Can someone explain why bleed-over occurs in true bypass loops?

[hallenj]

Hi,

I am trying to understand how a true bypass loop switch works.  I have one from Radial Engineering called the Big Shot EFX.  When one of the loops is selected but the receiving jack from the effects loop is left unconnected, I can hear a faint bleed-over from the input guitar signal.  I don't understand why this happens if the design is true bypass.  How does that guitar signal get passed through to the output?

For example, if I send my guitar signal through the loop into a tuner and do not send the signal back to the looper from the tuner, there is bleed-over of the guitar.  It's also distorted.

I noticed that on Loop-Master's FAQ, he acknowledges that there is a faint bleed-over in his true bypass switches under the same condition.  Again, can someone explain why?

Thanks very much!
Jason

[PRR]

> explain why?

Same as Radio. Have a strong electric vibration, a sensitive detector will pick it up at a distance.

When the distance is a few inches, you don't need a big transmitter or a super-sensitive receiver.

There is no perfect conductor, no perfect shielding, no perfect insulator. We can never get leakage to zero; we can usually get it down to be "not a problem", though sometimes it is tough or tricky.

Very often an input jack is wired "grounding", so if nothing is plugged in the path is connected to ground, which is nominally silent.

However there is no perfect conductor. When you have a signal flowing to "ground", it doesn't vanish, only get small. Jacks on a common ground wire often bleed one to another.

Usually such bleed and leakage is "negligible". If you have a useful signal at the return jack, the bleed is probably overwhelmed?

Sometimes it matters. Some medical electronics procedures, such as heart- and brain-waves, must be done inside a wire-screen room so they don't catch leakage from all the other electrical systems in the building. When the military first started using computers, someone in the next room noticed that the pulses could be picked-up and sometimes decoded; "TEMPEST" was a set of rules for shielding military data systems.

[Processaurus]

It isn't the micro resistance of the ground wire causing bleed, pedals don't deserve that much worrying.  Grounding is a wide and interesting subject, but for people starting in electronics it is most helpful to think of the rule, rather than the exception, that ground is a trustworthy and low impedance source of 0 volts.

What it is is like PRR initially said, that it is cross talk (two wires or traces running near each other), and you've connected what is probably a high impedance input (next effect or your amp) to a floating piece of wire (the receive jack with nothing plugged in), so you pick up all that noise, like when a cord is plugged into an amp and with nothing on the other end, you get all the stray noise in the room.  Inside your pedal it is shielded from noise from the world, but not from your guitar's little signal, so it picks that up a little.

Routing of inputs physically away from outputs can minimize this.  As can shielded cable, starting with the inputs, if one wants to get more involved.

You want your amp to be silent when tuning, so you could plug in a DIY'd special 1/4" plug  with the tip wired inside to the sleeve, or mod the Radial by using a "normaled" jack as the receive to connect the tip to ground when nothing is plugged in.

[hallenj]

Thanks for your responses.

I'm new to DIY electronics, so I've got a couple questions.  First, Processaurus, the way you describe the high impedance input of an amp makes me think that it is like a thirsty input trying to suck up a signal.  When a cable is connected into the amp but nothing is at the other end of the cable, the amp amplifies all the noise in the room.  Or, in the case of the bypass loop, it amplifies the "cross talk" from nearby components.  Is that a good way to put it?  I've read several explanations for input/output impedance, and I still can't really wrap my mind around what it is.  Does anyone know of a way to explain it that is easier to grasp?

Also, I see in the true bypass loop diagrams that the sleeves of the jacks are usually grounded.  I don't understand what is actually happening when the sleeves are grounded.  What signals and/or voltages are being grounded?

As you can see, I'm pretty clueless about these basics.  I'd appreciate anyone's help.  Thanks!

[Paul Marossy]

Thanks for your responses.
First, Processaurus, the way you describe the high impedance input of an amp makes me think that it is like a thirsty input trying to suck up a signal. 

That's not a bad analogy.

[Brymus]

One of my first pedals I made from my own idea...well with nothing plugged into it you could actually play wierd noises by twiddling the knobs and flipping the switches I could get several pitches from it.same when plugged in and bypassed it was kinda cool except the effect was a failure because of it.
Its what drove me here trying to learn more.
Im hoping after I get RGs book I will know exactly what happened, it was obviously a very bad layout as it worked on the breadboard.