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Old 17th December 2016, 08:51 PM   #1
xrk971 is online now xrk971  United States
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Default BF862 based SE Class A Headamp without the HEAT

I was looking for a small portable amp for my DT880-250 headphones. I wanted class A and I wanted longer battery life and low rejected heat. These sound like requirements that are at odds with each other right?

This project started in another thread and is based on the BF862 JFET single ended White follower design suggested by EUVL here. The design has a fairly low bias current of about 50mA, but because it used qnty 8 x BF862 JFETs on the output stage in a singel-ended arrangement with about 12mA in each device, the overall heat dissipation is very low but it is guaranteed to work in pure class A (it has to because of the topology). This amp is very simple to make following this schematic:

Click the image to open in full size.

I did my layout using a drawing program (MS PowerPoint in this case) and came up with this:

Click the image to open in full size.

I then made iron on transfers using laser printer and got this on the copper as the etch mask:

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Etched in HCl and peroxide (hardware store muriatic acid concrete sidewalk cleaner and hydrogen peroxide (3%) for 20 minutes:

Click the image to open in full size.

Etched board looks like this:

Click the image to open in full size.

Soldered it up (I had to switch 1R source resistors for 33R to keep my bias current low, it was pegged at 23mA ea wit 1R):

Click the image to open in full size.

I had some difficulty getting it to make sound at first and found out that the problem was a bad 6in length of wire from the output terminal to the headphone jack. Took hours to figure that one out after debugging the board components.

Initial tests were done at 12v supply and I boosted that up to 17v to get a little more headroom and keep heat down still. I am using a small DC to DC boost converter (400kHz switcher) and added 2200uF cap to filter it some more. Ripple is about 9mV at 17v but luckily the amp has some decent PSRR and output measures 0.3mV with inputs shorted. It is silent to my ears anyhow. Absolutely no hum or hiss. Cannot tell it is on.

I spend half a day listening and decided it was worthy to be boxed. So using this niftly enclosure. After drilling some holes and adding a 50k stereo pot, and power jack, switch, LED indicator, 3.5mm jacks IN and OUT, the amp is completed:

Click the image to open in full size.

Here is the (messy) inside. I am not one to have neat wiring harnesses as my time is very limited to work on it. The wires work fine and do not seem to pickup noises. All wires are at least twisted:

Click the image to open in full size.

Edit: I added a cap multiplier using IRF540 MOSFET with 10mF output ca[ and circuit from Juma's Easy Peasy Cap Mx thread:

Click the image to open in full size.

Here is the amp next to DT880's that I will be using them with. I am using a 2200uF 16v electrolytic cap for C2 bypassed with 1uF 230v MKT. I initially used 330uF OSCON but found the bass lacking. With the 2200uF I can hear deep deep bass now. There is a 270R 2W load resistor, so when I plug in the 250R DT880's, overall impedance is about 125R - which I think may help to draw more current out of the amp so it gets more impact with the bass (seems so subjectively).

Click the image to open in full size.

So in addition to sounding extremely detailed, neutral, and having excellent bass authority, they are absolutely cool. The case is ambient temperature after hours of playing. Also, since I am using a XL6009 switch mode step up regulator, the power supply input is very flexible. For desktop use I have a 12v 3A SMPS wall wart from my external disk drive. For portable use, I will use a 5v USB phone charger battery pack and let the step up take care of getting the 17v rail. I estimate a 10,000mA Li-ion battery pack will last more than 12 hours. That's a lot of pure class A play time for a small battery.

This amp sounds wonderful and I thank EUVL for a great design. It has indeed been tested and works very well. Give it a shot.

Edit: Dec. 22, 2016 - an attempt at a smaller 37mm x 26mm layout to fit in an Altoids tin with two 9v batteries.

Click the image to open in full size.

Edit: Jan 6, 2017 - portable battery operation using 5v USB charging battery pack. 10,000mAhr cell should last about 15 hrs.

Click the image to open in full size.

Sound is still great - using XL6009 DC to DC step up internally for 18v supply to amp (after cap Mx).

Edit: Jan 7, 2017 - Here is a new portable 2 FET design that sounds great - very easy to make. We will have a GB soon so stay tuned!

Schematic and THD prediction:
Click the image to open in full size.

FFT HD profile (sweet H2 and not much else!):
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Implementation as portable mint tin pocket Class A amp a la CMOY:
Click the image to open in full size.

Internals of above amp:
Click the image to open in full size.

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Attached Images
File Type: jpg BF862-HA-build-boxed-10g.jpg (67.9 KB, 2353 views)
File Type: jpg BF862-HA-build-boxed-internal-11.jpg (140.0 KB, 2352 views)
File Type: jpg BF862-HA-build-final-with-dt880-12.jpg (109.6 KB, 4979 views)

Last edited by xrk971; 9th January 2017 at 04:40 PM.
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Old 17th December 2016, 10:13 PM   #2
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Did you matched those jfet?
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Old 17th December 2016, 10:17 PM   #3
xrk971 is online now xrk971  United States
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You already asked that before and I answered here:

MOSFET follower headphone amplifier
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Old 17th December 2016, 10:39 PM   #4
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Sorry about that. I forgot where i put my question
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Old 18th December 2016, 12:29 AM   #5
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Quote:
Originally Posted by xrk971 View Post
Here is the (messy) inside. I am not one to have neat wiring harnesses as my time is very limited to work on it. The wires work fine and do not seem to pickup noises. All wires are at least twisted:
.
With audio its better not to harness the wires as close wires can interfere with each other.
Its worst with valve amps where there are high VAC and heater AC mixed in with high impedance audio signals.

I just got caught out recently with a USB scope circuit.
I ran a RAM address line close to a 1meg input resistor and every time the address line changed I got a glitch on the audio signal.
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Old 18th December 2016, 01:59 AM   #6
EUVL is offline EUVL  Europe
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I want to put the record straight.

I posted this schematics as a Spice file for the gentleman easphyx who started the thread " Tin of FETs headphone amplifier".
Tin of FETs headphone amplifier
It is largely his design so all credit goes to him.
I merely suggested a variant using alternative devices.
I have not built this and have no intention to, also in the future.

For this to work as in the simulation requires matching of both Idss and Yfs.
Gettinng sound out and having the circuit functioning properly as intended are two different things.
If any of you wish to find out please google SRPP and White Follower.
Or ask the originl designer at his thread, as a respect to him.

And the circuit is capable of fitting into the original footprint by easphyx, as he intended.


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Old 18th December 2016, 02:19 AM   #7
xrk971 is online now xrk971  United States
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Thanks for clarifying EUVL and my apologies to Easphyx - thanks for the original circuit that led to this one. Here is the schematic of the Tin of FETs:

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Old 18th December 2016, 03:04 AM   #8
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Good on you... looks like a fun project.

Did you measure or simulate the distortion? (Not that it really matters a whole lot)
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Old 18th December 2016, 04:41 AM   #9
xrk971 is online now xrk971  United States
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Quote:
Originally Posted by googlyone View Post
Good on you... looks like a fun project.

Did you measure or simulate the distortion? (Not that it really matters a whole lot)
If you have LTSpice, it's simulated in file provided by EUVL in linked post above.
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Old 18th December 2016, 12:04 PM   #10
xrk971 is online now xrk971  United States
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It was helpful to read up on SRPP.

The Tube CAD Journal: Mixed Mode Amplifiers

I will order some a bunch more BF862's and try to match them. Idss is easy. How to measure Yfs? Need a small rig with contact pads and press clamp in place to measure.

http://www.nxp.com/assets/documents/...tes/AN211A.pdf

Last edited by xrk971; 18th December 2016 at 12:07 PM.
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