I've constructed all sorts of filters from CMOS inverters and after a while the outcome becomes pretty predictable - they all sound like the 'real thing' just not as good! I found SVFs to be the same. It's not really a question of high Q, just 'higher'. That's probably what you were looking for, wasn't it? One of the things that has never been discussed, and I'm wondering if it is possible, is some sort of mod to the clock/switching circuit that would permit either:ī) LFO modulation as is found in the MXR/Dunlop Auto-QĬ) alternate envelope shapes like exponential rise Based on build reports, though, you're probably looking at something like a 50k pot with fixed resistors on each end to accomplish the same overall current going to the invertors in the HF clock. This could probably even provide fodder for a mod than included a tuning pot in addition to overall range-switching. If you've ever A/B'd a Doctor Q (or clone) and an MXR-EF you will immediately notice how fairly ripple free the EF is, even at faster settings.Ħ) As some build reports on Tonepad have suggested, R14 and R16 form a voltage divider that feeds a bias voltage in to more or less set the initial sweep point. That seems to get around a lot of the envelope-ripple issues that arise when one doesn't use something like an LDR to smooth things out. IC2E acts as a kind of lag generator permitting the envelope signal to be as fast as it is, or slower rise time than it is. On the other hand, one of the things that the EF does differently than other filters is that it extracts the envelope signal and then applies additional filtering to *that*., which may be a big part of what produces the (to my ears) extremely pleasing envelope control on this unit. Faster decay times should be feasible by reducing its value, and longer attack and decay times would be feasible by increasing the value of C9 itself. I'm speculating that varying R11 would permit one to play with the sensitivity range a bit for those instances where pickups or playing style seem not to evoke much sweep.ĥ) R17 (1meg) sets the decay time of the envelope signal by providing a path for C9 to discharge. I wonder if it is possible to introduce emphasis for BOTH filter sections for super resonant filter sweeps or even to play with that loop.Ĥ) Presumably, IC1d fills the same general role that one of the op-amps plays in the Doctor Quack, providing high gain for the audio signal to yield a reasonable envelope signal. There IS an overall feedback loop through R12 from the second filter section (IC1f) to the input section (IC1c). I find this interesting since the two filter sections really are in series. He has, however, corrected that from what I understand, but I have not seen it yet.ģ) The resonance/emphasis mod essentially applies variable feedback around the first filter stage. This also has an impact on the effect of the resonance control. Consequently, earlier A-Gua layouts produce a less obvious range-shift by only doing so on one, rather than two filter sections. For whatever reasons, Fransisco misunderstood and thought this only meant one invertor rather than both. The range of the filter is partly determined by the cap in the feedback loop of the invertor. Each switch section acts like a variable resistor in a manner sort of like the LDRs in a Mutron, except that these are not LDRs and they produce a changing resistance based on how much time they spend being maximum off resistance. Conceivably this is due to some blending of straight signal in the output via R12. I'm still perplexed by just what sort of filter these make, since they look like they ought to be lowpass but sound like they are bandpass in many respects. It is this feature that permits Steve Giles' clever and simple mod for reverse sweep by using the "leftover" invertor section.Ģ) There are two cascaded filter sections, formed by IC1e/f. The envelope follower voltage varies the mark/space ratio such that the amount of "on" time, relative to "off" time changes with picking strength. The clock is constantly switching the CD4066 on and off at a constant ultra-sonic rate. There is no thorough analysis presently, although it works pretty much the way any other envelope-controlled filter would with only a few small differences.ġ) There is an envelope follower (you can spot the traditional diode+cap confuguration with D2/C9 in the Tonepad layout) but it controls a high-frequency clock formed by IC2A/B/C/D rather than directly controlling some element like an FET, transistor, OTA, or LED.
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