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Old 1st May 2013, 02:22 AM   #61
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Richard Murdey
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The simulation follows the measured result closely, so its fair to say the model is good enough for further discussion and optimization.

The attached is the LTSPICE file for my version of the J-Mo II. It has higher gain than the regular version, and lower bias current, optimized for 300 ohm loads.

It's good for 70 mW at 1% THD into 300 ohms. Distortion falls away as the signal voltage gets smaller. At typical listening levels, its much lower.
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File Type: jpg jmo2.jpg (383.0 KB, 280 views)
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File Type: asc J-Mo2 11m 5484 RJM personal.asc (3.2 KB, 5 views)
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Old 1st May 2013, 07:16 AM   #62
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Technical discussion:

The Mosfet source follower output stage used in this and all Szekeres-based headphone amplifiers basically sucks in terms of distortion, and the problem goes from bad to worse as the output load increases. If high THD figures bother you, this is terrible circuit element to use.

So, similar to single-ended tube amp design, isn't spending inordinate effort finessing this circuit to minimize the distortion is a bit ... backwards? Well, yes and no I guess. Its human nature to try and optimize the chosen circuit, even for a choice which is inherently poor. Even if we chose it for the way it sounds, part-and-parcel of which is the very harmonics we are busily trying to get rid of!

My compromise position runs something like this: yes, single-ended MOSFET source followers are going to have substantial harmonic components, and yes this is part of the reason we went with this topology, but it's not the whole reason (others being low noise, no crossover distortion, and very simple distortion characteristics) and within the given power supply voltage and current/thermal limitations its desirable to tweak the circuit to keep the distortion as low as possible, at as high an output power as possible.

Fiddling around with the simulation, I conclude that the cleanest signal is generally obtained with the Mosfet gate voltage biased well above half the power supply. This is done by adjusting the trim pot on the jfet drain. Overreach however and the jfet current drops below a critical threshold... if this happens the Sziklai element has to be reconfigured, perhaps substituting a jfet with higher Vgs0, or alternatively increasing the gain.

This kind of optimization can drop the second harmonic component 5-10 dB, so its not trivial. How much different it makes to the sound though is something I'll discover shortly.

At the moment, at -70 dB THD at typical max listening levels, I would not characterize the sound to be at all "fat", like you might expect for excessive 2nd harmonic. The amp though does have a very definite character. It's very quiet, for one. At the same time the music itself sounds slightly hushed, even when playing very loud. Like being in the midst of gently falling snow.

I now have four high-quality headphone amps in house. Two with push-pull bipolar output stages, two with single-ended mosfets. Some weekend I'll have to do a shoot out...
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Last edited by rjm; 1st May 2013 at 07:19 AM.
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Old 3rd May 2013, 12:56 AM   #63
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What the previous post says, in graphical format. This is real data, not simulation. By way of comparison the sapphire amp has no measurable distortion, i.e. no points above the black dashed line. However it's worth noting that real world listening power levels are not even represented on the chart, they are down at 0.01 mW where the THD is in any case about 0.03~0.05% and almost entirely 2nd harmonic...
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File Type: png jmo2.png (32.1 KB, 251 views)
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Last edited by rjm; 3rd May 2013 at 01:07 AM.
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Old 14th May 2013, 05:26 AM   #64
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Simulated results with LTSpice, for the standard BOM version driving a 16 ohm load.The BOM version is optimized at about 40 ohms, the idea being it will be reasonably comfortable driving anything from 15 to 120 ohms. 16 ohms is a "worst case" situation, more power / less distortion will be available into 30 or 60 ohms for example.

As shown, it will do 40 mW into 16 ohms at 1% THD.
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File Type: png Jmo2 THD Circuit.png (43.1 KB, 216 views)
File Type: png Jmo2 THD Sim.png (26.9 KB, 211 views)
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Last edited by rjm; 14th May 2013 at 05:30 AM.
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Old 14th May 2013, 02:01 PM   #65
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I will try this p2p.... any special recomendations from you ?
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Old 14th May 2013, 11:59 PM   #66
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The general layout of the PCB (which is single-sided) can be used for a P2P build also.

The heat dissipation by the MOSFET is quite low, 1~2 W. You could, however, arrange the MOSFET to bolt to the case instead of using a heatsink.

In line with the original Szekeres, this is a "fun" circuit, more for how it sounds than for being the last word in fidelity, so feel free to change values as you want.
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Old 15th May 2013, 05:44 AM   #67
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Cool

What is the power consumption (each channel) ?

I will power the rig using a BiB (Salas shunt) and need to set the shunt current accordingly

Can I use IRF610 and 2N5457 instead ? (Or a 2sk170 at 8mA)
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Last edited by RCruz; 15th May 2013 at 05:57 AM.
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Old 15th May 2013, 06:46 AM   #68
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@RCruz You can use IRF610 and 2N5457 for sure

I've built the Szekeres amp with LM317 as current source (at the place of R9) and there is something special about single ended class A sound. It has been reported many times that the SQ in this topology greatly improves upon using a low-impedance buffer before the MOSFET and I had the idea to try an OpAmp + MOSFET but I'd better try J-MoII first.

@RJM - I hope you don't mind etching our own boards at home for the sake of non-scientific measurement experience, do you?
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Old 15th May 2013, 07:34 AM   #69
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I will do it p2p... do not like etching at home
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Last edited by RCruz; 15th May 2013 at 07:36 AM.
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Old 15th May 2013, 11:50 AM   #70
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@Gloop

You can use and or modify the .brd files to make your own PCBs. That's fine with me.

I think the op-amp + MOSFET has merit, but it is best paired with a CCS on the MOSFET source. If you want to stick with the resistive load of the original Szekeres, then the jfet-bjt input stage of the JMo2 is a nice matching front end. Another route is the triode + MOSFET mu-follower.

@RCruz

"Can I use IRF610 and 2N5457 instead ? (Or a 2sk170 at 8mA)" Yeah, whatever takes your fancy, really. The pnp transistor, too, can be substituted with just about anything. In the worse case you find that the trim resistor is insufficient to properly adjust the source bias current. In that case have to adjust the values of R2,R3 up or down (keeping their ratio R2/R3 constant). High values reduce the current in the transistor, increasing the current through the jfet and lowering the MOSFET source current, lower values do the opposite. If you are unsure and can't run it through LTSpice to check give me a shout and I'll sim it for you.
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