Sunday, April 26, 2015

Bias Stability in a Quad 303

I had set the bias in my own Quad 303 in 2006 or 2007 with the aid of a high-quality Tektronics AA501 distortion analyzer and oscilloscope. This technique clearly shows cross-over distortion spikes when present, so it is easy to arrive at the minimum bias setting required to eliminate cross-over distortion. By this means I usually arrive at somewhere around 10-12mV across the emitter resistors.

Out of curiousity I checked this 303 over the weekend using the same distortion analyser. This check showed that after 8-9 years of daily use for several hours a day (about 7200 hours in total) there was absolutely no need for any re-biassing. This shows that the Quad 303 'triples' design really does work.

This 303 has a set of very early Dada Electronics 303 boards in it, which also goes to show that they have been entirely trouble-free over that quite long period.

Esmond Pitt, Dada Electronics Australia

1. The scope picture shows the signal in the upper trace and the distortion residual in the lower trace. Some tiny cross-over spikes just emerging from it, corresponding to the zero crossings of the signal.
Note that there is no 2HD or 3HD visible at all: they would show up as waves at double or triple the signal frequency.

2. The THD+N is .0096%.

3. The signal-to-noise ratio is -83.9dB.

All measurements taken at 1kHz, about 6V RMS across 9 ohms, or about 4 watts. For lab reasons I don't go much higher than this, but I also don't consider it relevant, as this is about the maximum level I listen to. It should be noted that at full power the THD+N would increase a little, but the S/N ratio would improve.

Equipment used: Tektronix SG505 oscillator, AA501/01 distortion analyzer, SC504 oscilloscope.