When you hear the name STAX headphone amplifier, first things comes in mind is the electrostatic headphone which is made in Japan and expensive.
It is all make sense since STAX never selling their unit cheap, same with many of the Japanese product as well.
Speaking for the STAX headphone amplifier, I have one long time ago here, but the entry level STAX headphone amplifier and I can say it sound really good. The sound is so much open, larger soundstage, so much detail and very natural music presentation. I may called it the STAX sound signature.
But if I get a cheap old STAX headphone amplifier that intended use for the professional, will it sound better then what I have before?
Before jump to that conclusion, lets see the unit that I just got here first.
You might curious what will you get when you get offered for this used STAX SRM1 MK2 Professional headphone amplifier along with their lambda headphone driver.
What can be wrong for the unit that sold cheap and having the ages like mine?
All my curiosity are paid off when I test this unit. It doesn't sound right! No bass at all, imbalance sound between right and left channel and it just like a broken unit.
I put my bet on and let's see if I can fix this unit.
The STAX named this amplifier is the driver/energizer unit. This is simply because the electrostatic headphone are require high voltage to vibrate the plate inside the driver unit.
At the glance, the unit I got here is considerably in a good shape, just a few minor scratch on the amplifier casing but still in one complete piece.
Open up the amplifier case is quite easy, just unscrew the side panel and I can see the internal components.
Fortunately it is all original inside, no sign of any repair has been made to this unit before.
This is a good start for me.
The only things I take a note is the input signal cable has been cut before. It looks like someone before me trying to connect the chassis ground to the signal ground for no logical reason.
This caused the cable at the RCA terminal looks messy.
The person who did this for sure don't have a better knowledge for troubleshoot the unit like this.
Lucky this can be repair easily so I can continue.
I put the amplifier unit aside for a moment and I want to have an inspection to the lambda headphone driver.
The headphone looks normal from outside. The plastic is still in a good shape with the leather earpad are also in a good condition. Thumbs up for the Japan product like this that can last forever.
There is no sign this headphone unit has been repaired before. Another luck for me to get this unit in the nice condition.
Then I want to have an inspection to the driver unit inside. To open up this headphone is quite easy, just lift up the earpad corners and I find the small 4 screws to open up the pad.
Then this what I found.
On the one ear cup, there is a white paper cover with a yellow sound absorber. But on the others ear cup, there is only a paper cover. This might be the cause of the sound imbalance between right and left channel.
The others common problem from the old headphone driver unit like this is the glue that attaching the driver to the housing are already dry and in this unit, they are already apart.
I start repair it by realign the front cover wire mesh so it become flat. I also remove the paper cover at the back of each driver. Then I clean up the housing as much as I can with a wet cloth, so they will looks like a new unit.
Once all parts cleaned, then I attach the driver into the housing using CA glue.
They are now attached together but it is not stop there.
As you can see, the CA glue doesn't fill the gap between the driver and the housing.
This gap should be closed otherwise it will cause sound cancelation, resulting no bass if there is a leakage from the gap.
That is why for the next steps, I am using the hot glue to sealed them up.
It is done and next step is assembly all parts together.
I prefer not to using any paper cover or the sound deadening at the back of the headphone. Later on I found this makes the sound better. Especially for the soundstage is more open and the mid range presentation more natural and airy without it.
The most important things, the bass is now coming back again.
This headphone driver restoration is now complete.
Pretty easy right?!
But this is still not finished yet.
I need to back to the amplifier unit. It still need to be check to make sure this unit will perform optimal like it should.
First I repair the RCA connector and I also clean the board as much as I can.
I checking all components and find that one of the small Mylar type capacitors is cracked. This capacitor is 10nF for low pass filter label C101 and C201 that connect in series with the electrolytic capacitors above it.
Once I remove it from the board, one capacitor is break apart. The other one is also not in a good condition. I checked with capacitance meter, it only show less than 1% of the original value.
Those capacitors need to replace.
I also changes all the small electrolytic capacitors around it.
Although some of them still in a good shape with normal value, but it is not harm to replace them with the better one.
Except for the main power supply capacitors. I leave it as is.
The original capacitors use Hitachi 100uf 400v and the size is very big. I don't find any good replacement for it. I was trying to compare between the original with others new capacitors I have on my desk but I don't think the quality are comparable with the original.
So I put back the original capacitors with additional WIMA MKP10 as bypass capacitors at the bottom of the board.
The one last steps is clean the old thermal paste on each power transistors heatsink then apply the new one.
This is an important steps for every class A amplifier that naturally will produce a lot of heat. The power transistors need sufficient heat transfer to the heatsink.
After all components in placed, I retrim the bias and offset to get almost zero DC output between + and - output pin. Also between + and ground output pin for each channel.
Finished with all of those steps, then this amplifier is ready to rock again.
I put the headphone on my head and start listen to music.
The sound of this headphone amplifier unit is so much better than when I got it on the first time.
Not only that, I can say STAX SRM1 MK2 with lambda headphone has it's own sound characteristic and one of the best sounding headphone amplifier combo.
What I can describe for the sound signature of this unit is very aggressive sound without missing the smoothness, lots of details with good separation, very large soundstage and wide frequency response without sibilance sound and very good low frequency attack.
No wonder this STAX SRM1 MK2 is featured in the Hi-fi books published by Phaidon.
Disclaimer: Any statement and photos in this article are not allowed to copy or publish without written permission from the writer. Any injury or loss from following tips in this article is not under writer responsibility.
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