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M2 Output Stage Analysed

XEN Audio

September 2022

Earlier, we published some distortion figures when comparing different MOSFET pairing

[1]

. For that,

we used the M2 Output Stage (OPS) as our platform. It is a nice and simple output stage with auto

biasing. And it is supposed to operate well in Class AB. So can’t we put it to better use ?

In the official M2 schematics, there are some components, namely D1,2,3 and Q6,7 that will enable it

to operate in Class AB, according to Nelson. There were a few hints dropped by him in

[2.3]

To understand the master’s brilliance better, I did the following analysis, aided by LTSpice simulations.

A few boundary conditions to make life easier :

Rail voltages +/-25V

DC bias 1.3A

Vgs MOSFETs 5.1V for both

Signal source impedance 100R

Benchmark load case +/-20V, 10Hz into 4R

(We normally need more power for bass.)

* Note: in simulations, the circuit behaves much better in Class B with the 4N26 coupler. This is what

applies to all the results below, and will be explained later.

With a DC bias of 1.3A, the maximum Class A output current is ~2.6A. In order to deliver enough

current for 20V into 4R, the maximum current must be 5A. At 5A current (Class B, one side

completely off), the voltage across R13 or R14 will be 2.35V. In real life, the current will be less as the

gain will drop to 0.9 at 4R load with Zout ~0.45R.

Without the LM385’s, the optocoupler LED will see an increase current due to the increased voltage

across R13-R14. This will cause the photo-transistor to conduct more, thus pulling down the MOSFET

bias. The LM385 locks the voltage across the LED at 1.2V simply by taking current away from the

optocoupler LED, thereby preventing the pull-down by Class B output current.

Summary of content (7 pages)

PAGE 1
M2 Output Stage Analysed XEN Audio September 2022 Earlier, we published some distortion figures when comparing different MOSFET pairing [1]. For that, we used the M2 Output Stage (OPS) as our platform. It is a nice and simple output stage with auto biasing. And it is supposed to operate well in Class AB. So can’t we put it to better use ? In the official M2 schematics, there are some components, namely D1,2,3 and Q6,7 that will enable it to operate in Class AB, according to Nelson.
PAGE 2
The MOSFETs have a Yfs of ~2.3S. To increase current from 1.3A to 5A, it needs a Vgs of ~6.7V. Add to this the voltage drop across the source resistor; that gives 9.1V. The normal DC voltage across C3 is 11.4V. So in theory at least, this is sufficient. Even after a few hundred cycles, simulations show that C3 does not lose voltage. And no current flows through any of the 1N4148 diodes. Some input current is required though to drive against R6//R7, but not unexpected.
PAGE 3
Fig. 2 LM385 Clamp If we push this even further, with an input of 24V into 1.5R, the output starts to clip at 17V. And the gate voltage of Q4 actually rises above the output during the positive half-cycle. D3 conducts (in a short pulse) some 2.6mA peak, and the same peak can be seen through C2. One can argue that D1~D3 is not a necessity, since it seldom sees 1.5R load at max. voltage. However, current in the LM385s exceeds the rated 20mA for loads below 3R.
PAGE 4
What if we, just for fun, reduce C3 to 10µF ? It still works, only that THD increases slightly from 0.75% to 1% (for 20V input 10Hz into 4R). We have mentioned above that the optocoupler has been changed to one with a lower CTR (4N26 instead of 4N35). What if we now put back in the 4N35, as specified ? You can see how it looks like in Fig. 4. Here, the input is only 20V into 1.5R, compared with 24V in Fig.3. So for Class B operation at least, a high CTR is not an advantage. Fig.
PAGE 5
R11 R13,14 C3 120R MPC74 5W 0R56 100µF Schematics of the Original M2 Output Stage MOSFET Choice In our earlier MOSFET pairing comparison [1]. The best result by far is from a pair of curve-tracer matched 2SK3497 / 2SJ618, both in H2 & H3. They also have the highest transconductance and best pentode characteristics. Normally they are quite tricky to use in Class A, as they have a very large positive tempco and can easily run away thermally.
PAGE 6
LT1084 / LT1033, but they are limited to 3A. Alternative solution can be a simple Cap Multiplier using Sanken 2SD2390 / 2SB1560 Darlington’s. The base should be driven by an On-Semi TL431 or Vbemultiplier configured for 26.2V, resulting in 25V rails. This should be fed by a J113 degenerated as a 5mA CCS. The schematics below is borrowed and adapted from Geoff Moss’s excellent JLH Class A site.
PAGE 7
References 1. 2. 3. 4. https://www.diyaudio.com/community/threads/complementary-power-mosfets.378024/ https://www.diyaudio.com/community/threads/official-m2-schematic.281520/post-4854929 https://www.diyaudio.com/community/threads/official-m2-schematic.281520/post-5281512 https://www.diyaudio.com/community/threads/complementary-power-mosfets.