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MJR9. Mosfet amplifier with feedforward.


The method used to apply feedforward was described on an older page, The explanation started with "Suppose the distortion at the output of the original MJR-7 was 0.0002% at 1kHz". In fact the Mk5 version is even better than this, which is why there is really no serious point to this improvement other than as a demonstration of the application of feedforward. The MJR9 is based on the earlier experimental MJR8-Mk2 which worked well but had insufficient feedforward current for lower impedance loads driven at high level. This problem can be solved by using a class-AB buffer stage. A LED is used to bias the feedforward stage and the variable mosfet bias is also derived from this. I have added a LM234 current source, with 12R current setting resistor to give 5mA. The buffer stage transistors need small heatsinks. The feedforward output circuit adds high frequency trimming with a 22n capacitor, which a simulation suggests is about optimum if we want to minimise distortion components up to 20kHz. The 2u2 in series with the feedforward resistor reduces the effect at low frequencies, where it is not needed, and anyway phase shifts would make it less effective.

The range of bias voltage for the mosfets is in theory too low, the range needed to ensure 100mA quiescent current can be set is about 0.3V to 3V, but all the mosfet pairs I tried so far needed around 1.2V, so the LED voltage, about 1.8V, is probably high enough in practice. If the full 100mA could not be set this may not be a serious problem, the feedforward would have more work to do, but the overall result should still be good.

The feedforward effect depends on the open loop gain of the whole amplifier, so it is not just a feedforward output stage, and it is not possible to just add this output section to any alternative input stages unless they match the gain of my own input stage.
One problem is that the distortion nulling will only be accurate for one value of input signal source impedance, so driving the amplifier direct from a volume control is not ideal, and a unity gain input buffer stage would be one option.

Initial tests showed that for best distortion nulling the feedforward resistor needed reducing to 46R, which is lower than my simulations suggested. I had assumed the output inductor L1 to be 0.5uH, but the resistor value needed would be explained if it was 0.4uH. Calculations and more recent measurements had given values greater than this, but it appears that the earlier value I measured could be about right. The problem then is that the feedforward buffer stage would need to operate at a higher quiescent current to ensure it stays in class-A at high outputs. The transistors already run hot, so better heatsinks are needed, but another option is to increase the inductor a little. Using 20swg wire instead of 18swg and using 17 turns instead of 13 the dimensions can be kept unchanged while increasing the value to 0.68uH. I will add these changes to my own final version, but the distortion tests have been completed with the lower value inductor.

Here is a photo of the MJR9 board, showing just one channel. It can be seen that there is not much room for the heatsinks, and some redesign of the layout would be needed to make any big improvement. Painting the heatsinks black could help, but probably not enough. This is the most recent version with a 17 turn inductor.

DISTORTION TESTS.

Here is the test setup using a signal generator I made myself many years ago, plus my EMU-1820M. The small white circuit board is the distortion extraction circuit which nulls the test signal by typically 40dB. An oscilloscope and voltmeter are also used, primarily to setup signal levels. The signal generator can produce sine and square waves and also tonebursts. A single frequency oscillator is also included for intermodulation tests, and was originally 19kHz but has drifted upwards a little over the years.

As previously the test signal is partly nulled, and the -10dB level is calibrated by a 100mV rms signal. The distortion relative to the 4V output level is therefore given by adding 22dB to the scales.

The feedforward only affects the distortion, and other test results and specifications from the earlier MJR7 versions should still apply. Harmonic distortion at 1kHz, 10kHz and 20kHz test frequency was measured with 4V rms output into a 7R5 load. The aim with all my recent class-B or class-AB designs has been to reduce crossover distortion, and the signal level is what I have found most clearly reveals this. At lower levels the amplifiers can stay in class-A and the distortion then falls under the noise level, making measurement difficult. At higher levels the signal spends less time in the crossover region and the effect can become less significant. Intermodulation between 19kHz and 20kHz was also checked.

First at 1kHz, only the second harmonic is visible at -126dB (0.00005%). Carefully trimming the feedforward and adjusting the supply wiring to reduce pickup I could get the figure down under -130dB, but I wanted to keep the same settings for all test frequencies.

Next at 10kHz two results are shown to illustrate a problem with feedforward adjustment. If I wanted the lowest distortion within the audio range I would only worry about the 20kHz harmonic and get the first result. If I wanted the best THD specification I would want to keep all harmonics low, and the second result could be better because the 3rd harmonic has been reduced lower. The high frequency phase trimming has been optimised in a simulation to give best distortion nulling up to 20kHz only, so I decided to use the first of these options.

Next at 20kHz the 2nd harmonic is at -123dB and 3rd at -120dB. The 4th is higher at -116dB, as expected the feedforward is becoming less effective at such high frequencies. Lower level components unrelated to the test signal were picked up from the nearby computer or other sources of interference.

Next, the 1kHz intermodulation produced by 19kHz plus 20kHz is -123dB. This is for a peak to peak test signal output of 11.2V, the same as the 4V rms sinewaves used previously. The rms ratio has been used rather than peak ratio (This is why the -120dB level marked on the picture is at a different level compared to the previous results.) The peak ratio would have been -126dB. There are higher order products at 2kHz and 18kHz around -130dB, plus a 15kHz spike from a nearby tv. (The ratio of distortion rms levels to total signal rms voltage has been given here, which I have since learnt is incorrect, the CCIF standard appears to require just one of the test signal components to be used as the reference level, so the 1kHz product should be specified as -120dB.)

The square wave tests and clipping tests on the MJR7-Mk5 test results page also apply to the MJR9, the feedforward has practically no effect on these. I am not entirely sure how crosstalk is affected, I have only made one channel into a MJR9 so far, but I can think of no reason why this would be much different to the Mk5 results.

Here is a later distortion result at 10kHz, with the output inductor increased to about 0.7uH. The 20kHz second harmonic has now dropped to somewhere under -130dB, but for some reason some of the higher harmonics have increased. I guess the increased inductor should have a higher parallel resistor, but trying to reduce components so far beyond the audio range seems pointless.

CONCLUSIONS.
I previously specified the MJR7 distortion as 'all audio frequency distortion components under -100dB (0.001%)' which is not the conventional way to specify distortion, but does at least describe the only distortion which could be audible if it was at higher levels. A similar specification for the MJR9 would be 'all audio frequency components under -120dB (0.0001%)' provided the feedforward is adjusted for minimum distortion components up to 20kHz. The distortion specification limit is probably set by supply breakthrough, input stage distortion, and the limitations of the test equipment I have available.

To avoid losing contact with reality I have marked the 300uV level on the harmonic distortion traces to show the lowest signal level I found to be audible to someone with excellent hearing at 3kHz at 1 metre from one of my Mordaunt-Short MS20 speakers (87dB per watt at 1m. so 300uV is theoretically a 7dB sound level, which is about the lowest level we could expect to hear with masking from typical domestic background noise.) At much higher or lower frequencies, or with the masking effect of the music, or at a normal listening distance, even distortion at this level will be inaudible. Whether the second harmonic of 10kHz is 35dB or 45dB under the 300uV level is clearly of no real importance. With conventional CD music sources at moderate levels with speakers having a reasonable impedance even the original MJR6 has distortion well below audibility and I have been using one myself for some time, and have no plans to replace it with any of the 'improved' versions. Most DIY enthusiasts are unlikely to have test equipment suitable for adjusting the distortion nulling of the MJR9, so my current recommendation has to be the MJR7-Mk5, which gives excellent results without any fine adjustments.

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