Balancing volumes on an OD with LED and silicon diodes

[mysticaxe]

I'm building a Game of Tones board that will have 2 clipping options - LED and silicon (I'm leaning towards assymetric, but that's beside the point).  The diodes are switched with a SPDT.  The issue is that the LED's are substantially louder. 

Would it be possible/reccomended to use a DPDT instead of the SPDT and use the other side to bring in a resistor that would be on the volume pot to increase it's resistance?  I know I'd have to tweak the value of the resistor.  Which lug would be best to put it on?  Ground?  Out?  In?

[jkokura]

I doubt it's a linear matter, so bringing in one resistor isn't likely to solve the issue alone. Two questions:

1. Why do you need the volume to be equal between settings? Is there a reason you can't flip the switch and turn the volume knob?

2. Consider using 4 LEDs for clipping, not just two (two per direction in series). This might bring the volume down to the level you're looking for.

Best solution here is experimentation on a bread board.

Jacob

[mysticaxe]

I don't think they need to be equal, as the EQ would also have to be switched around between the sets of clippers.  I figured that if I could equalize the volumes a little, that would be convenient (as it stands now, the silicon diodes with the volume knob fully up is about the same as the LED's at about halfway).

I'll try the 4 LED idea as I have the clipping section already breadboarded.

[midwayfair]

I'll try the 4 LED idea as I have the clipping section already breadboarded.

Two LEDs in series have a HIGHER Fv. Jacob meant to say two back-to-back pairs in parallel. Or two silicon diodes in series to get closer to the LEDs.

There's a sonic difference between LEDs and silicon and germanium, but the biggest difference is the clipping threshold. A higher forward voltage will make the effect louder because the diodes aren't clamping down on the signal. LEDs have a pretty high Fv, 1.7v or thereabouts, as opposed to .55-.65v for most generic silicon diodes. That's a HUGE difference in voltage going through to the output, and a great deal more headroom before the signal is clipped.

The point of using LEDs is getting more clean gain out of the circuit (without some problems that a diode lift might cause). Before you do this, you need to think about what lowering the collective forward voltage does and why you would do it. Most of the times you need a lower forward voltage is if you're dissatisfied with the amount of distortion you're getting overall, or the effect has so much gain that it's impossible to get it low enough for your purposes (e.g. can't get down to unity volume before the effect's off), or occasionally to prevent oscillation or distortion in a following gain stage. If you want the LED side to sound the same as it does now but not as loud, you need to turn down the volume knob.

There's another thing you CAN do, though, if you haven't boxed the pedal. Add a second control, and make the diode selector a channel switch. Have it swap out the volume pot and the diodes at the same time. You just hook up the wiper of both volume pots to the switch on opposite sides, and the common lug goes back to the board. Now you can set the volume independently for the LED side and the silicon side. At least, I think this works. You *might* have to double the size of the volume pots due to parallel resistance.

[mysticaxe]

Thanks for the clarification - I was wondering how two series LED's would lower the volume...  I'm concerned that two in parallel will alter the tone more than I want (I like how the LED's sound), but I'll try it since that part of the board is already socketed.  I'm planning for the silicon side to be a Si and Ge in series one way and either 1 or 2 Si the other way.  That should bring Fv up a little.

I haven't boxed it yet, but don't care enough to add a second volume knob.  My thought was that a series resistance is easy to do and I have a DPDT (and the called for SPDT) handy, if it would do the trick.

[oldhousescott]

I would put a resistor between the Vb and the ground lug of the volume pot, and short it out in the LED position. You'll have to experiment with the value to find the best match when you have the volume set where you normally would for LED operation. I wouldn't expect the levels between the two settings to track with the volume pot. Also, in the silicon mode, you won't be able to turn the volume all the way off, but that should be a non-issue.