Update 02-Dec-2009.
I bought an EMU1820M soundcard on eBay so that I can do some more accurate distortion testing. Soon when I have the new two channel board finished that will be the first to be tested. I will make a new distortion extraction circuit to include input and output network effects instead of the easier method measuring the input stage signal, which is relatively inaccurate.
Also I will add a page of pictures and test results from those who bought boards and did their own measurements. So far I have only one set of results, but I hope more will follow. I have a few comments from listening tests, and so far they have been entirely complementary, so that is most encouraging.

For now I have to replace a few capacitors in both the EMU and my computer. Both use the infamous G-Luxon LZ electrolytics which it has been reported were made with a missing ingredient, and consequently they generate hydrogen gas. This is revealed by the bulging tops, and in extreme cases leaking electrolyte. The EMU has just 2 of the bad caps, but my computer motherboard has 19. Some other makes of low ESR electrolytics are affected, and there is a list of suspect makes here. There are a few brands widely agreed to be reliable, the ones I have most often seen mentioned are Panasonic and Rubycon. I am ordering some Panasonic FM and Rubycon ZL, both types are available from Farnell.

Update 15-Oct-2009.
Regarding the MJR7 distortion specification, the 'under 0.001%' I sometimes mention applies only to distortion components up to 20kHz. I have now reworded the home page description to make this clearer. Using my usual measurement method of nulling the test signal and analysing the remaining output with a computer soundcard I can only measure components a little beyond 20kHz. My Santa Cruz soundcard is long overdue for an upgrade, but until then I can only give a very approximate estimate of 0.001% THD for a 20kHz input based on oscilloscope traces of a poorly nulled test signal. Some of those who bought boards have said they will do their own measurements, so I hope for some independant confirmation. Distortion components at 40kHz and beyond are inaudible to normal humans, but THD-20 figures are useful for comparing different designs.
The reason why such low distortion was achieved is that over 60dB feedback is used at 20kHz, and with open loop distortion around one or two percent a closed loop figure of 0.001% is quite possible. If it is a little higher it could probably be brought down to this figure just by increasing the quiescent current, e.g. to 150mA. Such a high level of feedback requires a rate of attenuation greater than -6dB/octave to keep the unity gain frequency low enough, and the consequent phase shift with a capacitive load can then exceed 180deg at some frequencies, which need not be a problem provided it is reduced before the unity gain frequency and also reduces near clipping. A 180deg excess phase shift does not necessarily create positive feedback as is sometimes stated; if the loop gain is still high the feedback is still negative with all the normal distortion reducing advantages. Feedback is positive if it increases gain, negative if it reduces gain. The phase shift becomes important only near the unity gain frequency. At other frequencies where the loop gain is high the phase shift primarily affects the phase of the closed-loop distortion. See my article here. The unity gain frequency is unfortunately variable, particularly near clipping, and it helps if the high frequency compensation is applied in such a way that its phase shift reduces near clipping.

Update 12-Oct-2009.
There will eventually be a two-channel PCB as mentioned earlier, and I have no plans to make more of the single boards. To encourage constructors to make their own boards for the single channel version I have rewritten the 'Part 1. Adding The Components' page listed further down this page under the heading 'HOW TO BUILD THE MJR7'. This now includes a high resolution image showing the PCB track layout and a few words of advice for anyone not already experienced in making boards. There are more complete sources of information, so please don't just depend on my own rather incomplete advice. The two channel board will not include a heatsink mounting bracket, which may be some advantage, it is difficult to get angle bracket in small lengths, my local supplier would only supply 5 metre lengths. The disadvantage is that fixing the mosfets direct to a heatsink may be more difficult, and also the PCB needs mounting holes and spacers to fix it in a case.

Update 31-Aug-2009.
My article about Slew Rate and TID showed, as Fig.1, the overshoot at an amplifier input produced by a filtered step function input. From this I concluded that extending the open-loop -3dB frequency to 20kHz by adding a resistor in parallel with the compensation capacitor would increase input stage distortion for both transient and steady state conditions, and so was an entirely bad idea. A similar analysis using a step function was used in an article by Daugherty and Greiner entitled 'Some Design Objectives for Audio Power Amplifiers' (March 1966, IEEE Transactions on Audio and Electroacoustics.) Subsequent articles have also tended to consider only a step input, and I had never thought this was a problem. Recently I decided to check my previous result using a Spice simulation. My original version was worked out around 1979 using a TI59 programable calculator. To my relief the Spice result for a step function was about the same.
In addition to the updated Spice result there was one small surprise, and this also is included in a new article TID - Part 2.

Update 21-May-2009.
I am making a list of alternative transistors to help anyone having difficulty finding those specified. For the Hitachi 2SC2547E and 2SA1085E alternatives are Hitachi 2SC1775E and 2SA872E, and also the Toshiba 2SC2240BL and 2SA970BL. The noise figure of the NPN device is important, it needs to be low at 10k source impedance and Ic 0.5mA. The types listed are mostly around 0.5dB. The 2SA872 was the input transistor specified about 30 years ago in the original Hitachi power mosfet application note, so it is rather ancient, as are some of the others, so I will keep a lookout for more modern alternatives. The 2SC2547 is available from RS Components in the UK, but they list the manufacturer as Magnatec.

The 2SA1209 (pnp) and 2SC2911 (npn) are less critical. The BF470 (pnp) and BF469 (npn) should work ok, as should other types rated at 120V or more, 50mA to 150mA Ic, and around 100mHz ft.

I have a final board design for the two-channel version of the MJR7, which has two channels on a single 6" x 4" board. To fit the reduced space I am using single 4700uf output capacitors instead of the parallel pair of 2200uF, and have left out the anti-thump circuit and the on-board mosfet mounting bracket. I will add the final circuit diagram when it has been tested, and may make the boards available.

Update 5-April-2009.
I have had a few requests to supply some of the transistors for the MJR7, and I know these could be difficult to obtain in some parts of the world. I don't keep any of these beyond what I need for current projects, so I have been suggesting a UK source which will supply worldwide, and this is Nikko Electronics, also known as Dalbani. Their prices are lower than most other UK sources, and their postage charges appear to be not much more for the rest of the world compared to UK, so there is probably no point in me buying and reselling. There is another reason I don't want to supply transistors which is the difficulty of identifying fakes or lower specification versions from different manufacturers. I have no transistor testing equipment beyond a multimeter, so I have no easy way to check. This is a serious problem, and as an illustration here are two pictures of the low power transistors used in the MJR7 (these are some I am about to use in my next design).
Compare and contrast:

The transistors on the left have brighter printing, and in the case of the A1209 there is an area with a rough texture which could be a result of removing the original markings. I checked the current gains and found all these transistors to be within the specification, and also all those I have used in different versions of my mosfet amplifiers have worked ok, so I have no idea whether this is just a normal variation in appearance or something more sinister. Fake transistors are a serious problem, and more information can be found on Rod Elliot's site, or just search on Google.

Update 31-March-2009.

I have another project I am working on, which is a cut down version of the MJR7. The idea is to see how far I can reduce the total number of components so that two channels will fit on one standard size 4x6 inches board without reducing track width and spacing too far. Using only 7 transistors is less of an achievement if the number of other components is excessive. The performance should be almost as good, but I will leave the MJR7-MK3 as a less convenient 'top-end' design and add the MJR7x2 as a slightly cheaper and easier alternative. One feature I regret needing to abandon is the heatsink mounting bracket, which was once a common feature on DIY designs and has some advantages, but I will conform to the more common practice of putting the power devices near the side of the board for direct mounting on a heatsink. The anti-thump circuit is also left out, so it may not be suitable for some high sensitivity speakers or active crossover applications. On the plus side the external wiring is minimised and there are no links needed on the board, and the star earth is also on the board, so the two channel earthing can be improved and simplified.

02-March-2009.
An address for enquiries concerning the MJR7 design is mike@renardsonx13z.freeserve.co.uk but remove the two letters x and z (this is to prevent automatic spam emails, I already get about 100 per week at this address). Use AMPLIFIERS as the subject, otherwise it may get missed and deleted with the spam. I am sometimes slow to reply so please be patient.

HOW TO BUILD THE MJR7.
Part 1. Adding the components.
Part 2. Parts List.
Part 3. Setup and testing.
Part 4. Photos of stages of construction.

I must mention that this amplifier is not a commercial design, it is just for the benefit of DIY enthusiasts, and is recommended only to those with some previous experience. Anyone who has not already successfully built other electronic projects should consider starting with something easier. Using the same component types and values I used myself there should be no problems.
Please check the cost and availability of the transistors in your location before deciding to make these amplifiers, they may be surprisingly expensive in some places. Buying from the cheapest sources may also be a problem, there are many fake transistors being sold with inferior ratings, I used one myself recently to repair a power supply, and it survived less than 5 seconds. Stick to well known and dependable suppliers if their prices are not too excessive.

11-Feb-2009.One small problem with the MJR7 is that setting the output stage operating voltage to half the supply voltage is difficult if this is done with no load connected. This is because there is then only a 1k load and the output capacitor time constant affects the overall feedback in such a way that adjusting the preset control has only a very slow effect on the voltage. I was turning the control from minimum to maximum and finding the voltage at the mosfet sources changed very little, and thought something serious was wrong, but being more patient and waiting a minute or two after each adjustment revealed that it was behaving exactly as expected. Using a small resistor load, e.g. 22R, during adjustment makes setup far quicker, as does connecting a speaker, but until everything is set up and confirmed to be working correctly it is safer to use the resistor.

01-Feb-2009. Testing the clipping performance of the MJR7 with a 2uF load revealed only a very small 'glitch' coming out of clipping. It also revealed that when clipping occurs the LED used to bias the current source flickers. This does not appear to be a problem, but it occurs to me that this LED could easily be mounted on the amplifier front panel and used as a clipping indicator. I have added square wave test and clipping test results to the amplifier page.

I have rewritten a few pages and moved some. The Introduction page is now listed at the top of the Designing Audio Power Amplifiers section, and includes a footnote about the use of high global feedback. The MJR6 page still includes some useful information, including extracted distortion results using music and speakers, but the home page link has been removed.

04-Jan-2009. I have a final MJR7-Mk3 board layout finished, but part of the problem remaining is to decide which components to use. For example a 2u2 capacitor can be anything from a small electrolytic with lead spacing 2mm up to a huge polypropylene over 4cm long. Having adopted a theory plus measurement based design approach it seems consistent to ignore the many differing claims about component sound, but in most cases it should do no harm to choose types reputed to have lower distortion, so for example I have used polypropylene capacitors for most of the low values, but chose others on purely technical grounds where these are important. The 2u2 input capacitor needs to be small to avoid interference pickup at this sensitive location, and low leakage to avoid changes to dc levels. This eliminates both polypropylene and electrolytic, and a small polyester is specified. Similarly the output capacitors are chosen primarily for current rating, which is of some importance in this application. Some time ago I wrote a piece about capacitor distortion which may be interesting to anyone who remains undecided about 'capacitor sound'. It includes a link to an article giving a different view, which some may find more convincing than I did. I have also tried to identify components widely available, or for which substitutes of similar size can easily be found. For this reason some component values have been changed, but the original values work just as well if they are obtainable and will fit the board layout.

I need to improve the input earthing arrangement. When I built a complete stereo version in a case there were small pulses at the amplifier output as shown here. I traced the problem to the toroidal transformer I used, and found that rotating it a few degrees round its fixing bolt would reduce interference pickup below the noise. Another approach is to reduce the field from the transformer. The interference pulses seem to start with a small pulse followed by a much bigger spike, suggesting that one happens when the rectifiers start to conduct and the other greater effect is when they switch off. In addition to taking steps to reduce this field we can reduce the pickup at the amplifier input, which I found was achieved by connecting the two earth inputs together, which causes an earth loop, but reduces a loop in the input signal circuit. This confirms my suspicion that earth loops are not the worst problem, as I suggested on one of the amplifier design pages: Earth Loops. (I was never entirely happy with this article and it is now in the archive section). Such a loop can have a current induced in it by a varying magnetic field through the loop, but not necessarily any potential differences between different points round the loop as we might expect. To avoid both earth and signal loops one idea is to take the input section earths and input source earths to a separate star earth near the input and connected to the main star earth via a single heavy gauge wire.

29-Oct-2008. I have added the final MJR-7-Mk3 via the link on the home page.

I have another project which may be worth supplying as a kit, and this is a class-A discrete component headphone amplifier. I have seen the sort of prices being asked for some fairly ordinary designs, and it should be possible to do better for much less. The current favourite version uses a jfet input stage with optional direct coupled input, and an active volume control to achieve something sufficiently close to a log response from a dual linear control with the advantage of better tracking compared to a typical log control. Having the PCBs made could be expensive, typical charges for small quantities are over £20 each, but I could make these myself, and I may have more spare time soon after Christmas. My designs have the advantage that only a few of the passive components could have any significant effect on performance, so anyone who believes component quality is a real problem can easily change these few parts to upgrade the designs.

The Physics section seems increasingly out of place on this primarily audio site, so I have removed the link on the home page. Anyone who is interested can still find the index page here: PHYSICS ARTICLES. INDEX. The articles about cables and conductors may be of some interest to audio enthusiasts. The 'Reflections on a Transmission Line' article is a much expanded version of an unpublished 'Letter to the Editor' submitted to Wireless World many years ago in response to one of their articles. It deals with the relationship between the electromagnetic field and the conduction electrons in a cable, and although somewhat flawed I still think it explains this better than most other treatments of the subject I have come across, so maybe it deserves a rewrite someday. 'Cable effect' enthusiasts are unlikely to find it either interesting or encouraging.

01-Oct-2008 It has become apparent that the MJR-7 is a very good circuit design, but although the original layout is found to work well there is room for improvement. To keep construction easy I want to do this without abandoning the single-sided board, for example by using external wire links for the high nonlinear current paths so that these wires can be twisted, or at least kept close together, to minimise current loops. One disadvantage is that the result may look rather untidy, but my layouts have never had neatness as a high priority, so nothing new here.
Some further thought is also needed about the 100n bypass capacitors added to the output and supply smoothing electrolytics. These at least appear to do no harm, but are probably not needed.

23-July-2008 I never really finished my MJR-7 mosfet design, I left the inductor and its damping resistor as just the original values I started with. My earlier simulation results, some of which are shown here suggested that a lower damping resistor could be a good idea, but were inconclusive. Then, as mentioned further down this page, I learned that quite small values of load capacitance could be a problem, and although a change to the internal compensation RC solved this there could be some variation if different transistor types or different layout is used, so I suggested a 100nF across the output for added safety to avoid smaller capacitance loads being a problem. After further thought and simulations I concluded that a 1R resistor in series with the 100nF is needed to damp any resonances with the load. Without this an inductive load could in theory resonate with the capacitor, giving the same impedance as the originally troublesome capacitor load near the unity gain frequency. Reducing the damping resistor across the inductor from 7R5 to 2R2 also helps.
I checked the older MJR-6 circuit, and the same output network is good for this design also, with the 100n plus 1R across the output, but here reducing the inductor damping resistor to 1R is better according to my simulations. Smaller damping resistors improve phase margin with high capacitance loads but higher resistance helps with smaller capacitance loads. Another benefit of including the 100n output capacitor is that by reducing the range of possible load capacitance this makes choosing the damping resistor easier.

A final version, the MJR-7-Mk3, will be added here eventually, there are just a few details to finalise. There will be only a few small differences compared to the MJR-7 and the Mk2, and if anyone has built either of these just adding the 100n plus 1R across the output and reducing the 7R5 inductor damping resistor to 2R2 are the only changes I would suggest.

03-Dec-2007. At the end of the MJR-8 page I have added a circuit I called the MJR-DC8, which is a version which includes a direct-coupled jfet input stage to reduce noise, but allows distortion to rise a little compared to the MJR-7. There is also a MJR-DC9 which is just another variation of the same approach. I have no plans to try these designs, they are just examples of alternative input stages, and are unlikely to be an improvement in any way other than noise level. If noise is a problem, e.g. if using horn speakers with high sensitivity, there are other alternatives such as reducing closed-loop gain.

Checking the level of the output switch-on pulse caused by the output capacitor charging through the speaker, I find levels up to 5V for the MJR-7. This is a fairly short pulse, and is unlikely to damage a typical speaker. There are, however, some high sensitivity speakers, and other applications such as bi-amp or tri-amp speaker drivers where this pulse may be excessive. My original measurement of only 1V was for the MJR-6 with a lower supply voltage and a much higher smoothing capacitor, giving a slower rise in supply voltage, which may explain part of the difference. There is a simple circuit addition which was once suggested to me, and I have simplified this and added it to the MJR-DC9 circuit. For most applications this is completely unnecessary, my own speakers make just a small unobtrusive thump at switch-on and nothing audible at switch-off.

1-Nov-2007. -Updated March 2008. The MJR-8 feedforward amplifier was originally added to replaces the abandoned MJR-7-Mk2 as 'work in progress', but I now have no plans to try this design. As with the step from MJR-6 to MJR-7 there is no real need for big improvements, but if it can be achieved by the use of little more than one extra transistor then why not? One reason I never tried this version is the need to adjust for a distortion null, which could be a problem when the distortion is already so low, but I have suggested an easy way to do this. An updated MJR-7-Mk2 page can still be found by following a link further down this page, but this is little different to the original MJR-7.

26-Oct-2007. I have now tried my MJR-7 with low capacitance loads, and found that with the original circuit it became unstable with capacitive loads from 1.1nF to 3.6nF. With 100nF added across the output terminals there was no problem, but I also tried changing the resistor in series with the 220pF compensation capacitor, and found that values less than 169R or greater than 285R lead to instability for some load capacitor values. A 220R resistor, about half way between these values, is now recommended, and I have updated the circuit diagrams accordingly. My test results are shown here. The 220R value is the optimum value for my own amplifier, but there may be some variation using different samples of the specified transistors, or with different layouts, so the 100n at the output is still a good idea for added safety.

I have also rechecked the clipping performance with a 2uF load, which I mentioned earlier is cause for concern because the phase shift at 100kHz is worryingly high at lower signal levels, but hopefully drops to safer levels near clipping where it otherwise could be a problem. Using a 10kHz sinewave with the 2uF load there are a few very small ripples when coming out of clipping, but not enough to worry about. Real speakers are never likely to be a pure capacitance, so this is even less of a problem in practice.

24-Oct-2007. I never tried the original MJR-7-Mk2 myself, but I have new information from Peter Schoellhorn, who has tried this 'improved version'. Serious stability problems were found, suggesting that I was over-optimistic about the possible increase in loop gain, so I have changed the circuit diagram to return to the original MJR-7 compensation. Even that may have problems with low capacitance loads, and the simplest way to avoid that problem is to add a capacitor across the load, initial simulations suggest 100n is a good choice. With low capacitance loads the series resonance with the output inductor may be above the loop unity gain frequency, and this adds extra phase lag at this frequency, and I believe this is the cause of the problem. I have also changed the Mk2 version in other ways, but again for now this will remain as untried 'work in progress'. Whether the MJR-6 also would benefit from a capacitor added across the output is not yet clear, but for now I suggest including this. This is not a complete solution to the MJR-7 stability problem, instead of being unstable with a range of capacitor load values there will now be a range of inductance values at the unity gain frequency which may be a problem, and simulations with 10n across the output are not so good because of this. Another change which should help is to increase the 100R in series with the 220p compensation capacitor. The 100R pulls back the phase lag by 45deg only at 7MHz, but the input stage already rolls off around 5MHz giving an extra 45deg, so to better compensate for this the resistor should be increased, e.g to 200R. My most recent simulation shows instability for loads from 500pF to 9nF with the 100R, but no instability with 200R. Many thanks to Peter Schoellhorn for letting me know his test results, I had previously assumed that low value capacitances would be no problem, and only tested with higher values.

19-Sept-2007. The more accurately I calculate feedback loop phase shifts for the MJR-6 mosfet amplifier the more likely it seems that shifts in excess of 180deg are possible with high capacitive loads in the 100 kHz region. This is generally regarded as a bad thing because if the loop gain falls near clipping it is possible to have positive phase feedback combined with unity gain at some frequency and consequent oscillation. There is no sign of such a problem with the amplifiers built, and the reason appears to be that close to clipping there is also a fall in phase shift so that long before the loop gain falls to unity the phase shift has reduced to a harmless value. This will not be the case for all amplifier circuits, for example using the more common Miller compensation capacitor method there can be feedforward through the capacitor near clipping, which will make the phase shift worse. The question of whether the driver or output stage clips first may also be relevant.
I was initially surprised by the level of feedback I could use in the MJR-6 circuit while still maintaining excellent stability. I had just finished reading the article by Baxandall which warned about the Miller capacitor effect, so my idea was to use a rather more predictable compensation method, which in the MJR-6 uses the mosfet capacitances, and in the MJR-7 improves on this with a more linear component. The use of a 10p capacitor in parallel with the feedback resistor plus a 390p capacitor from input base to earth is a useful circuit trick to accurately define feedback network gain and reduce the impedance at the input at high frequencies, and I believe this is part of the reason why such a high feedback level is possible.

20-Aug-2007. The 'Square Wave Testing' article mentioned that I find it more useful to check for problems near clipping, and now I have added oscilloscope traces showing examples of latch-up and instability.

20-Jun-2007. Updated further on 17-Aug-2007. The 'Output Inductor Problems.' article explains some of the problems involved when choosing the output inductor and its damping resistor. A negative resistance component at the input of the output stage when driving a capacitive load was also explained.

18-May-2007. A few months ago I listed an update about the value of output inductor used with my amplifier designs, which at first I thought was incorrect because a well known formula gave double the value I had measured. I now think my measured value is about right, and I have added a note about this to the end of the MJR-7-Mk2 section.

18-May-2007. I have probably written far too much about amplifiers, and for a change I have started a Speaker Design Section. I have more limited expertise regarding speaker design, and my current attempts are intended only as a simple low cost 'fun project'. I may eventually try one or two new projects when I have more spare time, but for now there are just two pages.

10-Feb-2007. The inverting amplifier configuration I used in all my recent designs avoids the problem of common-mode input distortion, but has a few problems of its own, and in a link on the MJR-7-Mk2 page I have illustrated these problems and my chosen solution, together with feedback network phase response plots for the latest version of the mosfet amplifier.

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