gdahl
100+ Head-Fier
- Joined
- Oct 27, 2001
- Posts
- 122
- Likes
- 11
After much anticipation, and an all-day solder-fest on Saturday, my new headphone amp played music for the first time last night at about 10:30. Much to my relief, it worked right away! This is a good thing, because I didn't use my usual first-time process of bringing it up slowly with a Variac.
The circuit has several interesting features. The amplifying tubes are 3A/167M's, which are the loctal equivalent of the rare and expensive Western Electric 437A. As it turns out, the 3A/167M's are even more rare and expensive, but I already have a good stash. The 3A/167M features high gain (about 41) and low plate resistance (approx. 1kohm).
Another feature of interest is the use of Peerless TL-404 autoformers. These were originally designed for MGM Studios for driving headphones. An autoformer has a single tapped winding, as opposed to a transformer's separate primary and secondary. As such, the TL-404 is designed to be operated in parallel-feed, with the DC blocked by a series capacitor. The operating current for the tube traditionally would be fed through a plate-loading choke.
Instead of plate-loading chokes, I used a pair of Gary Pimm's superb pentode-mosfet cascode constant current sources (PMCCCS), which I had already used successfully in the Aurora amps (see http://www.aloha-audio.com/triode1.html). The PMCCCS circuit has an impedance of several megohms through the entire audio band, which would be impossible to achieve with a choke. Best of all, it doesn't add any transistory colorations of its own. I have heard a direct comparison of the sound using choke loads, PMCCCS, and solid-state current sources. Pimm's circuit wins this one easily.
Input level is set with a DACT attenuator. The signal path goes from there through a 3A/167M, a 3.0 uF Hovland cap, and the TL-404 autoformer. The TL-404 has taps at 4, 10, 50, 200 and 500 ohms, selected by a DACT selector switch and taken to the headphone jack.
The power supply consists of an oversized power transformer and choke, a pair of 6CJ3 damper diodes used as a full-wave rectifier, and a Black Gate 47 + 47 uF/500 V capacitor. The whole thing runs with delightful silence. No background noise at all.
I have listened to two different Beyers so far (one borrowed), plus Senn 580's and 600's (borrowed). My W100's are supposed to arrive tomorrow, and the W1000's are also due early this week. It has been fun comparing the sounds on different impedance taps with each headphone.
So far everyone who has listened has been absolutely delighted. This is certainly the best sound I have ever heard on headphones. Can't wait 'til it breaks in, and my A-T cans arrive!
The circuit has several interesting features. The amplifying tubes are 3A/167M's, which are the loctal equivalent of the rare and expensive Western Electric 437A. As it turns out, the 3A/167M's are even more rare and expensive, but I already have a good stash. The 3A/167M features high gain (about 41) and low plate resistance (approx. 1kohm).
Another feature of interest is the use of Peerless TL-404 autoformers. These were originally designed for MGM Studios for driving headphones. An autoformer has a single tapped winding, as opposed to a transformer's separate primary and secondary. As such, the TL-404 is designed to be operated in parallel-feed, with the DC blocked by a series capacitor. The operating current for the tube traditionally would be fed through a plate-loading choke.
Instead of plate-loading chokes, I used a pair of Gary Pimm's superb pentode-mosfet cascode constant current sources (PMCCCS), which I had already used successfully in the Aurora amps (see http://www.aloha-audio.com/triode1.html). The PMCCCS circuit has an impedance of several megohms through the entire audio band, which would be impossible to achieve with a choke. Best of all, it doesn't add any transistory colorations of its own. I have heard a direct comparison of the sound using choke loads, PMCCCS, and solid-state current sources. Pimm's circuit wins this one easily.
Input level is set with a DACT attenuator. The signal path goes from there through a 3A/167M, a 3.0 uF Hovland cap, and the TL-404 autoformer. The TL-404 has taps at 4, 10, 50, 200 and 500 ohms, selected by a DACT selector switch and taken to the headphone jack.
The power supply consists of an oversized power transformer and choke, a pair of 6CJ3 damper diodes used as a full-wave rectifier, and a Black Gate 47 + 47 uF/500 V capacitor. The whole thing runs with delightful silence. No background noise at all.
I have listened to two different Beyers so far (one borrowed), plus Senn 580's and 600's (borrowed). My W100's are supposed to arrive tomorrow, and the W1000's are also due early this week. It has been fun comparing the sounds on different impedance taps with each headphone.
So far everyone who has listened has been absolutely delighted. This is certainly the best sound I have ever heard on headphones. Can't wait 'til it breaks in, and my A-T cans arrive!
