300W BJT AMP FOR HIGH CURRENT INTO LOW LOADS.

A guy asked me to rebuild his Phase Linear 700 amps.
These old bangers were famous for being "flame linear", and conking out when the party got really hot.

I made the case larger, increased the fins on the heatsinks to what they should have been originally, and used the following schematics...
I apologise about the hand drawn and scanned schematics, but I doubt there is great need for me to spend 2 days
redrawing Figs 1, 2, 3 , 4, 5 in MS paint for everyone to enjoy better. I doubt anyone would want to build
the schematic shown.

Fig 1.

I don't want to waste too much time telling everyone exactly what the finer poins of this amp are.
It sounded OK at completion after 3 long weeks of hard slog. ( Never again was my conclusion. I just don't like bjts in the output stage. )

The 6 x MJL21194 and 6 x MJL21193 power transistors were chosen because they were cheap and available, and have high voltage and current ratings, and have been used now in many high powered amplifiers, and they are flat pack types needing only one bolt to the heatsink.

I started off with the supposedly wonderful output stage with  as shown on the schematic and thought that the darlington pair arrangement wouldn't be too hard to drive with the MJE340/350 Q4,5 gain pair. The output stage base input resistance was still lowish and the NPN and PNP output devices have such a huge variation in hfe for each half positive and negative
wave cycle that thd was about 40% at the driver stage so when a lot of global NFB was added I did not get the
low distortion measurements which are so routinely easy and probably necessary with class B solid state.
This is because the MJE340/350 Q4/5 pair have high collector output resistance and the darlington output stage base input resistance is low by comparison to this collector resistance of the driver stage.
 I thought perhaps an arrangement as used in Crown amps with a darlington triple output stage could be used, but I don't like copying other designers.

So I added the buffer stage Q6/7 which is merely an emitter follower with CCS loading.

This reduced the effect of the low output stage input resistance and the open loop distortion dropped about 30dB
and all was well; I was able to measure less than 0.01% at near full power with the thd reducing about linearly
right down to low power, where at a few watts I couldn't easily measure the thd.

There  was no sign of any crossover distortion.

However, the main problem I encountered was with cross conduction when testing at highish power at 27kHz.
The output transistors appeared to be so sluggish about turning off that the devices on each side of the
PP circuit tended to stay on and seemingly ignoring the directions of the driver amp. This phenomenon
leads to considerable current flow from rail to rail rather than through the transistors and load only. What a bummer!
I finally settled for the above schematic which seemed to work fairly well to
reduce the very poor behaviour of cross conduction. Even without a load connected at 27kHz, the amp drew 10amps from each rail at high levels!
Mosfets never display this sort of poor operation, and they don't need to be set up as darlington pairs and they don't need a buffer stage.

I have to admit I did peruse the website of Douglas Self to get a clue on cross conduction, and how disastrous
it could be if it occured at lower F, but even his pages are largely free of "what to damn well do when smoke threatens".
Anyway, there was little trouble at under 20kHz, and the amp is stull running fine with continuous use for 18mths so far.
It happily and effortlessly drives a 15" subwoofer driver and the recording of the Space Shuttle launch
I heard was fraught with the fear that the windows might break walls would shatter, and the roof tiles could fly away.

Although it sounded fair with wide range music, tubes and mosfets are better.
The amp serves its owner well as a sub-woofer amplifier where it can easily churn out 300 watts in bridged mode into 8 ohms and a lot more right down to 2 ohms.

Power output with +/- 55V rails and without allowing for slight rail sag for sine wave testing due to high PTwinding resistance and some choke resistance in power supply:-
8 ohms, 180 watts per channel,
4 ohms, 360 watts per channel,
2 ohms, 600 watts per channel.
Bridged, 8 ohms :- 360 watts, 4 ohms:- 600 watts.

Fig 2.

The power supply for the  Turner Linear bjt amp used the original Phase Linear power transformer.
Unfortunately, the tranny hummed badly like many PA amps do and I didn't want to use the 62V windings to make
+/-86 volt rails because the possible 400w of power into 8 ohms from such high rails and from both channels wasn't wanted or needed, and may not have been reliable.
I built two carefully designed swinging chokes which are 0.35Henrys on 25mm tongue GOSS cores.
These were varnished and potted in mild steel boxes with roof pitch. This made two cubic boxes with about 90mm along each side. The the original 10,000 uF caps and power transformer were retained with the bridge rectifier.
Noise with the choke input set up was much less than with a cap input supply since there are not such huge
switching currents to excite motion in the transformer.
I added rectifiers from the 62V-0-62V CT windings to directly charge up a CRC filtered input amp rails for the driver amp and added simple emitter follower regulators and the final performance was quite pleasing.
There is some sag in the rails when continuous power is used, but with rock and roll taken up to occasional clipping
the rails don't sag much. There were no bad effects due to the chokes and 10,000 uF being series resonant at
about 3 Hz.

Fig 3.

This shows the arrangement for switching the amp from stereo to dual mono to bridged.

Fig 4.

This is the dc detection schematic for the large bjt amp. Its an essential item because if ever there is a failure
of a device and the fuses don't blow, 50V dc fed into an 8 ohm wooofer will have it in flames withing 30 seconds!

Fig 5.

This is my improvised  meter schematic using existing meters. They tended to stick a bit, and I didn't waste time trying
to fix 30+ year old meters designed by accountants. Originally they were horribly bouncy,
and very badly calibrated; they were never designed to indictate real power regardless of load.
They are very useful though, like a fashion statement by a fish swanning around in the sea with an umbrella.
To build a circuit that indicates power rather than just voltage takes a little more work.
The above schematic has some ability to indicate voltage logarithmically.
Amplifier voltage meters should be logarithmic. This means that they are calibrated for full output voltage level into
8 ohms for full scale, so then for 1/10 of the maximum power the meter needle should be about 1/3 of the way across the dial, and for 1/100 of max power the needle is at 1/10 the way across the dial. 

After all the work I did on the Phase Linear I concluded that I would never again use BJT output devices.
And I will never again reform such a recalcitrant amplifier such as one of these ancient horrors that would ideally
be better used as a boat anchor.

So if you have a bunch of dead Phase Linears, don't ask me to fix them; I will only build you a new
and better amp instead. I found there was more work involved with reforming a wayward and fault ridden
Phase Linear than building a brand new amp, and because it was a repair job, I could  not charge much for my work.

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