Stax SRM-323S

[Equaliser]

Some internal pictures of a modern in production Stax earspeaker amplifier unit. A far cry of difference in terms of parts and layout design compared to the old Stax stuff I've posted so far in my other threads. The SRM-323S was released in 2010 as an update over the SRM-323A/II released in 2006 which is an update over the SRM-313 released back in 1999, under the nature of the circuitry and schematics they all share similarities one way or another and same thing with the basic smaller Stax amps (Stax SRM-Xh, 3 , 212, 252II). 

 

Due to available information and saving my self the trouble of typing out the specifications here is a direct quote from the Stax website on this amp:

 

 

The emitter-follower circuit with high-voltage bipolar transistors adopted for the output stage of the SRM-323S enabled low output impedance resulting in increased drivability and wide high-frequency dynamic range. Moreover, the power transformer equal to that of the SRM-006tS has been adopted to achieve more powerful derivability. One simple RCA input is equipped with a parallel-out terminal directly providing the incoming signal. Two 5-pin output sockets are connectable to all pro-bias compatible earspeakers. The SRM-323S employs a double-axis volume controller to control left-and -right channel volume level independently without using a balance volume controller.

 

 

 

SRM-323S SPECIFICATIONS:

Type  

:All-stage semiconductors, Low-noise dual FET input Class A operation, Pure balance DC amplifier configuration Earspeaker driver unit

Frequency Response   :DC to 60,000Hz (with 1 SR-307) High Harmonic Distortion   :Max 0.01% (1kHz at 100V r.m.s. ) Gain   :60dB (x1000) Input Terminal  

:1 RCA with 1 RCA parallel out

Rated Input Level   :100mV / 100V Outputs Max. Input Level   :30V (at minimum volume level) Input Impedance   :50KΩ(RCA) Max. Output Voltage   :400V r.m.s. (1kHz) Standard Bias Voltage   :PRO 580V x 2 Power Voltage Frequency   :117V (50Hz / 60Hz) Power Consumption   :30W Temperature & Humidity   :0 to 35ºC / 90% max. without condensation Dimensions   :5.9 (W) x 3.9 (H) x 14.2 (D) in Weight   :6.4lbs Color   :Silver

 

 

Even though it is still operating in Class A formation, and thanks to modern parts and a re-design the heat output of this unit is minuscule compared to the heat put out of the older Class A Stax units (the ones I've posted, SRA-3S, SRM-1, SRM-1 MK2 and the SRA-12S). The output voltage is also higher on the 323S compared to the mentioned units so far.

 

Pictures:

 

 

 

Typical Bando transformer, Stax has been partnered with and been using them since the 60's. (Most Japanese manufacturers use Bando transformers).

 

 

Discontinued and well-regarded Sanyo 2SA1968 900v rated transistors (also used and needed in Kevin Gilmore's line up of DIY Stax amplifiers) but only Stax has access to these parts enough to keep the 323S in production. The rest of us consumers are left to buy surplus stock at fairly high prices for DIY applications.

 

 

 

Only available to Stax, custom Alps dual concentric potentiometer. 

 

 

All HV rated semi's.

 

 

This unit was purchased new from Japan and was wired for 100v only. Credits to Kevin Gilmore and Spritzer, the 323S can be re-wired to any primary voltage with just a change of the soldered jumper and re-soldering a cut off primary winding wire from the transformer. This is a way to stop grey imports but doesn't work so far.

 

 

 

Red wire is re-soldered where the original was cut off. Amp wired for 240V and ready for other voltages without use of a step down transformer.

 

How does it sound? Many others experienced including myself would rate the 323S as the current best in production amplifier you can buy from Stax. It has enough power to drive the power hungry Stax Omega 2's, the latest SR-009's and any of the older Pro-bias Lambda and modern Lambda framed earspeakers. I personally like it with my SR-404's but I still have a sweetspot for the older Stax SRM1 MK2 units I also have as they have more low-end in the sound spectrum paired with my older Lambda units. 

 

The price for these on the market is relative to the distributor selling it in your country, Addicted to Audio is one of the distributors in Victoria to selling these and is priced accordingly on their website. You can purchase for lot less from Japan but do note that as soon as the amplifier leaves Japan, it is no longer covered by warranty unless the amplifier is serviced in Japan and to avoid use of step down transformers the primaries need to be rewired for 240V operation.

Edited August 31, 2015 by DefQon

[Frank is here]

Thank You......would it be possible to get the pictures associated with this post....I have just purchased an SRM-323s and would like to do the voltage update as advised in this post....Again...Many Thanks