1970s Design Indulgence

It makes sense that they designed the amp around a 'friendly' power transistor and shows the risk of generic part swapping. Seems easy when an expert like Graham explains it.

Edited by Graham Slee - 29 Jul 2018 at 9:44am

Power Transistor Choice

I was doubly surprised to be reading a book in 1991 about Harry Nyquist's power amplifier criterion.

First surprise was it was nothing to do with digital audio, and the second surprise was his conclusion regarding the sizing of the device - valves in those days - but it applied equally to transistors.

He stated that the power of the device in a push-pull circuit (such as here) need only be one-fifth of the required power. I did the numbers game and had to agree.

Now this assumes none of the phase shifts from reactive loads or short circuits or impedance dips - but it's good to know.

So for 100 watts our starting point is 20 watts per output device. But I'd have need of some complicated protection circuitry to prevent blow-outs when the load isn't purely resistive, or when there's a short, or when somebody turns the volume up beyond clipping.

Rather than spending money on protection why not spend it on a better output device?

In the 70s we had the 2N3055. Rated at 115 watts it was good for nearly 600 watts according to Nyquist, but its current gain cut-off frequency was only 10kHz, and so it was problematic at higher frequencies.

Even so it needed little protection and a suitably rated fuse usually opened before the device failed upon applying a dead-short.

With the Motorola homotaxial version I remember applying a short with a screwdriver; the arc melted a chunk out of it, and the amp continued playing music on its mains transformer and output capacitor (very distorted) instead of the speaker. The fuse in this case never opened, and upon removing the short, the music continued to play through the speaker. Maybe it had lost some performance but I didn't have an audio analyser at the time to check it (roughly 1978).

To improve on performance over the 2N3055 used to mean using faster transistors which had poorer current handling, which meant paralleling devices, and paralleled bases could contribute more input capacitance, and often you would see two or more paralleled output stages driven by the one voltage amplifier.

The voltage amplifier then had to supply more current, and the basic bootstrap circuit was replaced by a current source, and more transistors led to more different and inconvenient phase shifts with the stability problems they present.

And in fact my memory of the kit amp is coming back to me, and it used a current source in place of the bootstrap (R9, R10, C5), but maybe the bootstrap will suffice? We will have to wait for the measurements, which means building one first.

So what am I going to use as output device?

I think it must be the MJL3281A (https://www.onsemi.com/pub/Collateral/MJL3281A-D.PDF).

At first glance its 25 amp peak collector current looks good, but that's tested for 0.005 seconds, and it would take between 0.01 and 0.015 seconds for the fuse to open at 25 amps.

It is 15 amps continuously, and the fuse would take 0.04 seconds to open. Will this be fast enough to open the fuse before the device is destroyed?

And this is where the word "compromise" comes into play, linked with the phrase "damage limitation". We see a 2 amp fuse opens in 0.008 seconds - closer to 0.005 seconds for 25 amps peak collector current, and there will never be an audio signal large and long enough to open a 2 amp fuse in normal use. On spec-test being driven continuously by a sine wave it could blow, but maybe not. Such a variable will need to be tested practically.

But I think the MJL3281A is a good choice. We see all other items of its specification are darned well superb, especially its current gain and transition frequency.

So how do we drive it?