Nurse Quacky "Quacks" but the decay .... !!

[petemoore]

  How about something simple like a compression diode [like Joe Davissons Antiquity Fuzz], or even a clipping circuit might make less voltage ripple when two notes are played [if the clipping threshold were set low]...of course there is the trade off between amount of sweep and steadiness of sweep when compressing the voltage swing of the EF's input source, but maybe some mileage could be gotten using something like this ?
  A series of delays [like reverb] into the EF, would smooth out 'two note ripple', you could still get good changes in amplitude [of course slowed by however many milliseconds the delay extends input to the EF]...I'think delays made short enough to allow the follower to still 'follow', and long enough to fill in the 'dips between the swells between interval harmonics [like when you try to harmonically tune a guitar with a buncha reverb or some echo on, you can't hear the pitch/notch/passover].
  Because the echo need not be in the signal path, a relatively low grade [inexpensive] chip [I think there are premade ones around for fairly cheep?] could be used without any degredation of audio signal, and improvement of sweep steadiness...also sweep slowness of course...DPDT...
  Am I rambling yet?

[MartyMart]

So here's a couple of envelope generators ( fairly complex ) ...

http://www.musicfromouterspace.com/analogsynth/ADSR.html
http://www.musicfromouterspace.com/analogsynth/adsr_new.html

Is there a way of generating a simpler but "stable" release portion for using
in a filter/auto wah type circuit ??
Do we need ADSR, or just "R"  ?
would one 4066 and a few parts do the job here ....

Marty.

[puretube]

pete: the end-result of that kinda envelope would be in the same realm as the "quadrature" one mentioned on bottom of this page: http://www.diystompboxes.com/smfforum/index.php?topic=37679.60
(the thread that is combinable with this one...)

[StephenGiles]

Mike - I'll repost the original Space Drum schematic this evening.
Stephen

[puretube]

So...
Do we need ADSR, or just "R"  ? ....

Marty.

or AD...
?

[MartyMart]

So...
Do we need ADSR, or just "R"  ? ....

Marty.

or AD...
?

Yup AD would do it !

[Vsat]

Looked at Steve's re-used  EH sweep schematic again. Easy to confuse the symbols for V(-) and ground on that schematic!  Re-draw with proper ground symbol is recommended...

The op amp (part of LM324) used for audio input has it's (+) input referred to V(-) through a one meg resistor.... not to ground! It is used a noninverting gain stage/buffer that clamps to V(-) on the negative input half-cycles, making it a sort of half-wave rectifier. The output is able to swing very close to the V(-) rail since the LM324 is used.  The output would consist of "half-wave rectified" positive half-cycles. This is fed into a second LM324 non-inv stage which provides additional gain for driving the OTA Iabc pin directly. Should be OK since the current into the Iabc pin will drop to zero when the output of the op amp  is less than two diode drops above the V(-) rail. The amplified positive half-cycles force current into the Iabc pin, the OTA is connected as a variable lag circuit, with the lag time becoming shorter as the audio input becomes larger and average Iabc increases. The "start" voltage is applied to the OTA (+) input, negative feedback forces the OTA to charge up the integrating capacitor to make the voltage at the (-) input equal to the "start" voltage, the larger the input signal the faster the OTA is able to achieve equilibrium. On the negative input half cycles the OTA output current becomes zero (the cap "holds" the most recent value, with slow discharge through the "release time" pot)...when the note finishes the integrating cap discharges fully.... not to ground, but to the "stop" voltage, which is present in buffered form at the output of the op amp connected to the lower side of the integrating capacitor. In summary,  the circuit is an "attack-release" envelope generator which tries to produce an output equal to the start voltage at the beginning of each note, while decaying to  stop voltage between notes. The louder the note, the faster the circuit is able to reach the start voltage, giving a dynamic response to playing style by providing a level-dependent attack time. A nice feature is that the start voltage can be set higher than the stop voltage, and vice versa, for normal and reverse sweeps.
Right, Steve?   
Cheers, Mike

[StephenGiles]

Absolutely Mike a brilliant synopsis. Just got back from a swim at a public pool on my way home from work. Funnily enough, in the public gallery was a young lady pouring over what looked like a huge  - I mean building plan size schematic as I swam past. So I just had to get out and take a look, but it was in fact a building plan, and she said that she was doing a little work whilst her kid was having a swimming lesson, and I had not been the first to comment on her huge piece of paper. The original Space Drum schematic with a CA3094 is here:
http://img.photobucket.com/albums/v317/StephenGiles/EHspacedrum.jpg
Upward & onward
Stephen

[Vsat]

Thanks Steve! The schemo is a bit different but the analysis is the same.
An aside: if you find a trashed EH SD with bad crystal mic, rubber-cementing one of those flat- circular brass piezo beeper plates from an old watch works even better than the original (and doesn't turn to powder on hot days, unlike the Rochelle salt slab in the crystal mic). And if you see a Marshall Time Modulator schemo while swimming, be sure to get a copy and post to the list!
Cheers, Mike

[StephenGiles]

Mike I used to have a couple of SDs which I bought for £2 each and sold for £100 each about 12 years ago!
Incidentally, for all you staunch Canadians, it was on this day in 1929 that women ceased to be"Chattels" and became "Persons" - heard it on the radio today.
Stephen

[Vsat]

Also should look at the original context of this circuit...   
Used in a drum synth, where the output voltage was used to sweep an oscillator up/down or reverse.
The input signal is a short impulse provided by a crystal mic attached to a drum pad. A sharp tap on the pad would provide a nearly instantaneous sweep up to the start voltage, followed by decay to the stop voltage. A light tap would provide a noticeably slower ramp up to the start voltage, which would then decay to the stop voltage. A very light tap would provide an even slower ramp up to an intermediate voltage.
Mike