MJR7 Constructor's Page
The first two photos are from Peter Schröder, who has used a wooden enclosure. In locations with no serious interference sources this is no problem, I have used wooden enclosures myself in the past. I particularly like the big heatsink. This is not essential, but keeping the amplifier cool will certainly improve long-term reliability, and it looks good too.
The next two photos are an amplifier built by by Peter Bergmann, who has used his own board design and a metal case. A heatsink is mounted inside the case. The mosfets can survive at high temperatures but other components, particularly the electrolytic capacitors, would have a shorter lifetime if allowed to get hot, so ventilation holes have been added to the base, back and top to maintain adequate air flow and keep the interior cool. An earlier version had problems with supply harmonics being picked up on one channel, but this was improved by a change to the wiring layout, with the positive and negative wires from the supply capacitors now bound together to minimise current loops.
The next photo is from Paul Bentz, who has also used his own board design. The amplifier is part of a tri-amped speaker system. The MJR7s are used for the mid and high frequency drivers, while the bass is provided by a Rod Elliot design included in the same case. This looks very impressive, with good solid construction. Paul mentioned that accidentally shorting an output blew the fuse, but otherwise did no apparent damage. The lateral mosfets are hard to destroy, but it is still reassuring to know that the simple fuse protection is effective.
The next pictures were sent by Anton Yankov, and are his own design circuit boards. These are double sided, and use a number of surface-mount components, which can be seen on the underside view. This makes smaller board size possible, which can help reduce unwanted stray inductance and capacitance which could adversely affect stability. The large electrolytics needed for the MJR7 limit the maximum possible size reduction. I have made my own designs single sided with wide tracks and through-hole components to make them suitable for constructors with a range of abilities. I have rarely tried using surface mounting components myself, soldering such small parts needs a steady hand and good eyesight.
Next is my own most recent construction using the latest and probably final version, the MJR7-Mk5. This uses a single board for both channels and a star-earth point on the board, which simplifies the resulting case layout and wiring. There are just the two pairs of speaker wires and the positive and zero supply lines to connect to the board, plus the input cable, which I attached via a terminal block. The transformer was taken from an old Cambridge ATAC3 amplifier, but gave a supply voltage slightly above the 63V rating of the capacitors, so I added a pair of rectifier diodes in series with the bridge rectifier output to the capacitor positive terminal, and this reduced the voltage to a safer level. The supply smoothing capacitors are a parallel pair of Epcos B41231-A8129-M , each rated at 12,000uF +/-20%, 63V and 6.83A. A total as high as 24,000uF is not essential, but this type were available from Farnell (UK) for only £2-32 each (+VAT), product code 1839291. The case is recycled from an older design I built over 30 years ago.
The only earth connection to the case is via a metal mounting post at the star earth point on the board. I never use supply earths, but for commercial designs this is only permitted provided adequate double insulation is used, and anyone choosing to omit supply earthing needs to be aware of the possible danger and have the necessary experience and ability to make everything safe. My own construction should not be taken as an example of the correct way to achieve this. I strongly recommend connecting all the equipment to the mains supply via an 'earth leakage' cutout device for extra protection.
Next is an extremely well made version of the Mk5. This is from Paul Bentz, who also built an impressive version of the Mk3, shown earlier on this page, as part of his tri-amped speaker system. My own layouts are always simple single sided boards with wide tracks designed to be easily built by less experienced constructors, but without that constraint there are definite advantages to using double sided boards, including the reduction of current loop areas and neater layout, as seen here.
Next is another version of the latest Mk5 amplifier built by Forr, using almost the smallest case size possible. I usually advise keeping the transformer far away from the amplifier board, but with a good transformer with a low external field this may not be essential.
Another application for the MJR7 is as an upgrade for an existing amplifier, and in the next example, built by Chuck Hicks, the starting point is a Dynaco SCA-80Q. This conveniently has a single 72V supply, and enough space in the case for the new amplifier board, as seen in the following photo. Some care is needed to avoid earth loop problems, the MJR7 is intended to have a single connection to the case via the central mounting post, but the Dynaco has its own earth link to the case near the supply capacitor. After solving an initial hum problem Chuck reports that there is now no audible noise or hum. The Dynaco uses a 250k volume control, which could be a problem for some power amplifiers, but the MJR7 is designed to maintain stability and low noise with a high source impedance, and although the control needs turning closer than usual to its maximum level the performance should be just as good as with a lower impedance control.
The next photo illustrates one of the greatest advantages of adapting an existing amplifier, which is that it would be difficult to achieve the same impressive appearance. Although commercial amplifier designers may fail to attain the level of performance of some good DIY designs at a similar price level, one thing they usually can be relied upon to do well is to make something that looks good. I adapted an old Cambridge ATAC3 for the pre-amp in my own system, but I think the Dynaco shown here looks far better.
The next photos show another way to make a good looking power amplifier, this one is made by Jean-Yves Coadou, and uses the case from a Phase Linear 2000 preamplifier. After stripping out all the components the case has many holes where the controls were fitted, and the solution is to add an anodized aluminium front plate. The wide case allows the amplifier board to be mounted well away from the power supply, which is always a good idea. The MJR7 has good rejection of supply line noise, but direct pickup from the transformer's external field is still possible, so it is good to keep a wide separation.
