the STAX Amplifier

by Andrea Ciuffoli

contact me for any problem or question: webmaster@audiodesignguide.com

Introduction

After many attempts to get the maximum performances from high quality dynamic headphones, the SESS Single Ended Single Stage amp is the my lastest creature, I have started to think about electrostatic headphones.
The electrostatic headphones are on paper, and in the opinion of many people the best for resolution and detail.
I have seen many schematics about amps to drive these headphones from the solid state solution to vacuum tube project and slso the original Stax products.
The first two steps have been search for the best supplier and to find the best product on the market.
I have compared specifications of many different models..
STAX is a leader in this field, because it is specialized in this kind of product and the models SR-303 or SR-404 seem to be the best choices.
I bought the SR-303 headset from Audio FX (see http://www.audiofx.com/) using the secure ordering system at a very good price, and the waiting time for delivery was very short.
The datasheet of this product can be seen at the official Stax website (see http://www.stax.co.jp - english version is available).
The SR-404 has a higher price but seem to be very similar to SR-303 in specifications.
After acquiring the headset I started to think about my design for a no compromise amp.
I made a list of target requirements:

- all tubes on the signal path
- no feedback
- min signal output voltage 200Vrms
- min signal output current 10mA
- max 2 stages
- only hi-end audio components on the signal path
- passive power supply
- vacuum diode on power supply
- no custom power supply transformer
- no regulator tubes or pentode with low linearity
- inductive power supply to prevent the generation of current peak on diodes
- no dc coupled of stage
- no dual power supply
- direct heating tube like output stage

The requirements are not few items but the design is very easy.

Components

All the audio transformers used in this project are Lundahl products and the distributor list is available here

2 x LL1660/25mA Lundahl output transformer
2 x LL1651 Lundahl power supply
2 x LL1638/8H 200mA Lundahl power supply inductances (price about 65$ each)
3 x MQA-3G3 Jensen 100uF double metallized paper and polypropylene

dielectric impregnated with mineral oil.

4 x 534 100ohm Spectrol precision wire wound multi-turn potentiometers
SR-303 Classic STAX Electrostatic Earspeakers

Lundahl is one of the world´s leading manufacturer of audio transformers used in many professional studio recording and radio stations so He have a very big experince and all these products use a custom audio C-core.

I am using a DACT stepper attenuator instead of the normal ALPS or NOBLE solution, because the the sound is clearer. A good review of these components can be found on the article published in Hi-Fi World.

The input stage is the very good Golden Dragon 6SL7.

The input stage is the incredible Golden Dragon 4300B.

The vacuum diodes used are the GZ34 Mullard.

datasheet of the GZ34 (pdf format)

About passive components types, I don't leave many choices: Caddock resistances on cathode, ELNA Cerafine capacitors on cathode.
In all my previous projects I have used in the power supply the ELNA Cerafine capacitors with a very high sonic performances and compact size.
If we skip any economic aspect and no size problem the first components is a paper in oil capacitor.
In comparation test the paper in oil are best of the ELNA Cerafine or other electrolityc type.
Searching a paper in oil capacitor is very important buy new product as the Jensen and not an old stock.

About wire forget the teflon ! use only stranded tinned copper wire with pvc isulator and for loudspeakers the same.

A very good quality stranded tinned copper wire could be buy directly from E-Z-HOOK that carries an extensive line of fine stranded and extra flexible wire.

Design

Digital Input stage

This project give a first class sonic performance but this can be easily compromised by a bad input signal so I suggest you to don't use preampl. and use one of my DAC projects.
Using one of my top DAC projects all the DIYer's will reach the same sonic result.
The DAC normally included into commercial CDPlayer (even these of significant price) have a rather low performance because the common usage of high NFB operational amplifiers "destroy" the sound given compression effects and poor soundstage stability.

Sonic results

Top performances but a little noise