I am no stranger to headphone amp design, as I've built several CMoy-based models in various mint tins for friends and myself, and eventually sold a couple on ebay for a good price. But there were always tough decisions to make about the actual design of the devices, due to the limited space in a typical Altoids mint tin, and compromises had to be made. Sacrifice battery life and voltage output for a current-follower stage? Or run dual 9V batteries for a stable power supply and low distortion, but run the risk of falling short on current for low-impedance headphones? Include a volume control and completely eliminate the dual-9V supply -and- buffer stage? I've tried all these scenarios, and none were to my satisfaction.
Enter the HA-1. A couple years ago I had decided to build a better mousetrap, as it were, and came up with the design for a dual-battery powered amp with a current-follower stage. It comprised of four opamps, two per channel, and had a gain of about 3. It was plenty loud, but it had always seemed... lacking. Perhaps it was because I had used rather low-end sillicon (OPA2134PA), or maybe it was the filter caps at the input to block any incoming DC, but one thing was certain: It didn't have the oomph I wanted, and distorted at high volume. Don't get me wrong, it was miles better than anything I had made in a mint tin, but it wasn't the best I could do.
I had the parts to make a better one, but after building the first version, I was rather disappointed with my work, and I didn't have any kind of an enclosure to put it in. I didn't want to get one of those cheap Radio Shack plastic project boxes, and I'll be damned if I'm gonna spend $55 after shipping on a nice aluminum enclosure. It wasn't until recently that I decided to have another go at it, and hence the MkII version was born. I would use the best opamps in my inventory (which was rather vast considering they were all samples), buffer the hell out of them, and have a power supply stiff enough to drive it all. Active Electronics recently started carrying the nice Hammond 1455-series enclosures, so I wouldn't have to sell a kidney to order one online.
Initially I had chosen Burr-Brown's OPA637BP opamps for the gain stage, but they had some problems. I got -bad- oscillation whenever I touched the metal case of my iPod during testing, and because the 637 is not unity-gain stable, I had to set the gain rather high. It was far too loud, and far too unstable. I swapped out the 637 in favor of the lower-bandwidth (and more stable) 627, and changed the gain from 11 to 3. I can now use the entire sweep of the volume knob, and the amp doesn't oscillate.
Most opamp design recommendations include a highpass filter consisting of a capacitor and resistor on the input of the opamp, to protect it from DC. I don't like capacitors in the signal path, because they introduce phase distortion and reduce low-frequency response. My amp is direct DC-coupled from input to output.
The power supply and buffer stage are probably a little overkill for an amp that is only driving headphones. 12,600µF total on the voltage rails and a sustained current output capability of ±250mA per channel from the BUF634T's. Texas Instruments (the manufacturer of Burr-Brown components) recommends running the 634's in high-bandwidth mode for stability when used with the 627 and 637, but this increases quiescent current draw up to tenfold from 1.5mA to 15mA. Initally, this was a concern for me because at first I expected to power the amp with two 9V batteries, so I tested it in low-current mode and I didn't have any audible problems. Unfortunately, upon connecting the output to a 'scope, I found there was some oscillation at about 500 KHz, and again at about 3 MHz. However, I am now powering the amp with a regulated AC power supply, so I can afford to run the buffers in high-bandwidth mode. This eliminated the oscillation I was getting.
Compare this all to Grado's RA-1, a very simple headphone amp with rather ordinary parts. They want $350 (in the US, nearly $600 in Canada) for a 6"x6" block of mahogany with about $25-$30 worth of parts in it. You can see one dissected. The opamps in my HA-1 MkII are US$26.63 each from Digi-Key. The buffers are US$8.10 each.
If you're still reading, you must really be wondering how it sounds. Is it everything I hoped for? Did I finally hit the nail on the head? I most certainly did. It's as distortion free at full volume as it is at more sane levels of output. It has authority and bass to drive even the most demanding of headphones, such as my Grado Labs SR225's, and practically blows my Bose Triports right off my head at full blast, yet is dead silent during those quiet passages.
I had the intention of selling this amp on ebay once I had completed it, but after listening to it for the first time, I knew I wouldn't want to let it go.
Copyright © 2006 Orcinus Orca