I have a technique which works very well.

Output transistors should be matched on two parameters for best results, not just one.

The matching process should be done at the intended bias current the amp is to be set up to; in my amps I choose 58mA. The devices should be matched for current gain and base/emitter voltage.

I built a simple jig with a current source which supplies this current to the transistor under test. A differential pair senses a reference voltage of half the Vcc (I use a 12V gelcel battery), and adjusts the bias on the base of the device under test (DUT) to ensure half the Vcc between collector and emitter. There is a 100R resistor in series with the base of the DUT so I can measure the voltage drop across it, giving me the current through into the base and thus permitting the operator to figure the beta of the DUT.

Once stable, I first measure the base/emitter voltage of the DUT.

I go through a sample of 100 or so devices, and categorize them in small piles by Vbe on a large sheet of butchers paper. Remember, this is at a prescribed current with around 6V across the collector/emitter.

You need to work reasonably quickly (less than two seconds) to avoid warming the junction too much, as the gain and the Vbe rises rapidly with temperature.

A fast sampling rate on the DMM is essential. In a sample of 100, you will get an approximately normal distribution of Vbe, and these can be sorted right down to the nearest millivolt, very accurate since 1mV in 600mV is around 0.16%.

Once you have multiple piles, all classified by Vbe, you then go through each pile searching for betas within 5%. For this you move the DMM probes to the 100R base stopper. The higher the reading, the lower the beta, given by 5800 divided by mV. (58mV is a beta of 100).

In this way you finish up with matched pairs which will turn on and off together and which will pass pretty much identical collector currents as well. This pays dividends in the final sound, as switching noise is much reduced. Generally I can accurately matched 60% of a batch of 100 Toshiba 5200/1943s in about forty minutes. The accuracy depends, of course, on the quality of your DMM. Make sure the battery is good!
_________________
Hugh R. Dean
Aspen Amplifiers P/L
Melbourne, AUSTRALIA
www.aksaonline.com