# 2 Night's Work



## keto (May 23, 2006)

Did one of these last night and another tonight. Nurse Quacky from http://www.home-wrecker.com/nurse-quacky.html . Dr Q variant, (with input from our very own Mark Hammer) envelope filter. Socketed the IC, the transistor, and 3 caps, plus sockets for the LED's on the one from tonight. One for me and hopefully, if my thinking is right and the socketed caps give *enough* adjustment, one for my bro who is a bass player.

Sad part is, I won't be able to verify them for a week or so as I have no pots left /cry.

As one might expect, the one from tonight is much cleaner in the soldering dept. I learned from some mistakes/misjudgements on some bridges last night. Made a MAJOR error right off the bat tonight, started with the IC socket 1 column over too far (thus the transistor socket on the far right being vertical rather than horizontal) but was able to adapt on the fly......I hope


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## Ti-Ron (Mar 21, 2007)

Cool, you're on a building spree!
Have fun with your new toys!


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## mhammer (Nov 30, 2007)

Nice work. Clean build. I have several suggestions to make this juuuust a little more enjoyable:

1) Get some heat shrink to cover the joint between the pot lugs and connecting wires. A great many of the "I don't get it, it worked before I boxed it up" inquiries I deal with are resolved when the builder discovers a short produced by something touching something it shouldn't or a wire fracture. Heat shrink over the pot lugs provides some protection against them inadvertently rotating and touching an adjacent pot chassis or the inside of the box when you tighten the nuts on the outside. It also adds strain relief to protect against wire fracture.

2) Though I did suggest the variable attack shown in the schematic, several years of followup experience have taught me that you get more bang for the buck playing with the decay that with the attack. That variable series resistance which produces the slower charge-up time of the 22uf cap also eats up envelope signal. This means you need to turn up the sensitivity as you slow down the attack, in order to compensate. It also means that there are some serious limits (at least using this circuit) on how much you can slow down the attack time before the envelope signal all but disappears. Variable decay also impacts on the "feel" of the circuit, but since it is not in series with the envelope signal, it does not detract from the sensitivity. It is also the sort of thing that can be shortened and extended more than the attack time can. You'll find, as well, that faster decays make an auto-wah sound more synth-like. That may or may not be your goal, but if it is, that's how to do it.

A suggested change to the circuit is to skip the 1k Attack pot. Instead, you can do one of several things:
a) replace the (already installed) 22uf cap with a 4u7 unit to get a faster discharge time, and use a toggle to add a 10uf cap in parallel with it for longer decays,
b) leave the 22ufcap intact and in place, and wire up a 3-position SPDT toggle with the middle lug going to the + side of the 22uf cap; then wire up a 22k from one outside lug to ground and a 330k from the other outside lug to ground. This will give you slow (middle position), medium (side position) and fast (other side position) decay times.

3) The filter can have variable gain and Q. Varying the value of the feedback resistor between the inverting (-) pin and output of the filter section (currently shown as 470k) alters the gain of the circuit and also the selectivity of the filter. A second byproduct is that the apparent sweep range changes a bit. A useful range of adjustment would involve a 330k fixed resistor in series with either a 470k or 1M variable resistor in place of the 470k. 

Alternatively, you could skip the pot and knob cost, and just wire up another 3-position toggle to get 3 different gain settings. For example, a 1.5meg fixed resistor would replace the 470k. The toggle would then switch in either nothing (middle position), an 820k fixed resistor in parallel (yielding 530k, which is close enough to stock) or another 1.5meg resistor, yielding 750k. So stock, higher, and much higher Q.

4) Since, increasing the gain of the filter also increases the output level, making the effect/bypass level-match off, you want to do something to offset that. One way is to replace the 470k fixed resistor on the output with a 470k log pot. A second way is to use the Q-changing toggle to automatically compensate level for Q.

Huh? Look at the junction of the 1uf cap and 470k resistor on the output. If another fixed resistor, let's say 47k, was placed between the 1uf cap and the 470k resistor, the 47k/470k combo would behave like a 527k volume pot that was turned down about 9% of the way. Make the 47k 100k, and it behaves like a 570k pot turned down 17.5% of the way.

Okay, so let's say our 3-position Q toggle-switch wasn't a SPDT type, but a DPDT type. We're only using one set of contacts to adjust the Q, leaving the other set of contacts to adjust the output level. When the Q setting is at its highest (1.5meg in the suggested values above), the output level is increased, which means it has to be padded down to get effect/bypass balance. So let's stick a 220k resistor between the 1uf cap and output/470k junction to do that padding-down, and wire the other centre lug of the toggle to the output of the circuit..

When the Q is set to medium-high gain, the output level won't be quite as high so we don't need quite as much padding-down, making the 220k padding value too high. So, let's use that other side lug on the Q-toggle to switch in a fixed resistor in parallel with 220k to drop the effective value down a bit and reduce the amount of padding down. With the filter gain set by 750k (again, following the suggestions above), that 220k can probably be dropped down to 100k or so. So wire up a second 220k resistor between the outside switch lug (on the side where you also connect the second 1.5M resistor), which, in parallel with the existing 220k unit gives us 110k. In the middle position , the gain is lower so no attenuation is needed. In which case the stock Q position also switches in a straight wire connection across the 220k so that there is no attenuation, only the 470k to ground.


Here is the redrawn version. Enjoy.


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