Elliott Sound Products

Project 101

High Power, High Fidelity Lateral MOSFET Power
Amplifier
Rod Elliott (ESP), Updated 21 Apr 2012

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Please Note:  PCBs are available for this project. Click the
image for details.

Introduction

In various parts of The Audio Pages, I have said that I am not a fan of
MOSFET power amplifiers. Well, this amp changed my views, and I consider this to
be a "reference" system in all respects. It uses lateral MOSFETs - not switching
types! The latter cannot be used in this circuit - they will self
destruct! The performance is extremely good, with vanishingly low distortion
levels, plenty of power, very wide full power bandwidth, and the "self
protecting" nature of the lateral MOSFETs themselves.

This is not to suggest that the amp is indestructible (no amplifier can make
that claim successfully), but it is much more tolerant of faults than a bipolar
transistor amp, and requires nothing more than a pair of zener diodes to limit
the current. Having said that, I would still recommend that you avoid shorted
output leads and the like - i.e. Don't push your luck
)

One thing that emerged during the design is that PCB layout is absolutely
critical. The layout of this new amplifier is similar to that used for the P68
Subwoofer amplifier, and this has some major benefits. P68 has no right to sound
as good as it does, and although designed for subwoofer use, it has proven
during listening and testing to be a very low distortion design - despite the
Class-B output stages. All PCB tracks in the input and driver section are as
short as possible, minimising the chance of noise pickup from other sections of
the circuit - especially noise/distortion generated by the half-wave signal
current handled by each output device.

This MOSFET amplifier is designed to be as flexible as possible, with no bad
habits. Indeed, it will operate stably with supply voltages as low as +/-5V
(completely pointless, but interesting), all the way to the maximum supply
voltage of +/-70V. The only change that is needed is to trim the MOSFET bias
pot!

With the full supply voltage of +/-70V (which must not be exceeded!),
continuous ('RMS') power is around 180W into 8 ohms, or 250W into 4ohms. Short
term (or "music") power is typically about 240W into 8 ohms and 380W into 4
ohms. Note that depends to a very great degree on the power supply, and a robust
supply is a requirement for the maximum output. The recommended supply voltage
is ±56V.

As noted, unless you really need the maximum possible power, I suggest
that you use a supply voltage of ±56V obtained from a 40+40V transformer. You
will get around 150W into 8 ohms from this supply voltage, but you also relax
the demands placed on the MOSFETs and heatsinks. The difference between using
±56V and ±65V is less than 1dB - for the extra peace of mind and relaxed
heatsink requirements that's a very small price to pay.

Since this amp probably has more power than you will normally need, even if
you do skimp a little on the transformer, the loss will be very small. I must
make one thing perfectly clear - this is a hi-fi amplifier. It is not designed
for professional PA use, although this is possible if the supply voltage is
reduced so that both peak and long term dissipation are maintained regardless of
how hard the amp is driven. Although it is quite ok to use with a sub, P68 is a better proposition in that role.

It is worth noting that a MOSFET amp will always produce less power than a
bipolar transistor version using the same supply voltage. Even using an
auxiliary supply will make only a small difference (one reason I elected not to
add the extra complexity). A bipolar design using a ±70V supply can be expected
to produce something in the order of 270W into 8 ohms, and well over 500W into 4
ohms. The specified MOSFETs have a rated Vds (saturated voltage, Drain to
Source) of 12V at full current, and that is simply subtracted from the DC value
of the supply voltage. Using a ±56V supply with a lateral MOSFET amp will always
give less power than can be obtained from a bipolar design (see below for
measured figures).

Photograph of Completed
Amplifier Board (Early Version)

The photo shows the simplicity of the PCB. The MOSFETs are mounted below the
board, and are bolted down in the same way as with the P3A and P68 boards. No
other mounting is needed. PCB pins or tinned copper wire pins are used as anchor
points for the power ground link (the green wire along the front edge), so that
the main current carrying tracks were not compromised by running a separate
track (which would have required a reduction in size of the positive supply
rail).

The entire front-end section is between the electrolytic caps, and is
deliberately as compact as possible. This improves performance, by ensuring that
there are no long tracks for the input stage, which will pick up noise and can
seriously degrade the sound of the amplifier.

Performance Figures

The performance of this amp is such that many measurements are very
difficult. Some of the more basic measurements are as shown below, based on my
custom made transformers which provide ±65V unloaded. Other than output power
(which will be ~150W into 8 ohms), the figures are virtually identical with ±56V
supplies ...

Parameter

Measurement

Conditions

Output Power

> 180W

< 1% THD, 8Ω

> 275W

< 1% THD, 4Ω

DC Offset

< 20mV

Typical

Noise

< 2mV RMS

Unweighted (-54dBV)

THD

0.015%

No load, 30V RMS output, 1kHz

0.017%

8 Ohms, 30V RMS output, 1kHz

0.02%

4 Ohms, 30V RMS output, 1kHz

Output Impedance

< 10 mΩ

1kHz, 4Ω load

< 25 mΩ

10kHz, 4Ω load

Frequency Response

10Hz to 50kHz

At 1W, -1.5dB

Basic Performance
Figures

In particular, the distortion figures show that amp loading causes only very
small variations, with any harmonics being predominantly from my audio
oscillator. There are no visible or audible high order components to the
distortion waveform. Output impedance was measured on a fully built amplifier,
including the internal wiring. This entails around 200mm of wire in all (per
channel), so the output impedance of the amplifier itself is obviously lower
than quoted. For an 8 ohm load, the damping factor at 1kHz is around 800 (8 /
10 milliohms) - completely pointless of course, since any speaker lead will ruin
that very quickly.

Noise was measured with inputs open-circuited, and at -54dBV may not look too
wonderful, however this figure is very pessimistic. Remember that this is the
unweighted measurement, with bandwidth extending to well in excess of 100kHz.
Even so, signal to noise ratio (referred to full power) is 86dB unweighted, and
the amp is completely silent into typical speakers. Indeed, even connecting a
pair of headphones directly to the amp outputs revealed that no noise was
audible. Naturally, your methods of construction will differ from mine, and you
may not be able to get the same performance.

Intermodulation distortion cannot be measured with the equipment I have
available, but I have included a screen capture of the three measurements I
took. Most of the harmonic content visible (not that there is a great deal
anyway) is present in the two generators I used, and the amplifier contributes
virtually nothing.

1kHz + 2kHz at +30dBV Output (8Ω)

1kHz + 2kHz at -25dBV Output (8Ω)

10kHz + 12kHz at +20dBV Output (8Ω)

Click on any of the images above for a full resolution version.

Description

The very first thing you will notice is that I have broken with tradition
with this amp, and there are no component values shown. Given the performance of
the circuit, and the fact that I have already sold a couple as completed,
finished amplifiers, I am not about to give away all my secrets for the design.
If you want the component values, you must purchase the PCB. There are
no exceptions, so don't ask.

The schematic of the amp is shown in Fig. 1, and it is about as simple as a
high power MOSFET amplifier can get - it is considerably simpler than most, but
lacks nothing in performance. The circuit diagram belies the ability of the
amplifier though, so do not be tempted to think that it cannot perform as well
as more complex designs - it does, and exceeds the performance of many (if not
most) of them. It will be seen that I elected to use a bootstrap current source
rather than an active version - there is negligible cost difference, but I was
unwilling to make such a radical change after testing the prototype and being so
impressed with the results. (If it ain't broke, don't fix it!)

The front end is a conventional long-tailed pair (LTP) using a current mirror
load and an active current sink in the "tail". Interestingly, adding the current
mirror made no difference to distortion, but reduced the DC offset to less than
25mV. The improvement was such that I elected to retain the mirror.

In tests thus far (both measurement and listening), I have been unable to
detect even a hint of what is commonly referred to as the "MOSFET sound". The
relatively high levels of low order distortion and susceptibility to crossover
(or "notch" distortion that plague many MOSFET designs are completely missing -
indeed, even with zero bias on the MOSFETs, crossover distortion below
10kHz is barely measurable, let alone audible!

Note Carefully:
The most critical aspect of the design is the PCB
layout, and it is very doubtful that if you make your own board, that you will
get performance even approaching mine. Power output is essentially unchanged,
but distortion and stability are achieved by a compact and carefully designed
layout for the front end and driver circuits, which minimises any adverse PCB
track coupling that causes much higher distortion levels, and may cause
oscillation.

This is not a ploy on my part to get people to purchase my PCBs - that has
already been taken care of by leaving out the component values. The simple fact
is that unless the PCB layout is done with the utmost care, any amplifier can be
made to have far greater distortion levels and reduced stability margins than
the published figures suggest.

Low Power Version
As shown in the schematics below (figures 1 and
2), the amplifier can be made in high or low power version, and although there
is a bit of vacant PCB real estate in the low power design, it is significantly
cheaper to make and will be more than sufficient for most constructors. If this
version is built (using only 1 pair of MOSFETs), IMO it is essential to limit
the supply voltage to ±42V so that it can drive both 4 and 8 ohm loads without
excess dissipation. With this voltage, expect about 100W continuous into 8 ohms,
and around 150W into 4 ohms. Naturally, dual MOSFET pairs may be used at this
voltage as well, providing much better thermal performance (and therefore cooler
operation), far greater peak current capability and slightly higher power. This
version may be used at any voltage from ±25V to ±42V.

Figure 1 - Standard (Low Power) Version

The MOSFETs used are Hitachi lateral devices, 2SK1058 (N-Channel) and 2SJ162
(P-Channel). These are designed specifically for audio, and are far more linear
than the (currently) more common switching devices that many MOSFET amps use.
Unfortunately, they are not especially cheap, but their performance in an audio
circuit is so much better than vertical MOSFETs, HEXFETs, etc., that there is no
comparison. Note that using HEXFETs or any other vertical MOSFET type is not an
option. They will fail in this circuit, as it was not designed to use them.

An alternative (and possibly marginally better than the 2SK/2SJ series) is
the Exicon ECX10N20 and ECX10P20 (available from Profusion PLC in the UK). These
have been used in most of the amps I have built, and they work very well. So
potential constructors can verify that the semiconductors are available before
purchasing a PCB, this information has now been included. All other parts are
quite standard. You may also use BUZ901P/BUZ905P devices.

High Power Version
The same PCB is used, but has an extra pair of
MOSFETs. Since the devices are running in parallel, source resistors are used to
force current sharing. Although these may be replaced by wire links, I do not
recommend this. This version may be operated at a maximum supply voltage of ±70V
(±56V is recommended), and will give up to 180W RMS into 8 ohms, and 250W into 4
ohms. Short term (peak) power is around 240W into 8 ohms and 380W into 4 ohms.
These figures are very much dependent on your power supply regulation,
determined by the VA rating of the transformer, size of filter caps, etc.

Figure 2 - High Power Version

Although not shown, the transistors and MOSFETs are the same in this version
as for the low power variant. The additional capacitors (C11 and C12) shown are
to balance the gate capacitance. The P-Channel MOSFETs have significantly higher
gate capacitance than their N-Channel counterparts, and the caps ensure that the
two sides of the amp are roughly equal. Without these caps, the amp will almost
always be unstable.

As noted above, the PCB is the same for both versions, but for Fig. 2 it is
fully populated with 2 pairs of power MOSFETs. The high power version may also
be used at lower supply voltages, with a slight increase in power, but
considerably lower operating temperatures even at maximum output, and
potentially greater reliability.

With both versions, the constructors' page gives additional information, and
the schematics there include an enhanced Zobel network at the output for greater
stability even with the most difficult load. This is provided for on the PCB,
and allows the amp to remain stable under almost any conditions.

The entire circuit has been optimised for minimum current in the Class-A
driver, while still providing sufficient drive to ensure full power capability
up to 25kHz. The slew rate is double that required for full power at 20kHz, at
15V/us, and while it is quite easy to increase it further, this amp already
outperforms a great many other amps in this respect, and faster operation is
neither required nor desirable.

Note - There are actually two caps marked C5, and two
marked C6. This is what is on the PCB overlay, and naturally was not found
until it was too late. Since these caps cannot be mixed up, it will not cause
a problem.

In both versions of the amp, R7 and R8 are selected to provide 5mA current
through the voltage amplifier stage. You will need to change the value to use a
different supply voltage ...

R7 = R8 = Vs / 10 (k)   (Where Vs is one supply
voltage only)
For example, to set the correct current for
±42V supplies ...

R7 = R8 = 42 / 10 = 4.2k (use the next lower standard
value - 3.9k)

Construction

As suggested above, I strongly recommend that you purchase the PCB for this
amplifier, or you will almost certainly get results that are nowhere near the
amp's real ability. The PCB also makes construction a breeze, with everything
except the power supply mounted on the board itself. Like many other ESP power
amps, the MOSFETs are mounted underneath the board, requiring only two (or four)
screws to attach the PCB and output devices. As always, full construction
details will be available in the ESP secure site when you purchase the board(s).

Heatsinks for an amp like this are always going to be an issue. Since the amp
is intended for hi-fi use, fans are undesirable, so the heatsink needs to be
substantial. I suggest that you aim for a heatsink with a thermal resistance of
around 0.4°C/W for the high power version. It can be somewhat smaller for the
low power version of course, but I recommend that it be no smaller than ~1°C/W.

The heatsinks used must have a perfectly flat back, without any
protrusions or anything else that prevents the MOSFETs from making perfect
contact with the heatsinks. The MOSFETs must be electrically isolated from the
heatsink, and you can use thin mica, Kapton (25um) or aluminium oxide
insulators. Do not attempt to use silicone pads - they have far too much thermal
resistance and will result in MOSFET failure.

The suggested power supply is completely conventional. Although a small
amount of additional power can be obtained by using an auxiliary supply (to
boost the rail voltage for the MOSFET drive stage), this is at the expense of
greater complexity and more things to go wrong. The transformer for the supply
should be matched to the expected power you wish to obtain from the amp. The
following table shows the recommended transformer voltage and VA rating for a
single channel - either use two transformers or a single unit with twice the VA
rating shown for stereo.

AC Volts

DC Volts

VA

Power (8Ω)

20-0-20

+/-28V

100

40

25-0-25

+/-35V

100

50

30-0-30

+/-42

160

80

40-0-40

+/-56V

200

150

(Recommended Supply Voltage)

50-0-50

+/-70V

300

240

(Can be used, but not really recommended)

Note that all powers shown are "short term" or peak - continuous power will
always be less as the supply collapses under load. Peak power levels are usually
achieved (or approached) with most music because its transients are generally
between 6dB and 10dB greater than the average power output. Transformer VA
ratings shown are a guide only - larger or smaller units may be used, with a
marginal increase or reduction of peak power. Always use at least the size shown
for subwoofer use! Values in bold are preferred, and will give enough power for
most systems along with optimum reliability and low operating temperature.

Figure 3 - Power Supply Circuit Diagram

Figure 3 shows the power supply circuit diagram for a ±56V supply, and there
is nothing new about it. As I always recommend, the bridge rectifier should be a
400V/35A chassis mount type, and should be properly chassis mounted using
heatsink compound.

Filter capacitors must be rated to at least the nominal supply
voltage, and preferably higher. If possible, use 105°C rated caps, and join the
earthed terminals very solidly to form the star earthing point.

Note - The fuse should be selected according to the size of
the power transformer. For any toroidal transformer over 300VA, a soft start
circuit is highly recommended. Use the transformer manufacturer's suggested
fuse - if this information is not available, ask the supplier - not
me!

The DC supply must be taken from the capacitor terminals - never from the
bridge rectifier. Using several small capacitors will give better performance
than a single large one, and is usually cheaper as well. For example, the
performance of 10 x 1,000uF capacitors is a great deal better (in all respects)
than a single 10,000uF cap, at between 50% to 70% of the cost of the large unit.
This lunch is not free, but it is heavily discounted

When you purchase the PCB, you will not only get all component values, but
will also have access to information for a power supply that is optimised for
the best possible performance for a conventional supply. There is nothing
especially innovative about the "advanced" supply schematic, but the overall
results will surprise you.

Testing

Connect to a suitable power supply - remember that the supply earth (ground)
must be connected! When powering up for the first time, use 10 ohm to 22 ohm
"safety" resistors in series with each supply to limit the current if you have
made a mistake in the wiring.

For a much more detailed description of the general test processes (as well
as troubleshooting information if the amp does not work), please see the Troubleshooting &
Repair Guide. That article has much more detailed information than I can
include in each project page.

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Copyright Notice.This
article, including but not limited to all text and diagrams, is the
intellectual property of Rod Elliott, and is Copyright © 2004.
Reproduction or re-publication by any means whatsoever, whether
electronic, mechanical or electro-mechanical, is strictly prohibited under
International Copyright laws. The author (Rod Elliott) grants the reader
the right to use this information for personal use only, and further
allows that one (1) copy may be made for reference while constructing the
project. Commercial use is prohibited without express written
authorisation from Rod Elliott.

Page Created and Copyright © Rod Elliott 07 Jan 2004./
Updated 01 Feb 2004 - added measured results./ 21 Apr 12 - minor page reformat
and added heatsink info.