Jump to content

Koss ESP/950 electrostat thread


deepak
 Share

Recommended Posts

after a bit more listening today I feel the smegma pros still have the edge on them for overall balance. The 950 is a bit shouty in the upper mids and are pretty bright. All from the E90 amp and SRD-7Mk2. I'll wait until the eXstata gets here for proper full evaluation but so-far the Stax still have it.

In the end I sold the ESP-950 and kept the Smegma Pro. You should be proud. :D

Link to comment
Share on other sites

  • 4 months later...

So, I know that it is pretty much public knowledge that the ESP950 bias is 600v, and Koss even says this, but has anyone actually looked at the bias circuit? I got my ESP950 back from RMA today, and noticed that is has the squeal that so many of us know just too well out of my SRM1/mk2, but is dead quiet out of the stock E/90. This has been really, really bugging me, so I finally went ahead and tore the E/90 open to get a look at the bias supply. Well, its pretty simple, a 10M resistor from the 600v (measured 612v) rail to the pin connecting to the bias, and another 10M resistor from that to ground, in parallel with a 0.022uf 630v film cap. My measurement of the bias voltage, using a DMM with a 10Mohm rated input impedance, gave me 202v, just what would be expected from a 10M-(10M||10M) divider. I dunno guys, this sure looks like the E/90 is really just putting out 300v bias, and if that is the case, it would really explain the all-too-common squeal these tend to develop and we have been attributing to dust. Thoughts?

Link to comment
Share on other sites

Well Koss does state the bias voltage to be 600V:

SPECIFICATIONS

ESP/950

Physical Specifications

Size of earcup: Approximately 150 mm h x 100 mm w x 40 mm d (including cushions)

Cushions: Large foam filled leatherette type for high ambient noise isolation and conformance to facial structure.

Headband: Extendable, stainless steel band with self adjusting, pivoting yokes and headband release mechanism.

Headband Cover: Wide leatherette style vinyl with 12 mm thick polythylene foam padding.

Yokes: Horizontal and vertical pivoting with integral headband lock and detach mechanism for convenient storage

Weight: 12.45 oz. (353 gm)

Cord Length: 47 inches (182.9 cm)

Extension cord length: 72 inches (182.9 cm)

Electroacoustical Specifications

Frequency response: 8 Hz to 35,000 Hz

Sensitivity: 104 dB, 100 Vrms differential input a 1 kHz

Diaphragm radiating surface area: 7 sq. in. (45.6 sq. cm)

Diaphragm: Ultra-low mass 1.5 micron C grade polyester film electronbeam coated with a proprietary semiconductive material 250 A thick.

Acoustic Design principal: Circumaural open air design incorporating minimal acoustic damping.

E/90 Energizer/Amplifier

Physical Specifications

Dimension: 65mm h x 110 mm w x 155 mm d

Weight: 17 oz. (482 gm)

Electrical Specifications

Frequency response: 1.6 Hz to 50,000Hz (-3dB points) 100 vrms differential output

Distortion (THD+NOISE): 0.001% at 1kHz and 100Vrms differential output

Input Impedence: 100k ohms

Input level: 1 Vrms (for full power output)

Voltage Amplification: 60 dB

Audio output voltage: 600 Vrms differential, 2300 Vrms push-pull with soft limiting

Channel Separation: - 80 dB at 1 kHz, 100 Vrms differential output

Signal to Noise: -100 dB at100 Vrms output

Bias voltage supply: 600VDC

But I've been working after similar lines that the Stax bias supplies simply aren't compatible. 10M/10M will half the voltage but it is strange that there is no ballast resistor. I'm going to ask KG...

Link to comment
Share on other sites

I know what the Koss spec is, and I am wondering if it is simply wrong. Both my analysis and measurements of the E/90 I have right here show 300v. Pretty obviously this isnt a recent change, as my PCB is dated 7-11-90, and has no obvious modifications made to it. I tried adding in a 5M ballast resistor inline with the ESP950 driven off the Stax amp, no change. Its just fine plugged into normal bias. I wonder if just some drivers are able to handle the voltage and some arn't. Also explains why Smeggy didnt have catastrophic results when he ran his ET1000 off the E/90.

Link to comment
Share on other sites

I know what the Koss spec is, and I am wondering if it is simply wrong. Both my analysis and measurements of the E/90 I have right here show 300v. Pretty obviously this isnt a recent change, as my PCB is dated 7-11-90, and has no obvious modifications made to it. I tried adding in a 5M ballast resistor inline with the ESP950 driven off the Stax amp, no change. Its just fine plugged into normal bias. I wonder if just some drivers are able to handle the voltage and some arn't. Also explains why Smeggy didnt have catastrophic results when he ran his ET1000 off the E/90.

Good find man. I never thought to measure this since I assumed it was fine but yes mine most definitely squealed out of the KGSS until I guess I lucked int a pair that worked after 3 rmas. All pairs except the first one were dead quiet on the E/90. This would explain an awful lot.

Link to comment
Share on other sites

Could be but could you do one thing for me, measure the amp output. If the amp sits at -300V then a +300V bias would result in +600. I seriously doubt that this is the case but it was something which KG did with his DC coupled tube amp.

Nope, all outputs measure under 1v offset.

Link to comment
Share on other sites

My meter is rated for 10M input impedance while measuring DCV, so that in parallel with the 10M resistor to ground in the bias circuit would give 5M. 600v through a 10M/5M divider should give 200v, which is what I measure when doing this. Also, the 300v figure is just from looking at the circuit (10M resistor to bias pin from ~600v rail, 10M||0.022uf from bias pin to ground.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.