Wha wha guitar effect - now working

585

06:41:47

Upgrade: in my spare time  (in the breaks & stops taken whilst driving across France ), I worked out what was wrong with the circuit. @hurz pointed out some major defects including the fact that the circuit first presented, attenuates the signal by 40db

NOTE: The current design does not suffer from this problem!

The current representation of the circuit does not solve all the problems which I believe is impossible with a single stage design but it does provide a nice tuneable tight band pass which elevates the input signal up to 10db above the zero volt line!

Having researched this, I see that almost all current thinking on this type of circuit moves away from the original  single stage idea toward either op amps or Jfet inputs stages. This reflects the  need to pump the volume up a little before hitting the lossy filter. Indeed a filter by it's nature is a lossy mechanism. Amplification is the only way around this. 

In the future, I  may design something using filters in the feedback path of op amps which is quite a powerful mechanism and although I do not like the sound of op amps, I will admit they have their place in audio circuitry!  

I have played around with the capacitor and resistor values so as to provide an effect that sounds great and is easy to build. (no huge rf receiving and therefor noisy inductor! )

The circuit is a tight bandpass filter with control over the centre frequency of the bandpass  (wha ) and also control over the depth of the filtering action. 

Mechanics: The simplest way to create a foot pedal wha control is to use a LDR  (Light dependent resistor ) and a LED. 

Additionally I would add a mosfet buffer on both the input and output sides of this circuit. This will prevent loading down the electric guitar and will allow the filter circuit to function better.

published 10 years ago

hurz

10 years ago

Hmmmm, almost the complete spectrum is below 40dB or even much more suppressed

zorgrian

10 years ago

It's true and although I've played with the values I cannot make it work any other way. The original wah wah pedals with one transistor used an inductor capacitor network. So either I put a preamp and an output buffer in or redesign the thing with op amps ( I don't like the sound of op amps apart from the expensive ones )

hurz

10 years ago

I'm a drummer, I've never played guitar ☺ .... why have you connected the input DC coupled. This way it shifts the operatingpoint quite badly for the transistor. I would place a cap as normal. Probably there is an idea behind like doing some intermodulation for sound effects and driving the bjt more in class C mode?

zorgrian

10 years ago

Again you are correct. Naturally it would have a cap. Actually when I'm building effects or other equipment I put caps rated at 400 volts. Not only does this provide DC isolation, it also affords some protection against unknown bad tube amplifiers with horrible power supply defects.

zorgrian

10 years ago

I sometimes drive transistors into class C mode. However the only other usual case for connecting with no cap is when I use a jfet as the input. I would not do this with mosfet transistors as they are more delicate.

zorgrian

10 years ago

OK so I just did an experiment with an input cap in EC. The result is the filter no longer operates as per my calulation values. It needs to see a low Z input or the whole thing changes.

zorgrian

10 years ago

A 1uF cap should present itself as about 150 ohms. This is low Z but no EC does not like the look of the cap and the circuit no longer provides a nice notch filter. What do you think is going on?

hurz

10 years ago

Problem with this circuit is the already given 180 degree phase shift by the bjt and as we saw the impedancies from filter inout output and amp input have to match. I saw a circuit some time ago with positive feedback were the filter ground is the feedback point. By having already a phase shift of the double t filter positive feedback from emitter to base case a behavior like a notch. Advatage is low impedance output from emitter to high base input. Search for 19khz notch @pip and I did some circuits for this. Thanx for inspiring me to think about those concepts again after a while 😉 i will check back.

hurz

10 years ago

Searching for circuits is a mess here on EC.... anyway i found this http://everycircuit.com/circuit/6570933766586368

hurz

10 years ago

And this one http://everycircuit.com/circuit/4622246974849024

zorgrian

10 years ago

I'll check out your links once I've got access to a computer.

hurz

10 years ago

Not bad. However, now its more a bandpass and not a notch. I don't know what this wha wha effect is based on. My guess was is cuts a peace of spectrum from sound tuneable with a pedal?

zorgrian

10 years ago

Yes it is more like a variable band pass filter. Both notch and tight bandpass produce a wha effect. The tight bandpass is often referred to as a hendrix or crybaby wha.

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