover your own beautiful reality. But it's very slow.

The race is not always to the swift nor the contest to the strong, but that's the way to bet.