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 Elliott Sound Products Project 68 

300W Subwoofer Power Amplifier (Updated)

© September 2000, Rod Elliott (ESP)

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


There are some important updates to this project, as shown below.  Recent testing has shown that with the new ON Semi transistors it is possible to obtain a lot more power than previously.  The original design was very conservative, and was initially intended to use 2SA1492 and 2SC3856 transistors (rated at 130W) - with 200W (or 230W) devices, some of the original comments and warnings have been amended to suit.

Jul 2003 - OnSemi has just released a new range of transistors, designed specifically for audio applications.  These new transistors have been tested in the P68, and give excellent results.  As a result, all previous recommendations for output transistors are superseded, and the new transistors should be used.

The output devices are MJL4281A (NPN) and MJL4302A (PNP), and feature high bandwidth, excellent SOA (safe operating area), high linearity and high gain.  Driver transistors are MJE15034 (NPN) and MJE15035 (PNP).  All devices are rated at 350V, with the power transistors having a 230W dissipation and the drivers are 50W.

Sep 2003 - The new driver transistors (MJE15034/35) seem to be virtually impossible to obtain - ON Semi still has no listing for them on the website.  The existing devices (well known and more than adequate) are MJE15032 (NPN) and MJE15033 (PNP), and these will substitute with no problems at all.  It is also possible to use MJE340 and MJE350 as originally specified (note that the pinouts are reversed between the TO-126 and TO-220 devices).

Note that some component values have been changed! The layout is the same, but the changes shown will reduce dissipation in Q7 and Q8 under light load conditions.

Having built a couple of P68 amps using these transistors, I recommend them highly - the amplifier is most certainly at its very best with the high gain and linearity afforded by these devices.  Note that there are a few minor changes to the circuit (shown below).

With ±70V supplies, the input and current source transistors must be MPSA42 or similar - the original devices shown will fail at that voltage! Note that the MPSA42 pinout is different from the BC546s originally specified.  Full details of transistor pinouts are shown in the construction article.

High power amps are not too common as projects, since they are by their nature normally difficult to build, and are expensive.  A small error during assembly means that you start again - this can get very costly.  I recommend that you use the PCB for this amplifier, as it will save you much grief.  This is not an amp for beginners working with Veroboard!

The amplifier can be assembled by a reasonably experienced hobbyist in about three hours.  The metalwork will take somewhat longer, and this is especially true for the high continuous power variant.  Even so, it is simple to build, compact, relatively inexpensive, and provides a level of performance that will satisfy most requirements.




Please note that the specification for this amp has been upgraded, and it is now recommended for continuous high power into 4 Ohms, but You will need to go to extremes with the heatsink (fan cooling is highly recommended).  It was originally intended for 'light' intermittent duty, suitable for an equalised subwoofer system (for example using the ELF principle - see the Project Page for the info on this circuit).  Where continuous high power is required, another 4 output transistors are recommended, wired in the same way as Q9, Q10, Q11 and Q12, and using 0.33 ohm emitter resistors.

Continuous power into 8 ohms is typically over 150W (250W for ±70V supplies), and it can be used without additional transistors at full power into an 8 ohm load all day, every day.  The additional transistors are only needed if you want to do the same thing into 4 ohms at maximum supply voltage! Do not even think about using supplies over ±70V, and don't bother asking me if it is ok - it isn't!

The circuit is shown in Figure 1, and it is a reasonably conventional design.  Connections are provided for the Internal SIM (published elsewhere on the Project Pages), and filtering is provided for RF protection (R1, C2).  The input is via a 10µF electrolytic (or you can use a 4.7µF bipolar cap if preferred).  A polyester cap may be used if you prefer - 1µF with the nominal 22k input impedance will give a -3dB frequency of 7.2Hz, which is quite low enough for any sub.

Figure 1
Figure 1 - Basic Amplifier Schematic

The input stage is a conventional long-tailed pair, and uses a current sink (Q1) in the emitter circuit.  I elected to use a current sink here to ensure that the amp would stabilise quickly upon application (and removal) of power, to eliminate the dreaded turn on 'thump'.  The amp is actually at reasonably stable operating conditions with as little as +/-5 volts! Note also that there are connections for the SIM (Sound Impairment Monitor), which will indicate clipping better than any conventional clipping indicator circuit.  See the Project Pages for details on making a SIM circuit.  If you feel that you don't need the SIM, omit R4 and R15.

The Class-A driver is again conventional, and uses a Miller stabilisation cap.  This component should be either a 500V ceramic or a polystyrene device for best linearity.  I've tested 50V NP0/ C0G ceramics at 500V without any sign of leakage or failure, but with up to ±70V supplies the cap will be subjected to as much as 100V RMS.  The collector load uses the bootstrap principle rather than an active current sink, as this is cheaper and very reliable (besides, I like the bootstrap principle). 

Fuses are shown as 10A, and this is enough for normal operation from ±56V.  If you use the dual board version and ±70V supplies, you'll need to increase the fuse rating to around 12A.  Feel free to use 15A fuses regardless of supply voltage, as they are only there to protect the power transformer from a short - they cannot protect the amplifier.

note All three driver transistors (Q4, 5 & 6)must be on a heatsink, and D2 and D3 should be in good thermal contact with the driver heatsink.  Neglect to do this and the result will be thermal runaway, and the amp will fail.  For some reason, the last statement seems to cause some people confusion - look at the photo below, and you will see the small heatsink, 3 driver transistors, and a white 'blob' (just to the left of the electrolytic capacitor), which is the two diodes pressed against the heatsink with thermal grease.

C11 does not exist on this schematic, so don't bother looking for it.  It was 'mislaid' when the schematic was prepared, and I didn't notice until someone asked me where and what it was supposed to be.  Sorry about that.

It is in the output stage that the power capability of this amp is revealed.  The main output is similar to many of my other designs, but with a higher value than normal for the 'emitter' resistors (R16, R17).  The voltage across these resistors is then used to provide base current for the main output devices, which operate in full Class-B.  In some respects, this is a 'poor-man's' version of the famous Quad current dumping circuit, but without the refinements, and in principle is the same as was used in the equally famous Crown DC300A power amps.

Although I have shown MJL4281A and MJL4302A output transistors, because they are new most constructors will find that these are not as easy to get as they should be.  The alternatives are MJL3281/ MJL1302 or MJL21193/ MJL21194.

Note: It is no longer possible to recommend any Toshiba transistors, since they are the most commonly counterfeited of all.  The 2SA1302 and 2SC3281 are now obsolete - if you do find them, they are almost certainly fakes, since Toshiba has not made these devices since around 1999~2000.

Use a standard green LED.  Do not use high brightness or other colours, as they may have a slightly different forward voltage, and this will change the current sink's operation - this may be a miniature type if desired.  The resistors are all 1/4W (preferably metal film), except for R10, R11 and R22, which are 1W carbon film types.  All low value resistors (3.3 ohm and 0.33 ohm) are 5W wirewound types.

Because this amp operates in 'pure' Class-B (something of a contradiction of terms, I think), the high frequency distortion will be relatively high, and is probably unsuited to high power hi-fi.  At the low frequency end of the spectrum, there is lots of negative feedback, and distortion is actually rather good, at about 0.04% up to 1kHz.  My initial tests and reports from others indicate that there are no audible artifacts at high frequencies, but the recommendation remains.

Power Dissipation Considerations

I have made a lot of noise about not using this amp at ±70V into 4 ohms without the extra transistors.  A quick calculation reveals that when operated like this, the worst case peak dissipation into a resistive load is 306W (4Ω/ ±70V supplies).  The four final transistors do most of the work, with Q7 and Q8 having a relatively restful time (this was the design goal originally).  Peak dissipation in the 8 output devices is around 70W each.

Since I like to be conservative, I will assume that Q7 and Q8 in the updated schematic shown contribute a little under 1A peak (which is about right).  This means that their peak dissipation is around 18W, with the main O/P devices dissipating a peak of 70W each.  The specified transistors are 230W, and the alternatives are 200W, so why are the extra transistors needed?

The problem is simple - the rated dissipation for a transistor is with a case temperature of 25°C.  As the amp is used, each internal transistor die gets hot, as does the transistor case - the standard derating curves must be applied.  Add to this the reactive component as the loudspeaker drives current back into the amp (doubling the peak dissipation), and it becomes all too easy to exceed the device limits.  The only way that this amp can be used for continuous high power duty with ±70V supplies and a 4Ω loudspeaker load is to keep the working temperature down to the absolute minimum - that means four output devices per side, a big heatsink and a fan!

Figure 1a
Figure 1a - Double Output Stage

Figure 1A shows the doubled output stage, with Q9, Q10, Q11 and Q12 simply repeated - along with the emitter resistors.  Each 1/2 stage has its own Zobel network and bypass caps as shown, as this is the arrangement if the dual PCB version is built.  When you have this many power transistors, the amp will happily drive a 4 ohm load all day from ±70V - with a big enough heatsink, and forced cooling.  Over 500W is available, more than enough to cause meltdown in many speakers!

A Few Specs and Measurements

The following figures are all relative to an output power of 225W into 4 ohms, or 30V RMS at 1kHz, unless otherwise stated.  Noise, signal to noise and distortion figures are unweighted, and are measured at full bandwidth.  Measurements were taken using a 300VA transformer, with 6,800µF filter caps.

Mains voltage was about 4% low when I did the tests, so power output will normally be slightly higher than shown here if the mains are at the correct nominal voltage.  Figures shown are measured with ±56V nominal, with the figure in (brackets) estimated for ±70V supplies.

Voltage Gain23 (27dB)23 (27dB)
Power (Continuous)153W (240W)240W (470W)
Peak Power - 10 ms185W (250W)344W (512W)
Peak Power - 5 ms185W (272W)370W (540W)
Input Voltage1.3V (2.0V) RMS1.3V (2.0V) RMS
Noise *-63dBV (ref. 1V)-63dBV (ref. 1V)
S/N Ratio *92dB92dB
Distortion *0.4%0.4%
Distortion (@ 4W) *0.04% (1 KHz)0.04% (1 KHz)
Distortion (@ 4W) *0.07% (10 kHz)0.07% (10 kHz)
Slew Rate> 3V/µs> 3V/µs

* Unweighted

These figures are quite respectable, especially considering the design intent for this amp.  While (IMO) it would not be really suitable for normal hi-fi, even there it is doubtful that any deficiencies would be readily apparent, except perhaps at frequencies above 10kHz.  While the amp is certainly fast enough (and yes, 3V/us actually is fast enough - response extends to at least 30kHz, but not at full power), the distortion may be a bit too high.

Note that the 'peak power' ratings represent the maximum power before the filter caps discharge and the supply voltage collapses.  I measured these at 5 milliseconds and 10 milliseconds.  Performance into 4 ohm loads is not quite as good, as the caps discharge faster.  The supply voltage with zero power measured exactly 56V, and collapsed to 50.7V at full power into 8 ohms, and 47.5V at full power into 4 ohms.

Photo of amp
Photo of Completed P68 Amplifier

As can be seen, this is the single board version.  The driver transistors are in a row, so that a single sheet aluminium heatsink can be used for all three.  Holes are provided on the board so the driver heatsink can be mounted firmly, to prevent the transistor leads breaking due to vibration.  This is especially important if the amp is used for a powered subwoofer, but will probably not be needed for a chassis mounted system.  You may note that the 5W resistors are 2.2 ohm and 0.22 ohm on this board.  These values can be used, but I recommend those shown in the schematic(s).

The driver heatsink shown is adequate for all power ratings with normal programme material.  The power transistors are all mounted underneath the board, and the mounting screw holes can be seen on the top of the board.

Deceptively simple, isn't it?

Power Supply

WARNING: In some countries, mains wiring may only be performed by a qualified electrician - Do not attempt the power supply unless suitably qualified and experienced.  Faulty or incorrect mains wiring may result in death or serious injury.

The basic power supply is shown in Figure 2.  It is completely conventional in all respects.  Use a 40-0-40 V transformer, rated at 300VA for normal use.  For maximum continuous power, a 50-0-50V (500VA or more) transformer will be needed.  This will give a continuous power of about 450W, and peak power of over 500W is possible with a good transformer.  Remember my warnings about using the amp in this way, and the need for the additional output transistors, big heatsink and fan.

Figure 2
Figure 2 - Basic Power Supply Circuit

For 115V countries, the fuse should be 6A, and in all cases a slow blow fuse is required because of the inrush current of the transformer.  For anything above 300VA, a soft-start circuit is highly recommended (see Project 39).

The supply voltage can be expected to be higher than that quoted at no load, and less at full load.  This is entirely normal, and is due to the regulation of the transformer.  In some cases, it will not be possible to obtain the rated power if the transformer is not adequately rated.

Bridge rectifiers should be 35A types, and filter capacitors must be rated at a minimum of 63V (or 75V if you use 70V supplies).  Wiring needs to be heavy gauge, and the DC must be taken from the capacitors - not from the bridge rectifier.

Although shown with 4,700µF filter capacitors, larger ones may be used.  Anything beyond 10,000µF is too expensive, and will not improve performance to any worthwhile degree.  Probably the best is to use two 4,700µF caps per side (four in all).  This will actually work better than a single 10,000µF device, and will be cheaper as well.

NOTE: It is essential that fuses are used for the power supply.  While they will not stop the amp from failing (no fuse ever does), they will prevent catastrophic damage that would result from not protecting the circuit from over-current conditions.  Fuses can be mounted in fuseholders or can be inline types.  The latter are preferred, as the supply leads can be kept as short as possible.  Access from outside the chassis is not needed - if the fuses blow, the amplifier is almost certainly damaged.


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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 © 2000.  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 25 Sept 2000./ 06 May - added photo of amp./ Jun 2012 - updated images.