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JFET-MOSFET headphone amplifier J-Mo 3
JFET-MOSFET headphone amplifier J-Mo 3
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Old 30th July 2017, 12:01 AM   #1
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Richard Murdey
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JFET-MOSFET headphone amplifier J-Mo 3
Default JFET-MOSFET headphone amplifier J-Mo 3

There are endless variations of this simple circuit changing the voltage, bias current, and JFET. I'm optimizing here for relatively low Z headphones and overall simplicity.

The gain is 6 dB. Output bias current is 100 mA.

The circuit board is 50mm x 50mm.

Note: the LTSpice output distortion FFT is a little misleading as the JFET and MOSFET 2nd harmonic tend to cancel and how much cancellation depends on the headphone load and output signal. I just happened to hit a minimum at the simulation conditions shown. Typically it's about 0.2 % at -20 dB output.
Attached Images
File Type: png jmo3-ltpice.png (64.9 KB, 1519 views)
File Type: png pcb-jmo30f-brd.png (54.9 KB, 1481 views)
File Type: png pcb-jmo30f-sch.png (17.7 KB, 1496 views)
Attached Files
File Type: asc J-Mo3-5484.asc (2.5 KB, 44 views)
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Last edited by rjm; 30th July 2017 at 12:06 AM.
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Old 30th July 2017, 02:20 PM   #2
BuckarooBanzai is offline BuckarooBanzai
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Dang! Just when I thought I was done building amps for the week
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Old 31st July 2017, 01:48 AM   #3
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Richard Murdey
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JFET-MOSFET headphone amplifier J-Mo 3
Apparently the 2N5484 is obsolete and no longer stocked at Mouser, so I'm switching to the trusty J113.

It changes little. All we have to do is re-adjust R2.

The way this all works is as follows: The higher the supply voltage, the more gain you can squeeze out of the circuit. The lower the pinch off voltage (-V_gs0) of the JFET, the more gain you can squeeze out of the circuit.

I'm working on parts lists for L,M, and H variants for low, medium, and high impedance headphones respectively.

The JFET you might use could have a slightly different operating point than the simulated result. That's where the TEST pad comes in. This measures the voltage on the MOSFET source, and if this is too low or too high then R2 can be raised or lowered respectively to bring the circuit back into balance. Most of the time the voltage will naturally be in the right range already, but it will catch any outliers.

Yes, you should roughly match the JFETs between channels. The tolerance does not have to be especially tight however.

Images attached for those who want to make a set of boards by transfer-printing method.

Gerber files attached if you prefer to let pcbway do the work for you. This is the single-sided version with no thermals.
Attached Images
File Type: png pcb-jmo30g-ss-mask.png (32.2 KB, 1404 views)
File Type: png pcb-jmo30g-ss-silk.png (40.7 KB, 1238 views)
File Type: png j-mo3L-ltspice.png (148.8 KB, 302 views)
Attached Files
File Type: asc J-Mo3-113-L.asc (2.7 KB, 23 views)
File Type: zip J-Mo30g-ss-gerber.zip (40.5 KB, 44 views)
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Last edited by rjm; 31st July 2017 at 01:55 AM.
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Old 31st July 2017, 02:21 AM   #4
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Richard Murdey
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JFET-MOSFET headphone amplifier J-Mo 3
BOM uploaded
Attached Images
File Type: png pcb-jmo30g-ss-sch.png (8.4 KB, 441 views)
Attached Files
File Type: zip pcb-jmo-30g-ss-bom.zip (107.8 KB, 38 views)
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Old 31st July 2017, 06:50 AM   #5
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Richard Murdey
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JFET-MOSFET headphone amplifier J-Mo 3
Quote:
Originally Posted by rjm View Post
BOM uploaded
fixed a few small errors, and revised the optional M and H configurations as shown below in the LTSpice sims.

"L" 5 dB gain, for 16-32 ohm headphones
"M" 12 dB gain, for 60-120 ohm headphones
"H" 18 dB gain, for 150-300 ohm headphones
Attached Images
File Type: png j-mo3 ltspice allversions.png (87.7 KB, 449 views)
Attached Files
File Type: asc J-Mo3-113-H.asc (2.7 KB, 33 views)
File Type: asc J-Mo3-113-M.asc (2.7 KB, 20 views)
File Type: asc J-Mo3-113-L.asc (2.7 KB, 21 views)
File Type: zip pcb-jmo-30g1-ss-bom.zip (107.9 KB, 34 views)
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Old 31st July 2017, 08:17 AM   #6
cyc1994622 is offline cyc1994622
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It is very nice to see the new design of j-mo. I am just wondering what is the difference between the j-mo 3 to its older brothers and sisters in terms of performance to >120 ohm headphone and will the power efficiency be a bit better? Would like to see one of the j mo sitting on my desk as the first attempt of a discrete headphone amp.
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Old 31st July 2017, 09:08 AM   #7
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JFET-MOSFET headphone amplifier J-Mo 3
The original J-Mo was JFET buffer + MOSFET buffer, it had no voltage gain which was, well, less than ideal.

The J-Mo II added voltage gain using a compound JFET-bipolar input stage. It didn't work all that well at gains above 10 dB and because the circuit values were inter-dependent in a nonintuitive and complex way it was a pain to reconfigure.

The J-Mo 3 drops the compound transistor for a JFET voltage amplifier. The circuit loses some PSRR compared to the Mk. II, but it is much simpler both conceptually and operationally. It works fine with different headphones, but unlike more complex and full featured headphone amplifiers a single configuration won't be ideal with all headphones, it needs to be built in a configuration optimized for the range of headphone impedance its going to be used with. The upside is it is really tiny, dirt cheap, and as pretty near idiot proof as they come.

The output stage is unchanged since the beginning, but the power efficiency will vary on a case-by-case basis depending on the voltage, bias current, and load impedance. As a class-A output stage with resistive current source it is not going to win any contests on that front, no matter what, but the latest version does at least make an effort to optimize for different loading.

****

BOM 30g2 uploaded, tweaked "M" configuration values slightly.
Attached Files
File Type: zip pcb-jmo-30g2-ss-bom.zip (108.4 KB, 36 views)
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Last edited by rjm; 31st July 2017 at 09:23 AM.
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Old 31st July 2017, 09:55 AM   #8
cyc1994622 is offline cyc1994622
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so can I understand this as a fun, educational, simple project but certainly not the ultimate solution to whatever the problem is? I am sold already.
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Old 31st July 2017, 09:59 AM   #9
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JFET-MOSFET headphone amplifier J-Mo 3
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Originally Posted by cyc1994622 View Post
but certainly not the ultimate solution to whatever the problem is
Exactly this!
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Old 2nd August 2017, 09:04 AM   #10
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Richard Murdey
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JFET-MOSFET headphone amplifier J-Mo 3
Regarding the power supply I have two suggestions, one easy, one less easy.

First the easy one: use a three-terminal fixed voltage regulator. It's enough to get the job done. (values in brackets refer to L/M/H versions of the J-Mo 3 respectively)

transformer with (9 / 12 or 2x6 / 18 or 2x9) VAC secondary winding, 25 VA or more
---> bridge rectifier
------>1000 uF
---------> 1-2 ohms
------------> 1000 uF
---------------> LM7809/7812/7818 with 10 W/C heatsink (will dissipate about 1 W)
------------------> 1000 uF
output is 9, 12 or 18 V DC.

maybe string an indicator LED off the output, use 1k~2k2 dropping resistor

if not an LED, 2k2 between output and COM as a bleeder resistor to drain off excess charge when the power is cut.

Yes, you can upgrade the LM780x ICs with LM317 or whatever you feel comfortable with.

The more complicated solution in next post.
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