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Old 24th February 2003, 02:22 AM   #1
diyAudio Member
Join Date: Nov 2001
Location: Mars
Default Where to put volume potentiometer for headphone amplifier?


This concerns my attempt to construct a step-up adapter a'la SRD-7 for my Stax SR-3 electrostatic headphones. I have a quite a few pairs of shielded UTC audio transformers knocking about, and I would like to see if I can improve upon the somewhat flimsy audio step-up transformers used in the Stax adapters.

The circuit I would like to build is quite simple, see for example the circuit attached below: I obtained this from an excellent webpage on making your own electrostatic headphones: http://homepages.ihug.co.nz/~wradford/esphones.htm

I will simply be using an autoformer for the HV bias supply (to enable a certain variability to be allowed in setting the bias - I have found the SR-3 sound character to change quite considerably based on the bias setting used in the range 210-250V, the factory nominal being 230V).

This circuit is going to be driven by the speaker outputs of a Hafler DH-220 amplifier - my dedicated Stax amplifier . To keep things simple, I would like to drive the amplifier directly from a CD player, and based on cable layout etc - I would like to add a volume control BETWEEN the amplifier and the step-up unit.

My question to the audio gurus here is: Would a 100K alps black beauty work at the input to this adapter?? (i.e. at the points labelled as Amp Channel-1,2)? Or possibly at the output of the adapter after the voltage has been stepped up (i.e. just before the connection to the headphones??
Would 100K be an appropriate log-pot value (my parts collection is currently limited to 100K and 30K).

Any inputs/suggestions would be highly appreciated!

Thanks -

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Old 24th February 2003, 02:30 AM   #2
diyAudio Member
Join Date: Nov 2001
Location: Mars
Default Almost forgot

The step-up audio transformer pair I am planning on using is going to be either:

A) A pair of UTC HA-108X transformers. These are actually mic-to-line transformers
with multiple alloy shields to prevent hum. They have a 20hz-50khz rated response
with a 500V 'hipot' rating. The choice of step-up goes from 50ohm:500ohm to


B) a pair of UTC LS-151 transformers. These are line-to-line transformers, with
a 16k:500/333/200/100/50ohm step-down (of course I will be using them as
step-up transformers! these have a 7hz-50khz response with a 500V hipot

I was saving the LS-151's for a tube amp project, so I will be trying option A first to see if that gives enough of a step-up.

Any comments on the choice of the transformers are also welcome - I am hoping that the response doesn't degrade too much when I use these transformers in the reverse ratio!

Cheers -

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Old 24th February 2003, 02:31 AM   #3
diyAudio Member
Join Date: Nov 2001
Location: Mars
Default Specs for the HA-108X

Found at :
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