Shaka Express Waveform Study

[Paul Marossy]

Yep, it's time for another waveform study. I'm having fun with my scope...

I was mainly interested in how that bite control works since it has such a large inpact on the sound. What I found is pretty interesting. Scope (Tektronix Model 453) settings are as follows: Volts/Div 10mV, Time/Div 0.5uS, Slope set to "+". Scope was in chop mode with no input on B to give a flatline to reference to. 600Hz sine wave at 20dB attenuation was fed into the input of the Express. Unless noted otherwise, the Express control settings were as follows: Tone 12:00, Vol 5:00, Gain 5:00, Bite control varied, as noted below. What you see here is at the output of the pedal.


Here is the sine wave as it came from the signal generator. Volts/Div switch was set to 1V for this shot only.


Here the bite control is at 7:00. This sounds the brightest/harshest to my ears.


Here the bite control is at 10:00.


Here the bite control is at 12:00. This sounds good to my ear.


Here the bite control is at 3:00.


Here the bite control is at 5:00. This sounds a little muffled to me.


This is with the bite control at 5:00 and the tone control set at 3:00. Volts/Div switch changed to 50mV. Asymmetrical clipping is evident.


This is with the same settings as the picture above, but with Ed Rembold's mod at T3 switched on. Asymmetrical clipping again. This seems to sound a little better to me than the one above, but the waveforms don't look that different, other than the sides have a little more slope to them.



Hope you found this to be interesting.  I did.

[petemoore]

Yupp Thanks...cool pics, very interesting and info laden to look at.
 The assymetric'ness' I see is the flats of the top and bottom of tthe wave are just slightly different length.

[Fret Wire]

Love those scope shots!  You gotta drag out the phase 45, and show us some trimpot adj. pics.

[WGTP]

That's what I'm talking about.  Love it.  We need a whole lot of waveform analysis.  Easy for me to say, I don't have a scope.  

[Peter Snowberg]

Thanks for posting that Paul!

I hope everybody that doesn't have a scope yet will take this as inspiration to start saving their pennies.

[Ed G.]

Great post Paul!
I've been getting acquainted with my scope and as soon as I build the signal generator from your site I'll be posting some pics as well, starting with the BSIAB II! Thanks for the inspiration!

[Fret Wire]

I hope everybody that doesn't have a scope yet will take this as inspiration to start saving their pennies.

Peter, I've been wanting one too for a long time. What should the minimum specs be:
Dual channel? What range should it cover signal wise. Probes: What kind? I see alot of good scope deals on ebay without the probes, the probes aren't cheap I see. How often do the solid state scopes need re-calibrating? It would be a shame to use one, only to find out it's off.

It seems they are a requirement to calibrate most modulation pedals. Are there a standard  set of signals to feed most effects?

[Paul Marossy]

Thanks guys, and glad to be of service. I think that we could use some more of this kind of circuit analysis around here. I find it to be very interesting! BTW, on that last picture, I think the waveform undergoes an inversion, that starting point on the left changes. Either that, or a 180 degree phase shift happens (I guess that's an inverted signal, isn't it?!).

Peter, I've been wanting one too for a long time. What should the minimum specs be:
Dual channel? What range should it cover signal wise. Probes: What kind? I see alot of good scope deals on ebay without the probes, the probes aren't cheap I see. How often do the solid state scopes need re-calibrating? It would be a shame to use one, only to find out it's off.

It seems they are a requirement to calibrate most modulation pedals. Are there a standard set of signals to feed most effects?

Dual channel, with chop mode. That way you can compare waveforms. 20mHz is plenty good for general audio use. Probes should be at least 1M. I use 10M, too. I got a pretty good deal on a 10M probe off of ebay, I think it was like $20 or something like that. It was brand new in the package, was part of a salesman's promo package thing at one time. Keep an eye out for a good deal, sometimes you'll get one. In the case of my scope, a Tektronix 453, it is totally solid state, and you can buy a manual on CD for it off of ebay pretty easily. It has schematics, how to calibrate it, use it, etc. My scope was give to me by my father-in-law, but you can get good working ones off of ebay for about $50 or less. Not a bad deal, they are still very good scopes, not digital, but still great scopes. I would stay away from the really ancient ones that need tubes. Some of those tubes that they use are hard to get now. I think for testing most effects circuits as a stand-alone, you would want to use a sine wave that would be about the same amplitude as that coming from your guitar.