TubeCAD two-tube OTL SE headphone amplifier.
So I have a lovely direct coupled Loftin White-esque 2A3 SET amplifier which sounds great through my Fostex FE208EΣ rear loaded horns. But I also have neighbours, and, if I'm completely honest, my better half really doesn't enjoy the footy commentaries on my local radio station in the same way that I do... So the next project was to be a headphone amplifier. After discussions with Philip at Bluebell Audio he suggested John Broskie's output transformerless single ended headphone amplifier, which appeared on John's website in December 1998.
My implementation of John Broskie's OTL SE headphone amplifier
According to the website, it's an amplifier capable of driving 300Ω Sennheisers as well as 32Ω Grados, and uses 2 number 6922 dual triodes per channel. Philip knocked up a power supply using a small Hammond 369AX 250V centre tapped mains transformer with two IN4007 diodes for rectification and a CRCRC smoothing arrangement after, the final RC being separate to each channel.
The plan was to use a black powder coated 13.5" x 5" x 2" Hammond chassis longways. Holes were drilled on the back for the input phono sockets, fuse holder, and cable gland. No more cutting IEC connectors for me!
Rear view of headphone amp. Note captive mains lead!
The mains transformer was mounted at the back, with an on/off toggle switch on the right side and a green led on the left. Then some holes to attach the power supply capacitors, and then the four miniature 9 pin Noval sockets. Philip had some concerns that the 6922s may be a little microphonic so supplied the sockets with skirts and spring-loaded cans. Finally, a Switchcraft 1/4" headphone jack was mounted on the front.
Internally, the tag strip was mounted on the fixings for the valve sockets or beneath the transformer to minimise visible fixings on the top surface of the chassis. This needs to be planned otherwise it's pot luck whether the tag strip fixings will mate up.
Close-up of audio circuit
Close up of power supply
The compact nature of the chassis helps to make the amplifier look really quite neat and smart, but I had a nightmare building the audio circuit as space and tag strip turned out to be in very short supply. If I was building it again I would try to get some more space for the audio circuit, though I'm very taken with the external look of the amp and wouldn't want to change its appearance greatly.
Well, this was the third valve amp I had constructed, but I was still nervous of things going bang. Fortunately nothing did, but, not thinking, I turned it on without headphones plugged in. Inserting the headphone jack with the amp switched on didn't seem to do it any good; I was greeted with loud rustling and then silence. It's probably testimony to the ruggedness of valve amps that after about five minutes of panic I switched on again and had sound! Phew, what a relief. I hadn't trashed the thing before I had started.
So the amp's working and my heart rate has started to settle down. The first thing I noticed was the dreaded hum. I checked the voltages which seemed reasonable, although the cathode voltages weren't particularly balanced between channels. I tried moving the four number 6922s around but this only had a small effect on voltage, and none on the hum.
Tackling the hum, I found judicious broddling of the cables inside sorted the worst of it out. Interestingly, it was the shielded cable running from the phono inputs to the input of the audio circuit at the front of the amp that was picking up the hum. Mental note, shielded cable doesn't provide complete immunity from hum. Much better to choose a cable routing regime that avoids the interference altogether. The remaining hum I could perhaps equate to AC heaters; I can hear it if there is no music playing, but as soon as there is (even footy commentary) the hum is no longer noticeable.
I only have a pair of 20 year old Sennheiser HD414s, not a pair of the 650s I fancy, but the sound is very nice. There's that wonderfully fluid grainless sound that valves produce, and that makes it very relaxing to listen to. There's no shortage of detail though, and there's plenty of texture too. I think the 414s are the weak link at the moment, but the combination is a very good alternative to my Loftin White and Fostex horns.
Thanks to Steve Shiels I replaced the original Chelmer 6922s with some nice NOS Mullard E88CCs. Very nice, a notable improvement over the Chelmer 6922s. I suppose I really should rebuild the audio circuit too and see if I can balance the cathode voltages, but the amp sounds pretty good as it is and I haven't felt the need to do this yet.
Chelmer 6922 and NOS Mullard E88CC flanked by spring-loaded screening cans
Reading John Broskie's TubeCAD blogs, JRB has published a few Aikido circuits including headphone amps, so I want to try one of these. It's a good excuse to start playing around with the Aikido topology. Then I'll treat myself to a pair of 650s too.
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