PWM Vibrato that sound Awesome!

Vibrato is the slight changing of frequency of a musical note. This can be accomplished by means of a varying time delay in a delay line or by phase shifting and analog signal over time.

The delay approach works fine but creates inherent latency and can sound clownish. So I wanted to create an analog design.

A modulated allpass filter can be used to change phase over time (which is frequency). This analog approach can sound lovely but to sound really good has to linear in its sweep and be modulated  in a sinusoidal manner. Both the design objectives are not trivial.

It occurred to me that my PWM phaser design does all this but simply mixes the original signal back in to create the phasing notches. So I modified the circuit and optimized it to simply modulate the filter and it works great. The PWM driven analog switches are very linear and the PIC DDS works great to create the sine wave modulation. I added some pre- emphasis/de-emphasis, adjusted some values and made an improved layout…so the pedal is super quiet. I left in the peaking filter adjustment to allow for some cool modulated filter effects along with the Vibrato. The range of PWM controls the extent(depth) of the effect at a given modulation rate and so the vibrato pitch bend level is easily adjustable.  Keep in mind the faster the modulation rate; the pitch shift is also greater because a faster the rate of change of phase over a given time, produces more pitch shift.  An alternate analog modulation circuit can be used if desired but the output swing needs to be limited from 0 – 2 volts.

Link to Schematic:


Brainwash Phaser converted into Vibrato pedal:

Coming Shortly Demo:



Stomp Amp with Improved Tone Stack

Have Just finished designing my new version of the Stomp Amp. I just love it. It has the same 100 watt Class D final as my Red Scare version:

but with an enhanced tone stack much like that of the Polytone Brute amps and a JFET driver stage for the final. The normal/bright selector is quite useful as well depending on the pickup type and location.

Here is a download link for the schematic and PCB in express PCB format(can be converted to gerber using freeware: copper connection)


Final Amp design in 1590BB enclosure:


Populated home brew circuit board:


Schematic: ( click here for larger view):

Demo coming soon!