Koss Model 2 stats

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#3
Hi! Bevo, I wouldn't swear on a stack of bibles but I seem to recall that the koss stats were made under contract by Acoustat for a very short time. They are indeed very rare birds! Do they have dome tweeters? That seems to ring a bell also.
 
#4
Koss Model Two Speakers

Hi,

I recently bought a pair of Koss Model Two Speakers. I tried to get some Information about those speakers on the internet but failed. Also I am looking for some data in regards to the speakers before I put my hands on (Datasheets, manuals).

I would appreciate any hints...
 
#5
I've owned both Koss !A's and 2's back in the 70's. The Model 2 originally had a dome tweeter but I replaced them with JVC ribbon leaf tweeter and they blended much better. They were made in WI. I used to run them stacked. I've also owned several Acoustat models, ML CLSII's and others and now use DIY ESLs. Out of all the commercial ESLs I've owned I liked the sound of the Koss ESLs best. The downside was lots and lots of stator problems. About every 6 months you needed to take a heat gun to the diaphrams to retighten them. Koss had a guy that did nothing but rebuild these panels for a few years but then discontinued servicing them at all. The stators are simple and if rebuilt with a more modern diaphram material they would probably be much more reliable. Except for an arc that burned a hole in the HV board I didn't have any other problems. They were also capable of a very respectable SPL level.

Dave
 
#7
I've found them to be high manufacturing quality - no problems with diaphragms either - but a deeply flawed overall concept/design.

They were sold as an (expensive) Electrostatic but had a hidden (grille attached w/ rear screws) Philips dome tweeter to take over from the E/S HF module that rolled off above 3khz !
 

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#8
Even tho the Koss factory is near me, they would not give me any info on the 3 old speakers I found, so I traced out the schematic myself.
 

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#9
Does anyone know what the bias voltages were? I found a 1975 BAS meeting report that says it was only 7.5 kV for the bass panels, whereas about 18kV would be optimal for their very wide air gaps (about 0.3" stator-to-diaphragm), so this would seem to have more than wasted the gain from using multiple diaphragms. The same report also indicated that the diaphragms were of polyethylene, which seems unlikely and contributes to my doubt about the reported voltage.
 
#10
As you can see from the schematic I posted, they used a step-up xformer & a 10X voltage multiplier to supply bias from the speaker drive signal. I found this to be inadequate as the 3 sample speakers I tested had weak output from the bass & MF panels making the speakers sound ridiculously bright from the dynamic tweeter. The bias generated peaked at less than 600v. This, coupled with the HF roll-off of multiple-layer panels made for a 'perfect storm' of failed performance.
When I separated the layered panels and applied 2.5kv bias from external PS, I got normal output and hi freq response as can be seen from my homemade version. Note additional dismantled HF panel elements next to TV set.
 

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