J-Mo Mk II headphone amplifier

[rjm]

Note on the prototype build

I'll be using the J202 and the circuit will be changed a fair bit to make it optimized for my 300 ohm HD-600 headphones. I will also substitute the 2SK213 for the IRF510. I mean, it's available at my local marutsu there's really no excuse not to.

The J202 works at around 1.2 V bias, and for the HD-600, I need as much voltage as possible, so the input to the mosfet should be [(V++/2) + 2] = 12 V Which means a gain of about 10x. I don't need so much current, so the mosfet source resistor will be increased to 150 ohms.

[rjm]

Parts placed on perf baord, before soldering.

[Avro Arrow]

I'm interested in your prototype results and listening impressions.
Good luck!

[rjm]

Thanks! Unfortunately those little boards are only half the story: unlike my usual modus operandi the regulators and all that are missing from the main board and have to be dealt with elsewhere.

Though for first run testing I'll just use a benchtop lab supply.

I'm working on developing a Z-reg circuit, tailored to my own high impedance version. It's sufficiently low current that I can use my standard 12x2 VAC transformer and drop the 36 V rectified DC though another IRF510/2SK213 using a pair of 12V Zeners --- all parts I have on hand. Output comes to 20 V, with 0.1 mV ripple. Turn on is 1s soft start, and the mosfet gate is protected from turn-off transients by a Zener stack.

See attached. (still kinda rough) R3 is a placeholder for the real load of the JMO2 circuit itself.

This will work for the regular high current version too, but you are looking at 5W dissipation in the IRF510, so a fairly big (~5 C/W) heatsink. It would probably be more sensible to use a lower secondary voltage of 15 or 16 VAC, a 20 V Zener, and a npn transistor as the pass element.

[rjm]

Everything is starting to crystallize now. Images are the first glimpse of the "public", standard version of the circuit including a suggested power supply.

[rjm]

Simulation results:

Voltage gain: 3 (9.5 dB).

Heat: 1W source resistor, 2W MOSFET, 2W Z-reg MOSFET (4W if shared). 10 W for the full stereo circuit.

Max output power before clipping: 100 mW into 16 ohms, 100 mW into 30 ohms, 90 mW into 100 ohms, 40 mW into 300 ohms.

"Nominal" clean power output for 0.1% THD: about 10 mW for 16 and 30 ohms, falling to 1 mW for 300 ohms.

The circuit is optimized for 16-50 ohm headphones, from the standpoint of both output power and gain.

To get better performance with 300 ohm headphones, there are a number of changes that should be made, starting by replacing the 2N5486 with the 2N5485 or J202, increasing the gain to 10, increasing the MOSFET source resistance to 150 ohms, and increasing the power supply to 20-24 V. I will publish the modification later.

[Avro Arrow]

Looks like I might need to change my board layout...

[rjm]

I'll try and clear all that up tomorrow. Please wait just a bit longer.

[rjm]

Project page is now up, work in progress.

RJM Audio - J-Mo Mk. II headphone amplifier

[rjm]

Matching amplifier and power supply boards, both are 5x8 cm.