Amplifier History: The JBL SA-600
The JBL SA-600 amplifier was launched by the James B. Lansing company in 1966, and this article was published in January 1967. I’ve included this in the hifisonix power amplifier library because it is one of the earliest – if not the earliest – example of an amplifier that addressed the potential for TIM/SID and that of Large Signal Non-linearity (LSN).
In the mid 1960’s, most engineers were still working with tubes with low loop gains and were not equipped to deal with the high loop gains that multi-stage solid state amplifiers offered. However, the designer, Bart Locanthi, had cut his teeth on military guidance systems in the 1950’s – the heyday of the analog computer – and would have been, as I have remarked elsewhere on this site, highly cognisant of things like slew rate, slewing distortion, loop gain, phase, overshoot, stability and so forth. Unusually for the time, the front end diff pair (Q7 & Q8) is degenerated with loop compensation provided by an 82 Ohm and 150 pF resistor across their collectors along with a 220 Ohm and 75pF network from the VAS output to ground. It operates in inverting mode, which has been tried on numerous commercial amplifiers over the years, but the modern take on this is that non-inverting mode offers advantages with respect to noise performance.
Locanthi’s EF3 output stage, nicknamed the ‘T’, has remained the go to circuit for high performance solid state amplifiers – it features wide bandwidths, very high current gain, and can easily be scaled up by adding output transistors in parallel. Because of the high gains involved, care is needed in layout along with good local decoupling. Although we do not have the rest of the power supply details, it seems this was well taken care of along with the base stoppers preceding Q3 and Q4.
Nowadays, we would do things a little differently insofar as compensation goes, and the VAS would certainly be loaded with a current source in the manner described by Douglas Self’s ‘blameless’ amplifier. But, this design is a classic and was years ahead of its time. Otala’s paper on TIM was still a few years away and he would regrettably draw the wrong conclusions from his findings i.e. TIM is the result of feedback. We now know this is emphatically not the case – its all about how it is applied.
It should be remembered that solid state devices were still in their infancy, and very expensive compared to todays prices – this required some creative engineering to minimize costs and still end up with reasonable performance which this design certainly does.
You can read Locanthi’s description of his design below:-