Circuit design Question: balance clipping diode output

[jhaneyzz]

I have built a killer Special Cranker pedal that has switchable clipping diode stages.

Currently switches between 4148 silicone and D9 germanium. (asymmetrical clipping in both cases)

I'd like to adjust the circuit to eliminate the volume jump when switching between the two. I naively thought of simply adding a 20k or so resistor to a leg of one or both channels of the 4148 clipping paths. For reasons I'm sure anyone with more ee experience than me (low bar to hurdle there) will likely tell me, that didn't work.

How could I make that work?

[idy]

There are ways to work around this, but they are not perfect. They may be good enough for you. There is a reason why "well designed" commercial pedals do this and the makers choose not to to mess with this "problem."
One solution is therapy or meds for OCD.
Another is to use a two pole switch to switch diode pairs. The second pole adds a "padding" resistor to cut the volume of the louder side. This resistor might go between the tone and volume pots. Maybe a trimmer or an extra pot you can tuck inside the enclosure. The downside is the "adjustment" will be perfect for one volume setting but not so for another. For example when the volume is at 12 o'clock, the two diode pairs are equal in volume... but when the knob is at 3 or 9 o'clock, not so much.

If you want to go whole hog you woulld switch between two different volume pots, "red" channel and "green: channel for example. Now your humble diode OD pedal is a "dual channel" pedal. In this configuration you might have both volume pots in parallel and just switch between wipers for the output.

There are other fancier solutions. One is to add another gain stage, maybe with trimmer(s) to adjust the gain. Since the diodes pairs differ by about a factor of 2, you adjust the stage for gain of 2.

Another possibility is to add more ohms to the emitter R on Q2.

[Vivek]

In an attempt to reduce volume differences, some circuits that swap diodes also swap a resistor near the volume control or elsewhere

[Mark Hammer]

In an attempt to reduce volume differences, some circuits that swap diodes also swap a resistor near the volume control or elsewhere

That's what I've done.  When people fritter around with diode-swapping, what they want is a different degree of clipping.  But they don't want to have to adjust volume every time they switch diodes.

So, imagine one wished to switch between a pair of silicon diodes, and a pair of germanium.  One might normally use a SPDT toggle or stompswitch to connect either pair to ground.  Let's say the circuit employed a 100k volume pot on the output.  Placing a 68k resistor in series with the input to that pot would make it functionally equivalent to a 168k volume pot that could never be turned up more than 60% of the way.  If one used a DPDT switch to select between diode pairs, the second set of contacts could be used to bridge across that 68k resistor when selecting the germanium diodes, and lift that contact when using the silicon diodes.  Depending on the Vf of the diodes you happened to pick, you'd end up with approximately the same output volume for the two diode types, by compensating for the higher output level of the silicon over the germanium.

[stallik]

Same circuitry but slightly different application, when switching between NO clipping and diode (pairs), I like to pop the resistor only in the 'clean channel'. That way, there's no volume loss when switching in the diodes

[jhaneyzz]

Ahhhh.... yes.... i see it now...

And is if fate had a hand in it it just so happens that I didn't have an SPDT switch when I built this and happen to have an SPDT already installed....

heat up the soldering iron!

In an attempt to reduce volume differences, some circuits that swap diodes also swap a resistor near the volume control or elsewhere

That's what I've done.  When people fritter around with diode-swapping, what they want is a different degree of clipping.  But they don't want to have to adjust volume every time they switch diodes.

So, imagine one wished to switch between a pair of silicon diodes, and a pair of germanium.  One might normally use a SPDT toggle or stompswitch to connect either pair to ground.  Let's say the circuit employed a 100k volume pot on the output.  Placing a 68k resistor in series with the input to that pot would make it functionally equivalent to a 168k volume pot that could never be turned up more than 60% of the way.  If one used a DPDT switch to select between diode pairs, the second set of contacts could be used to bridge across that 68k resistor when selecting the germanium diodes, and lift that contact when using the silicon diodes.  Depending on the Vf of the diodes you happened to pick, you'd end up with approximately the same output volume for the two diode types, by compensating for the higher output level of the silicon over the germanium.

[idy]

The reason it "offended" me when switching between diode pairs is that the more distorted group were quieter when esthetically it seemed like those should be louder! A nice crunchy rhythm sound and when you want a creamy sustained tone...it drops in volume! So close to a perfect thing....

[m4268588]