Stax SRM-1/Mk2 Bias Modification

This article originally was published by Psychaudio at the Audiophile-id forum. The original article was written in Indonesian and so I have obtained Psychaudio’s permission to translate it into english.

It all started when a local forum member obtained a SRM-1/Mk2 amplifier that comes with normal bias on both headphone outputs. A normal bias amplifier has very limited use, since most Stax headphones in circulation are on Pro Bias. After some discussion, the modification into Pro Bias is executed.

This article will outline how you can change any normal bias driver into a pro bias, or even up the bias into the 620V bias often recommended for the Koss ESP-950. The tutorial is also applicable on other normal bias Stax amplifiers.

Word of caution: You’ll be working at very high voltages, so be very careful.

Step 1: Layout

Please click on the image to see a larger size.

The layout above has been designed to minimalize arc and heat on the resistors. Arc happens when a high voltage path lies too close to ground, enabling an electrical discharge from the high voltage path to ground. Heat level on each resistor is minimalized so that the voltage going through each resistor is not more than 350V.


Step 2: Parts List
VR1: 500k Ohm, Single Turn
R1: 470k Ohm, 1/2 Watt
R2: 33k Ohm, 1/2 Watt
R3: 2.7M Ohm, 1/2 Wtt
R4: 2M Ohm, 1/2 Watt
D1, D2: 1N4007 (or other diodes with ratings above 800V)
C1: 0.1uF 630V, AC Rating
C2: 0.1uF 1000V, DC Rating
Breadboard: Independent pad holes, 5mm distance between holes
Spacer: 25mm, metal



Step 3: Assembling The Pro Bias Module
Solder the components on the breadboard, according to the layout. Red lines are connecting lines below the breadboard, blue lines are legs of each components.

For the ground hole, make a 3mm hole on the breadboard. I’m using a cable pad to make a circular trace on the ground hole, as illustrated below:


Then install metal spacer on the hole, as illustrated:

Make sure that the spacer is installed tight!

Step 4: Installing The Pro Bias Module
On the SRM-1/Mk2 amplifier, AC input can be obtained from one of the yellow cables which is a cable from the transformer output. Ground can be obtained from the black cable or one of the ground path from the PCB.

To simplify the process, only the AC input will be connected with a cable. Ground will be connected with a metal spacer, which will also serve as mounting points for the pro bias module.

The next step is to drill two holes on the amplifier PCB:

Make sure that those holes intersect the paths for the AC and the ground, as appropriate.

To ensure good contact on the ground hole, lay some solder around the ground hole.

Then, solder connecting cable on the AC input hole (~4cm).

The next path is to install the module in position. First, solder the AC input cable for the pro-bias module. And solder an output cable for the pro bias (~ 30cm). Then install the pro bias on the ground hole using a metal spacer.

The board underside should look like this:

Lastly, connect the pro-bias output on one of the headphone socket in the front panel. The easiest way is to use the right side socket. Cut off the normal bias cable that links the two sockets (blue color), then solder the pro-bias output cable from the pro-bias module to the pin for the bias.


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  • M H

    Would this design work with SRD-6/7?

    • http://www.headfonia.com Mike

      Aren’t the SRD transformer boxes?

      • M H

        Yes.

        • http://www.headfonia.com Mike

          I think you should be able to, it’s based on the same principles and basically you just want to up the bias voltage right? However I’m not quite an expert in this and so I can’t make any guarantees.

          • M H

            That’s what I think as well, though I am not an expert on the issue either.