Saturday, September 01, 2007

Ultra high quality headphone amplifier

I read Alastair's post about high quality headphones with great interest. 

After finding the excellent Headwize site, I rushed down to Jaycar this morning for bits, and built the cmoy headphone amplifier (in an Altoids tin, with space to spare, as shown above right).

What really got me was the data sheet for the OPA2134 operational amplifier from Burr-Brown which says, in part, "The distortion produced by OPA134 series op amps is below the measurement limit of all known commercially available equipment."

Further: "THD+Noise is below 0.0004% throughout the audio frequency range, 20Hz to 20kHhz".

My experience.. 
  • This is easy to build. I did a hack job, as you can see above, and it worked perfectly first time.
  • I'm driving Sennheiser HD 212Pros.
  • Ripped some CDs at AAC 256kbps as sample content.
  • iPod shuffle has noticeable hiss
  • iPod 30Gb has less noise
  • Switching the headphones between the iPod direct and via the amplifier makes the iPod sound relatively dull.
It's hard to explain, but this little amplifier gives a real sparkle to the sound. It's not artificial or boosted in any way, but things like cymbals sound quite different - better. Sometimes there is too much bass for my liking.

Other notes:
  • Jaycar don't advertise the OPA2134, I went to buy the NE5534AN which is in their catalog and they gave me what I really wanted as a substitute - excellent! (AU$3.95)
  • This chip is broadband, when I touch the input it picks up all sorts of hum, I expect that it will be susceptible to RF from things like mobile phones and will need to be in a well shielded box (Altoids tin for example) with filtering to avoid picking up hash.
  • I used "ugly construction" which works well for RF projects so it's probably pretty stable as it's all ground plane.
  • Didn't bother with gain control as I figure whatever is driving it has it's own volume control. Mine has a bit too much gain for my listening levels.
So, one more thing to carry on the train.. thanks Alastair.

Update

I've spent a very entertaining evening re-importing some of my old favourite CDs at 256Kbs/AAC and of course listening to tracks in different headphones. There is a story around about how your brain works harder listening to music which has been compressed for space (no, not level - that's another topic). Basically masked parts of the audio are removed to reduce the data rate, but in fact you miss those parts of the signal and have to imagine them yourself.

In my youth I was very interested in "hi fi" and well remember the arms race that would follow the upgrade of one component in the system: a new moving coil cartridge would show up the noisy amplifier, upgrading that would show up the speakers, and so on.

Recent years have seen my music move totally on to computers and mostly in to headphones. Loud speakers are always a compromise existing as they do in a room that resonates to some extent. Little computer speakers have advanced tremendously in recent years and work damn well at low level. Headphones can reveal detail and texture in an audio track that will be missed on even the most high end speaker system.

With the falling cost of storage it's now time to re-import my CD collection at a higher bit rate - or perhaps I should just bite the bullet and go loss-less at last.

Update 2

Hmm, not sure if the higher bit rate AAC is worth it for me. I created this test which chops back and forth between 128Kbps and 256Kbps every ten seconds and I can't tell the difference. Of course I'm getting old and probably not listening in the best equipment.

3 comments:

Alastair said...

Great work Marxy, looks like a great little toy.

I'm also playing around with different encoding, will let you know how it goes.

marxy said...

Thanks, yes do let me know how it goes.

Anonymous said...

Very nice your blog.
do you know planet ham??
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To add your blog to the Planet Ham feed please email the web address to webmaster@planetham.com

73 and thanks