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xrk971 Desktop Class A (DCA) Headphone Amp

Founder of XSA-Labs
Joined 2012
Paid Member
Edit Aug 11, 2020: annotated BOM in MS Excel format here.

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:
679489d1525836497-xrk971-pocket-class-headamp-gb-xrk971-desktop-amp-schematic-v2-production-portrait-png


Actual PCB:
679262d1525754963-xrk971-pocket-class-headamp-gb-desktop-amp-pcb-photo-jpg


Stuffing Guide:
679339d1525781867-xrk971-pocket-class-headamp-gb-0bc0cbc4-0d1b-41b9-9c0c-04ed6985b5a3-jpeg


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:
679919d1526006465-xrk971-pocket-class-headamp-gb-607a5f68-c10e-43c5-aa9b-984bbdb911e7-jpg


Rear View
679735d1525938938-xrk971-pocket-class-headamp-gb-build-desktop-hpa-03-jpg


Measurements:
20.2v Vcc and 125mA bias
679967d1526039467-xrk971-pocket-class-headamp-gb-dca-20-2v-125ma-1vpp-270ohms-fft-4-png


21.3v Vcc and 0.0066% THD
679969d1526039508-xrk971-pocket-class-headamp-gb-dca-21-37v-135ma-1vpp-270ohms-fft-1-png


Power Measurement Test into 42ohm load:
680143d1526117185-xrk971-pocket-class-headamp-gb-build-desktop-hpa-max-power-dmm-05-jpg


Corresponding O-scope shot:
680144d1526117185-xrk971-pocket-class-headamp-gb-build-desktop-hpa-max-power-oscope-04-jpg


I was able to get 10.5vpp without clipping for 544mW into 42ohms.

BOM:
https://www.diyaudio.com/community/...class-a-dca-headphone-amp.322638/post-7509042

PartValueDevicePackageDescription
C10110 µ0CPOL-EUE3.5-8E3,5-8POLARIZEDElectrolytic 3.5mm pitch x 8mm dia
C101AC22/10C22.5B10CAPACITORFilm 22.5mm pitch x 10mm width
C101BC27/17C27.5B17CAPACITORFilm 27.5mm pitch x 17mm wide
C101C0 µ1MKC1860-0U1MKC1860-0U1Wima part number
C101D10 µ0MCAP250V10UMCAP250V10UFidelityMundorf Mcap part number
C10210 µ0CPOL-EUE3.5-8E3,5-8POLARIZEDElectrolytic 3.5mm pitch x 8mm dia
C102AC22/10C22.5B10CAPACITORFilm 22.5mm pitch x 10mm wide
C102BC27/17C27.5B17CAPACITORFilm 27.5mm pitch x 17mm wide
C102C0 µ1MKC1860-0U1MKC1860-0U1Wima part number
C102D10 µ0MCAP250V10UMCAP250V10UFidelityMundorf Mcap part number
C1111 µ0C5/5C5B5CAPACITORFilm 5mm pitch x 5mm wide
C111AC10/6C10B6CAPACITORFilm 10mm pitch x 6mm wide
C111BC15/8C15B8CAPACITORFilm 15mm pitch x 8mm wide
C1121 µ0C5/5C5B5CAPACITORFilm 5mm pitch x 5mm wide
C112AC10/6C10B6CAPACITORFilm 10mm pitch x 6mm wide
C112BC15/8C15B8CAPACITORFilm 15mm pitch x 8mm wide
C1132200uFCPOL-EUE7.5-18E7,5-18POLARIZEDElectrolytic 7.5mm pitch x 18mm dia
C113ACPOL-EUE5-10.5E5-10,5POLARIZEDCAPACITOR,Electrolytic 5mm pitch up to 18mm dia
C1142200uFCPOL-EUE7.5-18E7,5-18POLARIZEDElectrolytic 7.5mm pitch x 18mm dia
C114ACPOL-EUE5-10.5E5-10,5POLARIZEDCAPACITOR,Electrolytic 5mm pitch up to 18mm dia
C131220 µCPOL-EUE5-13E5-13POLARIZEDElectrolytic 5mm pitch up to 13mm dia
C1331 µ0C5/5C5B5CAPACITORFil 5mm pitch x 5mm width
C1342200 µCPOL-EUE5-13E5-13POLARIZEDElectrolytic 5mm pitch up to 13mm dia
C1352200 µCPOL-EUE5-13E5-13POLARIZEDElectrolytic 5mm pitch up to 13mm dia
C1412200 µCPOL-EUE5-13E5-13POLARIZEDElectrolytic 5mm pitch up to 13mm dia
C1422200 µCPOL-EUE5-13E5-13POLARIZEDElectrolytic 5mm pitch up to 13mm dia
C1431 µ0C5/5C5B5CAPACITORFilm 5mm pitch x 5mm wide
C1441 µ0C5/5C5B5CAPACITORFilm 5mm pitch x 5mm wide
R101RK09L12BRK09L12B
R102100kCMF-55CMF-55Dale 1/2W metal thin film 1%
R103100kCMF-55CMF-55Dale 1/2W metal thin film 1%
R10422k0CMF-55CMF-55Dale 1/2W metal thin film 1%
R10522k0CMF-55CMF-55Dale 1/2W metal thin film 1%
R1061k1CMF-55CMF-55Dale 1/2W metal thin film 1%
R10768R0CMF-55CMF-55Dale 1/2W metal thin film 1%
R108250RCMF-55CMF-55Dale 1/2W metal thin film 1%
R1091k1CMF-55CMF-55Dale 1/2W metal thin film 1%
R11168R0CMF-55CMF-55Dale 1/2W metal thin film 1%
R112250RCMF-55CMF-55Dale 1/2W metal thin film 1%
R11347R0CMF-55CMF-55Dale 1/2W metal thin film 1%
R11447R0CMF-55CMF-55Dale 1/2W metal thin film 1%
R115270RCMF60CMF60Dale 1W metal thin film 1%
R116270RCMF60CMF60Dale 1W metal thin film 1%
R117270RCMF60CMF60Dale 1W metal thin film 1%
R118270RCMF60CMF60Dale 1W metal thin film 1%
R119270RCMF60CMF60Dale 1W metal thin film 1%
R121270RCMF60CMF60Dale 1W metal thin film 1%
R122270RCMF60CMF60Dale 1W metal thin film 1%
R123270RCMF60CMF60Dale 1W metal thin film 1%
R124270RCMF-55CMF-55Dale 1/2W metal thin film 1%
R125270RCMF-55CMF-55Dale 1/2W metal thin film 1%
R13110k0R-EU_0207/100207/10RESISTOR,2mm dia x 7mm long body, 10mm pitch axial
R132220RR-EU_0207/100207/10RESISTOR,2mm dia x 7mm long body, 10mm pitch axial
R1331R5R-EU_0207/100207/10RESISTOR,2mm dia x 7mm long body, 10mm pitch axial
R1340R47R-EU_0309/120309/12RESISTOR,3mm dia x 9mm long body, 12mm pitch axial
R1350R47R-EU_0309/120309/12RESISTOR,3mm dia x 9mm long body, 12mm pitch axial
V101BF862SST201SOT23TEMICSOT23 SMT package
V102BF862SST201SOT23TEMICSOT23 SMT package
V103JFET2SK170TO92-DGSFieldTO92 package
V104JFET2SK170TO92-DGSFieldTO92 package
V111IRF610IRF512TO220BVN-CHANNELTO220 package
V112IRF610IRF512TO220BVN-CHANNELTO220 package
V1311N40041N4004DO41-10DIODEDO41 package axial 1A diode
V132IRF610IRF512TO220BVN-CHANNELTO220 package
X10122-27-2021-0222-27-2021-026410-02CONNECTORMolex KK 2-pin 2.5mm pitch connector
X10222-27-2021-0222-27-2021-026410-02CONNECTORMolex KK 2-pin 2.5mm pitch connector
X11122-27-2021-0222-27-2021-026410-02CONNECTORMolex KK 2-pin 2.5mm pitch connector
X11222-27-2021-0222-27-2021-026410-02CONNECTORMolex KK 2-pin 2.5mm pitch connector
X13122-27-2021-0222-27-2021-026410-02CONNECTORMolex KK 2-pin 2.5mm pitch connector
X132AK500/2AK500/2CONNECTORunknown2-pin 10mm pitch screw terminal
X1011SK104SK104SK104HEATSINKFischer Heatsink SK104
X1012SK104SK104SK104HEATSINKFischer Heatsink SK104
X1031SK104SK104SK104HEATSINKFischer Heatsink SK104
 
Last edited:
Founder of XSA-Labs
Joined 2012
Paid Member
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
 
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

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

Thanks for the tips, X!
 
Founder of XSA-Labs
Joined 2012
Paid Member
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.
 
Last edited:
Founder of XSA-Labs
Joined 2012
Paid Member
Haha, that’s 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.
 
Founder of XSA-Labs
Joined 2012
Paid Member
The Aksa Lender HPA has lower THD but similar sonic profile of SE Class A. The Aksa Lender also has more gain, if that’s what you like. But I would say that a no global feedback SE Class A like the DCA, will have better soundstage and imaging. The DCA can be driven to very high bias (tested to 150mA which means it can drive 300mA peak current) as it has provision for larger heatsink. So can drive lower impedance cans with more power. I think you will find that both sound similar in that they are very dynamic and natural sounding with smooth response and low low noise. They are cheap enough to enjoy many times. Don’t need to settle on one. DCG3 is a great amp but I have not listened to it side by side so can’t compare.
 
Founder of XSA-Labs
Joined 2012
Paid Member
Here is the BOM:

xrk971 Desktop Class A HPA v2 May 9, 2018
BOM

Part Value Device Package Description
C101 10µ0 CPOL-EUE3.5-8 E3,5-8 POLARIZED CAPACITOR, European symbol
C101A C22/10 C22.5B10 CAPACITOR
C101B C27/17 C27.5B17 CAPACITOR
C101C 0µ1 MKC1860-0U1 MKC1860-0U1
C101D 10µ0 MCAP250V10U MCAP250V10U Fidelity Component MCap
C102 10µ0 CPOL-EUE3.5-8 E3,5-8 POLARIZED CAPACITOR, European symbol
C102A C22/10 C22.5B10 CAPACITOR
C102B C27/17 C27.5B17 CAPACITOR
C102C 0µ1 MKC1860-0U1 MKC1860-0U1
C102D 10µ0 MCAP250V10U MCAP250V10U Fidelity Component MCap
C111 1µ0 C5/5 C5B5 CAPACITOR
C111A C10/6 C10B6 CAPACITOR
C111B C15/8 C15B8 CAPACITOR
C112 1µ0 C5/5 C5B5 CAPACITOR
C112A C10/6 C10B6 CAPACITOR
C112B C15/8 C15B8 CAPACITOR
C113 2200uF CPOL-EUE7.5-18 E7,5-18 POLARIZED CAPACITOR, European symbol
C113A CPOL-EUE5-10.5 E5-10,5 POLARIZED CAPACITOR, European symbol
C114 2200uF CPOL-EUE7.5-18 E7,5-18 POLARIZED CAPACITOR, European symbol
C114A CPOL-EUE5-10.5 E5-10,5 POLARIZED CAPACITOR, European symbol
C131 220µ CPOL-EUE5-13 E5-13 POLARIZED CAPACITOR, European symbol
C133 1µ0 C5/5 C5B5 CAPACITOR
C134 2200µ CPOL-EUE5-13 E5-13 POLARIZED CAPACITOR, European symbol
C135 2200µ CPOL-EUE5-13 E5-13 POLARIZED CAPACITOR, European symbol
C141 2200µ CPOL-EUE5-13 E5-13 POLARIZED CAPACITOR, European symbol
C142 2200µ CPOL-EUE5-13 E5-13 POLARIZED CAPACITOR, European symbol
C143 1µ0 C5/5 C5B5 CAPACITOR
C144 1µ0 C5/5 C5B5 CAPACITOR
R101 RK09L12B RK09L12B
R102 100k CMF-55 CMF-55
R103 100k CMF-55 CMF-55
R104 22k0 CMF-55 CMF-55
R105 22k0 CMF-55 CMF-55
R106 1k1 CMF-55 CMF-55
R107 68R0 CMF-55 CMF-55
R108 250R CMF-55 CMF-55
R109 1k1 CMF-55 CMF-55
R111 68R0 CMF-55 CMF-55
R112 250R CMF-55 CMF-55
R113 47R0 CMF-55 CMF-55
R114 47R0 CMF-55 CMF-55
R115 270R CMF60 CMF60 (for R115 to R123 recommended to use 1w parts here and elevate above board 5mm or more for cooling)
R116 270R CMF60 CMF60
R117 270R CMF60 CMF60
R118 270R CMF60 CMF60
R119 270R CMF60 CMF60
R121 270R CMF60 CMF60
R122 270R CMF60 CMF60
R123 270R CMF60 CMF60
R124 270R CMF-55 CMF-55 (can be up to 1k if using 600ohm cans)
R125 270R CMF-55 CMF-55 (can be up to 1k if using 600ohm cans)
R131 10k0 R-EU_0207/10 0207/10 RESISTOR, European symbol
R132 220R R-EU_0207/10 0207/10 RESISTOR, European symbol
R133 1R5 R-EU_0207/10 0207/10 RESISTOR, European symbol
R134 0R47 R-EU_0309/12 0309/12 RESISTOR, European symbol
R135 0R47 R-EU_0309/12 0309/12 RESISTOR, European symbol
V101 BF862 SST201 SOT23 TEMIC / Siliconix N-Channel JFET
V102 BF862 SST201 SOT23 TEMIC / Siliconix N-Channel JFET
V103 JFET 2SK170 TO92-DGS Field Effect Transistor Silicon N Channel Junction Type 2SK170 Audio Grade (optional instead of BF862)
V104 JFET 2SK170 TO92-DGS Field Effect Transistor Silicon N Channel Junction Type 2SK170 Audio Grade (optional instead of BF862)
V111 IRF610 IRF512 TO220BV N-CHANNEL MOS FET
V112 IRF610 IRF512 TO220BV N-CHANNEL MOS FET
V131 1N4004 1N4004 DO41-10 DIODE
V132 IRF610 IRF512 TO220BV N-CHANNEL MOS FET
X101 22-27-2021-02 22-27-2021-02 6410-02 CONNECTOR unknown unknown
X102 22-27-2021-02 22-27-2021-02 6410-02 CONNECTOR unknown unknown
X111 22-27-2021-02 22-27-2021-02 6410-02 CONNECTOR unknown unknown
X112 22-27-2021-02 22-27-2021-02 6410-02 CONNECTOR unknown unknown
X131 22-27-2021-02 22-27-2021-02 6410-02 CONNECTOR unknown unknown
X132 AK500/2 AK500/2 CONNECTOR unknown unknown
X1011 SK104 SK104 SK104 HEATSINK manufacturer Fischer/distributor Buerklin
X1012 SK104 SK104 SK104 HEATSINK manufacturer Fischer/distributor Buerklin
X1031 SK104 SK104 SK104 HEATSINK manufacturer Fischer/distributor Buerklin
 
Founder of XSA-Labs
Joined 2012
Paid Member