Failed attempt to build wah-clone.

[BN]

Hello!

The last days I've spent some (quite a lot in fact  ) time trying to build a wah-circuit out of these schematics;
http://www.geocities.com/CollegePark/Library/1355/voxwah.gif
http://www.geocities.com/j4_student/crybaby.gif

My first intention was to build the Cry Baby clone but later I decided to build the Vox one instead, still using the 2N3905 transistors.
Yesterday it was time for testing. I connected a tonegenerator at the input of the circuit and, at first, a small audio amplifier at the output. Maybe needless to say but there wasn't a sound!
Then I connected an osciloscope in paralel with the amp and tried to follow the signal's path through the circuit, when i got to the righthand side of the first 0.01uF cap the signal was almost dead (the amplitude of the input sine wave was about 100 mV when the input amplitude was somewhere around 10 V...) and thus not high enough to reach the voltage treshold of the base at the 2N3904 (I haven't checked the datasheet but I assume it's somewhere around 0.7 V).

When wiring it all upp I assumed that the negative pole of the power supply was supposed to be connected to ground so maybe I've just misinterpreted the schematic. If that's the case I would appreciate if someone would like to tell me how to connect the input signal and input ground.

Does anyone have other ideas of what the fault might be?

[johngreene]

The negative of the battery is usually connected to ground through the input jack. The battery is therefore disconnected if nothing is plugged into the pedal. If you wire it to ground, the effect will always be powered and will drain the battery.

10V is a lot of signal to be pumping into the input of any guitar effect. A typical guitar signal is about 100mV p-p.

The input signal doesn't have to reach the voltage threshold of the input transistor because the transistor should already be biased on. If you could measure the voltages at the pins of the transistors and post them, it would provide a much clearer picture as to what is going on.

--john

[BN]

Ok, I'll try to measure the voltage at the transistor pins but I'll have to do it with the inductor shorted (or rewind the inductor) cause the copperwire broke at the beginning of the windings (guess I should have used thicker wire...)  Or will I be able to obtain the same values with the inductor compeletly out of place?

Thanks for the information about the grounding, it worked the same way as I thought. But I'm not using any jacks for the moment though I haven't used the circuit with any guitar yet, that's why i connected the negative pole directly to ground (in fact I'm not using any battery either so it doesen't really matter whether the circuit will be broken when unused or not).

Maybe I should tell that i started the testing with much smaller amplitude of the insignal. The 10V p-p was more a, pretty stupid I guess, matter of extreme measurements to see if I would get any output signal at all (I was pretty frustrated at that time  )

Tanks for the shown interest, I'll provide more information as soon as I have time to do some more measuring.

[Paul Marossy]

Try about 500mV P-P, that's about the "average" level of a hot humbucking pickup. It sounds to me like your inductor may be causing a problem if you have a broken wire in it...

[BN]

I'm almost certain that the inductor was fully functional when I began the testing at least (I noticed the lose wire when i was putting the PCB back in my locker) but anyway I started to wind a new one at the train home from school earlier today. But would an interrupt in the circuit where th inductor i supposed to be cause such problems? It seems to me like the problem rather is that I dont get any amplification from the transistors... But of course, replacing the inductor is worth a try (guess I'll have to do it anyway...)

But before putting any more time into this project  I'll have to study some solid state physics and some transform theory in order to pass the upcoming examinations next week. It feels a bit like a paradox, I'm supposed to know what makes a semiconductor work but I don't know the theory behind using a transistor as an amplifier yet...

Thanks for the tip on suitable voltage levels for stompbox testing. By the way, what output terminal of the tone generator is the best to use in order to "simulate" an electric guitar? The one with high output impedance or the one with low?

/BN

[Paul Marossy]

You want to use high impedance.

[BN]

Hello again,

now I've been able to do some measuring in the transistor pins. I still haven't soldered any new inductor to the board but I figured it wouldn't matter as long as I'm only measuring the DC-levels (I got the same voltage readings both with the inductor replaced by a jumper wire and with the inductor completely out of place).

As I said before this is the schematic I used - http://www.geocities.com/CollegePark/Library/1355/voxwah.gif - but with both transistors replaced by 2N3904, and with the 510 ohm resistor between the emitter of the first transistor and ground replaced by a 470 ohm to fit the E12 standard. Maybe I also should mention that don't have any "real" wah-pot yet so I tested the circuit with a standard 100k pot and then with a 100k trimpot. For power I used a Mascot Type 719 0-30V power supply for experimental use instead of a battery, but it isn't a potentially malfunctioning wallwart either (I'm certain that tthe power supply was working during my measurements).

Here is the results I got from measuring with the DMM's COM-lead clipped to signal ground (= negative pole of the power supply), supply voltage at 9V:

Q1 (the lefthand transistor):
C - 9V
B - 0V (or rather 0.1mV but I assume this is due to the tolerance of the rather cheap DMM, it the same reading as I get when measuring with both probes connected to ground)
E - 0V

Q2:
C - 9V
B - 0V
E - 0V

Would it be legal to post a redrewn copy of the schematic showing how I put it together. I've got an .sch-file created with Cadsoft Eagle that I used to make my PCB and it shows exactly how I connected everything (in theory...), what component values I used etc. and it also includes indices of the components.

[johngreene]

The circuit isn't biasing up. The collector of Q1 is supposed to be connected to the base of Q2 through a 470K resistor but you are showing 9V on one and 0V on the other. I'd check that connection for starters.

--john

[BN]

Thanks for all your help.

I found the thread about debugging at this forum and followed the instructions I found on geofex.com. I found out that the transistors, just like John wrote, weren't biased at all. Later on when I compared the original schematic with the one I drew in the cad-program I saw that I had missed to draw one wire in my schematic, and thus it was missing on my PCB as well. Pretty embarassing...  For now I've solved the problem by soldering a jumper wire to the board. Guess I'll etch a new board later on.

But now am I at least able to hear the sine wave that I tried to send through the circuit even though it still doesn't make any wah-sounds. Got to get that inductor finished soon (winding 300 turns of wire round a toroid can seem like a never ending job when you just want it to be finished...)

/BN

[Paul Marossy]

Guess I'll etch a new board later on.

Why? If it works, it works. If you plan on sharing the PCB layout, then by all means make that is correct!