Dunlop - GCB-95F Crybaby Classic Wah Wah  [schematic]

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Goodrat
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Post by Goodrat »

I did some testing today with a scope and a signal generator. I don't like how there is some distortion when I switch my HSS Strat to humbucker at certain positions of the wah. There is too much gain..
After working on a few tweaks with the first op amp and the Q1 emitter resistor, I find that changing R18 (the 47K into the wah section at the Q1 base) from 47K to 100K, solves this. At first I concluded changing R15 to 850 ohm, which was ok, but that may affect the response of the wah and I couldn't decide if for the better.
So changing R18 seems best.

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Goodrat
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Post by Goodrat »

Dirk_Hendrik wrote:Thanks Rick! :thumbsup
Personally I'd remove and jumper R20 straight away. First create a solid 4.5 volts virtual ground, then screw it up by giving it a 1k source resistance and filtering that off again with a cap.. totally nuts :scratch:
I never tried this for a bias voltage, but I would think you need some kind of resistance there.

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Post by Dirk_Hendrik »

Why? There's 2 points this bias feeds to and they both feed that voltage through a 1Meg resistor. The DC voltage on the opamps output is AC-clean due to C14. What's the need of R20?
Sorry. Plain out of planes.

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alexradium
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Post by alexradium »

Vref going through the opamp is better filtered in conjunction with the RC filter in the output,take out that R20 and that filter won't work as well.

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Post by Goodrat »

Concerning R20, I guess I would first of all test it with a poorly filtered 9V power supply and see if made an improvement.
Maybe they were trying to get rid of an occasional oscillation under certain circumstances.
Or, maybe, since there is no diode on the input of the 9V to the op amps, it protects them from reverse polarity and at the same time adds a little over kill to the filtering at the same time.
That's all I could think of.

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Post by Goodrat »

Oh, wait, I was referring to R19. As far as R20, maybe an oscillation, I don't know. If the C2 cap is needed, then maybe the op amp wouldn't like the cap hanging there.
If the C2 cap is needed at all, I don't know, but I would wonder why.

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Post by clevelandrocks »

i see that your knowledge of the 95f is way better than mine. im trying to get a LED in the pedal but have only found a way to turn it on when the wah is off. any help there?

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Post by matt239 »

So, if I understand what I'm seeing correctly;

C7 can be removed and several caps put on a switch connected at JP1 for a range switch.

R17 can be removed and a pot inserted at TP3 with a resistor at R12 for a variable Q control.

Right?

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Post by matt239 »

I could also use some help calculating values for the first op-amp buffer/filter.

I can do the basic resistor-only op-amp gain, and there are online calculators for Sallen-Key filters, & basic Bessel filters,
but there is no calculator or explanation for a setup with only the one cap in this position.
I get the basic concept, but I don't know how to do the calculation.
I'm not an EE, so my filter kung-fu is not strong.

I'd like to reduce the gain to 9dB or 12dB instead of 21dB, and lower the corner frequency to about 1k.

clevelandrocks wrote:i see that your knowledge of the 95f is way better than mine. im trying to get a LED in the pedal but have only found a way to turn it on when the wah is off. any help there?
It's got a 3PDT switch, so can it be done in the "usual" way? Sorry I can't be more help, I haven't added any LEDs to anything. Try searching stompbox off-board wiring?

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Post by matt239 »

Can any of you help get me pointed in the right direction on calculating values for the first filter?

I know it's basic electronics, and the information is out there, but I'm having a hard time finding what I need in a format I completely understand.

I can do R-C filters, and I can do op-amp gain, but I don't fully understand how to calculate op-amp filters with gain.
There are several calculators for filters online, but this filter is missing a cap to be a typical configuration.
I don't know what to enter for the missing cap; 0? infinity?

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Post by Goodrat »

Image

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Post by Victor Nery »

Greetings,

Have you guys tried another opamp in place of the MC33178? I got the datasheet and the specs of this one is far different from the common ones I have available in my country. The most hi fi opamp I have is the OPA2134. I know it is a noob question but I'm not sure if due to the specs of the MC33178, the circuit would show any issues.

Thanks in advance.

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Post by astrobass »

Victor Nery wrote:Greetings,

Have you guys tried another opamp in place of the MC33178? I got the datasheet and the specs of this one is far different from the common ones I have available in my country. The most hi fi opamp I have is the OPA2134. I know it is a noob question but I'm not sure if due to the specs of the MC33178, the circuit would show any issues.

Thanks in advance.
The op amps are being used as buffers here. You shouldn't be able to ear the difference between the stock op amp, your 2134 or a 4558 or any other op amp.

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Post by soupbone »

Goodrat wrote:I did some testing today with a scope and a signal generator. I don't like how there is some distortion when I switch my HSS Strat to humbucker at certain positions of the wah. There is too much gain..
After working on a few tweaks with the first op amp and the Q1 emitter resistor, I find that changing R18 (the 47K into the wah section at the Q1 base) from 47K to 100K, solves this. At first I concluded changing R15 to 850 ohm, which was ok, but that may affect the response of the wah and I couldn't decide if for the better.
So changing R18 seems best.
Will this mod work for the Dunlop GCB-95?(the gain mod)

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Goodrat
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Post by Goodrat »

I also simulated the basic wah portion and the changes in red would work to stop distortion and have plenty of gain. If a musician who plays on stage and knows what he/she wants would agree it was better, I can only guess. But I would make the changes.

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Post by Bernardduur »

I currently am servicing one of these wahs (rev F I guess) and find the schematic is pretty the same. I also hear the distortion going on when I play with a humbucker equipped guitar and play in the toe down position. When I add a delay behind it I can isolate the distortion pretty nice (as I can hear the sound back)

It this THE distortion the mods are fixing? I've isolated the wah circuit (for testing use) and found that the main distortion is generated in this circuit; the buffers and gain stages all add something but not the extra unwanted distortion. ALso, only changing the 'input' resistor of the circuit (47k to 68k or 100k) does lower the volume and the overall distortion but not completely eliminates it.
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Post by champ222 »

Hi All

i realise i'm resurrecting an old thread, but the info is all here, so it seems silly to start a new one.

I'm currently trying to fix one of these wahs.

Ive notice that C2 and C4 (both 4.7uF according to the schematic above) have been broken off, so the first job is to replace those.

Are these values critical? I have some 4.7uF caps, but there are not electrolytics, so there huge. id have to mount them off board (the surface mount pads have gone, so surgery is required anyway. i do have some 10uF electolytics, and some 3.3uF tantalums which would be easier to fit in.

Thoughts?

Thanks

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Post by champ222 »

Update

I put a 10uF in C2, and the 4.7uF Film cap in C4. Now wah-ing as it should :)

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Post by Victor Nery »

Greetings,

Sorry ressurecting this topic. Could someone check C12 again? 1nF plus 4k7 sets bass rolloff frequency at 33kHz. Is that correct?

Thanks in advance.

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Post by Dr.Detroit »

GBC95FrevB.jpg
I don't think Mr. Viola will mind me posting this to the forum. Please delete if causes trouble, etc. etc.

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