303/x0x VCO tuning - the EASY way

This is a quick guide on how to tune the VCO of your x0xb0x, TB-303 or equivalent 303 clone.

Before you start

Now for the actual VCO tuning, there are 2 trimpots which affect the VCO pitch:
- TM4 sets the pitch offset, this is equivalent to what the TUNE knob does.
- TM5 sets the octave “width” or “scale”

Obviously there are two possible ways, one of them is easy, the other one is hard because it’s “iterative”.

The easy way:

Step 1. TM5 - adjust untill low-c and high-C are one octave apart (the exact frequency doesn’t matter, as it’ll be fixed in step #2)
Step 2. TM4 - adjust untill you get the right frequency for an A note against a reference (that’s equivalent to adjusting the TUNE knob on the front)

Tuning with the AS_x0x_Tune tool

as_x0x_tune is a small win32 VST plugin (made with SynthEdit/SE_SDK2) which aims to help with the VCO tuning.

Download: AS_x0x_tune.zip (965 KB)
Quickstart video:

The plugin requires a pattern of alternating C and C(UP) notes (that is the upper C key on the keyboard, 3V and 4V). Or you can simply load this pattern: as_x0x_tune_pattern.nsp using c0nb0x’s pattern editor and save it to the x0xb0x.

Quick (and rough) tuning using just a Spectrum Analyzer

As an alternative way, you can roughly tune your VCO with the help of a Spectrum Analyzer. I’ve tried it with the Signal Analyzer plugin by rs-met so any similar spectroscope would work.

If the individual harmonics aren’t clearly visible (due to the scope) you can try boosting the TUNING knob and TM4 in order to push the VCO into higher frequencies, this will help the scope a bit.

Step 1. Adjust TM5 while watching at the 4th harmonic of the low note and the 1st harmonic of the high note, try to bring them as close as possible to minimize the “error”, see the picture below:

Step 2. Set the TUNING knob centered, program a long A note and play it. Adjust TM4 untill the first harmonic is at roughly 110Hz.
You can also play an A note on another synth as a reference, and tune by ear.