The step up transformers were a simple way of getting the audio
signal to the required voltage to drive the headphones, but had
the disadvantage of needing the audio amplifier in the first place,
as well as not being espscially HiFi. To this end, I built a high
voltage tube amplifier, based on a design detailed on
www.tubecad.com. The amplifier connects directly to the
ESL headphones and audio source (CD player/preamp).
The idea of using tubes instead of transistors was not one
of sonic preference, but simply because tubes are naturally
high voltage/high impedance devices; so they suit this task
especially well. Transistors are available that can produce
the 500V+ voltage swing needed, but they are specialised,
and I was not confident with them.
2 12AX7 tubes driving the 6SN7 grids. The 6SN7 plate
volatage is close to the maximum of 600V, and is a lot
higher than is normally used with a 6SN7, but the tubes
are biased well within limits. Both the tube types are
dual-triode parts, so each stator on the headphones receives
its signal through 2 triodes. This is just enough gain, and will
max out the headphones at full volume as long as a relatively
high output source is used. Any less and another gain stage is
required, and I did not want to complicate things with another
gain stage.
A lot of the part selection was dictated by what I had on hand,
as you can see the power transformer is ridiculously oversized.
The switch next to it is the HT switch, this is switched on once
the heaters are warm, to prevent cathode stripping since there
is a solid state rectifier. The coiled wire is an RCA lead, and the
ugly looking DB25 plug poking out the bottom is the connector
for the ESL headphones. This was supposed to be a temporary
measure until I could get a nice looking high voltage CPC connector…
The sound from this amplifier is quite a bit different from the
transformer driver, the treble is much more prominent, althought
at present teh bass response is somewhat lacking. No doubt this
could be rectified with improvements to the amplifier though,
where as the transformer driver has met its limits right ‘out of the box’
The result
I built these headphones for fun, and to replace the
awful cheap ‘ear buds’ that I was previously using.
But these headphones do work exceptionally well.
Having never listened to a high end commercial
offering like Sennheiser or Stax, I cannot tell you
how they compare. However they are clearly
superior to any ‘affordable’ headphone, and for
price you can’t really complain (especially considering
Sennheiser Orpheus ES headphones are US$14,990!!).
Bass is very refined and DEEP. Not like most cheap
headphones, which either have no bass at all or
augment the bass and end up muddled (”Now with
Bass Boost System”).
The highs are especially clear. You can listen to these
headphones for long periods with far less fatigue than
ear buds. Cymbals sound like Cymbals, not like hissy-fits.
However, I found that with the transformers, midrange
was somewhat recessed, a trait that the valve amp did
not have. This is most likely due to the mid-fi quality
of the transformers. With equalization it was easily
boosted to more appropriate levels.
Volume is acceptable, when you turn it up too loud
the diaphragm hits the stator and gives a very audible
snap crackle and pop which is VERY annoying.
The headphones still almost make it to what I would
consider maximum listening level. They are by no
means quiet. They could be made to go louder by
increasing spacer thickness (and hence excursion
limit), but that would also mean an even greater
increase in voltage swing to achieve the same output…
it quickly gets out of hand.
can hear what you are listening to, but it is not intrusive.
It is like when someone turns up there walkman REAL
LOUD. But it also means any outside sounds go
straight through the diaphragm and you can hear
them easily. They don’t block out the outside noise
like normal headphones, so they are best listened
to in the dead of night…
