Koss Esp-9 to ESP-9b Conversion... Help needed with grounding scheme - diyAudio
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Old 3rd February 2013, 06:31 PM   #1
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Default 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

Click the image to open in full size.

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....

Click the image to open in full size.

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?

Last edited by justblair; 3rd February 2013 at 07:32 PM.
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Old 22nd April 2013, 08:01 PM   #2
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Old 26th January 2014, 06:54 PM   #3
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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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Old 1st February 2014, 07:12 AM   #4
RNMarsh is offline RNMarsh  United States
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Originally Posted by NottsLad View Post
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.

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Old 13th October 2015, 08:20 PM   #5
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Not to engage in thread necromancy, but I was wondering if anyone had done this mod and how it turned out.
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Old 4th November 2015, 02:02 AM   #6
Struth is offline Struth  Canada
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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
London Power / Power Press Publishing
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