HeadLine

Headphone Tube Amplifier (OTL)

Page 4, Listening and More. December 20, 2006

Conclusion

This amp is HOT! And I do mean it, after 30 minutes of listening the enclosure gets more than 40 degress Celsius. But who cares as long as the silicone kit holds and plastic components don't melt. I added a heatsink to the amp at the backsite and since then the amp can run for hours without me burning my fingers. The cheap silicone kit I used does indeed NOT hold up to the challenge and therefore I had to take some other measures like using tie wraps for critical components (400V caps) in combination with a fresh run of Silicone kit.

This was a typical first-time-right project. Everything worked right the first time and I needed only small modifications to get the amp where it is now. And though I did not do any extansive measurements yet in order to determine important things like distortion etc. I do think that this amp is a pleasure to listen at.

The two 12SN7 tubes in parallel are not quite the right tubes for building a headphone based on specs alone. But other than specs, they sound OK, and thats very important for me. Also, the main headphones I will use for this amp are the set of Sennheiser HD424 cans that I own for more than 20 years now, and that are not as difficult to drive (600 Ohms?) as the HD650. I know, the new HD650 sounds probably better, but it is different. The old Sennheisers are open and do allow conversation with other people in the room and as such they are quite useful in the living room where I like to work while others are watching TV.

The output impedance of this amp is not very low, and since I did not use any feedback (maybe I will and make it user selectable) it will stay at around 160 Ohms. Still a reasonable value I think but not OK for Grado cans as I understand.

Listening

Listening impressions were very positive. I've listened to Eagles' Hell freezes over and Classical Music from Cecilia Bartoli and I was impressed with the sounds of voices etc. After that I listened during the Xmas holidays while writing this up to the combinatyion of the Headline and the Sennheiser HD424 and I must say that it is a good combination.

• The amp is dark and quiet between music, I had to change the step-up transformer to a toroidal version in order to get rid of all the residual humm, but that's gone now. Only when putting the amp in the highest volume setting there is some hiss, but I think this is nothing to worry about as the amp with it's 12ST7 tubes in the driver stage will provide more gain than needed anyway. Listenings to these levels of music is unhealthy anyway.
• It reveals recordings where there is hiss and noise of microphones etc. Bad recordings or badly sampled music on the iPod is unbearable.
• There is an enourmous sense of space when listening to headphones in general and with this amp in particular.

I'll probably do an extensive one-on-one comparison with the Little Dot amp, but already now I'm sure this amp is going to be here for a while. Meanwhile my conclusion is that the amp sounds sweet, and is has the typical airy tube sound in combination with ear-friendly distortion characteristics.

Construction etc.

Regarding the mechanical construction there are a few points for improvement:

• The case gets hot, and I mean HOT. That's because of the voltage regulator that I connected to the chassis at the back. The regulator will use the complete enclosure as it's heatsink. It works, but I'll have to keep in mind that it could lifetime of influence connectors etc. A next version should preferrable include external heatsinks (PC heatsinks are cheap).
• Glueing the step-up transformer to the case works very well from a functional point of view. However, it still doesn't feel the same as using ol' bolt'n'nuts.
• The amp could use a more powerful powersupply. The trick with cheap 20V AC transformer and step-up works great, but for a next version I might return to the old "all in one box" approach.

Conclusion

The main conclusion might be that since I constructed this amp with relatively cheap components, I might start an improved version soon. Is it better than a commercial model such as my own Little Dot 2++? Maybe .. but I'll have to find out in a later stage.

Room for improvement:

• Adapt to headphone impedance (make selectable?)
• Better capacitors (Black Gate etc.)
• Maybe DC coupled? (eliminate caps in the signal path)
• More Powerful tubes? (EL84?)
• Better cooling of the 12V power regulator (or an open air cabinet)
• May need a slow-start heater supply. The 12SN7 tubes have a nasty load when cold. Once the heater reaches the right temperature everything is OK.

Links:

• Audioforum article on MF X-Can V3
• HeadWise design of Andrea Ciuffoli
• HiFi.nl article MF X-Can V3
• Illuwatar OTL design
• Balanced OTL in TubeAmp

© Maarten&Annemarie, November 2006

last modified 21 January, 2007