Electret headphones amplifier

[spandrel]

I've just bought a pair of Stax electret 'earpeakers' on a whim. There's no adapter box with them. The box consists of a pair of step-up transformers driven from an amplifier output, with a ratio of about 1:25. It also has a thermistor on the inputs acting as a limiter. I'm going to try and buy a box but wondered if anyone knew of a circuit which could drive the headphones directly. I'm assuming this would improve the performance. They need a high voltage and are a capacitive load. Would a bridge circuit be suitable? Any ideas?

Another issue is that the outputs need to be floating to avoid shocks. This could be a problem to arrange.

 

[Externet]

No place for 'bridges' here.
Look in your junk box for a 230V to 3, 5, 6 V linear adapter and canibalize the transformer alone.
Feed audio into the secondary and the earphone to the primary. If satisfactory, repeat for the other channel.

 

[spandrel]

Good idea , though a 12v DC (~9AC) one would be nearer the correct ratio. It would need a thermistor as well to limit the current. I've got a 12v brick so I'll give it a go. Thanks. I'd still like to make an amp for them though. It's got to be better than a transformer.

 

[Externet]

An amplifier ? A transformer is a voltage amplifier. You do not need a power amplifier. That is already in your 'stereo' to feed the transformer. The transformer alone will nearly 'match' the impedance to your earphone needs.

 

[Hopup]

You could search or ask in https://www.head-fi.org/ there is probably lot of info about/related your phones.

 

[spandrel]

The up-market Stax electrostatics have amplifiers now, probably because it's better but also because it's now cheaper than transformers. This is from the blurb for the SRM-353X

"The SRM-353X is the exclusive driver unit (amplifier) produced in order to drive STAX electrostatic type Earspeaker much more ideally. Carefully selected high-quality parts of an audio grade further improved the tone quality, and re-examination of circuit details realized much wider frequency response.
Original low-noise dual FET at first stage as well as all-stage direct coupling class-A DC amplifier configuration with no coupling capacitor has realized natural tone quality with much amount of information.
Further improved emitter follower at output stage has enabled wide-range reproduction resulting in both the extended dynamic range at high frequency and the improved tone quality."

I know the transformers work but I fancied building an amp. Not sure about Class A DC-coupled though, could be tricky. I doubt the electret ones are worth the bother to be honest.

 

[Hopup]

I would rather get good DAC if you lack one. I have some very transparent headphones and having good DAC is very important for the reproduction.

 

[Audioguru]

You do not want a mains transformer or an audio transformer in a good audio circuit. You also do not want a thermistor limiter.
The head-fi.org thread has an acoustical frequency response curve that might have no bass due to poor coupling/sealing or a lousy mic. Some transformers cut bass and highs.

 

[spandrel]

head-fi is a great site but they don't go into electronics design. This is an article in the 1979 Wireless World, not only an amplifier design but the actual headphones as well!! A pretty simple Class A bridge arrangement but running on 800V DC. Scary.
http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1979/Electrostatic Headphones.pdf

 

[BobK]

Original low-noise dual FET at first stage as well as all-stage direct coupling class-A DC amplifier configuration with no coupling capacitor has realized natural tone quality with much amount of information.

Electrostatic drivers are capacitors.

If the transformer was 25:1, the driving voltage is probably somewhere in the range of 100 to 400V. Do you have any specs for the headphones?

Class A is okay because the currents involved will be very small.

Bob

 

[spandrel]

No spec for the electrets but the electrostatics are about 150pF and 150Kohm at 10KHz. I may be worrying too much about DC offsets but even more about having a few hundred volts near my ears referenced to earth. Optical input maybe?

I'll play around with transformers for now and do some measuring.