Two Channel OD / Fuzz

[AM]

Hi,
I've made a two channel OD sometime ago. Based on the Llama/Anderton Fuzz, I replaced various cap and resistor values and also replaced the gain knob with a footswitch. This was to switch between two gain pre-sets while having just one master volume knob. At the time the volume boost I was getting when switching from low to hi drive was welcome. With my current band I need to go from a slight O/D to a fuzzy sound without the volume boost.
Is there a way to wire a trim pot to the same 3pdt switch to set a master volume reduction when activating the higher gain setting? I guess a trimpot in series with the master volume knob activated or bypassed according to the gain channel I'm selecting should do the job. I'm trying to keep only one external knob on the box (the master volume knob). Thanks in advance for your input. Here is a link to the schem:
https://picasaweb.google.com/harm.on.x/RLSwitch#5589277358141001362

[Mark Hammer]

On one of my adapted tube sound fuzz units, I have a boost switch that pretty much does exactly what you describe.  It gooses the gain but keeps the output level about the same under higher and lower gain conditions.

The switch is a DPDT rocker switch.  One set of contacts is used to change the gain of the input stage, and the other is used to alter the output volume.  Let's look.  Here we have an op-amp based input stage, using a non-inverting configuration.  The gain of 22.36x is set by the 47k feedback resistor, but also by the 2k2 resistor to ground.  If I were to place a 3k9 resistor in parallel with that 2k2, making their combined parallel resistance 1.407k, the gain of that stage goes up to 34.4x.
Looking at the other end of the circuit, we see a 10k volume pot.  If I were to stick a 4k7 resistor in series with that pot, it would behave similarly to a 14.7k pot that could never ever be turned up more than about 70% of the way.  You see where I'm going with this?
So if we have a toggle, stomp, slide or rocker switch that simultaneously places a 3k9 in parallel with the 2k2, AND lifts a wire shunt that would normally bypass an added 4k7 resistor in series with the pot, you would simultaneously increase the boost at the front end, and reduce the output volume in complementary fashion at the back end.  The actual component values to be placed in series or parallel would vary, depending on a bunch of things, but the principle is the same.  You can easily figure where to insert the level reduction.  The tricky part in any given pedal is figuring out where the best spot to insert the gain change.

[AM]

Thanks for your help Mark. I really appreciate it. In the diagram below is what I've done so far (ignore caps and resistors values. Everything has been modified).
This was a solution that gave me popping-free channel switching.
Should I solder the new resistor for the master volume at pins 3 and 6 of the switch,  and break the circuit after the output cap and before the 10k vol pot? Then wire pins 3 and 6 to those points. Kind of a diagonal mirror image of the gain wiring?

[Mark Hammer]

Because you are using the feedback loop to adjust the gain of that first stage, and shunting the extra resistor to reduce gain, you will need to use to use contacts 6/9 (or 5/8).

If you have perfed this, or even if you have PCB'd it, I'd suggest using a trimpot in series with the output pot since you may have to do some trial and error for identifying a suitable resistance value for equal loudness.

That being said, you have a 3PDT switch with two sets of contacts available for duty.  Why not use them to select between one level pot for higher drive and one for lower drive?  After all, there may well be situations where you'd like to have the same general tone quality, but have a little more push AND level for solos.

To do this, you'd have the - side of the output cap go to contact 5 and the output jack (well, the stompswitch contact from the output of the circuit) connected to pin 6.  Pin 2 goes to the input lug of one pot and pin 3 goes to the wiper of that pot.  Pin 8 goes to the input lug of the other (10k) volume pot, and pin 9 to its wiper.  The ground from each pot remains tied to ground (which is why you only need 2 contacts to do it).

Since the 10uf cap will be left "hanging" during the switchover, you'll need to stick a fixed resistor between the output cap and ground,for it to drain off.  Start with 1M and see if that does the trick.

[AM]

Thanks a lot Mark!
I will try your suggestion. I had actually wired two gain pots the way you describe but I was getting a loud POOOOOOOOOOOPPpppppp so I went for the option I have drawn in the schematic.
Thanks again. You are always very helpful!

[AM]

Mark, one more thing...
Would it be possible to use some of the unused pins of the CD4049 for an input buffer stage??
Thanks

[Mark Hammer]

You'll need to ask someone more knowledgable about invertors for that, I'm afraid, but my gut says you should not expect any improvement.