Two years ago, Sennheiser sold, through Meier Audio, around twenty brand new HE90 units.

After several buyers mentioned they had problems (because they used the HE90 on other amps than the Sennheiser HEV90 amp), Sennheiser agreed to modify a bit the headphones so that they can work on any electrostatic amp on the market (because, if I understood correctly, the HEV90 amp had a very different bias than most of other current electrostatic amps).

Does anybody know exactly what were the modifications made by Sennheiser at this new batch of Orpheus headphones?

In case you're considering buying one, I had a second batch Meier group buy HE90 that was originally purchased by headfier carlosgp (not sure) and still had the channel drop out problem using Stax amps. Sennheiser subsequently fixed the problem for free.

The main changes AFAIK were done to the diaphragm and how the coating interacts with the charge put on it. The HEV90 has a 10meg loading resistor while Stax never goes above 5meg with any of the energizers. Now whether Sennheiser just installed a 5meg resistor inline with the bias or altered the coating on the diaphragm I don't know but it must have been something along those lines.

I had Sennheiser Staxify my set and I had no problems with it though the diaphragm would stick to the stators when forced.

Thanks!

Now how would perform those "special batch" HE90s when fed by a HEV90???

I mean, having them adapted to STAX amps would have as a consequence a bad performance with the HEV90?

Thanks!

Now how would perform those "special batch" HE90s when fed by a HEV90???

I mean, having them adapted to STAX amps would have as a consequence a bad performance with the HEV90?

I tired mine with a HEV90 and there wasn't much of a difference and nothing that can't be attributed to the difference between each and every HE90.

The main changes AFAIK were done to the diaphragm and how the coating interacts with the charge put on it. The HEV90 has a 10meg loading resistor while Stax never goes above 5meg with any of the energizers. Now whether Sennheiser just installed a 5meg resistor inline with the bias or altered the coating on the diaphragm I don't know but it must have been something along those lines.

I doubt that it was the resistor, as I tried the resistor myself before sending the headphones to Sennheiser to be "Staxified". AFAIK Sennheiser has not publicly said to anyone exactly what the issue was. The person who would know would be Jan Meier, but he waffled when I asked what the issue turned out to be.

In theory, the mod was not supposed to affect sonics at all, just get rid of the stator sticking issue with Stax amps.

I doubt that it was the resistor, as I tried the resistor myself before sending the headphones to Sennheiser to be "Staxified". AFAIK Sennheiser has not publicly said to anyone exactly what the issue was. The person who would know would be Jan Meier, but he waffled when I asked what the issue turned out to be.

In theory, the mod was not supposed to affect sonics at all, just get rid of the stator sticking issue with Stax amps.

It would have been much wiser to alter the coating to make it hold less charge then to add a resistor in line. That would make it play well with a 5m bleeder but also works fine with the HEV90 bias supply so that's probably what they did. The stators are very close to the diaphragm so if the charge on the diaphragm is too big it gets attracted to the stators making that nice sucking sound. It's comparable to the "electret effect" where the a channel imbalance springs out of nowhere due to an alien charge of the wrong polarity.

Was waiting to see if someone other than myself knew the right answer...

guess not.

Its actually 3 resistors added per side.

A series 5 meg resistor to the diaphram and 2 x 100 meg resistors from

the diaphram to each stator. Takes about 9% off of the bias, so down

to about 525 from 580. Works fine with any amp with a stable output

voltage of 0 volts dc, otherwise creative things might happen.

If you plug those headphones into a real hev90 plug set for 500 volts the

bias will now be 10% too low.

Ok, thanks for clarifying that. When I opened up one of the capsules on my former set (to check if the polarity was off) I didn't see anything that was out of place and certainly no resistors on the tabs. Now they could have hidden them inside the driver assembly but that would have been a lot of trouble for something like that. I checked the plug as well and no resistors.

Works fine with any amp with a stable output

voltage of 0 volts dc, otherwise creative things might happen.

now, who will be the first to try them out on Ray's new amp?

Was waiting to see if someone other than myself knew the right answer...

guess not.

My set had the driver units replaced by new drivers sent from Germany. Sennheiser US did the assembly, and I doubt that Germany would have sent new drivers if all that was needed were resistors, unless those resistors are in the driver assembly itself. The headphones themselves never left the US.

Mikhail has taken apart both old and new HE90's. He believes the "Staxification" to simply be spacers between the stator and the membrane. Apparently, the new batch were built with a smaller gap than the older ones, which he believes was the source of the problems. Simply increasing the gap to the proper distance may have been the fix.

I attempted the bias mod that you posted on Head-Fi for the 007t a couple years back, in addition to altering the bias loading resistor, but neither had an effect on the membrane sticking to the stators. Accordingly, I'm inclined to believe that bias voltage was not the problem, but I'm keeping an open mind on this one.

If photos exist of a resistor mod, I'd love to see them.

How much did that second batch cost? (I know, if you have to ask...)

How much did that second batch cost? (I know, if you have to ask...)

Around $4500 I believe if not $5000. The HE90 commands about 8 to 8.5k in a used market.

How much did that second batch cost? (I know, if you have to ask...)

The second batch was $5000 from Meier Audio (the only seller). IIRC this was actually less than the cost of an old stock HE90 (without amp). I believe that Jan sold out in two or three days.

now, who will be the first to try them out on Ray's new amp?

lolz ray's new amp.

Sold the headphones only for 8k + shipping but that was almost a year ago.