mutron/neutron as a manual wah

[9 volts]

I found this info in the archives

'HOWEVER.....the reverse is a very different story.

It is plum easy to take an autowah and turn it into a wah.  For example, if you had a Dr Q /Quack / Nurse Quacky, all you would need to do to turn any of those into a foot controlled wah would be to switch the transistor for a simple foot-controlled variable resistance to ground.  Turning a Mutron/Neutron into a foot-controlled lowpass/highpass/bandpass filter is as simple as sticking a variable resistance between the positive supply voltage and the LED that shines on the LDRs.  Make the resistance higher and the light gets dimmer, causing the LDR resistance to go up and the filter frequency to go down.' M Hammer

Question is- Is there a simpler way to do this. Eg could I replace the ldr's with a duel pot. If I did this what else could I remove from the circuit. I've built the neutron a while ago and would Iike to use it as a wah also. Thanks

[Mark Hammer]

A footswept state-variable filter makes a really nice wah.  I know because I had one many years ago.  The Mutron/Neutron/Q-Tron/who-tron/foo-tron/etc is also a state variable filter, except that the key resistances to be varied are varied with a pair of LDRs.  Those LDRs, in turn, are in parallel with a pair of fixed (220k) resistors, which set the maximum interstage resistance (the LDRs on their own would have a much higher resistance).

Varying the value of those fixed resistors does essentially the same thing as varying the resistance of the LDRs.  So, if you had a dual-ganged 250k pot, and a shell with sufficient space to accommodate it, plus the appropriate rack-and-pinion hardware to move the pot shaft around, you could conceivably just pull the 220k resistors off the board, and run leads from the pads where the resistors used to be out to the lugs of a 250k (220k in some parts of the world, so I'm told) dual-ganged pot.

If you wanted to lift have this work on its own, lift the + end of C8 ( http://files.muziq.be/schematics/musitronics_mu-tron3.jpg ) so that no signal enters the envelope follower section.  If you leave that cap connected, then the footsweeping will work in conjunction with the automatic sweep, enabling you to effectively move the range of auto-sweep around a bit with your foot.

While this sounds feasible on paper, do note that not every shell lends itself to use of a dual-ganged pot, and not every dual-ganged pot likes being swept in that manner.  This is why I initially suggested use of a single pot and variable light source.  That being said, I like what state-variables have to offer as foot-operated wahs.  A smooth downward-sweeping highpass filter is very neat.  A song from a band from a quarter-century ago provides a nice example of this ("One thing leads to another" by The Fixx).

[9 volts]

Thanks Mark, I've got a shell so I'll give it a try. The two batteries may be tricky but I'd like to test this.

[SonicVI]

Varying the value of those fixed resistors does essentially the same thing as varying the resistance of the LDRs.  So, if you had a dual-ganged 250k pot, and a shell with sufficient space to accommodate it, plus the appropriate rack-and-pinion hardware to move the pot shaft around, you could conceivably just pull the 220k resistors off the board, and run leads from the pads where the resistors used to be out to the lugs of a 250k (220k in some parts of the world, so I'm told) dual-ganged pot.

Resurrecting an old thread.    Mark, alternatively, rather than replacing the 220K resistors with a dual 250K pot, would replacing the LDRs with a dual 1M pot work as well?   To answer why, in case you are wondering, I have plans for another mod which involves switching the LDR's, so it would just be more convenient than replacing the 220K's.

[candidate]

In my experiments, one of the 220Ks had way more influence over the sweep.  I'd recommend at least trying a fixed resistance on one side and variable on the other, and possibly avoiding the need for a dual ganged pot or matched ldrs.

[zyxwyvu]

Resurrecting an old thread.    Mark, alternatively, rather than replacing the 220K resistors with a dual 250K pot, would replacing the LDRs with a dual 1M pot work as well?   To answer why, in case you are wondering, I have plans for another mod which involves switching the LDR's, so it would just be more convenient than replacing the 220K's.

That would most likely work fine. The 220k's and the LDRs are in parallel, so only the combined resistance matters, and you can change either one to get an effect. You might not end up with the sweep you want though, since the taper you get in that setup is reverse log.

In my experiments, one of the 220Ks had way more influence over the sweep.  I'd recommend at least trying a fixed resistance on one side and variable on the other, and possibly avoiding the need for a dual ganged pot or matched ldrs.

This surprises me. Both of the resistors affect the frequency, and the better they are matched (along with the LDRs), the better the filter works. Changing one would make the filter act strangely.

How I would do foot control is to disconnect the envelope detector (at the left side of R21), then use a low value (~10k) pot as a divider between the power rails to feed signal into the opamp (A6). This will imitate the action of the envelope detector, and it needs no modification of the LDRs or the associated resistors.

[SonicVI]

I'm not going to use an expression pedal, but just a big knob to control the manual wah, so the pot taper won't be AS important, but I've got linear and log pots to try.  The reason I want to replace the LDR's rather than the 220K's is I'm planning on adding an LFO with dual opto-isolators from something like the Tremulus Lune or Ross Phaser.  If the only thing I'm switching out is the LDRs between the auto/manual/LFO control then I can wire it all up to a single 4P3T rotary switch, rather than two or three toggles.

[zyxwyvu]

I'm not going to use an expression pedal, but just a big knob to control the manual wah, so the pot taper won't be AS important, but I've got linear and log pots to try.  The reason I want to replace the LDR's rather than the 220K's is I'm planning on adding an LFO with dual opto-isolators from something like the Tremulus Lune or Ross Phaser.  If the only thing I'm switching out is the LDRs between the auto/manual/LFO control then I can wire it all up to a single 4P3T rotary switch, rather than two or three toggles.

If you include opamp A6, the filter is effectively voltage controlled. You can just use R21 into A6 as the control input, and switch between envelope, LFO, or manual CV inputs. That way, you can avoid switching the LDRs at all, and use a 1P3T switch instead of 4P3T.