debugging MXR Envelope Filter

[tambek]

I have breadboarded the thing. I have a common problem - the output is very low and it doesn't wah. even if i put my distortion pedal before the MXR EF, it doesn't wah. I have triple-checked the connections, i have replaced all the ICs and i also changed C4, C2, R16 as suggested at tonepad.com before...  i also checked for power, it was all good. One thing I thinki better mention, is that I have 20M resistors instead of 22M and 1,2M instead of 1M... could this be a possible problem? the schematic and layout are available at http://www.tonepad.com/getFileInfo.asp?id=5

any ideas?

[tambek]

bump

[Bill_F]

I had the same problem when I first built mine. No wah at all, just a straight signal.

What corrected my problem was changing the IC's. I used some from Texas Instruments at first and they didn't work.  Fortunately somebody had suggested I buy a couple of brands when I was purchasing the parts for this build, so I also had some from Fairchild. I put them in and they worked fine. Perhaps others have used the Texas Instrument products with no problem in this circuit, or maybe I fried one with static electricity, all I know is soon as I changed them the circuit fired right up. Plus, I believe I read somewhere that this circuit can be a bit picky about what ICs are stuck in there.

HTH,
Bill

[tambek]

ok, gonna try others then, thanks

[tambek]

ok, my ic's were ok, what corrected my proble was increasing C5 from 0,05 to 0,47uf, that boosted up my circuit which made me able to hear how does it wah. Well, it does wah, but i was excpecting a continous wah, like wah-wah-wah-wah-wah, not waaaaah, like it goes right now. I know usually people step on their pedal to achieve what im trying here, but isn't it possible to have this circuit a repeating wah?
and it basically wahs only for the first note i pick, the others are just filtered, but you can't hear the actual wah going on.. would it be possible to have it wah for every note i pick? (even if there's a fast solo or something?)


secondly.. the attack control doesn't seem to do much... if i take it out, there is no wah, but by turning the pot, well, nothing happens... i have a 500k lin pot there... would it be possible that by changing it to say 100k or maybe some 1m or 2m would help to correct this (or maybe from lin to log?)

[tambek]

the reverse sweep and filter range mod, however, work great! (also the emphasis mod)

i'm still kinda worried about the attack control since it really does nothing...

[Chris R]

sorry.. can't answer your question.. but i gonna start building this one early next week.. just wondering if i need to socket those big ic's ?  would they be easily damanged by heat ?

C

[Mark Hammer]

I hope this doesn't seem like scolding, but from your comments it seems like you are unfamiliar with how such pedals work.  As a result, it may be that the attack control *does* work, but you are unfamiliar with what it is you are supposed to be listening for.

The difference in attack time is NOT huge.  This will not produce anything that sounds like a very slow sweep.  If anything, when working right it should sound like something you would normally do with a wah by foot to something much faster than you could do with your foot.  Just as an aside, you CAN replace the existing 500k pot with a 1meg pot for a little more attack time.  

The other thing is that the variations in attack time are not always perceptible if the sensitivity is not up high enough.

Hopefully, the things that don't seem to be working for you now are just a question of growing pains and getting comfortable with the effect itself, rather than something that needs to be repaired.

[tambek]

Mark, well, uh, not that I'm not familiar with wah pedals, I'm not familiar with any pedals.. I'm just a guy who wanted to build the effects instead of buying them.. so yeah.. maybe i should put that in my signature

I undesrtand what you want to tell me about the attack. Well, yeah, I understand that, but however, it doesn't sound like it can wah much faster than with doing this with my foot. And actually, this is what I'm looking for. So, if i would like it to wah faster, then the attack time should be quite short, right? in that case, is there a way i can make it happen, because the attack time with the pot turned down or fully up, is not very short. Maybe i need a resistor before the pot?

And is there a way to increase the sensitivity.. or does that concern the IC's?


To Chris: I don't know about frying those IC's when soldering, but i do know, that they fry up very quick when you have a wallwart with the wrong polarity.. especially the CD4066. I had to replace it...

[Mark Hammer]

A properly-functioning EF does have a pretty whip-crack fast attack when set to its fastest attack time.  Indeed, it is the ability to have a smoothly variable attack time from very fast to seductively slow that attracted me to it in 1978 and has kept me faithful to it all these years.  So, something is wrong, with either the components, the board, or perhaps the wiring of the pot (it would not be the first pot in history to have a non-functioning wiper or have connecting wires with intact insulation but a metal fracture inside the insulation).

As for repetitive filter-sweeps, the recently produced MXR Auto-Q - itself a very nice unit that has won lots of praise - includes a mixable LFO.  The LFO will cause the filter to sweep repetitively for the bubbly wah-wah-wah-wah sound you noted.  The old Electro-Harmonix Polyphase and Polyflange both had this capability to some extent, although the MXR unit does not allow you to envelope-control the LFO rate the way you could with the E-H pedals.

In any event, it would be technically possible to do what you want, but that would be outside the scope of the existing design of the pedal you have cloned, and certainly well outside the scope of the board layouts that have been posted.

The envelope sensitivity of the EF is actually pretty wide range.  I have yet to encounter many circumstances where the sensitivity control could not be set to generate an audible sweep over a wide range of pickup outputs, or a circumstance where it could not be set low enough to tame a too-hot envelope signal (though increasing the pot value from 500k to 1meg helps there).

One more demeaning and condescending question:  You mention a wallwart.  Are you sure you are powering up the pedal appropriately?

[tambek]

well, i'm using a battery at the moment, so it should be powered OK...

I'll check out any other circuits you have suggested, thanks  

[gez]

well, i'm using a battery at the moment, so it should be powered OK...

I haven't built this circuit, but I should imagine that all those inverters would suck the life out of a battery fairly quickly.  Might be an idea to use a wall wart till you get your problems sorted out?

[Mark Hammer]

The current consumption of the EF is actually pretty modest.  I never had problems with batteries in any of mine lasting as long as batteries in other effects.

[gez]

The current consumption of the EF is actually pretty modest.  I never had problems with batteries in any of mine lasting as long as batteries in other effects.

Out of interest Mark, what is it?