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.