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-   -   xrk971 Pocket Class A Headamp GB (https://www.diyaudio.com/forums/group-buys/302859-xrk971-pocket-class-headamp-gb.html)

xrk971 27th January 2017 12:48 PM

xrk971 Pocket Class A Headamp GB
 
2 Attachment(s)
Edit (July 24, 2017): schematic with DC setpoints for actual standard build amp using suggested values in recent BOM. Note that adjusting R4 (as high as 51R for higher bias) gives room for tuning desired bias current vs battery life. Voltages here are useful as reference for debugging a circuit that is not working:
https://www.diyaudio.com/forums/atta...8-voltages.png

Edit (June 2, 2017): A review of this amp on Headfonics.com

https://www.diyaudio.com/forums/atta...ase-dtr880.jpg

This project started over in this thread:

BF862 based SE Class A Headamp without the HEAT

but I thought it would be good for it to have a dedicated GB thread here. It's a superb sounding single ended (SE) Class A topology that has a predominantly a second harmonic distortion profile with just a tad bit third harmonic and not much else. The noise floor is about -125dB and dynamic range is about 96dB with THD measured to be about 0.029% at 1kHz and driving a 270ohm load at 700mV RMS. Note that most of that HD is H2 so the sound is very much like a nicest SE tube amps. Frequency response with a 47uF output coupling cap was measured at -1.5dB at 20Hz and 45kHz (probably limit of measurement as ADC sampling was 96kHz). The sims show that it can go beyond several MHz on the high end. Best part is that it fits in an Altoids tin and looks like a CMOY headamp - but it's no op amp. It relies on just two FETs. The ever popular BF862 JFET for the input and a ZVN4306 MOSFET for the output stage. The bias runs between 50mA and 60mA which allows it to last about 4 to 5 hours on a pair of 600mAhr Li-ion rechargeable 9v batteries. The amp dissipates about 2w so never gets hot but makes a nice hand warmer for your pocket in the winter.

This is the basic circuit - very simple and it's what gives it a natural and very musical sound that is engaging, lively, never sterile.

https://www.diyaudio.com/forums/atta...5306-ha-v1.png

Here is the predicted HD profile:
https://www.diyaudio.com/forums/atta...1-fft-1khz.png

Here is what the hand made prototype looks like:
https://www.diyaudio.com/forums/atta...new-oscons.jpg

You can listen to this amp as recorded with UCA202 connected to a 270ohm load resistor. Change the file name extension from .asc to .mp3 to listen.

http://www.diyaudio.com/forums/attac...class-clip.asc


This is the layout for the actual board being sold for this GB:
https://www.diyaudio.com/forums/atta...-gb-layout.png

This is the associated schematic that goes with this layout:
https://www.diyaudio.com/forums/atta...-schematic.png

Many thanks to BabyDontHertzMe (BDHM) for making the layout and to Agdr for providing the initial Altoids footprint Eagle file.

Here is the measured HD using Focusrite Solo 2nd gen at 96kHz and 24bit:

https://www.diyaudio.com/forums/atta...b-img_6145.png

The HD measured using a UCA202 at 16bits and 48kHz looks like this:

https://www.diyaudio.com/forums/atta...ft-via-rca.png

Edit Feb. 18, 2017 - Here is the latest measured frequency response and THD from the GB PCB:
https://i.ebayimg.com/images/g/lg4AA...Fe/s-l1600.jpg

Edit Feb. 18, 2017 - Here is the latest measured FFT from the GB PCB measured for 1kHz excitation at 700mV into a 270ohm load:
https://i.ebayimg.com/images/g/PMAAA...Fn/s-l1600.jpg

This headamp can drive headphones with impedances as low as 30ohms no problem. The GB has already started in the other thread and is currently at 6 people and 11 boards. We have enough critical mass but I want to keep it open a bit longer to make sure everyone who wants one gets to place an order.

Please send me PM for pricing of bare PCB's or matched FETs.

Pre-made amps and pre-soldered SMT-only boards and full kits available here:

XRKAudio designs for superb natural sound. by XRKAudio on Etsy

Please use the quote feature to add your name and desired number of boards to grow the list:

Quote:

Pocket Class A GB Order List:

(As of 02:00 EST, Jan 29, 2017)
xrk971 - 2 boards
pcgab - 2 boards
zman01 - 2 boards
stellarelephant - 2 boards
samoloko - 1 pcb
lui04 - 2 boards
Morde - 2 board
cyteen - 2 boards
funch - 2 boards
syyma - 2 boards
av-trouvaille - 2 boards
Mituisho - 2 boards
bogde - 2 boards
faltinsen - 2 boards
vanofmonks - 1 board
M_Balou - 2 boards
dwpeterson - 1 board
agdr - 1 board

(add your name and number of boards here. Please use quote feature to grow list.)
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Latest BOM is post 15:

xrk971 Pocket Class A Headamp GB - Page 2 - diyAudio

BOM errata for C9 and C11:

http://www.diyaudio.com/forums/group...ml#post4971339

Edit - Jan 9, 2017: The GB PCB works! Sounds fantastic.

https://www.diyaudio.com/forums/atta...mp-test-04.jpg

https://www.diyaudio.com/forums/atta...mp-test-01.jpg

Final as-built schematic:

https://www.diyaudio.com/forums/atta...hematic-v8.png

Final as-built BOM:

Quote:

All resistors 0805 package, metal thick film, 1% on specified parts, else 5%. Yageo 1% 0805 SMT from Digikey are $0.77 for 100
R1 100k (5%)
R2 47k (1%)
R3 1k (1%)
R4 33R (1%) ** try experimenting with 39R and 47R to give higher bias current but more H3 and less H2 but overall lower THD **
R5 270R (1%)
R6 47R (5%)
R7_1 470R (1%)
R7_2 470R (1%)
R7_3 470R (1%)
R7_4 470R (1%)
RL 270R (5%) User select (optional, use for higher current draw on amp)
RLED 15k (5%) Current limit for LED, 15k or value according to supply voltage

C1 Input cap 5mm pitch up to 5.5mm wide (Wima 2.2uF MKS2B042201F00JSSD)
C2_1 Output cap 3.5mm pitch, 8 dia (390uF 20v Panasonic OSCON 20SEPF390M) or Nichicon AK 470uF if you can fit it Edit: I highly recommend a 1000uF 16v OSCON here for low impedance phones.
C2_2 Output cap 5mm pitch, up to 9mm wide (Wima 1uF MKS2C041001F00KSSD) Edit: I highly recommend a second 1000uF 16v OSCON here, if you have 30ohm or lower phones, and place Wima on SMT side.
C3 Rail cap 100uF 3.5mm pitch, diameter whatever you can fit in the tin (100uF 20v Panasonic OSCON 20SEP100MX) - or Nichicon 2200uF 16v lay on its side
C4 Rail cap 100nF 1206 package (50v X7R)
C5 Additional 100nF rail cap 1206 package (50v X7R)

Q1 BF862 (771-BF862-T/R)
M1 ZVN4306GTA *** a great alternative is ZXMN10A08GTA now that 4306 seems to be out of stock or EOL? ****

Switchcraft 3.5mm stereo jacks x2 (35RAPC4BV4)
Alps 10kOhm stereo pot with power switch (RK0971221Z05)
3mm round LED (red or green)
4-pin 2.54mm pitch power connector (see post #44)
Edit (Dec 22, 2017) - here is the latest BOM that I use for low impedance cans with a smooth sound signature:

http://www.diyaudio.com/forums/group...ml#post5284641


Here is a Digikey shopping cart for all SMT parts. Good to have assortment of R4 values ranging from 33R, 47R, 51R, 56R, 68R to tune bias current on M1. Start with 47R and increase R4 for higher bias - aim for 50mA to 55mA across R7.

Here is a ONE-STOP shopping cart for Digikey put together by Sodacose. It doesn't have the nice red Wima's but should be fine if you are not a capacitor snob :)

http://www.digikey.com/short/3wzbtt


https://www.diyaudio.com/forums/atta...mp-test-05.jpg

Here is the template for drilling the holes in the mint tin, courtesy of Agdr (same pattern as his Super CMOY 1688 as the pot, LED, and 3.5mm jack locations are the same).

http://www.diyaudio.com/forums/group...ml#post4985588

Desktop version with IRF10's and 115mA bias here:
http://www.diyaudio.com/forums/group...ml#post5014544

https://www.diyaudio.com/forums/atta...1-build-02.jpg

Edit (July 28, 2017): DSP Convolution Files for OB-1 and DT880-250 headphone!
Here is a neat thing to try if you want to hear what your headphone would sound like if it followed the ideal Harman target response. Byrtt posted some directions on how to do this in DSP and kindly provided convolution files for DSP processing of the OB-1 and DT-880 headphones (I happen to own both :D). Sounds very very nice - definitely worth a try.

http://www.diyaudio.com/forums/group...ml#post5134580

Edit Aug 28, 2017: a very nice USB LiPo charger and load balanced with cap multiplier mod by RaptorLightning.

http://www.diyaudio.com/forums/group...ml#post5168688

https://i.imgur.com/muypU8W.jpg

Edit May 9, 2018: Schematic and BOM for the Desktop Class A Headphone Amp:
http://www.diyaudio.com/forums/group...ml#post5428007

PCB layout courtesy BDHM and JPS64:
https://www.diyaudio.com/forums/atta...-pcb-photo-jpg

https://www.diyaudio.com/forums/atta...n-portrait-png

Stuffing guide:
https://www.diyaudio.com/forums/atta...c61c49d779-jpg

https://www.diyaudio.com/forums/atta...d6985b5a3-jpeg

Update Jan 8 2019: New ZXMN10A08GTA to replace ZVN4306GTA provides higher transconductance for lower output impedance and higher damping factor. xrk971 Pocket Class A Headamp GB

Morde 27th January 2017 06:32 PM

Count me in. Do we have a BOM somewhere already existing for Mouser or DigiKey etc.?

Quote:

Pocket Class A GB Order List:

xrk971 - 2 boards
pcgab - 2 boards
zman01 - 2 boards
stellarelephant - 2 boards
samoloko - 1 pcb
lui04 - 2 boards
Morde - 1 board

(add your name and number of boards here. Please use quote feature to grow list.)
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xrk971 27th January 2017 07:18 PM

Below items are all available from Mouser (circa $20)

Description Part No Qnty
n-channel power MOSFETs ZVN4306GTA 2
n junction JFET BF862 771-BF862-T/R 2
Panasonic OSCON 390uF 20v output caps 20SEPF390M 2
Output cap bypass WIMA 1uF MKS caps MKS2C041001F00KSSD 2
Input bypass Wima 2.2uF MKS caps MKS2B042201F00JSSD 2
Panasonic OSCON 100uF 25v rail caps 20SEP100MX 2
Switchcraft 3.5mm stereo jacks 35RAPC4BV4 2
Alps stereo pot with power switch RK0971221Z05 1


Plus you will need the following 0805 1% metal thin film SMT resistors (qnty):
33R(2), 47R(2), 270R(12), 1k(2), 15k(1), 47k(2), 100k(2), and qnty 4 x 100nF X7R 0812 MLCC bypass caps, and basic 3mm red or green LED.

cyteen 27th January 2017 07:29 PM

Quote:

Originally Posted by Morde (https://www.diyaudio.com/forums/group-buys/302859-xrk971-pocket-class-headamp-gb-post4966921.html#post4966921)
Pocket Class A GB Order List:

xrk971 - 2 boards
pcgab - 2 boards
zman01 - 2 boards
stellarelephant - 2 boards
samoloko - 1 pcb
lui04 - 2 boards
Morde - 1 board
cyteen - 2 boards

(add your name and number of boards here. Please use quote feature to grow list.)
Report Post


Morde 27th January 2017 11:46 PM

Quote:

Originally Posted by xrk971 (https://www.diyaudio.com/forums/group-buys/302859-xrk971-pocket-class-headamp-gb-post4966974.html#post4966974)
... and qnty 4 x 100nF X7R 0812 MLCC bypass caps...

Do you mean 1812 X7R caps perhaps? I couldn't find the 0812 case size from Mouser. Or maybe I just don't do the search correctly. All the other parts were quite easy to source from Mouser. Thank you very much for the BOM!

xrk971 27th January 2017 11:59 PM

Sorry I meant 1206 size 100nF X7R caps.

funch 28th January 2017 04:25 AM

Any boards left? I'd like at least one, maybe two. Thanks.

xrk971 28th January 2017 05:56 AM

I haven't ordered them yet so I will put you down for two
Quote:

Pocket Class A GB Order List:

xrk971 - 2 boards
pcgab - 2 boards
zman01 - 2 boards
stellarelephant - 2 boards
samoloko - 1 pcb
lui04 - 2 boards
Morde - 1 board
cyteen - 2 boards
funch - 2 boards

(add your name and number of boards here. Please use quote feature to grow list.)

syyma 28th January 2017 07:11 AM

Pocket Class A GB Order List:

xrk971 - 2 boards
pcgab - 2 boards
zman01 - 2 boards
stellarelephant - 2 boards
samoloko - 1 pcb
lui04 - 2 boards
Morde - 1 board
cyteen - 2 boards
funch - 2 boards
syyma - 2 boards

av-trouvaille 28th January 2017 07:56 AM

xrk971 - 2 boards
pcgab - 2 boards
zman01 - 2 boards
stellarelephant - 2 boards
samoloko - 1 pcb
lui04 - 2 boards
Morde - 1 board
cyteen - 2 boards
funch - 2 boards
syyma - 2 boards
av-trouvaille - 2 boards


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