I have used a simple DN2540 depletion mode MOSFET and sense resistor as a CCS on the Aksa Lender HPA and it works very well in the role of CCS. But then it needs to go on top at the Vcc rail and the active output needs to be a P channel and you get SEPP action. But then might as well get the Aksa Lender HPA. The beauty of the DCA is the simplicity of two transistors.
There is more surface area to cool with 4 than a single resistor body so it stays cooler. It is also cheaper to use four lower wattage resistors. These resistors also determine the harmonic distortion to a certain extent in that they heat up with the AC content in the signal and having lower thermal coefficient on these local “feedback” resistors reduces distortion from thermal heating effects.
You can certainly try say a single Panasonic 68ohm 3w ERX metal thin film, or your favorite brand: Dale, Mills, Caddock, Takman, etc. Four will always be better than one.
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As far as I'm aware this one's only available in the SOT23 package (see the datasheet: https://www.nxp.com/docs/en/data-sheet/BF862.pdf). It's large enough to solder by hand.
Edit: oops, didn't see the thread continued on the next page; ignore this one.
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Anyone else build it yet??
1) What chassis are you guys using?
2) I'm looking to go the DC-DC converter route (with a 12V 1.8A smps). Which ones everyone using?
My board should be here in a few days
High quality CNC aluminum cases are available from China at very reasonable cost of circa $30 to $40. You will pay $40 in shipping but still less than anything comparable locally in the US. Search on Aliexpress or eBay for aluminum CNC headphone amplifier case.
For power, my experience has shown that a good Class 2 linear transformer wall wart such as this one is the quietest thing to use to drive the DC step up. It doesn’t add any switching noise.
GCi technoLogies AM-12800 12VDC 800mA Charger / Power Supply Amazon.com : GCi technoLogies AM-12800 12VDC 800mA Charger / Power Supply : Everything Else
Dang, I just hit buy on my cart a couple days ago. 
Some hand adjustments of resistor values is giving more balanced output. The R106/R109 1k1 resistor was producing early negative clipping. Changing to 910R improved things a lot.
Adjustments of R107/R111 to 56R seems to give a good balance of overall low THD and good proportions of H2 vs H3.
Vcc is set at 20.07v at the MOSFET drain.
For 1,0vpp into 270ohms, this gives 0.023% THD and H3 is -18dB below H2.
Sorry about that but it’s easy to fix. Just parallel about a 5k1 or even 4k7 with your 1k1 to drop the value combined value towards 910ohms. It doesn’t t have to be exact as variations in your JFETs vs mine may require adjustment. To find if it works, measure output Source pin voltage at MOSFET. Adjust until voltage is about 1/2Vcc.
Another thing is to buy a complete set of all values of 1/4w metal thin film resistors from China. They are cheap quality, but metal thin film resistor values will be correct and distortion performance is only maybe 5% worse than premium Dales that cost 50x more. Seriously, $7 gets you 20 pieces of every value from 1R to 10M. Handy to have on hand and you can upgrade later.
Another thing is to buy a complete set of all values of 1/4w metal thin film resistors from China. They are cheap quality, but metal thin film resistor values will be correct and distortion performance is only maybe 5% worse than premium Dales that cost 50x more. Seriously, $7 gets you 20 pieces of every value from 1R to 10M. Handy to have on hand and you can upgrade later.
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If there is enough interest in full-on DCA "Kits" which include everything you need except for RCA jack, headphone jack, power switch, DC-DC step up, 12vdc wall wart power adapter, and amplifier case... I can offer them in my shop. Just chime in and express of interest for such a kit and I will see if I can get it listed.
Included with the kit will be Nichicon 2200uF 25v electrolytic caps for the CRC, 6800uF Nichicon 25v caps for the power rails, and Panasonic FC 3300uF 35v output couplers, Vishay Dale 1% metal film resistors, heatsinks, Alps 10k RK09 pot, Vgs matched Vishay IRF610, matched set BF862, 5 sets of 2-pin JST connectors, 2pin screw terminal connector, 10uF Silmic II input cap, 2.2uF Wima output cap, 0.1uF Wima bypass caps, and hardware and silicone spacer for MOSFETs.
Included with the kit will be Nichicon 2200uF 25v electrolytic caps for the CRC, 6800uF Nichicon 25v caps for the power rails, and Panasonic FC 3300uF 35v output couplers, Vishay Dale 1% metal film resistors, heatsinks, Alps 10k RK09 pot, Vgs matched Vishay IRF610, matched set BF862, 5 sets of 2-pin JST connectors, 2pin screw terminal connector, 10uF Silmic II input cap, 2.2uF Wima output cap, 0.1uF Wima bypass caps, and hardware and silicone spacer for MOSFETs.
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Nice component selection! I was planning to do my own shopping but the kit is sounding awesome.
X, have you tried the FC caps already for headphone coupling? I have only heard good things about them, and it was one of the few electrolytics I did not audition in the PCA.
What's the word on using off-board MoFo-style chokes? Worth it for playing with difficult cans? I think it would be fun to try it, if i had an idea of what to get.
X, have you tried the FC caps already for headphone coupling? I have only heard good things about them, and it was one of the few electrolytics I did not audition in the PCA.
What's the word on using off-board MoFo-style chokes? Worth it for playing with difficult cans? I think it would be fun to try it, if i had an idea of what to get.
You can use the Hammond 193T at $39ea or you can get some microwave oven transformers like I did (search eBay for microwave oven transformers). The MOTs are actually better as they can pass 10amps steady-state and give great bass authority. I got some little toroidal inductors that are 20mH and may try it to see if I can get anymore swing in the output.
193T Hammond Manufacturing | Mouser
I am using FC caps now. They sound quite good. Actually that amp is now on its way to JPS64 as a gift for doing the layout. The other one is for BDHM forthe layout work as well.
Hi XRK, also definitely interested in a full kit. Just sent PM to BDHM re getting a board, but may go for complete kit. Already have a 22V HQ linear PSU handy so can just use that for the amp.
Regards, Kevin
Parts arrived today, totally forgot about the power socket though, and not ready to sacrifice my laptop power supply.waiting is half the fun right? Right??
Also after the Aliexpress resistors comment I remembered a stash of 1/8W resistors I got years back. Should be fine for fiddling around to nail the correct values
Also after the Aliexpress resistors comment I remembered a stash of 1/8W resistors I got years back. Should be fine for fiddling around to nail the correct values
I have tons of those 5.5mm barrel jack connectors - must have been one of those Aliexpress bag of 50 for $3 orders. I would have thrown one in for you if I knew.
I am surprised no one is finishing their builds yet. This amp is a quick and easy build. Pretty much guaranteed to work the first time.
I am surprised no one is finishing their builds yet. This amp is a quick and easy build. Pretty much guaranteed to work the first time.