SOLID STATE AMPLIFIERS 1,
MOSFETS.
The contents of this page
include :-
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 specification,
NFB, thd, output current limiting, input voltage limiting,
class A operation.
Schematic of 600VA power
supply for stereo 2 x 300W amp.
Picture of amp underside.
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I
have not quite avoided
solid state completely. In
1993 after a lapse of many years
began to get interested in learning all
I could about audio hi-fi amplfiers. My first
project was to upgrade a solid state Linear Design receiver I had
bought in the late
1970s when I had forgotten how fine tube amplifiers could be. That led
to building a "better" dedicated stereo
class AB 100W/channel amp using hitachi mosfets in a kit designed by
David Tilbrook and published in ETI magazine
in the early 1980s.
I learnt that the mosfets and the input driver transistors were far
easier to destroy than tubes by careless workshop practices and
accidental short circuits. After fusing a few transistors I quickly
learnt to be very careful with everything I did.
That first SS amp had 2 cascaded differential input stages instead of
just one that was based around
the BF469/BF470 video transistors and a mosfet output stage in source
follower.
It was very easy to get excellent thd measurements, and the cicuit open
loop gain was so great that it could not be tested without the global
NFB connected. It tended to be unstable, and I wasn't skilled then
about critical damping stabilisation
around a loop where open loop gain was extremely high and so applied
NGB also was high. The sound was no better than
the sound I had enjoyed from my all bjt Linear Design receiver.
So I read all the Wireless World and Electronics World articles on
audio from 1917 onwards.
The local Australian Nationional University had all the old
magazines in their basement archives.
Mr Tilbrook's design was abandoned because the second differential gain
stage was simply not required
to get thd and Rout low enough, ie, below 0.01% and Rout less than 50
milliohms.
I set about designing my
own 100 watt per channel amp which worked well
with just 3 input transistors in a differential pair with CCS, 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 which
covered all my listening.
That amp wasn't something I could be entirely proud of so I dismantled
it all and built a 300 watt per channel amp.
2 x 300W amp with mosfets.
300W AMP WITH MOSFET OUTPUT STAGE.
Fig1, one 300w amp schematic of
one channel.
There are six
mosfets per channel in the output in complementary source follower mode.
The driver stage is also a complementary pair but with BF469/470 bjts
in common emitter mode for high gain.
The collector load of each bjt is the load for the other of the pair
and
gain is very high in this simple driver stage.
There are two differential input stages, one npn and the other pnp.
Each are coupled together to work in parallel to give wide bandwidth,
low thd, and symetrical squarewave performance to a high frequency. I
found BF469 and BF470 were quite OK to use in the input and driver
stages.
Each of the aluminium extruded heatsinks for the flatpack mosfets are
300mm long, 150mm high with 30 fins 40mm
long,
good for a 2 x 300 watt amp with no need for a fan even when left
running at 300 watts into 4 ohms with a
sine wave.
I have thermal fuses fitted to each heatsink to shut the amp down if
the temperature exceeeds 76C, but I have never
been able to trigger the fuse.
I biased each channel it for about 42watts of idle dissipation (300mA
idle current ) which gives about 1.4 watts of class A before resorting
to class AB for the next 250 watts into 8 ohms.
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 above the board to keep them all at about the
same temperature.
There is limiting of the input voltage to prevent excessive accidental
input voltage being applied.
There is a decent protection circuit on the output used which isn't
shown, 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 wire wound types to
prevent oscillations and force the
mosfets to share the the current evenly 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 ohm gate
"stoppers" on each mosfet
eliminated the RF oscillations. I also have four bypassing caps. The
power supply has large 75V rated Sprague 100,000uF
capacitors on each +/- 70V rail, but the leads from the caps have
enough inductance to prevent
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 kHz
with only 180 ohms plus 100pF
( R23/C12 )
needed from the driver collectors to bases to tailor the open loop
phase and gain to stop overshoot and HF
instablity.
In addition there is the C15 across R25, 47k to compensate for the open
loop phase 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 abobe 200kHz,
the load on the output stage is 10 ohms
if there is not other load connected so the amp is more likely to
remain stable.
The value of C15 isn't given sunce in my case it was a pair short
insulated wires twisted together until the stability
became excellent. A trim cap of 3 to 20pF could be used.
With 6 ohm load, each mosfet sees 36 ohms while in class A and then 18
ohms when in class AB.
So while in class A the gain reduction in the output stage with
follower action is about from 25 to about 1,
and the local source follower series 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 distortion of a
pair of mosfets in complementary pair might be
10% at 250 watts into 8 ohms. The 27 dB 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 77dB is
operative. The driver stage, Q7/Q8, has to produce a
voltage slightly higher than the 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%
with no global NFB.
Most of the crossover distortion artifacts are reduced by the local
output stage NFB.
The 53dB of global FB reduces the 4% open loop thd by 53dB, or by a
factor of 0.00223, so 4%
becomes 0.0089%, or just under 0.01%.
The thd is approximately better than proportional to the output
voltage, and at 3watts, thd = 0.0007%,
and typical of many solid state amplifiers.
However, excellent, thd, imd, 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 sound better.
And by the way, if one could operate the 300 watt amp as a pure class A
amp by having more output devices
and dissipating about 700 watts at idle, the voltage gain driver stage
would still have 4% open
loop thd, and need
some global NFB to reduce it. It would not measure any better than the
low bias class AB amp.
Any type of load can be connected and an 8 uH 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 20kHz.
5uF is a difficult load, but there may be some electrostatics, possibly
Lartin Mogan, which present such an awkward
load to an amp, but usually all awkward loads do have a series
resistance in their equivalant impedance 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
most 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 going
to always be the case with an amp capable of 300 watts.
There could be a problem 5uF or with with insensitive electrostatics if
the amplifier was only 10 watts, and high levels were
wanted. Quad ESL57 are equivalent approximately to 1.6 ohms in series
with 2uF with 16 ohms also as a load.
The response peaking effects caused by 2uF as the sole load are usually
prevented by the series R in front of the
C; the other 16 ohms has little loading effect at all, and in fact
ESL57 are very easy loads to drive by this amp or any other.
There is current limiting that operates to prevent the output stage
from developing 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 area.
Tubes have large metal areas of perhaps 12 sq.cms, ( EL34 ) and
temporary adverse heat disipation
can be tolerated for longer than in a solid device.
The maximum theoretical peak load
current = rail voltage / ( load + Ron + Rs )
where Ron is the minimum 'on' resistance of the mosfet when fully
turned on and Rs = the source resistor.
In this case if RL = 3.5 ohms, load on 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
mosfet with a 10.5 ohm load plus 0.5 source R ohms
will have a voltage change of 0V to +63.8V, and 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 effect the 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 than
when load = 3.5 ohms or lower, and the output power = 350 watts.
So with 1 ohm load there is still 14 amps peak produced, and maximum
output power = about 100 watts.
The amplifier would become overheated with a load of 1 ohm and worked
hard but then the fuses in rails or at the
output ( not shown ) would blow.
The amp is meant to be used with above 4 ohm loads, but will
tolerate normal home use with loads down to 2 ohms where
power is unlikely to average more
than 1 watt per channel, ever.
The only way to increase low value load power handling is to use
devices with higher ratings and more 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 at home.
The amp has been reliable and trouble free and was a great learning
exercize to build.
I have surrepticiously substituted it for a 50 watt PP class A tube amp
during a demonstration about 10 years ago and the
3 other guys present did not hear any noticable change. However, I had
much poorer
speakers in those times
using cheap asian made drivers. The speakers I have built since then
use far better sounding SEAS drive
units and much better enclosures and perhaps I now would not get
away with playing such skullduggerous tricks on unsuspecting
audiophiles.
Maximum output voltage is about 45Vrms. It is possible to use such an
amp in pure class A.
The bias current rail to rail may be increased to about 500 mA, so that
about 70 watts is comfortably
dissipated in the heatsinks.
Using a toroidal output
transformer with 3 : 1 step down ratio, there is a possible 15Vrms
which will give
35 watts of pure class A into 6.4 ohms, and all power into loads above
6.4 ohms will be pure class A,
and there will be 75 watts into 3 ohms with 13 watts of pure class A.
When I wound the power transformer I used 4 x 25vrms secondaries all in
series but which could be
paralleled to allow +/- 35V rails. The six mosfets could still
dissipate 70 watts but be able to put
32 watts of pure classA into 16 ohms,
60 watts class AB into 8 ohms, including 16
watts of class A,
100 watts of class AB into 4 ohms with 8
watts of class A.
The distortion would be very low at audiophile levels of power, and
perhaps the class A would convey
some better subjective fidelity.
People have told me why my tube amps and mosfet amps both sound
well...they run warm,
have some class A working rather than none, and are made by the same
guy.
One guy I know has some Duntech Soverign speakers which take some
driving. With a recording of
20 africans beating away on drums and percussive instruments, we were
able 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.
Some Lieder music from olde europe also sounded detailed and sweet, and
methinks the 'Elephant' treads softly
when required.
300w amp power supply.
Underside 2 x 300W
amp.
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