Following the outstanding success of our SC480 power amplifier
module published in the January & February 2003 issues, we've taken the lessons learned there and from our Ultra-LD series published in 2000 and 2001 and applied them to a much bigger power amplifier.
There is no doubt the publication of the SC480 triggered off a
lot of interest and since then we've had readers suggesting we update the 300W amplifier from the February 1980 issue of ETI. Others have asked about the possibility of upgrading the SC480 with bigger transistors and higher supply rails or variations on that theme. So the seeds were sown. A bigger amplifier was called for. But how much bigger? And using which transistors?
Looking at the SC480, for example, you can't increase the power
output by simply substituting bigger output transistors and increasing the supply rails to some likely value. If you were to take that approach, other transistors in the circuit would blow up. And if you're driving low impedance (ie, 4-ohm) loads, the output transistors could easily expire as well.
Our first approach was to decide on the target power output,
given a likely supply rail. Given that we have already published amplifiers capable of delivering 100 watts into 8-ohm loads (ie, the Ultra-LD series), the next likely step would be to aim for 200 watts into an 8-ohm load. A few back-of-an-envelope calculations show that we would need supply rails of about ±70V or a total of 140V.
