Head-Fi.org › Forums › Summit-Fi (High-End Audio) › High-end Audio Forum › Can I drive Koss ESP-9 phones with a Stax SRD-7 energizer?
New Posts  All Forums:Forum Nav:

Can I drive Koss ESP-9 phones with a Stax SRD-7 energizer?

post #1 of 26
Thread Starter 

Not sure if I read it or imagined I read it, but can it be done?


The Koss E/9 Energizer is not well respected.  I own it too, but I think the Stax energizer is more respected, so I'd like to try to mate them up if it won't hurt me or the phones.



post #2 of 26
Thread Starter 

I guess the bias voltage is the problem.  Stax "normal" is 230 and Koss is 630 (not sure exactly).  Maybe what's described in this link will do what I want?





post #3 of 26

Stax has two kinds of bias: "normal" (old) is 230V, "pro" (new) is 580V. The Koss ESP/950 (the new one, not the ESP/9) uses 600V, so that one can use any Stax energizer with a simple plug adaptor.


I have no idea what the ESP/9's bias voltage is. The plug seems similar to Stax's, but don't even try to connect them unless you make sure the ESP/9's bias voltage is equal or lower than Stax's!

post #4 of 26

Will it work without major mods?  No and can cause damage to any Stax gear you try to plug them into.  The plug may be the same but electrically they are very different. 


The Koss bias is +360V but it is used very differently from the Stax setup and how the phones are driven is very different.  Part of the circuity is in the cups and the Koss aren't fully balanced like the Stax models. 

post #5 of 26
Thread Starter 
It might be helpful for me to explain.  I have just acquired both the Koss E/9 and Stax SRD-7 energizers plus some Koss ESP-9 phones.  And I already have Stax SR-40 + SRD-4.  Of course I think I should be happy, but I've heard the Koss E/9 is the weak link mostly because of a PS hum, but it also seems to get general overall bashing.
So my goal is to either improve the Koss energizer or modify the SRD-7 to run any headphone.  And if I can modify both, I will be able to run either headphone in any of 3 energizers (I think - please let me know if I'm missing anything).
So what are your thoughts on all the above?  Is it effort well spent?
OOPS!!  I created this post before refreshing and missed Spritzers and Wildstar's replies.  Very sorry - I think I was asking questions that were answered.  I will leave my comments so everyone will have the context of this thread, but please forgive my mistake.

Edited by bigbargain - 6/14/13 at 12:15pm
post #6 of 26
Thread Starter 

I suspect the previous owner plugged the Koss ESP-9 into the Stax SRD-7 (not Pro) as they commented that "they could, but only the left side played".  What kind of damage if any should I be checking for when they arrive?

post #7 of 26

I don't see anything wrong with the E.9 design.  It's overly complicated and Koss had their own crazy ways of doing things but if it hums then it's probably the power caps that are well past their useful age.  The grounding was a mess in these which is why there are so many versions.  The first model is an amp killer as all the grounds are tied together with the - inputs so if you amp has the - floating (bridged amps) it will burn out very quickly.  The A model might be this way too but the B has this finally fixed.  Stax never did this though, even on the first boxes when there were no bridged amps.  


The components inside the cups could be damaged and so could the diaphragm.  Depends on how much voltage the parts saw.  


As for the original question, the ESP9 can be run off a SRD-7 but you need to modify it so it works as a Stax headphone.  That means new cable and 3 wires to each cup.  I did this to a Koss ESP6 where I removed all the active stuff from inside the cups and hooked a 6 pin cable directly to the drivers.  I then moved all the active bits to its own chassis so they could still be used as intended.   

post #8 of 26

To get rid of the hum from the E.9 box, it may be well worth trying to insert an isolating transformer between the wall outlet and the box. There may be some inductive interaction between the circuits (the amplifier driving the signal step-up transformers on one side, and the-very closely mounted- transformers delivering “bias” voltage to the diaphragm through the voltage multiplier on the other). The current here is very small, so the isolating transformer need not be large. A good idea in my opinion would even be to reduce the AC voltage at this point, since the bias factory setting seems to be on the high side. An adjustable auto-transformer followed by an isolating transformer would in theory be a nice solution, allowing you to easily find the “sweet spot” for the bias voltage.  In practice this would mainly mean a voltage which reduced the sporadic (discharging) noises from the drivers to acceptable levels..
In any case, with the suggested use of an isolating transformer, it  will be important also to experiment with the orientation of the plugs in the wall outlet.

(myself i chose another route with an outboard setup to deliver AC to the voltage multipliers, not  having isolating transformers at hand when I began experimenting, but their use was the “final touch” in my set-up also, hence my suggestion).

Cheers, Olaf

post #9 of 26
Thread Starter 

Thanks!! Spritzer and Olaf.


Been a few days, as I'm waiting for the Koss to arrive.  Guess they haven't shipped yet.  I was hoping to take them to this Saturday's event here in Chicago.  I don't think i'll have them in time.


Spritzer-  You mention the potential for damage was dependent on how much voltage.  Based upon the description I got from the seller, ESP-9 were plugged into a normal bias SRD-7.  I don't have them yet, but what damage should I be looking for and any suggestions for how to diagnose any resulting problems?


I am getting the Koss E.9 and Stax SRD-7 energizers, and the Koss ESP-9 phones from the sister of the original owner.  Sadly the brother died a couple years ago, which is why she didn't know not to experiment with mixing the Stax phones with the Koss energizer.


Likewise, I'm not too familiar with them either - I just know them by reputation, so have some studying to do.


Once I have sorted everything out, I will rebuild the Koss energizer and/or maybe try the trick Olaf describes and use the Stax energizer to run the Koss phones.


Because I also tinker with tube amps, I am intrigued by direct drive (drive the stats directly from the tubes without OPT's) so could see myself experimenting with that also.


At the end of the day I will continue my pursuit of perfect sound - I have such a good imagination; I'm certain I will see improvements with each iteration.

post #10 of 26

I'm more worried about the AC audio voltages than the bias.  Plug them in and turn the volume to max with something playing and the parts would see very high AC voltages. 

post #11 of 26
Thread Starter 
Thanks. So if she ddidn't hear something does that imply no problem? And how can I diagose the problem that may have resulted?
Originally Posted by spritzer View Post

I'm more worried about the AC audio voltages than the bias.  Plug them in and turn the volume to max with something playing and the parts would see very high AC voltages. 
post #12 of 26

That is the hard part, parts may just become marginal and show very odd behavior.  No good way to test them either as nothing tests under the same conditions as the circuit exposes the parts to.  This is the truly hard part about electronics... 

post #13 of 26
Thread Starter 

OK!! the ESP-9 + E/9 Energizer arrived today along with a Stax SRD-7.


The previous owner did some testing, but I not elaborate, so I'm hoping she just didn't let the phones change up using the Self Energized mode or maybe simply tried it on the AC energized mode. So I will try to get them going in Self Energized mode.


In the mean time, I will need to start looking for a power cord.  If anyone here has a Koss E/9 power cord, I'm interested.   And Most Important - what is the configuration of the 3 prongs in the power plug on the back?? I suspect I will be making one soon - not sure there's enough room to install an IEC plug, and for now I'm going to avoid cutting it up.

post #14 of 26

In the picture you can see that the leads of the mains cable go directly to the component board (brown and blue lead, respectively), The ground lead (yellow/green) goes to a soldering tag attached to one of the screws holding the top transformers in place.






The two small transformers on top are supplying about 200V AC to each channels voltage multiplier. The connections here are very simple, as you can see in the schematics.


(Never mind the red marks, it is where I cut and re-connected the secondary leads when making an outboard AC supply instead. This I did to get rid of hum and "magnetic interference" -and also to be able make the bias voltage adjustable, as mentioned earlier ).

Back to the mains leads. Electrically one of the leads is connected through the fuse to the primary winding of the two small transformers, the other goes to the other end of the same winding. (The route through the component board is electrically un-essential, except for the connection to the (un-essential) lamp (L).


Good luck! Olaf




May I add that in my opinion the ESP-9 is extremely well suited to the use of EQ, due to the surprising big headroom in the bass. (actually I mostly use a 3-band conventional but high quality tone-control unit). Otherwise the dominating mid tends to give the sound a rather tiresome, hard and somewhat metallic character. Corrected –and with the cans supplied with fresh foam- these become fabulous sounding headphones, if you ask me: Very objective, powerful and engaging (unless if airiness and big spatial presentation is primarily what engages you, that is). I am tempted to use the word "masculine" about the sound. They are my favorite headphones nowadays!


They ideally need a decent amp to feed the step up transformers, but interestingly the power section of an old and inexpensive NAD 3020 suites them quite well, adding a tad air, mild upper bass boost and a little (feminine) softness.


(In any case their high weight and low comfort will give ones masculinity a certain boost..).


post #15 of 26
Thread Starter 
So I think I see my game plan coming together. But 100% of the times people say to clean out the rotting foam from the ear cups, so that should be my first step. So how do I disassemble the cups?? I think I read somewhere I may have to remove the pads. I really don't want to as they're in ok shape and not leaking and I fear that if I have to stretch them to expose screws or something, they will start leaking. Help! Thanks in advance. P.S. - and thanks for all the help so far...
New Posts  All Forums:Forum Nav:
  Return Home
  Back to Forum: High-end Audio Forum
Head-Fi.org › Forums › Summit-Fi (High-End Audio) › High-end Audio Forum › Can I drive Koss ESP-9 phones with a Stax SRD-7 energizer?