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Old 27th January 2017, 11:48 AM   #1
xrk971 is offline xrk971  United States
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Location: Metro DC area
Default xrk971 Pocket Class A Headamp GB

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

Click the image to open in full size.

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.

Click the image to open in full size.

Here is the predicted HD profile:
Click the image to open in full size.

Here is what the hand made prototype looks like:
Click the image to open in full size.

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:
Click the image to open in full size.

This is the associated schematic that goes with this layout:
Click the image to open in full size.

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:

Click the image to open in full size.

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

Click the image to open in full size.

Edit Feb. 18, 2017 - Here is the latest measured frequency response and THD from the GB PCB:
Click the image to open in full size.

Edit Feb. 18, 2017 - Here is the latest measured FFT from the GB PCB measured for 1kHz excitation at 700mV into a 270ohm load:
Click the image to open in full size.

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:

https://www.etsy.com/shop/XRKAudio

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

BOM errata for C9 and C11:

xrk971 Pocket Class A Headamp GB

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

Click the image to open in full size.

Click the image to open in full size.

Final as-built schematic:

Click the image to open in full size.

Final as-built BOM:

Quote:
All resistors 0805 package, metal thin 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%)
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
C2_2 Output cap 5mm pitch, up to 9mm wide (Wima 1uF MKS2C041001F00KSSD)
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

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)
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


Click the image to open in full size.

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).

xrk971 Pocket Class A Headamp GB
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Last edited by xrk971; 12th June 2017 at 04:20 AM.
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Old 27th January 2017, 05:32 PM   #2
Morde is offline Morde  Finland
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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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Old 27th January 2017, 06:18 PM   #3
xrk971 is offline xrk971  United States
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Location: Metro DC area
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.

Last edited by xrk971; 27th January 2017 at 06:23 PM.
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Old 27th January 2017, 06:29 PM   #4
cyteen is offline cyteen  England
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Quote:
Originally Posted by Morde View Post
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.)
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Old 27th January 2017, 10:46 PM   #5
Morde is offline Morde  Finland
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Join Date: Oct 2014
Quote:
Originally Posted by xrk971 View Post
... 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!
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Old 27th January 2017, 10:59 PM   #6
xrk971 is offline xrk971  United States
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Location: Metro DC area
Sorry I meant 1206 size 100nF X7R caps.

Last edited by xrk971; 27th January 2017 at 11:01 PM.
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Old 28th January 2017, 03:25 AM   #7
funch is offline funch  United States
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Any boards left? I'd like at least one, maybe two. Thanks.
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Old 28th January 2017, 04:56 AM   #8
xrk971 is offline xrk971  United States
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Location: Metro DC area
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.)
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Old 28th January 2017, 06:11 AM   #9
syyma is offline syyma  United States
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Join Date: Apr 2008
Location: SF Bayarea
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
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Old 28th January 2017, 06:56 AM   #10
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Join Date: Aug 2003
Location: Near Rotterdam
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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