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Old 9th July 2012, 11:30 PM   #51
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Richard Murdey
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For the boards, there are several options: Screw terminals, a length of wire lead, gold pin, as well directly soldering the wire to the pad. For perf-board, I usually make a loop of wire around two adjacent pads and either solder a lead wire to the loop or, for example, attach test leads by alligator clips.
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Old 15th February 2013, 07:20 PM   #52
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Hi, I wonder if you could post the values you use for components, different to those stated on your website for use of this amp with your 300ohm 'phones? (including the power supply if any different)

I have had a look through the thread but the component names seem to be different to the final design on the site (might just be me).

Also what values would you use for 600 ohm phones?

Thanks!
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Old 14th April 2013, 11:03 AM   #53
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Sorry for taking such a long time to get back on this.

I built a prototype on perf board, to match 300 ohm headphones and my available power supply (24 VAC), in post 34. The LTSPICE file is attached.

Quote:
Q1 = J202
Q2 = BD140
Q3 = 2SK213
R1, R8 = 75k Welwyn
R2 = 475R||475R = 237.5R Vishay 1/2W
R3 = 4k75 Vishay 1/2W
R4, R6, R9 = 47R Kamaya carbon
R5 = Vishay 1k trim
R7 = 150 ohm 2 W, Vishay PR02
C1,2,3 = Nichicon KZ 100u 50V
The parts numbering are consistent with the web page, but the values are changed to fit not just my headphones, but also the different jFET and MOSFET transistors I used. The 2SK213, in particular, has very different Vgs from the IRF510.

It's a drawback of this circuit that it does not suffer modifications gladly. Change one component, and all the others have to be adjusted slightly to match. This is most easily done in LTSPICE.

The general aim when adjusting for higher impedance headphones is to increase the voltage rail, increase the gain, and decrease the bias current. The source resistor R7 should be increased to approximately the value of the headphone impedance, and the voltage rail increased until about 0.5 W is dissipated in both R7 and the MOSFET.

I should emphasize that this is an optimization, it is not necessary to change the values from the circuit shown in the web page even when using 300 ohm headphones, it just increases the available output power into the specified load.
Attached Files
File Type: asc J-Mo2 11k rjm proto.asc (3.6 KB, 19 views)
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Old 18th April 2013, 04:50 PM   #54
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simple amp but very interesting
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Old 25th April 2013, 09:27 AM   #55
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Prototype should be up and running in a day or so. I just have to finish the chassis wiring.
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File Type: jpg DSC_0378 1600.jpg (196.6 KB, 274 views)
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Old 26th April 2013, 09:13 AM   #56
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Up and running. Passed the burn in test, the bias stability test, and the playback test, but the amplifier gain has an 8 dB channel miss-match.

Investigation ongoing.
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File Type: jpg IMGP6178.jpg (489.0 KB, 254 views)
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Old 27th April 2013, 04:47 AM   #57
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This is proving to be more pernicious than usual.

Changed out jFET, pnp, and MOSFET on the channel with abnormally high gain (and unstable bias). No effect. Still getting +8 dB (2.5x), and a gain that varies wildly with the trim resistor position. Does not behave like the "good" channel, yet the DC voltages / currents are all nominally correct (but consistently Vgs is about 100 mV less). Triple checked the board layout for mistakes and cold solder joints, no joy there.

I'm coming to the inevitable conclusion that the circuit operating points are borderline, and that the amplifier section can slide out of the "control region" where the behavior is described by simulation to some other state...
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Old 27th April 2013, 11:36 PM   #58
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Fault traced to a bad solder joint, position shown in the schematic below.

Fixed! (big sigh of relief...)

I'll let them burn in (I replaced most of the silicon before I finally clued in to the problem) and trim the bias currents before reporting the final channel balance, but it looks to be very, very close.

Should be able to do trial listening tonight...
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File Type: png jmo2-amp cold solder.png (36.9 KB, 222 views)
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Old 28th April 2013, 12:25 PM   #59
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J-Mo Mk. II development: Feb 2012 - Apr 2013.

Finally, up and running properly.

With the bias currents matched, the AC gains are matched to within 0.5%, or 0.04 dB. If really want, you can trim the bias to match the AC gains even further. Channel balance is in any event a non-issue.
Attached Images
File Type: jpg IMGP6321 1280.jpg (307.3 KB, 211 views)
File Type: jpg IMGP6323 1280.jpg (331.2 KB, 193 views)
File Type: jpg IMGP6324 1280.jpg (240.3 KB, 40 views)
File Type: jpg IMGP6326 1280.jpg (270.1 KB, 37 views)
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Last edited by rjm; 28th April 2013 at 12:32 PM.
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Old 28th April 2013, 12:48 PM   #60
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Measurements.

330 ohm load, since my optimization of the circuit is for my 300 ohm headphones. The circuit gain is 23 dB (14x).

With no input, the noise baseline is a ridiculously clean -130 dB.

With 100 mV (1 kHz) into 330 ohms, the 2nd harmonic is at -90 dB, or -70 dB below the fundamental. 0.03% distortion. The large second harmonic component was expected for this single-ended output topology. And it's not even that large. 3rd harmonic and all higher harmonics are below -110 dB, the peaks shown in the plot are most likely artifacts of the signal generator.

The results are actually pretty nice...
Attached Images
File Type: png jmo2 noinput.png (23.7 KB, 74 views)
File Type: png jmo2 -20dB.png (22.9 KB, 65 views)
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