EHX Electro-Harmonix Doctor Q

[michael_krell]

welp, the impedances are right. I dont know what could be the problem here. I am just about at the end of my rope. anyone have any suggestions?

also, how come in the schematic the sens and range pots are not the same as whats on the pedal?

[niklas_d]

Hey Michael, Hey Forum-members,

More than one decade after your discussion, I discovered this topic. I have excatly the same problem, did you ever find the cause of the problem?

I've cahnged every part that could be broken, OpAmp, transistor and the two PolCaps. Power Supply at the OpAmp is fine (Vcc- 0 Vcc+ 9V), the Sensitivity-Pot and the Trim-Pot seems okay (also tried to bridge the pot), Diodes work unidirectional at round about 0,5V, I/O connections are also fine.
So, I really have no more ideas where to search for the problem. Any suggestions anyone?

Greeting Niklas

[Mark Hammer]

Make sure that the pinout of the transistor you used is the same as what you thought it was.  Some are ECB, some are EBC.  Some are ECB looking at the flat side, and some BCE, looking at the flat side.  Same with EBC pinouts.

[ElectricDruid]

Make sure that the pinout of the transistor you used is the same as what you thought it was.  Some are ECB, some are EBC.  Some are ECB looking at the flat side, and some BCE, looking at the flat side.  Same with EBC pinouts.

Good point. It's been easy to get that wrong for a lot longer than just one decade...

T.

[StephenGiles]

Make sure that the pinout of the transistor you used is the same as what you thought it was.  Some are ECB, some are EBC.  Some are ECB looking at the flat side, and some BCE, looking at the flat side.  Same with EBC pinouts.

You beat me to it Mark! I remember having the same problem when building one of these - before internet of course, so I had to visit our local library (remember them?) to look up the relevant transistor!

[niklas_d]

(Sadly) the transistor is built in correctly: emitter is connected to ground, base (over the 22k) to the trim poti and collector to the input signal to be amplified.

Strange thing is, when I remove one of the diodes I still do not get a good amplified signal at the output of the OpAmp in the envelope-detection-path. That means that the problem would be in before that part, right? But there is only the poti, a cap and a resistor...I've tried 4 new OpAmps, unliky that they are all damaged. The power-supply of the OpAmp is working correctly....I don't have a glue..

[michael_krell]

Wow. i cant believe this is still going! Since my last post I got married, lived in 3 different states and had 3 children. All that to say i can't remember for the life of me what I ended up doing with that Doctor Q. Hope you find your answer.

[Mark Hammer]

(Sadly) the transistor is built in correctly: emitter is connected to ground, base (over the 22k) to the trim poti and collector to the input signal to be amplified.

Strange thing is, when I remove one of the diodes I still do not get a good amplified signal at the output of the OpAmp in the envelope-detection-path. That means that the problem would be in before that part, right? But there is only the poti, a cap and a resistor...I've tried 4 new OpAmps, unliky that they are all damaged. The power-supply of the OpAmp is working correctly....I don't have a glue..

If you built it on perf or vero, I will suggest using an existing PCB layout, since all the connection of components will be correct.  Consider using one of the layouts for the Dr. Quack ( http://www.generalguitargadgets.com/effects-projects/filters-envelope/dr-quack-ehx-doctor-q/ ) or Nurse Quacky ( http://home-wrecker.com/nurse-quacky.html ).

[niklas_d]

Ah, sorry, you got me wrong. I try to fix a pretty old EHX Doctor Q. So there should be no layout issue with that. But because I have no hint where to search for the problem, I need to retire the pedal.

[ElectricDruid]

What op-amp did you use for the replacement? The word is that this circuit is extremely fussy about op-amp choice, with some of the common candidates (TL072, 4558) not working at all. NE5532 and 1458 are both supposed to work.

In my view a circuit that is that fussy about something as basic as an op-amp is a badly-designed circuit. I want something where I can chuck in any old op-amp that I've had kicking around in my parts bin since forever and it still works fine. I hate having to tune stuff, and I hate things where only "selected" parts work - that's just lazy designers not working out why some work and some don't and building it better.

Ok, rant over. Sorry.

T.

[niklas_d]

What op-amp did you use for the replacement? The word is that this circuit is extremely fussy about op-amp choice, with some of the common candidates (TL072, 4558) not working at all. NE5532 and 1458 are both supposed to work.

Indeed that could be the problem! Probably the old OpAmp was busted and I had just some RC4558Ps. I will get some NE5532s and have a try! Thanks so far.

[Fancy Lime]

Hi Tom,

without checking the datasheets I'm going on a limb here and guess that the fact that the circuit is picky with op-amps has to do with the op-amps ability to deliver current in the envelope follower half. The filter section should work with any old op-amp. But since the envelope follower needs to pump up the 10µ while Q1 drains it, a TL072 or 4558 may just have too weak outputs. They aren't exactly famous for high current applications. Adding a buffer may solve that problem but then you have more parts. I would argue that there is something to be said for few and select parts over generalized but more parts. So I would not dismiss this as "lazy" design right away, although it might be. But it may have been a deliberate choice to make it simpler than it could otherwise be. Generally speaking, envelope followers in many designs seem to be picky with op-amps (at least way more picky than the audio paths), some demanding much more exotic types. I would be very interested in someone more informed than me chiming in on this. Would like to improve over my current "I'll just make it work with a NE5532"-approach.

Cheers,
Andy

[Mark Hammer]

For a stock Doctor Q it has to be a 1458, or else you won't get sweep.  That was one of the virtues of the Dr. Quack, it compensates for different op-amp types.

[ElectricDruid]

without checking the datasheets I'm going on a limb here and guess that the fact that the circuit is picky with op-amps has to do with the op-amps ability to deliver current in the envelope follower half. The filter section should work with any old op-amp. But since the envelope follower needs to pump up the 10µ while Q1 drains it, a TL072 or 4558 may just have too weak outputs. They aren't exactly famous for high current applications. Adding a buffer may solve that problem but then you have more parts. I would argue that there is something to be said for few and select parts over generalized but more parts. So I would not dismiss this as "lazy" design right away, although it might be. But it may have been a deliberate choice to make it simpler than it could otherwise be. Generally speaking, envelope followers in many designs seem to be picky with op-amps (at least way more picky than the audio paths), some demanding much more exotic types. I would be very interested in someone more informed than me chiming in on this. Would like to improve over my current "I'll just make it work with a NE5532"-approach.

Yeah, fair comment. The 5532 has a famously powerful output (often used as a headphone driver, for example) so that'd make sense. Having a quick look on TI's site, the 1458 manages 45mA output current, the 5532 manages 38mA, and the 072 only manages 10, so that supports your theory.

I'm not adverse to using a specialised op-amp if the situation demands it, and in the synth world it definitely sometimes does if you want your VCO to play in tune or your filter to track properly. In stompboxland, I guess I don't find that need for precision quite so often, and most stompbox designs *are* extremely forgiving, so perhaps that comes to be expected.

Tom

[Mark Hammer]

In the case of the DQ, what matters is how wide the voltage swing is.  I've tried 6 or 8 different dual op-amp types in the circuit.  Some yield no discernible sweep.  Some deliver a sweep that goes so wide it is unusable. The 1458 is not too hot and not too cold.