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previous: Mark Hammer It was an envelope-triggered filter... -- 8/6/1999 3:31 PM View Thread

Re: BassBalls schematic - seek no more

8/7/1999 12:50 PM
Mark HammerRe: BassBalls schematic - seek no more
Looking at it a bit more, it is essentially two parallel Doctor Q circuits, with the range setting capacitors (.022 in one case, and .0047 in the other) set roughly two octaves apart. The envelope detection circuitry is pretty much like the Dr. Q, with attack and decay set by the usual R/C combination. The "fuzz" switch selects betwen two sources for filtering: the buffered input signal (which is what the output sees in bypass mode), and a boosted signal coming from the stage used to drive the envelope detector. The fuzz is not explicitly clipped in any manner other than running out of headroom, and is padded down to provide signal balance betwen straight and fuzz when switching.  
The schematic comes from EH, so I shouldn't challenge its accuracy, but I will note that the 2.2M input resistor on the the envelope detector op-amp stage seems ridiculously high. Moreover, since the circuit is essentialy a double Dr. Q, I will point out that the feedback resistor for the same op-amp element in the Dr. Q is 2.2M not 220k. Bottom line: possible there is an accidental reversal of parts here (input resistor should be 220k, feedback should be 2.2M). As always, there are probably design quirks that I'm naive about, so take these suggested changes skeptically.  
Areas for mods....  
1) A true fuzz might give a livelier sound, so try sticking the textbook diode pair to ground, just ahead of the 100k resistor going to pad "E". Given that the diode pair will reduce the signal level to a half volt or so (depending on diode type selected), the padding at E will need to be adjusted to provide balance between straight and fuzz. This will likely involve reducing the 100k resistor to reduce the overall attenuation.  
2) As with the Dr. Q, the attack/decay parameters can be tweaked by varying the resistor and capacitor following the diode. Values greater than 100 ohms will slow the attack a bit, and capacitors greater than 4.7uf will result in longer and smoother decays.  
3) The trimpot in the Dr. Q can also be used to "tune" the range of the filter by attenuating some of the envelope-triggered control voltage coming from the first 2N5088, and the same rule applies here. This also means that the staggering of the two filters and sweep width of each can be played with by tweaking the two trimpots. Anyone who builds one of these may want to chasis mount them for playing with, although you should note that only part of the range is actually useful.  
4) STEREO, BABY, YEAH!! There are two versions. One involves running the direct signal available at G to one amp and the effect signal available at H to another. A second involves having the 2.7k resistor from each filter section connect to its own output via a 1uf cap and 47k resistor to ground. Send each effect out to a different amp and voila. Naturally this introduces the question of effect bypassing, which I haven't quite figured out yet.  
5) Single or multiple sections. Piece of cake, here. If you're running in mono, run each 2.7k resistor to a SPDT on-both-on toggle switch, and you'll be able to have the lower, upper (standard Dr. Q sound) or both filter sections.  
See, now THIS is why I like analog.