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Koss ESP/950 electrostat thread


deepak

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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

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  • 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?

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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...

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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.

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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.

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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.

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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.

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