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xrk971 Desktop Class A (DCA) Headphone Amp
xrk971 Desktop Class A (DCA) Headphone Amp
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Old 14th May 2018, 01:11 PM   #1
xrk971 is offline xrk971  United States
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xrk971 Desktop Class A (DCA) Headphone Amp
Default xrk971 Desktop Class A (DCA) Headphone Amp

Announcements & Updates: I have been getting a lot of reports of builders experiencing sparks with connecting the power supply or melting the power switch at first turn on. Please make sure your MOSFETs have an insulator pad and insulator washer/bushing around the screw head. Check for no continuity between the tab(pin 2) of MOSFET and GND of board before startup. Also make sure polarity of your power supply input is correct. Positive goes to pin next to JST power switch jack.


This is a spin-off from the PCA (Pocket Class A) amp. It’s the same topology but I wanted a separate thread to discuss the technical aspects which are different. This amp is a joint effort between myself, BabyDontHertzMe (BDHM), and JPS64 whom both did a fantastic layout for the PCB. The DCA is designed for through hole components with lots of room for capacitor rolling options, a built in capacitance multiplier and CRCRC power supply section. There are 3 ground planes (PSU,Left, Right) that come together at a star topology. The board has almost identical high current traces on both sides that are joined with thousands of vias for a true low impedance path. Nominal design is for 20v Vcc, 125mA bias current and can be pushed as high as 25v Vcc for driving low impedance cans. Typical performance is 0.015%THD all H2 and a little H3 for 1vpp into 42ohm load. Can drive 42ohms as high as 544mW without clipping. Sound is powerful, and being zero global feedback, has an impressively wide and accurate soundstage. The signature is engaging and natural with no hint of fatigue. Tested with DT880-250, OB-1’s, and HE-400i. All sound fantastic and can get louder than you can handle comfortably without strain. In summary, it’s an easy to build amp and and designed for the DIYer who likes to tweak. The nice low noise PSU built in simplifies the power supply logistics.

Schematic of Production Board:
Click the image to open in full size.

Actual PCB:
Click the image to open in full size.

Stuffing Guide:
Click the image to open in full size.

Please note that polarity of 4 jacks on front from left to right is +ve -ve / -ve +ve / +ve -ve / -ve +ve

Due to mirror symmetry.

Built up Amp:
Click the image to open in full size.

Rear View
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Measurements:
20.2v Vcc and 125mA bias
Click the image to open in full size.

21.3v Vcc and 0.0066% THD
Click the image to open in full size.

Power Measurement Test into 42ohm load:
Click the image to open in full size.

Corresponding O-scope shot:
Click the image to open in full size.

I was able to get 10.5vpp without clipping for 544mW into 42ohms.
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Last edited by xrk971; 26th August 2018 at 10:22 PM.
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Old 14th May 2018, 02:38 PM   #2
pcgab is offline pcgab  United States
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xrk971 Desktop Class A (DCA) Headphone Amp
X,
I'm putting parts orders together now and seems most suppliers are out of my favorite coupling cap (Clarity CSA 10uf/250v). Any reason a 12uf would be an issue there? I can't imagine it would, but thought I should ask.

Cheers,
Gable
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Old 14th May 2018, 02:42 PM   #3
xrk971 is offline xrk971  United States
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Hi Pcgab,
Actually, anything above 4.7uF is enough for practical bass extension needed. The larger caps have a bit more EMI pickup but only noticed when my fingers touch the body. 12uF is fine as long as room on board permits. I have 6.8uF purple Clarity Cap on my Aksa Lender HPA and find that sounds great. That’s a good price too.

Truth be told, the $0.35 10uF 35v Silmic II sounds darn good.

X
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Old 14th May 2018, 02:49 PM   #4
pcgab is offline pcgab  United States
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xrk971 Desktop Class A (DCA) Headphone Amp
Quote:
Originally Posted by xrk971 View Post
Hi Pcgab,
Actually, anything above 4.7uF is enough for practical bass extension needed. The larger caps have a bit more EMI pickup but only noticed when my fingers touch the body. 12uF is fine as long as room on board permits. I have 6.8uF purple Clarity Cap on my Aksa Lender HPA and find that sounds great. Thatís a good price too.

Truth be told, the $0.35 10uF 35v Silmic II sounds darn good.

X
Maybe I'll try an 8.2 instead, the 12's are likely a bit big for the board.

Thanks for the tips, X!
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Old 14th May 2018, 02:53 PM   #5
xrk971 is offline xrk971  United States
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This might be useful for those who don’t have JST crimp tool:

JST XH 2.5mm 2-Pin Connector with LOCK Plug Wire Lengh 300mm x 10 Sets 726811952092 | eBay

10 sets is enough for 2 DCA’s
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Old 14th May 2018, 03:58 PM   #6
Idiosyncrasy is offline Idiosyncrasy  Europe
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Very nice board and good distortion profile too!

What was the reasoning for going with open vias (no soldermask)? Better thermal dissipation?
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Old 14th May 2018, 04:27 PM   #7
xrk971 is offline xrk971  United States
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Thanks, Idiosyncrasy. The “Swiss Cheese” traces and vias are a JPS64 signature layout technqieue. The extra vias connect two planes of essentially identical traces on both sides to increase metal and surface area for low impedance electrical paths. Side benefits are enhanced cooling and strength of the traces and pads from being pulled off due to rework. Board is 2mm thick and copper is double thick 2oz with ENIG finish. Board is very sturdy and substantially built. Fab houses do not charge any extra for extra vias.
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Last edited by xrk971; 14th May 2018 at 04:54 PM.
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Old 14th May 2018, 04:42 PM   #8
Idiosyncrasy is offline Idiosyncrasy  Europe
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Quote:
Originally Posted by xrk971 View Post
Board is 2mm thick and copper is double thick 2oz with ENIG finish. Board is very sturdy and substantially built.
Haha, thatís like all the premium options checked.
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Old 14th May 2018, 04:59 PM   #9
xrk971 is offline xrk971  United States
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Quote:
Originally Posted by Idiosyncrasy View Post
Haha, thatís like all the premium options checked.
Yes, plus the cool blue color vs usual green. Note that parts are all easily sourced and low cost (BF862 JFET and IRF610PBF MOSFET). No matching needed (unless you really are picky about balanced left right then match the JFETs to within 5% Idss). Easy to do with low cost SMT JFETs. The performance to price ratio is extraordinary.
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Old 14th May 2018, 07:09 PM   #10
badboygolf16v is offline badboygolf16v
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How does this compare to the AKSA Lender HPA and Salas DCG3?

I think we can all agree that you can never have too many Class A headphone amps!
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