Koss ESP-9 to ESP-9b Conversion... Help needed with grounding scheme

Hi There.

Not posted here for a long time, but perhaps someone here will be good enough to help me out?

I purchased a set of Koss Esp-9 from E-bay a while ago and have finally got them out of the packing box they went into during a move a while back.

They work reasonably well, though I think I will need to give them a decent renovation to get the most from them. Before I do I thought I would turn my attention to how I am going to drive them. Ideally I would like to drive them from a decent little digital amplifier module that I have a few of. But that currently is not possible due to the grounding arrangement of the Esp-9. The problem can be seen in the schematic below


You can see in the schematic, the left and right channels of the energiser box share a common ground. The diagram shows this ground tied to Earth. Though my multimeter tells me that it is not. I may be reading the schematic wrong. I have always hated Earth wiring! This shared ground for the left and right channels is a problem for a lot of amplifiers as not all can tolerate having their negative terminals connected to a shared ground. Digital amplifiers certainly don't like it.

Now I know from my internet research that the later Koss model the Esp-9b came with an updated energiser box that had a new grounding arrangement. This was apparently introduced because of compatibility issues with some amplifiers. The only other difference I have read is someone suggested that separation improved on the later model (possibly linked to grounding differences). I have hunted high and low, but I cannot find a schematic for the Esp-9b anywhere. But I would like to try and find out what the differences are and document a conversion from 9 to 9b. It would let me connect more amplifier sources to my headphones and I reckon might be of interest to other Koss Esp-9 owners.

I am guessing that Koss would want to solve the grounding problem without altering too much the construction of the energiser box or headphones, so I have made a guestimate as to how they may have separated the grounds....


Now here is where the help comes in...

First things first, can someone with a good knowledge of grounding schemes just check what I have done and tell me if it will function safely. I am especially interested in whether I need to tie the shared ground to earth or not. I guess so for safety reasons. I guess that on my energiser it is not tied to earth to avoid a ground loop between the energiser and the amplifier

Secondly. If anyone out there owns a Esp-9B set and a multimeter... any chance that you would be prepared to sketch out the schematic for the Esp-9b?
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When I got the same headphones 20+ years ago I connected them to an older valve amplifier and they worked fine. In my career moves they ended up in a box never to see the light of day until my son started buying 'expensive' headphones. I remembered I actually owned a good set myself, so I connected them up to a newer amplifier. Initially, they worked fine but then the would sound would cut out. I found I had to disconnect from the amplifier (or receiver) and power cycle the amplifier for the speaker output to start working again. This basically supports what you mention about the newer amplifiers not liking the negative output to the speakers connected. In fact when I checked the inputs to the /E.9 energizer I was surprised that the negative speaker inputs were connected.
I have looked at your proposed modification and I think your logic is sound and it should work. My question to you is did you try your modication and did it work? If so, I will modify my /E.9 accordingly.
By the way, thanks for this schematic.
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My question to you is did you try your modification and did it work? If so, I will modify my /E.9 accordingly.
By the way, thanks for this schematic.

Ditto here... I have a pair in a box for a long time also and will do mods and re listen if it works well.
I would replace all the ceramic caps that are not part of the power supply with film type.

Hi Guys

It's surprising that Koss would bother to provide two bias supplies but then not keep the channels entirely separate? One more wire through the headphone cable would have allowed this and made the dual supplies "sensible" AND allow much better connection to any amplifier.

Stax uses a shared bias supply so some link between the channel grounds is required in their setup, too.

However, I believe that if one adopts the modern ground-isolator-link approach, then the shared bias supply or shared bias reference can be made nonproblematic. In the Koss schematic above, R9, R10, C15 and C16 provide the ground linking demanded by the shared connections imposed. if the Rs are 100R, the Cs 100nF and these are parallelled with anti-parallel diodes, then we have the mnodern ground-isolator links. The shared reference or bias supply can still find a DC path to each channel but the resistance is high enough to break any ground loop between the channels.

I don't see why a digital amp would have any more problem with this than an analog amp? The problem could arise with a computer output since computers were not designed to be integrated with sound systems.

I also don't see why there are caps in the cups between the stators and the diaphragm.

I don't know how difficult it is to take apart the Koss headphones - Stax 307 is not difficult - but my preference would be to add the sixth wire and make the channels completely independent.

Have fun
I know this is an old post, but some may still be interested. I did the modification shown above as part of a rebuild of the energizer unit. I also removed the speaker / headphone circuit. The input wires are now connected directly to R15 and R16. I do not have any connection between the audio ground and earth.
The unit had a lot of hum from the mains voltage before the mod, now there is a barely detectable amount of hum. I am not sure which of the changes had the most affect, but I suspect that removing the common ground was the most important. Now these 50 year old headphones sound really good.
They don't have the power and presence of my ARC fed Quatros. But the detail and nuanced sound is pretty addicting.
I am trying to decide if I should do a complete rebuild with all new components.
Update after 2 months of listening. I am still happy with how they sound. Tons of detail and nuance.
I bought a set of Brainwave memory foam earpads on Amazon. They are pretty comfortable, much better then the Koss OEM earpads.
I always found the Koss head band pad to be uncomfortable. We took a flight on a small airplane recently and we had to wear head phones for well over an hour. I had no problem with the pad they used. I ordered a David Clark SuperSoft Double Foam Head Pad #40688G-36. It is very comfortable. I did have to make some minor modifications to make it work. I highly recommend both products.
I am trying to figure out why the left channel has a slight ticking sound. There is no noise in the right channel. I tested the rest of the system with speakers and there is no ticking sound. It happens at any volume setting. This one has me stumped. Could it be the film being charged?
I ended up fixing the energizer unit that I thought was bad. It turned out to be bad connections, not a bad transformer. I was still having trouble with the left can losing all volume for a while. I suspect that the old wiring is falling apart.
I bought ESP7's off of fleebay. The ad claimed they were from 1969. I cannot confirm this. All the wiring on the ESP7's is perfect. The cords are much more flexible then the ones on my 1973 EXP9's. I think the 7's might have been re-wired by Koss at some point. The wiring diagram in the 7 manual says that the circuit in the cans is different then the 9's. The 7's are not supposed to have C2 and C4. My 7's have those caps. The energizer that I got looks to have been modified with the addition of three parallel resistors in the audio stream. They were wrapped in packing tape. Kind of bizarre.
I was able to plug the 7's into the 9 energizer. The combo works perfect. I moved the headband upgrade to the 7's. Now they are my everyday headphones.