Simple Component Upgrade STAX SRM-252S Headphones Amplifier - Part 3

The previous part, I do the upgrade only on the power line capacitor. The effect is not audible improve. That is why in this part 3, I do a little components modification on the signal path and value increase at the secondary high voltage power supply section caps. The main goal in this upgrade is to improve dynamic range, clarity and widening the sound stage.

This is the filter capacitor rated 22uF 250 volt OEM STAX from Sanyo - Suncon FH series. It is just too small and I wish I could increase them as much as possible. But the space inside this STAX is very limited. I tried everything from the very big 110uF Rubycon, up to the 100UF Nippon Chemicon are not fit enough in this little STAX box.

Fortunately I bought many capacitor for this mod. Then I am using Nippon Chemicon KXJ 33uF 250 volt. A little bit value increase from the OEM Sanyo, but the ripple rejection based on specification is greater. This new caps also not too large compare to the OEM, and the fitment is just perfect over the board!

This is the original datasheet Nippon Chemicon KXJ versus the Sanyo Suncon FH series. Nippon Chemicon got the 240 point compare to Suncon only 184 point of ripple rejection.

The next target upgrade is the filter capacitor consist of C7 & C8.

This is the comparison between the OEM caps from STAX is only using 0,01uF, then I will using Wima MKS4 0,47uF.

This big red Wima capacitor has been well known for best ripple rejection especially for high voltage such as tube or like this STAX amp. But I need extend one of the lead to make a good fitment on the board. Please notice I covered the open lead with heat-shrink tube to avoid short circuit.

How sexy is it:)

Complete on the secondary high voltage section, then I jumping to the signal line section. This simple schematic maybe get you to understand in simple way.

I remove the R1 & R2. This resistor are located near the RCA plugs. This resistor act to lower the impedance which are not really needed since we already have the potentiometer as volume control.

The C1 & C2 is also remove. By removing this caps, the improvement in dynamic range will be noticeable better. You can have an option to lower the value up to 100pF but I prefer not to using any caps at all. This caps is for low pass filter.

Then the last things to do is changing the R3 & R4 to a better quality resistor. I also lower the value from 3k ohm to 1k ohm. The resistor that I am using is Takman carbon.

The OEM compare to Takman is quite different class. This is become very important due to the line signal is pass through this resistor at the first place. If you can obtain the Vishay Z-foil resistor in this section, I can recommend you to go ahead.

It is all done now. I put it back to the casing and ready to roll the music.

I am more than happy to hear the difference. The goal has reached now. This STAX sound signature remain the same but it is improve the dynamic range and widening the sound stage even more. After 100 hours new components break in, it will shows another improvement for sure.

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