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Envelope detection and filters

1/29/2001 7:00 PM
loauc Envelope detection and filters
Lately I've been thinking about puting a switch in the EH-DrQ (or DrQuack, 'cause it's a very good mod)so I could change the direction of the filter, just like the mutron does. For what I recall, in the envelope detection part of the mutron circuit you use a dpdt switch for adding a resistor to the inverting input and a voltage divider to the inv input of the opamp that drives the led.  
How could achive this in the DrQ/DrQuack circuit?  
Anybody tried modding this way?  
1/30/2001 4:55 AM
Mark Hammer

The circuitry you refer to in the Mutron adds a DC voltage and subtracts the envelope signal from that to get an envelope follower output that starts high, quickly goes low, and returns to high. You *could* do that with the DQ but it would obviously involve more parts to combine the DC offset with the envelope signal at a mixing stage, and a different PCB layout.  
My own feeling is that downward drive tends to sound better with lowpass and highpass filters, where it produces a terrific sucking sound. One instance where bandpass filters sound pretty good (in my view) with something LIKE downward drive is the EH-Baseballs. Although the Baseballs does not invert the envelope, it tends to sound like it does, because the attack time is so fast that the sweep spends most of its time settling back down from its highest point (i.e., if you slow down the attack, it starts to sound a bit more like a DQ). So, what you hear is primarily "ow", and very little (if any) "bwow". I mention the BB because it uses 2 bandpass filters, in contrast to the DQ's single filter. Upward sweep tends to sound pretty good with faster *and* slower time constants, but downward drive tends to sound best when the tone becomes "thicker" as the sweep descends, and especially if the sweep time is fairly quick. Because the BB uses two spaced filters, the sound remains thick, despite the use of downward sweep and bandpass filters.  
My hunch, though, is that the DQ probably would be kinda thin and wouldn't sound as exciting in the downward drive position as the Mutron. Feel free to tinker and prove me wrong, though. I am always happy to find out that a gadget has more up its sleeve than I first thought. You may also want to take a look at the component values in the BB for tweaking the envelope follower time constants in the DQ so they behave like the BB, since that would let you use the DQ board layout.
1/30/2001 7:30 PM
Thank you very much Mr. Hammer, I was thinking precisely in the circuit that adds the voltage and substracts the envelope signal but never thought about the filter being bandpass. I can try it, of course, because I have the two of them side-by-side in protoboards making comparisons and little mods.  
Maybe I can build an EH-Baseballs so these tests can go further and deeper.  
By the way, somebody posted a link to a pcb layout of MutronIII, is a parts placement drawing available?  
1/31/2001 11:01 PM

kind of off topic, but does anyone know a transistor based envelope follower? Seems like every envelope filter I see is an chip festival and I'm sure for good reason. Just wondering if a transistor version is possible to make and fit in a small box...  
1/31/2001 11:12 PM
Electro-Harmonix's DrQ or Jack Orman's DrQuack are transistor based and small enough for any standard stompbox enclosure. Have you seen the schems? Probably you can imagine the whole thing just seeing the drawings.  
1/31/2001 11:34 PM

wow, I'll have to check these out! Was ready to give up on a transistor follower and buy a moog, but good thing I waited, I guess!  
2/1/2001 1:09 AM
Mark Hammer
Not quite
Although you are correct in identifying transistors as components in each of these, the transistor simply serves as the control element to sweep the filter. The actual envelope detection IS done , more or less, by an op-amp. I say "more or less" because the op-amp simply provides the gain so that the *REAL* envelope extraction components (diodes, resistors and caps) have enough signal to work with. In principal, you could use a transistor gain element to drive the same diode/resistor/cap circuit. Personally, I can't see a transistor-based envelope-follower/extractor being more compact than an op-amp based one as in the DQ, although I suppose it is worth trying.

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