Need help with a failed bass-wah modification

[Derringer]

My bass player handed his GCB-100 over to me to work on because it was to subtle/quiet
http://fuzzcentral.ssguitar.com/schematics/basscrybaby.gif


I changed out the 1K power supply filter resistor for a 100 ohm resistor because I figured I'd get a little more headroom that way.

I probably should have just lowered the 68K resistor for more gain but instead I decided to add an opamp gain-stage at the output because i figured that this way, he would have a buffered output as well.

here's the design I used:


I'm getting my 1/2 Vcc from a voltage divider that is fed power from the circuit side of the new 100 ohm PS Filter R into a voltage divider made of 2 47K resistors and a 10 uF cap.

I did this about a month or so ago, and as far as I know it worked fine. But now, when it first powers up it sounds right, but then maybe a minute later it loses gain and becomes very fuzzy and glitchy.
I tried another chip and the outcome was the same. I changed the input capacitor from a 1 uF electro to a 0.22 uF and the outcome is the same. I was thinking that the electro might be causing the problem since it works fine at first and then misbehaves.

Before I go swapping the output capacitor I wanted to know if any of you saw any kind of intrinsic flaw in my design and implementation.

Thanks.

Bill

p.s. and in looking at it drawn out the way it is now, I do realize that my RC cutoffs are absolutely ridiculous

[Gus]

 first find out what is happening
Take out what you added and find out if the wha still works stock.
 AND/OR
Measure the circuit voltages before and after it changes sound.  measure at the "legs" of the opamp.  What are you doing with the other 1/2 of the opamp?

You could try removing the divider network, input cap and 1meg and connect the + input of the opamp amp to the collector directly and change the gain setting resistors to maybe a 10K and 47K and add a cap from the "bottom" of the 10K to ground.  Do you really want 4.7/1 + 1 gain you can make it a gain of 2 with the same value resistors maybe two 33Ks.  What are you doing with the other 1/2 of the opamp?  You might run out of output headroom measure the collector to ground before trying the direct coupling

Where you adjust gain in a wha makes a difference.  Find the patent for the wha to understand how it works

[Derringer]

thank you for the reply

what you said first " first find out what is happening Take out what you added and find out if the wha still works stock." is what I realized later that evening what I should do.
My friend said he didn't like the quietness of the wah and I just jumped to the conclusion that all it needed was a booster slapped onto the end .... didn't even consider that there may in fact be something wrong with the actual wah circuit. ...bad engineering on my part ... but lesson learned

Disconnecting my circuit and testing the stock circuit is the first thing that I'm going to do when I go at it again.

The second half of the opamp is doing nothing. I don't have any single IC's on hand so I used the dual. I originally had pins 5, 6, and 7 connected to nothing, but when this weirdness started happening, one of the things I tried was grounding all of them. It didn't help.

gotta run now .. but i do have ssome q's about your suggestion

thnx

[Derringer]

I disconnected my circuit and the stock wah worked, although quietly, without any of the issues I was having before.

I tried to clean up my circuit a bit first. I changed the input capacitor to 0.01 uF and changed the output cap to 0.068 uF. These changes did not seem to help.

I am powering my booster section from a lead that connects directly to the "circuit side" of the PS filter resistor. When my circuit is connected in this fashion, using a battery with about 8.7 V, I get 8.7V on the battery side and around 3 volts on the circuit side of the PS filter resistor. The voltage divider section of my booster reads around 1.8v.


Here are the pin readings with my booster section being powered from the lead I have hooked to the circuit side of the PS filter resistor (I did change this resistor to 470 ohm from 100 ohm ... thinking maybe something was wrong with the 100 ohm R but the results are pretty much the same.)

1 - 1.9
2 - 1.8
3 - 1.3
4 - 0
5 - 0
6 - 0
7 - 0
8 - 2.9

the divider is at 1.8 v

Now, when I disconnect this lead that powers my booster section, the voltage on the circuit side of the PS filter R reads 8.36V

When I power my booster circuit individually, hook it up to the 8.7V battery directly through the lead that I previously disconnected I get these voltages

1 - 4.95
2 - 4.84
3 - 1.8
4 - 0
5 - 0
6 - 0
7 - 0
8 - 8.59

and the divider is at 4.8 v



You suggested a different circuit as well. Is this what you intended? ( in other words, did I understand you correctly)

Just what value cap should go from the 10k to gnd?

thanks so much

Bill

[zombiwoof]

When you say the wah is too "quiet", do you mean there is not enough intensity to the "wah" sound?.  If so, just look at the various pages on modding wahs, that will tell you how to change certain components to tune the response.  I don't think any active circuitry is needed at all.  Read the GEO pages on "The Technology of Wah Pedals" to start.

Al

[Derringer]

When I had the "stock" circuit out by itself, I did try replacing the 68K resistor with a 33K ... but it didn't make enough of a difference.
I didn't mess with the emitter resistor because even though that affects gain, it also affects bass response.


When I first added this booster, it sounded great. My bass player was really happy with the extra kick in the pants it gave when engaged. I actually started with a 5k trim pot in the feedback loop of the opamp so that the gain could be adjusted, but he liked it all the way up. So when it started acting up, swapping the trim for a fixed resistor was one of the first things I tried, hoping that maybe the trim was causing noise ... nope ... not the problem.

I just can't figure out why it worked once, and for a while, and now not at all.
And why am I getting this power supply problem?

I should run a signal through just the booster to make sure it's working properly, but I just can't wrap my head around why the supply voltage gets all screwy when I feed my circuit from the stock circuit's power supply.

thank you for your suggestion though.

[zombiwoof]

So in other words he wants a boost in gain when the wah is engaged?.

Al

[Derringer]

yes

however when engaged, the volume of the stock wah circuit is less than unity

[zombiwoof]

yes

however when engaged, the volume of the stock wah circuit is less than unity

That problem can be fixed by adjusting the first resistor as you noted, but if he wants a boost when it is engaged, then yes the added buffer is a good idea.  I was just not sure what "too quiet" meant, it could have meant that he thought the effect wasn't pronounced enough.

Al

[Derringer]

well .. I tested the booster section by itself on my scope and the top of the waveform was clipped. I checked all soldered points, touched some up, and same result. I swapped in a new 4558 and no more clipping.

So then I wired the circuit back onto the end of the wah circuit just like I had before and then got the same old problem. I was getting about 3 v just aft of the PS filter resistor.

So, what the hell, let me connect the power supply of my boosted directly to the incoming 9V. Now I get full power to the booster and full power minus whatever the 470 R resistor eats up going to the stock wah circuit.
It works fine now.

This pedal will always be powered by a filtered PS (right now it's a VS 1-spot) or a battery, so I'm not concerned about the lack of proper filtering on my booster circuit.

But, can anyone explain to me why I was getting the voltage readings on the PS filter resistor like I was? The two circuits worked fine separately when separately powered. But then when they were both powered from the same point aft-the PS filter resistor, the voltage was basically third of Vcc. And now, taping the booster circuit direct from the incoming 9V, it works fine.


thanks,

Bill