Zendrive Tone Control

[sonnyboy27]

Hey guys,

I was looking at Aion Electronic's schematic for the Zendrive and had a questions. I was wondering if there's a reason that we tie lug 1 to the wiper in the tone controls. From what I understand, the signal is looking for the path of least resistance which will always be from the wiper to lug 3 since the path from 1 to 2 is essentially an infinite resistance since it doesn't go anywhere. Is there a reason we tie lug 1 to 2 instead of leaving lug 1 floating in these types of circuits? I noticed that the gain pot has a similar thing going on too.

[culturejam]

You could instead connect R4 to lug 2 and it would function the same. The reason you see the wiper tied to one of the other lugs is because if the wiper dies, you'll still get signal through the pot. Otherwise, the signal would terminate at the Tone pot and you'd get no (or a very little) output.

And the connection between lugs 1 and 2 is not infinite resistance. It's actually the opposite: zero resistance (or as close as you can get to zero).

[sonnyboy27]

You could instead connect R4 to lug 2 and it would function the same. The reason you see the wiper tied to one of the other lugs is because if the wiper dies, you'll still get signal through the pot. Otherwise, the signal would terminate at the Tone pot and you'd get no (or a very little) output.

And the connection between lugs 1 and 2 is not infinite resistance. It's actually the opposite: zero resistance (or as close as you can get to zero).

So what is happening in that zero resistance loop then? I'm assuming something must go on that causes the signal to flow through the other half of the potentiometer. Is it because the signal is trying to get to ground but will never reach it by going through that loop?

Please feel free to go as technical as you want. I'm trying to bring back some of my circuits knowledge from college that is quite rusty at this point.

[culturejam]

So what is happening in that zero resistance loop then?

Unhindered electron flow. 

It's not really a loop. It's just an electrical short.

By shorting Lugs 1 and 2, the pot becomes a variable resistor instead of a variable voltage divider. As I said, you could accomplish the same thing by leaving Lug 1 unconnected and routing the signal straight to Lug 2. So then why bother to short lugs 1 and 2? Well....

The underlying thing to keep in mind is that a pot always has it's full rated resistance across Lugs 1 and 3, so from that perspective it looks like a resistor. It's Lug 2 (wiper) that is the variable element.  So if you have a pot configured as a variable resistor (input to wiper, output to one of the other lugs) as in the Zen tone control, if the wiper "dies", it usually dies "open" (infinite resistance). In that case, no signal would pass and the pedal would make no sound, and then you'd have to figure out what the hell is wrong (could be any of several problems). If you short Lugs 1 and 2, and the wiper dies open, the signal still has a path through to the output via the constant (non-variable) resistance across Lugs 1 and 3. And if that happened, you would immediately notice the change in tone and could quickly figure out that the tone pot was not working, and it would be faster to troubleshoot and repair. Otherwise, you'd have to start checking voltages and probably audio probe until you found the spot where the signal stops.

Does that make sense?

[sonnyboy27]

Yes! That makes total sense now.

I was looking at it from the wrong point before and wondering why when the signal reaches the wiper it doesn't look back at the resistance between the first and third lugs and choose the lesser. But now, if I am understanding this correctly, the signal reaches the wiper because it's less resistance than traveling from lug 1 to 3. So there's essentially no current flowing across the resistive portion of the pot where the lugs are connected.

So as you put it

It's not really a loop. It's just an electrical short.

[culturejam]

But now, if I am understanding this correctly, the signal reaches the wiper because it's less resistance than traveling from lug 1 to 3.

Yes!