The MJR7-Mk5 Mosfet Power Amplifier


Latest updates:

9April 2017). In the circuit diagram I added a 5k6 resistor across the output of the 220R quiescent current adjustment preset. It is not entirely obvious what happens if the preset wiper becomes open-circuit, we don't want the quiescent current to then increase, and I am almost certain it will not, but for added security the 5k6 may be added. I have not altered the layout diagrams, it is easy enough to solder the resistor on the back of the board. I haven't added this to my own amplifiers. If the mosfets have zero drain to gate leakage there is no point, but I am not 100% certain about this.

(July 2012). I have added a link above to a page of 'design notes'. Most of that already existed somewhere, distributed around the various versions of the MJR7 on the old website. The Mk5 is almost certainly the final version, so a summary of the design philosophy is a useful addition.

(June 2012). The original 2SA1209 and 2SC2911 are becoming less easy to find. I have used the slightly better KSA1381 and KSC3503 in my own final version. I got these from Mouser in USA, they are actually KSC3503D and KSA1381E but the gain group is not critical. The 2SC2240BL are hard to find, a possible substitute is BC550C (ON-Semi), but the pin layout is different. 2SC2240 is E-C-B but the BC550 is C-B-E. The pnp transistor is now specified as a BC560C, made by ON Semiconductors if possible, some others have a lower specified fT. A high fT and current gain are the requirements. The mosfets could be any lateral type with similar case and specifications to the Renesas 2SK1058 and 2SJ162. The Renesas types have internal gate protection zeners, but as far as I know none of the alternative makes do, so care is needed to protect the gates from static charge until connected to the board with the external zeners in place, e.g. keeping the gate and source connected together by a clip or metal foil. The external zeners are still a good idea even for the Renesas mosfets, the data sheet fails to mention the maximum current rating of the internal zeners.

The compensation capacitor has now been reduced from 220p to 100p and its series resistor increased to 470R. To keep adequate stability margins with reactive loads two other changes then became necessary. The capacitor from input base to earth is increased from 330p to 470p (this is part of the compensation, not an input filter - the impedance at that point is just a few ohms because the feedback creates a 'virtual earth' - the other 470p is the only input filter), and also the resistor in parallel with the output inductor is reduced from 2R2 to 1R5 - this also reduces 'ringing' with a 2uF load.

I have included an anti-thump circuit, at the top left of the diagram, which consists of BAV20 diodes plus a resistor and capacitor. I reduced the capacitor across the LED biasing the 2SA1209 current source to 1n to improve recovery from clipping, and this makes the switch-on pulse at the output worse, typically 8V. With 470uF in the anti-thump circuit the pulse is then not much over 1V, which makes just a small unobtrusive sound with my own speakers. ( Even this may still be a problem if directly driving mid-range or high frequency drivers in a bi-amp or tri-amp system. For this purpose the 4700uF output capacitor can be reduced, but then the 10uF in the feedback network and the 2u2 at the amplifier input must also be reduced in the same proportion. e.g. the values could be 470uF, 1uF and 0.22uF giving a -3dB low frequency gain around 60Hz.) The pulse can be reduced further by increasing the 470uF anti-thump capacitor, but there is then a longer delay before the amplifier reaches normal operation, it is about 15sec for each 470uF. At switch-on the capacitor starts to charge, which slows down the amplifier output capacitor charging pulse and so reduces the charging current through the speaker, but once fully charged the BAV20 diode connected to the 2SA1209 is reverse biased, so there is no further effect on the circuit. The capacitor needs a voltage rating greater than the supply voltage used because in normal operation it is charged close to the full supply voltage.

The NPN input transistor Tr1 was originally a 2SC2547E, but this is becoming increasingly hard to find, and those I have bought recently include some obvious fakes far below the specified current gain range. More readily available types such as the 2SC2240BL can be used instead. The important specification is noise figure at 0.5mA collector current and 10k source impedance, and a figure under 1dB is good. If all else fails the BC550C should work well enough.
The PNP is shown as a BC560C, which I thought would have higher current gain than the original 2SA1085E, but testing a few samples I found little difference. The BC560C made by ON Semiconductors have a higher specified fT than some others, and this is useful because the pnp is the limiting factor in input stage bandwidth, which is relevant to overall feedback stability. The npn is effectively operating in common-base mode at very high frequencies as far as the local feedback loop is concerned, so it has a much higher bandwidth. For both input stage transistors avoid types with high base-spreading resistance rbb', this may add significant phase shift affecting loop stability.

Gain and Phase Shift

A Spice simulation gives the following result for closed-loop gain and phase shift. There is an almost perfect linear phase response from 1kHz to 20kHz, having the same effect in that frequency range as a constant time delay of 3.3usec, so having practically no effect on wave shapes. The -1dB frequency range is about 12Hz to 30kHz.

Board Design

The Mk5 includes two channels on one standard size board (6" x 4") with a single star earth included on the board.
To avoid the mosfet mounting bracket used in earlier versions the mosfets are connected near the edge of the board so that they can be fixed directly to a heatsink. They could be soldered onto the board, but then there is some difficulty in ensuring they all line up correctly with the heatsink surface, and so the layout has been changed to allow the option of using terminal blocks as shown in the next photos. The blocks should be soldered in place before the 300R gate resistors so that these can be routed correctly to avoid obstructing the blocks. The inputs can also be taken to a terminal block if required.

The coil, shown next, is air-cored, and is made using 18-gauge enamelled copper wire (1.2mm dia.) which can be made by winding it on a 1cm dia former, for example an AAA battery as shown in the photo. There are 13 turns, giving an overall length about 17mm. The other photo shows how the gate protection zeners are fitted. Only two holes are provided and the zeners have two ends soldered together off the board. The connected ends are the same polarity, in the photo the cathodes (indicated by a dark band) are connected, but it works just the same if anodes are connected together instead.

The next photo shows the final MJR7-Mk5.

Footnote: One problem with the inverting amplifier configuration is that with an open-circuit input it becomes a unity gain feedback amplifier, and the compensation needed to ensure stability can severely limit the performance. The 470p capacitor from input base to earth is the trick used to avoid this problem, and how this works is explained in Inverting Amplifier Feedback. The damping resistor across the output inductor is also important for stability, as explained in Output Inductor Damping. More details of the output network design are included in Output Network.