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Envelope Controlled Filter using tubes?

7/28/2000 1:34 AM
jon	Envelope Controlled Filter using tubes? Hi Reading thru the recent thread on the autowha, I was wondering if someone would be kind enough to explain how envelope controlled filters work, and if anyone knows of any tube based circuits that will do this? I've been going nuts trying to get a tube frequency divider to do the octave below thing...I can get the guitar signal translated to a sawtooth that will reliably trigger the flip-flop, but when the note decays, I get unpredictable oscillation and degradation of the signal. If I could modify the envelope before or after, might clear things up a bit. Thanx, Jon

7/28/2000 1:34 AM

jon

Envelope Controlled Filter using tubes?

Hi
Reading thru the recent thread on the autowha, I was wondering if someone would be kind enough to explain how envelope controlled filters work, and if anyone knows of any tube based circuits that will do this?

I've been going nuts trying to get a tube frequency divider to do the octave below thing...I can get the guitar signal translated to a sawtooth that will reliably trigger the flip-flop, but when the note decays, I get unpredictable oscillation and degradation of the signal. If I could modify the envelope before or after, might clear things up a bit.
Thanx,
Jon

7/28/2000 4:03 PM
Don Symes	Compress before the divider?

7/28/2000 6:38 PM
jon	Hi Don, I tried compressing with a stage after the sawtooth before the flip-flop by several means: A) using voltage dependent resistor (VDR or varistor) as a constant-voltage cathode resistor to stabilize negative grid voltage. NG. 2) Constant output cathode follower with the VDR, in an attempt to limit the output signal voltage with wide swing of the input (tried pentodes and triodes) NG. 3) diode bounding. NG. 4) Split rail to the stage before the divider. NG. My next step is trying a split rail approach on the flip-flop, or negative grid bias. Am I missing the obvious? thanx, Jon PS. The varistors have specs like the old silicon carbide ones, not the new MOV's.

7/28/2000 6:38 PM

jon

Hi Don,

I tried compressing with a stage after the sawtooth before the flip-flop by several means: A) using voltage dependent resistor (VDR or varistor) as a constant-voltage cathode resistor to stabilize negative grid voltage. NG. 2) Constant output cathode follower with the VDR, in an attempt to limit the output signal voltage with wide swing of the input (tried pentodes and triodes) NG.
3) diode bounding. NG. 4) Split rail to the stage before the divider. NG. My next step is trying a split rail approach on the flip-flop, or negative grid bias.

Am I missing the obvious?

thanx,
Jon

PS. The varistors have specs like the old silicon carbide ones, not the new MOV's.

7/31/2000 1:00 AM
Don Symes	I was thinking compress before the whole pedal. At least before the sidechain. But then, you lose the envelope function. Zach sounds like you man for this.

8/2/2000 1:26 AM
jon	Don, Can you recommend any good texts or other references that deal with this kind of thing? I'm kinda falling between the cracks with my own limited background, several old design books, and EE texts. thanx, Jon

7/29/2000 9:16 AM
zachary vex	there's a schematic for the doctor quack on this page at the amz site: http://www.muzique.com/schem/quack.gif the signal is buffered and sent two places... one is an envelope generator (lower) whose output is applied to the base of Q2, which serves as a variable resistor (you could do this with a tube too)in the filter circuit above, which also receives guitar signal from the buffer. the guitar strike triggers the env gen, which sends a changing dc level to Q2, which changes the center point of the filter, and it wahs once for every strike on the guitar. you could even build a simple wah circuit based on the crybaby, substitute tubes for the transistors, and use another tube or vactrol or lamp-driven photocell to act as the wah's control pot. you could make the envelope generator from a one-shot version of your multivibrator circuit you are using for the octave divider. as far as the octave, the groove tubes manual has a gibson tube bass amp schematic which has a compressor built-in. perhaps this circuit will work as your compressor. remember that you can use the "tube as a variable resistor" for this too. if you buffer and rectify the signal and store it on a cap, you can create a control voltage to drive the grid of a tube being used as a variable resistor, much like a fet, in a voltage-division scheme, or of course you could also drive an incandescent lamp-driven photocell and that would remain pretty old school. there's an amazing pitch-shifting vacuum tube circuit in the groove tubes book also, the magnatone vibrato, which uses those varistors you were talking about. might be some ideas there. the maestro octave box used a scheme where the octave portion of the signal only was "gated" using an envelope control and steering diodes... it only sounded the octave for a moment after you played a note. sounded like a tuned kick drum or an upright bass played pizzicato. when you defeat that envelope control, the octave becomes completely insane. it's wonderful that way, like a cloud of low end burbling around in and out of time with the music. zachary vex

7/29/2000 9:16 AM

zachary vex

there's a schematic for the doctor quack on this page at the amz site:

http://www.muzique.com/schem/quack.gif

the signal is buffered and sent two places... one is an envelope generator (lower) whose output is applied to the base of Q2, which serves as a variable resistor (you could do this with a tube too)in the filter circuit above, which also receives guitar signal from the buffer. the guitar strike triggers the env gen, which sends a changing dc level to Q2, which changes the center point of the filter, and it wahs once for every strike on the guitar. you could even build a simple wah circuit based on the crybaby, substitute tubes for the transistors, and use another tube or vactrol or lamp-driven photocell to act as the wah's control pot. you could make the envelope generator from a one-shot version of your multivibrator circuit you are using for the octave divider.

as far as the octave, the groove tubes manual has a gibson tube bass amp schematic which has a compressor built-in. perhaps this circuit will work as your compressor. remember that you can use the "tube as a variable resistor" for this too. if you buffer and rectify the signal and store it on a cap, you can create a control voltage to drive the grid of a tube being used as a variable resistor, much like a fet, in a voltage-division scheme, or of course you could also drive an incandescent lamp-driven photocell and that would remain pretty old school.

there's an amazing pitch-shifting vacuum tube circuit in the groove tubes book also, the magnatone vibrato, which uses those varistors you were talking about. might be some ideas there.

the maestro octave box used a scheme where the octave portion of the signal only was "gated" using an envelope control and steering diodes... it only sounded the octave for a moment after you played a note. sounded like a tuned kick drum or an upright bass played pizzicato. when you defeat that envelope control, the octave becomes completely insane. it's wonderful that way, like a cloud of low end burbling around in and out of time with the music.

zachary vex

7/29/2000 2:31 PM
jon	Hi Zachary, Looks like my weekend work is cut out for me... I've approached several designs just as you suggest...build it up with SS components, then figure out how to make tubes do the job. I'm a lot more comfortable with tubes. On the Dr. Quack, will a TL072 work here? Those Magnatone vibrato circuits are wonderful...I've restored a 280 and a 410 which got me very interested in voltage dependent resistors. Built several multistage vibrato circuits that I use in stomp box format..... I've built a Vox type wha using tubes, then tried subbing in a vibro-champ type tremelo circuit for the wha control pot... tried the Magnatone-type vibrato mixed with some of the dry signal(more of a chorus than phase shift) and drive this signal into the wha control pot, also tried both these approaches superimposed on the normal signal path...nothing all that useable, short of the wha and vibrato/chorus/phase shift circuits alone. After I got burned out with these, started on this octave divider....What you're describing with the Maestro sounds very much like what I've been getting with my circuit, as soon as the signal decays, there's all this "low end burbling" which I attributed to oscillation that I've yet to filter out. I've been reading up on the one-shot's, seems like they should be able to provide attack delay?? I haven't used LDR's with incandescents, just ne-2's. Is there an advantage here due to the lower voltages? Thanx for the Gibson GA-100 reference, looks like I've gotta start reviewing some of the guitar amp stuff again. Jon

7/29/2000 2:31 PM

jon

Hi Zachary,
Looks like my weekend work is cut out for me...
I've approached several designs just as you suggest...build it up with SS components, then figure out how to make tubes do the job. I'm a lot more comfortable with tubes. On the Dr. Quack, will a TL072 work here?

Those Magnatone vibrato circuits are wonderful...I've restored a 280 and a 410 which got me very interested in voltage dependent resistors. Built several multistage vibrato circuits that I use in stomp box format.....

I've built a Vox type wha using tubes, then tried subbing in a vibro-champ type tremelo circuit for the wha control pot... tried the Magnatone-type vibrato mixed with some of the dry signal(more of a chorus than phase shift) and drive this signal into the wha control pot, also tried both these approaches superimposed on the normal signal path...nothing all that useable, short of the wha and vibrato/chorus/phase shift circuits alone.

After I got burned out with these, started on this octave divider....What you're describing with the Maestro sounds very much like what I've been getting with my circuit, as soon as the signal decays, there's all this "low end burbling" which I attributed to oscillation that I've yet to filter out.

I've been reading up on the one-shot's, seems like they should be able to provide attack delay?? I haven't used LDR's with incandescents, just ne-2's. Is there an advantage here due to the lower voltages?

Thanx for the Gibson GA-100 reference, looks like I've gotta start reviewing some of the guitar amp stuff again.
Jon

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