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Help with Hemmo's Auto Wah


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7/16/2000 7:19 AM
Mark Hammer
Help with Hemmo's Auto Wah
I built this thing from the schematic posted on Aron's site, because it looked like it had some nice features.  
 
Well, it works, more or less, but mine sounds awful. The Release function also doesn't seem to work as intended (rather, it doesn't do anything noticeable). The sweep is really unpredictable, even for an autowah. The tone is distorted, even with the Gain/Q control at minimum.  
 
So, I need some help debugging.  
 
Here are some parameters for what I've got on my bench:  
1) I'm using a TL072  
2) I'm using 2N5088's for all transistors, except for the 2 just before the Range control. These are 2N3904's.  
3) I'm providing Vref using what every floating ground circuit seems to use - two 10k resistors and a 22uf cap between Vref and gnd. All polarities are fine.  
4) For the two 4.7uf caps coming off the rectifier, and just before the diode pair, I'm using some electrolytics that don't have polarity marked, but since every other cap in my bin has the value/voltage closest to the negative lead, I assumed it was the same on these. I may be wrong, but can't tell.  
 
So, assuming Hemmo's hand drawn schematic reflects sound thinking, some questions...  
A) Are there any parts of the circuit that strike you as implausible or quirky? (e.g., the junction of the 2 collectors just before the Range control)  
B) Are all indicated polarities on caps and diodes likely to work out fine? If I had my 4.7uf caps oriented wrong, would that screw things up?  
C) What difference would variations in transistors make? (My guess is none)  
D) What is the Release control supposed to do vis a vis attack?  
E) Is this thing supposed to mimic "down" drive on a Mutron in the bandpass setting?  
 
There is a hint of something interesting in this design (thanks Hemmo!) so I'd like to get it functioning. I'd also like to field test another pedal so thst people will know what the schematic corresponds to.  
 
Any ideas?
 
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7/16/2000 8:30 AM
Ivan
Hello,  
 
I could not get that Hemmoīs Auto Wah link working on Aronīs site.  
 
Thanks,  
Ivan
 
7/17/2000 10:12 AM
Hemmo P.

Are there any parts of the circuit that strike you as implausible or quirky? (e.g., the junction of the 2 collectors just before the Range control)  
 
This schem was a scratch, but i posted it anyway because it worked VERY well on the breadboard..  
I drew it straight from my notes, and itīs been a while ago, so i donīt remember so much stuff i came up with this  
And to the quote;  
The MXR dyna-comp had another transistor before the Range pot, but the BOsSīs slow gear attack delay doesnīt have that tran, so that shouldnīt be the problem... So the envelope is actual copy of the  
SGAD  
 
I'm providing Vref using what every floating ground circuit seems to use - two 10k resistors and a 22uf cap between Vref and gnd. All polarities are fine.  
 
I marked separately all the points that go to either +1/2Vcc and Gnd, the transistor that acts as a variable resistor could be connected to either one where Gnd gives deeper wah.  
 
The Release function also doesn't seem to work as intended (rather, it doesn't do anything noticeable).  
Are you sure you connected the pot correctly?  
3rd pin connects to base of the upper tran via 100ohm R and 1st goes to lower tran via 100ohm R,  
and wiper to GND?  
The Release controls the time the WHA takes, full down and it makes WWWWWWWWWWWWWWHA (the īHA comes just when the note fades away, and full up then the effect is very minimal..  
(disconnect the Gnd from the Range pot, if that might help somehow..)  
 
Itīs weird by the way, how the circuit worked differently when i offhandedly tried to make something out of it at a later time.. The WHA turned other way around where the tran was closed and striking a chord opened it..  
 
I guess some of the guruīs can help you out in possible īmechanical errorsī, Iīll take the schem off, until iīve confirmed itīs actual correctness..  
 
Hemmo P.
 
7/17/2000 4:03 PM
Mark Hammer

Don't pull it until I've confirmed the accuracy of my perf-boarding! :)  
 
I will recheck my construction tonight or tomorrow. As far as I know, the release pot is wired in the manner described, although I will try lifting the GND and see if that makes a difference.  
 
Functioning or not, thanks for the circuit. I love envelope controlled filters, and the more controls they have, the happier I am.
 
7/18/2000 8:38 AM
Hemmo P.

I already took it off, sorry, but I promise that  
I go back to the circuit maybe today, i didnīt yesterday cause i was busy building green ringer..  
iīll send a īneaterī schem and a bit fixed one soon..  
 
Hemmo P.
 
7/18/2000 3:59 PM
Mark Hammer

Thanks. If I can offer some response from a consumer....  
 
I tried mine again last night, and the distortion is still there, although I could get it to work better. The "release" control is interesting, and takes some time to get used to. What makes this a potentially interesting design is that the circuit does not respond the way one is accustomed to with other envelope-controlled filters, and I like that. The circuit does appear to be very hungry for current, though, and brought a battery down to a weak state fairly quickly, so I plugged in an adapter.  
 
I still can't see anything wrong with the board I made, though. Everything works the way it is supposed to; all controls produce the changes intended.  
 
One or two modifications you might consider are to the filter section itself. It may be a good idea to have a fixed resistor between the emitter and ground, to set the highest filter frequency. With the existing pot set to the highest range (lowest resistance), the filter is up in the "insect noise" range. A 470R resistor between emitter and ground might tame things.  
 
I suspect that the Gain/Q pot probably doesn't need to be more than 500k, either. I have a 1meg pot in there, as per the schematic, and pot settings much greater than 500k tend not to produce desirable changes in tone. Perhaps when I figure out the source of the distortion, this may change, however.  
 
When it comes to envelope-controlled filters, what gives many units their character is not the filter section itself, but the envelope extraction circuit. It is not so much the filter tone, but how the filter changes with your playing that gives the uniqueness of the unit. For instance, in the case of the Mutron, it is the breadth and speed of sweep and choice of sweep direction, that gives it a distinctive sound, compared to other units. The filter itself is a plain vanilla state-variable filter.
 
7/19/2000 7:11 AM
Hemmo P.
couldnīt make it happen..
Morning,(or is it night over the sea?)  
I went out of my mind last night because I couldnīt get it work the way it worked when i first came up with it.. freaks me out.  
 
quote:
"I tried mine again last night, and the distortion is still there, although I could get it to work better."
 
 
If thereīs still distortion(thatīs a good thing in my opinion) increase the resistor before the filter(donīt remember the value), or decrease the resistor in series with the G-pot.  
 
quote:
"The "release" control is interesting, and takes some time to get used to."
 
Does it work?  
quote:
"I still can't see anything wrong with the board I made"
 
I donīt doubt it.  
 
 
I also thought about limiting the range with a resistor from the emitter, but the transistor i tried(BC-something) had a good minimium resistance.  
 
But itīs good if you liked it..  
 
BTW, i came up with something else with the circuit,  
with a few resistors and a cap over the base of the īvari-res.ī tran, you can get it to work with a momentary push-button switch to go wahwah by pressing the switch.. Iīll build a proto and if it works as intented, then iīll put the schem on  
 
Hemmo P.
 

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