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Hi all,

I'd like to share a modification to make the Q106's pulse output a bit smoother. I realize that this is purely a matter of preference, so check out the mp3 clip (see below) of the original and modified pulse waves to get an idea of the difference in sound. In the clip I switch between original and modified about every 2 seconds, starting with original.

With PW set at 50%, the square wave sounded a bit rough to me, especially in the lower ranges. In the very low, sub-audio range, instead of two snappy steps per cycle, it was a snap followed by a thump (or vice versa, depending on phase :). Looking into it, I discovered that the rise-time at low frequencies (~1Hz) is about 1mS, while the fall-time is about 50uS, and rise-time decreased as frequency increased.

Looking a bit into the circuitry, the Q106's pulse wave is generated by feeding the sawtooth wave and PW signal into a high-gain amplifier. When the sawtooth + PW are above 0, the amplifier saturates positive; below 0, the amp saturates negative. During the transition period between saturations, the output is just a highly amplified sawtooth wave. The pulse's fall-time is always fast because the sawtooth's fall-time is also always fast. However, the sawtooth's rise-time is frequency dependent, so it follows that the pulse rise-time will also be frequency dependent. The effect becomes audible once the rise-time is slow enough that it's high-frequency content falls below the upper limit of our hearing range (probably around 50uS). Increasing the amplifier's gain too keep longest rise-time above hearing range should solve the problem.

My fix uses the square-wave output amplifier to boost the gain of the pulse output. The square-wave output is normally not connected to anything unless you're using a Q141 oscillator aid, so this should have no impact in most circumstances. The modification involves lifting an IC pin and soldering a wire. Then moving the pulse output's connector to the adjacent square wave output header (see photo / synthesizers.com documentation).

Instructions:

With the PCB text "SYNTHESIZSERS.COM" as north, lift pin 3 of the northern-most IC (TL074) and connect that pin to the southern lead of the 4th resistor away from pin 3 (see photo). Move pulse output connector to adjacent square wave output header. (I wish there were reference designators in the documentation!)

Here is a photo of the modification: Q106 pulse mod photo

Audio clip of before and after: Q106_pulse_mod_AB.mp3

With the mod. installed, both edges of the pulse are about 30uS. If you want to go back to the OEM pulse, just move the pulse connector back to the pulse header.

Cheers,

Cobey