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Stax SRD7 SB MK2 small treble issue

post #1 of 11
Thread Starter 

Hello everyone


This is my first post on this forum but I read it for a long time ; english is not my first language (I am french) so I hope you will forgive some bad writing...


I like Stax headphones a lot (I can tell I am an addict...), I own one Lambda normal bias, one Lambda pro bias, one SR-X . My amps are : Stax SRM1 MK2 pro, SRD7 SB MK2, SRD7, and SRD6 SB


Source is a Sony CD player and writer RCDW100 (I have a Teac VRDS too, but the tray mechanism is broken and I did not had the time to fix it yet...and I found the Sony sound good enough for the price)


I use SRD7 SB MK2 energizer with a Dual CV 1460 amp (a gem) , the sound is stunning, even better than with SRM1 MK2 pro (yes! no wonder for those who know the cv1460, very well known among vintage amps in Europe for its great sound). The SRD6 SB recently came with an Ebay purchase of an SR Gamma that I don't like (defective headphone, unfortunately...), but itself works fine so I keep the small box. The SRD7 (normal bias) came too from another Ebay purchase with the Sr Lambda, but I prefer the sound I get form the MK2 version...


However I just discovered a little problem : despite the SRD7SBMK2 is the only one I purchased new in a store (all others are Ebay or second hand purchases) in 1995, I discovered today that it lacks a bit of treble compared to SRM1 and the old (!) SRD6 SB. I can hear it, and check it , since I have an Audio Source EQ-One/spectrum analyzer+its own calibrated microphone : for instance measuring Lambda frequency response using the pink noise generator (I use a carboard box with a hole for the mic : simple and effective tool), 16KHz is always 2dB lower on SRD7 SB MK2 than on SRD6 SB and SRM1 MK2 pro (these two having the exact same frequency response on my Lambdas. 8kHz and lower frequencies are all OK on SRD7SBMK2, only above 8kHZ does this problem appear )


Since, despite this issue, my SRD7 SB MK2 has definitely the better sound of the 4 amps (I wonder why, but this is very obvious on midrange, even a bit more "textured" than on SRM1 MK2 pro ! the soudnstage is also a bit better, more "3D", if possible), I still want to listen my Staxes with it first, and think about a worn component that could explain this little trebble issue ??


I did not open it yet, I think since it is self biased there are not many components inside, so anyone has some idea about what I should replace to get a flat frequency treble response ? Some capacitor(s) ? I know nothing about electronics...I just know how to use a solder iron, but nothing more...should I send it to Stax ? 


Of course :

-I made all possible combinations between the gears listed (and between "A" and "B" speakers outputs of the Dual CV1460, the result was not changed at all)

-Irepeated several times the measurements


and finally always found the same result : my SRD7SBMK2 has a 2dB audible high frequencies loss :-(


Thank you for your help

Edited by Benoit62 - 10/14/13 at 10:58am
post #2 of 11

The SRD-7 Mk2 and a normal SRD-7 are pretty much identical.  The bias supplies are naturallty different but in the same vein.  Not much to go wrong in them as the audio is routed through the front switch and to the main PCB where there are some resistors to protect both the transformers and the amplifier.  From there it goes to the transformers and then back out again to the sockets. 


Now on the SB units the bias supply is powered by the HV audio signal coming off the audio transformers put through another transformer to step it up even further.  If anything were to limit HF response then that would be my culprit. 


What you could do is remove the bias wire on the SB unit temporarily and feed the phones with a Pro bias from something like the SRM-1 Mk2.  I made a special cable to do just this and to test bias supplies. 

post #3 of 11
Thread Starter 

Thank you very much Spritzer :smile:, this is an excellent idea and easy to do, I will do this the next weekend and then post the results !

post #4 of 11
Thread Starter 

Spritzer, I need your help again...


I just opened the SRD7MK2SB to look how it is made inside. I took some pictures (unfortunately I have not posted enough messages yet to show pictures here...).


Well, there is not much room behind the headphones sockets without removing the transfos, so I think the best way is to disconnect the proper wires not fom sockets, but from the card itself, it is very easy (I think there is too risk of damaging the sockets because several wires are linked together between the Normal and Pro sokets, re-soldering them would be difficult to me)


So, I guess I have to remove the big green (normal bias) and grey (pro) one (these two wires go direct to the sockets), and lead them to the SRM1 MK2 ouptout sockets, am I wrong ?


I have two other questions :


- watching the SRM1 MK2 amp from outside face, in which hole should I put the wire to get the bias voltage from it ? (I hesitate between down right and down left ; I saw drawings on the web, but do not know if they show the part from inside or from outside....I dont want to destroy my amps and/or headphones!)


- ont the Pro socket I can see 4 small grey components attached (with "471.....31" written on it), do you know what they were designed for ??? That's strange, I tested my very old SRD7 (not SB) today, and he does not have the treble weakness....nor this small components since it has two normal bias sockets



post #5 of 11

You can host the pictures elsewhere and just provide links.  I use imgur.com but there are dozens and dozens of sites like that. 


Removing the wires from the PCB would be a good idea and just run a wire directly to the Stax pro socket on the SRM-1 Mk2.  The bias connector is the one in the lower right hand corner.  This is the socket as seen when standing in front of the Stax amp:


If you have an old computer PSU sitting around then the male pins in the old molex connectors work perfectly as pins for Stax sockets. 


Those parts are zener diodes which are supposed to clamp the output from the transformers.  For normal bias headphones they should only see twice the bias voltage which is 460V.  Any more than that and they can arc. 

post #6 of 11
Thread Starter 

Thank you very much, your pic is very clear.


I'll purchase an cheap mini amp to make the tests and preseve my Dual from any wrong manoever


Here are links showings the card pics, I think, too, that removing the thick green and grey wires from there is the easiest and safier way to do :






I have plenty of old stuff from various parts, i'll check them and if I don't get somethnig the right diameter for Stax plug I think a simple wire with solder at one end can do....


A have also an old pair of Lambda drivers (my very first, bought new in 1988, drestroyed 1994 in a stupid accident ; I think I would still use these headphones otherwise...this bad experience learnt me how vulnerable electrostats are) I could use the cord removing drivers from it and using the correct wire would be an intersting way to prevent any form of electric schock :D 

post #7 of 11

Removing green and gray would be a good plan.  :)

post #8 of 11
Thread Starter 

Hi Spritzer and sorry for being so long, I had been kept away from Hi Fi for a while by some personal issues...


I made the external bias test and had surprising results (I used an old Lambda wire to "feed" the headphones from a SRD7 box, this giving me the perfect plug...)


1) I was very surprised hearing almost no pink noise when feeding the headphones (SR Lambda normal bias) direct from the SRD7 (I did not made any wiring error : I could hear a sound, but so muted and etched than I gave up this way, turning my Dual amp too lod drove me afraid to break it...)


2) Then, as you advised me, I took the bias from the SRM1 MK2 : it worked fine this way (I still don't understand why the bias from SRD7 was so week, since used by itlselfs to listen to headphones it works normally) but I don't know why, "feeding" the SRD7Sb bias with the SRM1MKII, the general sound hedphones level decreases pretty fast...


the result was : setting the exactly the same midrange reference levels, I still get the treble (8K and,more : 16 K : -2dB, but listening to music it seems to be more...) attenuation trough the SRD7SBMKII vs : SRD6SB, SRD7, SRM1 MK2, all these tree giving exactly the same treble level...


Wouldn' this come from SRD7SBMKII audio transformers themselves ?? I seems than I can see on the + and - sockets of each channel, thin red and blue wires going straight to them...

Or, if the problem comes from the self bias transformer, is it the same model on SRD6 and SRD7 SB ? Since frequency response is OK on my SRD6 SB, may I swap it to put it in my SRD7SBMK2??

Edited by Benoit62 - 12/1/13 at 7:37am
post #9 of 11

How the bias is generated in the SRD-7's is slightly funky so they need an amp to power them while trying to use the bias supply. 


So the bias decreased rapidly when fed from the SRM-1 Mk2?  That shouldn't happen unless there is a break in the line somewhere or it is drawing too much current. 


The transformers could be damaged but so could the compansation network in front of them.  It's a combo of resistors and a thermistors which could be to blame here. 

post #10 of 11
Thread Starter 

Thank you again


yes it decreased pretty fast out of the SRD7SBMKII fed by the SRM-1 Mk2 , I don't think there was a break in the line since it is an old SR Lamba wire (very useful!) and it worked perfect when it was on the headphones ; I repeated it several times


I don't know how to locate the cause if there can be more than one...maybe I should look after another SRD7SBMKII on Ebay, but mine was purchased new and I never hurted it...my two other boxes (SRD7, and SRD6/SB) are normal bias and both do not sound as great (especially in midrange) as my  SRD7SBMKII... (I don't know why, but it is so obvious!)


Can the SRD7 transformers (or SRD6SB) be put inside the SRS7SBMKII ? It could help to check if they are the reason or not...anyway it would be a big job to me ! 

Edited by Benoit62 - 12/15/13 at 3:41am
post #11 of 11

The SRD-7 transformers should all be interchangeable but over such a long production run there might be small changes. 


I would also try to connect them directly to both the amp and the Stax outputs.  Less variables that way...

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