I had been ignorant...

[Kursun]

I’m 62 years old. I have been a music and hi-fi addict since the age of 6.

I have never been a headphone man. I just don’t like listening through the headphones.
But I do have two headphones:
Philips SBC HP 1000 and AKG K701.

I like the Philips HP 1000 a lot. Its sound is very good for its price. It is a pity it’s discontinued.
I had found the AKG K701 quite bright and had listened to it only for “dim” sounding recordings.
Lately I thought of selling the AKGs as second hand but before doing that I thought of giving it a second chance…

I had been listening top my headphones through my HT receiver. I decided to buy or build a headphone amplifier before deciding to sell my AKG K701s.
I thought,

1. Headphone diaphragms produce much less distortion than loudspeakers as they have to move much less air mass than speakers. Because of this fact they show the deficiencies of the driving amplifier much more than speakers.
2. Headphones require milliwatts to produce their maximum output. Unfortunately 99.9 % of the amplifiers we use at home are class B/AB designs. Class B/AB designs produce their highest distortion (especially crossover type distortion) in this milliwatts range…

This information is probably elementary stuff for hard core headphone listeners. It’s a pity that I had thought of such things after so many years. As I said I was ignorant… Well I have never been a headphone man…

In the end I bought parts from e-bay to build a class a headphone amplifier originally designed by good old John Linsley Hood. Actually my first pre-amplifier I had built many, many years ago was also a John Linsley Hood design published in Wireless World. Unfortunately the semiconductors in those days were much too noisy…

If you write “JHL class A headphone amplifier” in e-bay you’ll see many such kits. (BTW I don’t know why it is spelled JHL and not JLH) My kit has Nichicon filter capacitors and Alps potentiometer.

Last week I finished the project. My first listening material was “Randy Crawford & Joe Sample - Live (2012)” album. Though a live recording the sound quality is first rate. And I love to listen to Steve Gadd on drums…
Pictures of the project:
Top: http://img20.imageshack.us/img20/6798/image00001tn.jpg
Back: http://img690.imageshack.us/img690/1...ge00002cse.jpg
Front: http://img507.imageshack.us/img507/1...age00003kh.jpg

On the chassis though there is no magnetic shield between the r-core power transformer and the circuit board there is no hum on the output. The circuit is very quiet too.

What is the difference compared to listening through the HT receiver's headphone output?
There is more and better controlled bass. The midrange has more immediacy of a live feed. Output from the HT receiver’s headphone output has slightly more distortion in the midrange reminding you that this is a recording. The differences are slight but important.
I’m completely satisfied. The results I got is slightly better than what I expected to get. I have decided not to sell the AKG K701s and keep them for critical listening.

Rest in peace John Linsley Hood.

[Kursun]

Here is the photo of the finished project with top cover and "feet" attached.
http://imageshack.us/a/img580/3446/image00001si.jpg

[Hyfi]

What did the project cost you to build and how long did you spend? Looks very good.

[Kursun]

Well, I paid
$24.80 for circuit board (assembled board)
$ 8.00 shipping
$25.00 for r-core transformer
$14.00 shipping
$ 9.90 for aluminum feet
$ 7.00 shipping

Total $88.70
Plus the aluminum chassis, power switch, volume control knob, input plugs and the power plug with fuse I bought locally probably cost about $30. So the total cost should be around $120.

I was lucky to find a relatively good looking enclosure with just the right size (especially its height). The aluminum amplifier feet are not a necessity of course but it does look good and professional with them. I had to decide where to put the tiny power on led. Putting it above the on/off switch would have been much too common. I decided to gamble a bit with the design and modified the volume control knob, drilled it and inserted a blue led instead of the supplied red led on its back.

The links for the components I used:
Assembled circuit board
Assembled Test JHL Headphone Amplifier Class A Preamp DIY | eBay
R-core transformer
115V 230V 30W R Core Transformer 15V 15V for Headphon Amp Preamp | eBay
Aluminum amplifier feet
4 Pcs Aluminum Feets for Power Amp with Rubber Ring D 30mm H 12mm | eBay

Same circuit is also offered in a ready-made form in a chassis from various vendors.
But I enjoyed building it myself. It’s got a personalized touch.
BTW, I live in Izmir, Turkey. In my country personal imports are duty free up to €75 (~$100, including shipping I suppose). Buying it ready-made would go beyond that limit. I wouldn’t mind paying duty tax but bureaucracy to do that would kill me.

Building it shouldn’t take more than a few days. I mounted the circuit board on four pc mainboard spacers. Drilling a hole on the front panel at the right point for the potentiometer shaft was a bit critical. Also making a square hole for the power plug was a little time consuming. I drilled tiny holes along the lines of the square and filed the hole afterwards.

One important note: The heat-sinks on the board do really get hot. I plan to find a way to provide better air circulation inside the chassis. It may be a problem in hottest days of summer. When I first started the project I was startled when I saw a 30W transformer was recommended for a headphone amplifier. But then, this is a class A amplifier with relatively higher consumption and heat dissipation.

[Feanor]

That's a very nice, well-executed project, Kursun. Congradulations!

[Feanor]

I'd guess that a hole of 7-8 mm drilled through the top cover directly over each heatsink, and maybe two-three more through the back plate, would adequately solve the heat problem: what do you think?

[Kursun]

Thank you.
I was thinking of drilling holes about the size you have mentioned above each heat sink (4 output transistors and 2 voltage regulators) on top plate and also below, on their projections on the bottom plate. But I am not sure about its aesthetic consequences.

I have also considered installing a tiny, 4 cm 12V computer fan inside but that would not be an elegant solution at all.

BTW I have just picked a cheap non-contact IR thermometer at a local electronic parts store for about $25, a nice toy for the curious…
The thermometer’s accuracy is specified as ±2°C/±2%

After keeping the unit on and idle with top cover off for about 30 minutes at an ambient temperature of 23.5°C (74.3°F) the heat sink with the highest temperature was 59°C (138.2°F). Not too low but not too high either…
You might say “the situation is critical but not serious.”
But good air circulation is surely necessary.