A few weeks back we managed to meet at Jon’s with a few of the London Audio Circle members. I haven’t seen the bunch for a long time due to family and work travel commitments, however it was a great opportunity I didn’t want to miss this time round.
One of the intended purpose of the meeting , which for obvious reasons these meetings never goes as planned due to the random nature of DIY, was to shootout two headphone (HP) amplifiers over modded HD800 headphones.
Jon’s HP amplifier was a simple stage formed by a C3g (triode-strapped) valve loaded with a nice amorphous-core Valab choke. Output cap-coupled to a TVC to his main system when he uses it as preamp. Otherwise cap couple to HD800s headphones.
On the other hand, we had Geoff’s HP amplifier. Geoff undertook the crazy quest of building my 2P29L HP amp design in a single enclosure! After a lot of tinkering with the grounding layout and physical layout and orientation of transformers and other components, he managed to get it dead quiet. An amazing achievement for a DHT in a single enclosure! After discussing it with him, obviously regretted having done so as it was a big headache which left his metal case with a several holes from moving stuff around.
Geoff fitted a pair of Kemet 100μF/500V large Polypropilene capacitors. He also had the option to hook in a custom-made (by himself) of 4:1 output transformers.
His design is very close to my original circuit here:
The only differences are obviously the output cap and OPT and Geoff only has 6 SiC diodes. He runs the stage at about 17mA which provides just enough the necessary current for a 300Ω HP.
Listening to the HP amps
We had a long series of unplanned technical bits during the day which deviated us from the intended purpose of the day, however it was fun.
Listening to both amps was more than a pleasing experience. Source was the Starlight CD player with the discrete DAC. In this case we tried a low-impedance DAC board which Tom managed to develop recently. A great achievement as it can drive the TVCs directly and sounds superb.
Everyone agreed that the 2P29L sounded slightly better than the other one. Perhaps is the subtle sound transparency and detail that the 2P29L brings and also the benefits of the hybrid mu-follower, which has proven many times is a real winner.
I have to admit personally I preferred Geoff’s amplifier with the OPT, rather than the direct drive (which is very convenient). This was my personal preference
I have a pair of huge Mundorf EVO Oil 100μF/350V to try on my new HP breadboard. So stay tuned!
Modding the HD800 headphones
Here’s Geoff’s take on his HP modification:
The mods on my hd 800 as first the “anaxillus” modification and second the “Dupont”. They are both designed to reduce a sharp peak in the phones response at 6khz. Its highly probable that this peak gives the phones its imaging and detail retreival strengths but at the expense of excessive brightness. So those who were trying to reduce the peak were careful not to reduce it too much and rob the phones of their strengths. I think both mods have succeded with this objective. The “anaxillus ” mod tries to deal with this peak by adding a ring of dampening foam in the main ear cup of the phones. The type and thickness of the foam was arrived at after considerable measurement and experimentation. It does indeed succeed in reducing THD 6kHz peak but also has some effect on other resonances within the ear-cup. The overall result was widely acknowledged as a success. There is a thread on the web describing how to make and fit this mod. The “dupont” modification tries to deal with the peak in a different way. It uses a Helmholtz absorber to damp the 6Khz peak. It is made from rings of specific thickness and density foam to produce a small chamber with a small port which fits into a small cylinder in the ear-cup. The damping effect comes from frictional losses in the port. Again this was arrived at by lots of measurement and experimentation. Sennheiser themselves use something similar in the hd800s but which appears to be a little more effective probably because they they can use bespoke materials. Again there is a thread describing how to make and fit this mod plus a ready made one can be bought from Dupont. When I first bought mine with the anaxillus I compared them to Jons which only had the Dupont and thought Jon’s had less of the 6Khz peak problem. So I orderedand fitted a Dupont one but kept the anaxillus fitted. Mine now does have a reduced 6khz peak but also now sounds slightly softer. Its difficult to know which is best.I think it depends on what the headphone amp sounds like. I quite like the double mod with the direct o/p on the 2P29L.
Geoff Mead (2019)
