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Brief note about my experience with solid state amps,
Picture of 2x300W amp.
Schematic of 300W AB amp
channel with mosfets.
Schematic operation, topology, heatsinks, performance
NFB, thd, output current limiting, input voltage limiting, class A
Schematic of 600VA power supply
for stereo 2 x 300W amp.
Picture of amp underside.
Although I try to specialize in tube amps, I have not always
fine solid state amps and repairing them.
1993 I became seriously interested in all audio hi-fi amplifiers
at a time
when I thought I should change trades from construction work to
My first project was upgrading a solid state Linear Design
I had bought in 1977 after I had forgotten how fine
tube amplifiers could be.
But later in 1993, I thought I should build a "better" dedicated
I used two 100W mono amp kits designed by David Tilbrook who had
design published in ETI magazine in the 1980s. These kits were
in 1993 from a Jaycar outlet which had just set up a Canberra
I very quickly learnt that the output mosfets and the input driver
easier to destroy than tubes by careless workshop practices and
accidental short circuits.
After fusing a few transistors and mosfets I learnt to be very
everything I did. Direct coupled
solid state amps with many PCB tracks
close to each other are
prone to instant smoke with a careless short
somewhere such as a splosh
of solder, or from soldering two points
which are not meant to be soldered, or from tiny wire offcuts and
sitting on soldered up boards. The speed of transistor failure is
greased lightning, and unlike tube amp stages there is no R&C
which allows each stage to be more easily measured before anything
overheats to badly.
That first SS amp had 2 cascaded differential input stages instead
Both used BF469/BF470 video transistors. The mosfet output
stage was in
It was very easy to get excellent thd measurements, and the
gain was so high that it could not be tested without the global
It tended to be unstable above the AF band and in 1993 I
was not well skilled
about critical damping stabilization networks around a gain
loop where open
loop gain was extremely high which meant the amount of applied
was also high.
The sound was no better than the sound I had enjoyed from my
all bjt Linear
Following these efforts I read all the Wireless World and
magazine articles on
audio from 1917 onwards.
The local Australian National University had all the old
magazines in their
Mr Tilbrook's design was abandoned because the second differential
was simply not required to get thd and Rout low enough, ie, below
just under clipping, and Rout less
milliohms. If the THD = 0.005%
at 100W, then at 2W the THD was difficult to measure, ie,
used for most music. I set about designing my own 100 watt
worked well with just 3 input transistors in a differential pair
then a single MJE340 gain transistor loaded by an MJE350 CCS
followed by an
NPN and PNP power mosfets. I found it very easy with
such a simple circuit to
get 100 watts at 0.005% thd, and utterly negligible thd at 2 watts
all my listening. The build quality of that amp was too amateurish
something I could be entirely proud of so I dismantled
it all and built a 300 watt
per channel amp with 6 mosfets per channel.
2 x 300W amp with mosfets.
300W AMP WITH MOSFET OUTPUT STAGE.
Fig1, One 300w amp schematic of
There are six
mosfets per channel in the output in complementary source follower
The driver stage is also a complementary pair but with BF469/470
emitter mode for high gain. The collector load of each bjt is the
of the pair
and gain is very high in this simple driver stage.
There are two differential input stages, one npn and the other
But they are not cascaded, but coupled to work in parallel to give
low thd and symmetrical square wave performance to a high
found BF469 and BF470 were quite OK to use in the input and driver
Each of the aluminium extruded heatsinks for the flat pack
150mm high with 30 fins 40mm long, good for a 2 x 300 watt amp
for a fan even when left
running at 300 watts into 4 ohms with a sine wave.
The total surface area of the heatsink is 30 x 15 x 4 x 2 = 3,600
The required surface area needed to keep T rise less than 40C
about 40 sq.cm per Watt, so the extruded heatsinks I used are good
of dissipation, or a lot more if one allows for a higher T rise.
anyone follows my
simple rules here they should find their amps won't run hot, but
MUST be vertical, and not within a box which restricts air flow,
Devices should be fixed to the heatsink with machine screws and
and be spread over the area of the backing plate of the sink.
The 150 x 300 x 30 fin sink would be good for a 40W pure class A
needs to dissipate 90Watts at idle to allow 40W of class A.
I have thermal fuses
to each heatsink to shut the amp down if
the temperature exceeds 76C, but I have
never been able to trigger the fuse. I biased each channel it for
42watts of idle
idle current ) which gives about 1.4 watts of class A before
to class AB for the next 250 watts into 8 ohms. The high bias
in the mosfets stops all tendencies of the mosfets to oscillate at
currents are low, as used in BJT amps.
The schematic is about as simple as a good 300
watt solid state amp can be.
All the input driver bjts are mounted on a strip of aluminium
keep them all at about the
There is limiting of the input voltage to prevent excessive
There is a decent protection circuit on the output used which
turns off the amp if there is a dc offset fault that
exceeds about +/- 1V for longer
than 4 seconds. The 0.5 ohm "N" source resistors are non magnetic
prevent oscillations and force the mosfets to share the the
and remain evenly biased
without great need for the mosfets to be accurately matched.
Anyone trying to build an amp with
mosfet output devices should anticipate parasitic
oscillations at RF.
I found that 56pF from gate to drain with short leads plus 560
"stoppers" on each mosfet eliminated the RF oscillations. I also
bypassing caps. The
power supply has large 75V rated Sprague 100,000uF capacitors
+/- 70V rail, but the leads from the caps have enough inductance
silent rails when testing with square waves so I placed
1,000 uF electros + 0.47 uF
with short leads from the drain rails to
0V, then one 0.01uF from each drain to the
chassis, and finally there
was no more rail signal present at HF.
The open loop bandwidth is maximum at 200Hz but -3dB at about 5
180 ohms plus 100pF ( R23/C12 ) needed from the driver collectors
to bases of
Q7&Q8 to tailor the
open loop phase and gain to stop overshoot and HF instablity.
addition there is the C15 across R25, 47k to compensate for the
lag at HF. The RC zobel network R41&C28 from the
commoned sources to 0V act
to provide the amplifier with a resistive
load above 100kHz when the 0.22uF cap has
an impedance of 7.3 ohms. At
F above 200kHz,
the load on the output stage is 10
ohms if there is not other load
connected so the amp is more likely to
The value of C15 isn't given because in my case it was a
pair of short
wires twisted together until the stability became excellent. A
could be used.
With 6 ohm load, each mosfet sees 36 ohms while in class A and
when in class AB. So while in class A the gain reduction in the
follower action is about from 25 to about 1, and this local source
voltage FB totals about 27dB.
The output stage works mainly in class AB and in fact not much
different to a class
B amp where there is no idle current at all. With
no source follower or global NFB
the THD of a pair of mosfets in
complementary pair might be 10% at 250 watts
into 8 ohms. The 27dB of
local NFB used in the source follower connection reduces
this to less
than 1%. There is about 50dB of global NFB which means a total of
operative. The driver stage, Q7/Q8, has to produce a voltage
speaker output voltage and its open
loop THD at about 45Vrms is about 3% of which
most is 3H followed by
2H. This adds to the output stage THD to make a grand total
of about 4%
THD with no global NFB.
Most of the crossover distortion artifacts are reduced by the
output stage NFB.
The 53dB of global FB reduces the 4% open loop THD by 53dB, or by
0.00223, so 4% becomes 0.0089%, or just under 0.01%.
The THD is approximately proportional to
voltage, and at 4.5Vrms output
which is 2.5Watts/8ohms, THD < 0.001%, and rather difficult to
Such low THD is typical of many solid state amplifiers. However,
excellent THD, IMD
and TID measurements do
not always tell the
whole story about how an amplifier will or
will not alter the sound
signals than pass through it.
Tube amps with ten times the THD can
(( And by the way, anyone could operate the 300 watt amp design as
pure class A
amp by having say 2 more output devices and dissipating about 80
and then using +/- 35Vdc rails instead of +/- 70Vdc. One is then
to get up to
36 Watts of pure class A, depending on the load value. The same
driver stages would produce about 3% open
loop thd, and need
some global NFB to
reduce it. It would not measure much better than the
above low bias class AB amp.
Subjectively, The class A operation of the output mosfets may
having a 300W capability, which is never fully used. ))
With the class AB 300W amp as it is, any type of load can be
8 uH inductance in parallel with 8 ohms
protects the output stage from over heating
if it ever had 5uF
connected and the output signal had a high level signal above
5uF is a difficult load, but there
may be some
"Lartin Mogan", which present such an awkward load to an amp, but
awkward loads do have a series resistance in their equivalent
characteristic so that the worst of phase shifts and peak
currents at above 10kHz will
not bother the amplifier, and in any case
the % of audio F energy above 7 kHz is
usually very small. 5uF = 4.5
ohms at 7 kHz, and 2.27ohms at 14 kHz, and in fact
amplifiers of any type will
cope quite well with such a load providing the
output voltage levels
remains well below the maximum possible which is probably
always be the case with an amp capable of 300 Watts if it is used
"normal hi-fi" and with average speakers, which most ppl find
with never any more than 10 Watts.
There could be a problem driving 5uF or with with insensitive
electrostatics if the
amplifier was only rated for 10 watts, and if
high levels were wanted. Quad ESL57
are equivalent approximately to 1.6 ohms in series with 2uF
with 16 ohms across
the 1.6R&2uF also as a load. The response peaking effects
2uF as the
prevented by the series R in front of the C; the other 16 ohms
little loading effect at all, and in fact ESL57 are very easy
my 300W amp or any other amp. However, Quad's later model, the
following models need more than a 20watt amp.
There is current limiting that operates to prevent the output
excessive current which is dangerous to solid state
devices which can very rapidly
fail if the excessive current lasts for
long enough to heat them up and fuse the small
pn junction size of the
active devices which are not much bigger than a 4mm x 4mm
large metal areas
of perhaps 12 sq.cms, ( EL34 ) and
temporary adverse heat dissipation can be
tolerated for longer than in a
The maximum theoretical peak load
current = rail voltage / ( load + Ron + Rs )
where Ron is the minimum 'on' resistance of the mosfet when fully
Rs = the source resistor. In this case if RL = 3.5 ohms,
each mosfet = 10.5 ohms,
Ron = 1
ohm and Rs = 0.5 ohms, I max = 70 / 12 ohms = 5.8 amps peak.
The rating for the mosfets is 7 amps peak.
The voltage gain of the mosfets reduces with RL value so a single
a 10.5 ohm load plus 0.5 source R ohms will have a voltage change
the Vg-s change needed
is about +8V. There will be 2.9V across the 0.5 ohm Rs,
and the 9V
zener diode plus
other diode will conduct to prevent any increase in signal
at the gates
from over driving the mosfet. To cut a long story short, in
maximum current able to be
produced by the amp = 14 amps peak for 3 mosfets,
and in fact my
measurements indicate maximum load current cannot be any higher
when load = 3.5 ohms or lower, and the output power = 350 Watts.
1 ohm load there is still 14 amps peak produced, and maximum
output power is
about 100 watts.
The amplifier would become overheated with a load of 1 ohm and
then the fuses in rails or at the output ( not shown ) would blow.
to be used with loads above 4 ohm, but will
tolerate normal home use with loads
down to 2 ohms where power is unlikely to average more
than 3 Watts per
The only way to increase low impedance speaker load power handling
more of the same output devices or use devices with higher ratings
of them on a bigger heatsink.
Exicon flat pack mosfets rated for 16 amps each,
which would allow
40Vrms into 2 ohms giving 800 Watts.
I have never ever needed more than 50 watts for hi-fi
There is an input voltage limiting circuit included which prevents
voltages from being applied to the input transistors.
Diodes D4 and D5 act to
shunt excessive input voltage just before the
amp output stage clips.
Below clipping the input limiters have zero
action on the input signal because each
diode which is a fast type with
low forward turn on voltage <0.25V is biased by
The input limiting may be omitted if really wish, but if used it
careful setting up to avoid any input clipping below high
The amp has been reliable and trouble free and was a great
build. I have surreptitiously substituted it for a 50 watt PP
during a demonstration about 10 years ago and the 3 other guys
hear any noticeable change. However, I had
much poorer speakers in those
times using cheap asian made drivers. The
speakers I have built since then
far better sounding SEAS drive
units and much better enclosures and perhaps I
now would not get
away with playing such skullduggery on unsuspecting
Maximum output voltage is about 45Vrms. It is possible to use such
pure class A. The bias current rail to rail may be increased to
about 70 Watts may be comfortably dissipated in the heatsinks.
Using a toroidal output
transformer with 3 : 1 step down ratio, there
is a possible
15Vrms which will give 34 Watts of pure class A into 6.6
and all power into loads above
6.6 ohms will be pure class A.
There will be 75 watts class AB into 3 ohms with 13 watts of pure
A suitable toroidal is made by the man at www.zeroimpedance.com in
When I wound the power transformer I used 4 x 25Vrms secondaries
to give 50V-0-50V which produces +/-70Vdc rails. The 25V rails
paralleled to allow +/- 35V rails. The six mosfets could still
dissipate 70 watts but
be able to put 32 watts of pure class A into 16 ohms, 60 watts
into 8 ohms,
watts of class A, 100 watts of class AB into 4 ohms with 8
The measured distortion would become extremely low at audiophile
and perhaps the class A would convey some better subjective
People have told me why my tube amps and mosfet amps both sound
One reason is that they run warm but not stinking
hot, they have some class A
working rather than none, and were designed and
made by the same
an uncompromising attitude about how things should
One friend I had, now deceased, had 4 ohm Duntech Sovereign
He had a vinyl recording of 20 Africans beating away on drums and
instruments thus generating a very difficult and transient rich
to just get the 300 Watt amp into clipping on the transients but
the outcome sounded
exactly like 20 Africans
belting away furiously on leather and wood. Of course such
levels could not be sustained lest the street full of people call
police for noise
Some Lieder music from olde Europe also sounded detailed and
the 'Elephant' treads softly when required.
300w amp power supply.
Underside 2 x 300W
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