Headphone amp build


#1

This is just a quick overview of a project I’ve been working at on and off for a while now and finally got finished thanks to the pillar drill (and the impressive array of drill-bits in Farset).

So, a few years ago I got a pair of Seinnheiser 595’s which are excellent. This immediately got me to thinking about how to get the best out of them. I’m not an “audiophile” so I didn’t realise how deep the rabbithole went when it came to the sheer array of digital to analogue converters (DACs) and amplifier variants. I was also not willing to spend the frankly terrifying asking price of many of the devices available so I did a bit of research on the best price to performance ratio and stumbled onto a headphone amplifier design by a respected amp-design veteran named Pete Millet (http://www.pmillett.com/) called the “Starving Student”.

Two things immediately struck me about the design - it was a tube amp with a low number of parts and it had rave reviews from those who had built one. When Pete first released the amp design the tubes he used - 19J6’s - were available for a couple of quid each on ebay. Unfortunately I jumped on the bandwagon a little late and found that not only were 19J6’s a rarity, those that were available were quite expensive thanks entirely to the popularity of the amp design. Thankfully a few users on the head-fi forums (http://www.head-fi.org/t/319231/millett-starving-student-hybrid-amp) had been experimenting with alternative, more plentiful tubes resulting in a variant using 12AU7 (alternatively, ECC82) tube. As well as being relatively inexpensive - around £6 a tube - they also are still manufactured. Of course, being audiophiles additional parts were also added to the design to improve the sound including high-quality output stage capacitors.

I prototyped the amp using stripboard and mounted the hardware in an old project box:

It was ugly but it worked. However, thanks to the difficulty in shielding the inside of the project box (and probably parts of the stripboard acting like an aerial!) it was a bit noisy. An aspect of the design that I was concerned with was mismatching of the tubes - due to their design no two tubes are electrically identical which can result in audible differences in output between the left and right channels. Thankfully I was fortunate and I got two that were very well matched due to being from the same batch and the same factory. One can pay for a matched pair but those tend to come at a premium.

One of the head-fi members had done a run of PCB’s and was selling off the boards at close to manufacturing cost so I grabbed two (http://www.head-fi.org/t/664191/starving-student-pcbs). Real life got terribly interesting at that point so I shelved the project for a while. Some time later when I had a bit more free time I started thinking about finishing the amp. I love wood and have collected a fairly decent bunch of tools since my guitar-building days so I decided that I’d make or modify a wooden enclosure. I scoured ebay and found trinket boxes for less than £10 made out of pine plywood that were of suitable dimensions to fit the components into. My only concern was drilling large, perpendicular holes that would allow me to recess the parts, a job that pretty much demands a drill press; yay for Farset! The inside is shielded using aluminium foil and the entire circuit is star-grounded which was very effective in attenuating the noise to a barely audible hum at 50% volume with no input.

tl;dr: I now have a really amazing sounding amp that didn’t cost a small fortune and looks cool.


#2

That, sir, is a sexy little box.


#3

You should post this on hackaday, it looks cool


#4

Have you thought about staining the wood?


#5

I think it would take stain pretty well but I’ll have to put it off until I can afford to pick some up. If I do I would rather go for a subtle stain as I generally prefer light woods to dark woods.