Stax and other electrostatic amp information

[deepak]

Copying KG and Birgir's posts from the Stax Huddler thread

The 727 is a local feedback version of the 717. The 727 has a current source, the 717 has a resistor. Otherwise virtually identical.

The 727 runs the output stage at higher power.

The kgss and the 717 are similar amplifiers. (4 stage amplifiers)

The kgsshv and the 727 are similar amplifiers except the kgsshv uses global feedback. (4 stage amplifiers)

If people want it, the next revision of the kgsshv boards will have the local/global feedback option.

Personally i like the sound of the global feedback version better.

The 313 and 323 are 3 stage amplifiers. Same or similar to all the transportables (srmXh,srm151,srm252)

There are the 2 ac coupled single power supply amps, sr-001 and srm300

the srm-t1, t1s and 006t are virtually identical. (solid state/tube hybrid)

the 007 has twice the number of output tubes and runs the output stage at twice the power. (solid state/tube hybrid)

The T2 is a tube/solid state/tube hybrid.

The BH and BHSE are essentially the T2 output stage with the kgss input stage configured for 800 volts.

For stuff older than the srm-t1, things get complicated. spritzer has more of the really old schematics.

While we are on the subject, Stax amp history lesson.

First SRA1-8 amps were all classic tube units of varying complexity made in the 60's.

SRA-3S was the first deviation from this with a transistor front end and AC coupled tube output stage. Preamp and phono amp on plug in cards similar to the old ISA units used in PC's.

SRA-10S/12S were the first SS amps with a push pull front end and AC coupled outputstage. Run fully in Class A they get stinking hot and feature a nice regulated PSU for the low voltage circuit and a stacked PSU for the +640VDC B+. Similar plug in system as on the SRA-3S but now everything except the basic PSU is on plug in cards and the build quality is far better.

SRD-X was the first portable Stax amp and features an automotive chip amp driving a pair of transformers. Think Darkstar but with much better specs done in 1979....

SRM-1 Mk1 was pretty much the same circuit as the SRA-12S but turned on its head. Similar AC coupled output stage. I recently refurbished one of these and it is a nice little amp but the SRA-12S is better over all.

SRM-1 Mk2 is where things get tricky since there are so many variations of it. The first ones were some weird Mk1/Mk2 hybrid though fully DC coupled but later units are much more similar to the current crop of Stax amps like the 313. Now the amps are fully balanced from input to output though most only have RCA inputs.

SRA-14S is pretty much the SRM-1 Mk2 circuit but turned on its head. Better parts and some tweaks but most of the price difference is spent on the preamp capability.

SRM-3 is just a SRM-Xh with a full size power supply. It even uses the same circuit boards.

SRM-600LTD is the same basic circuit as the T1/006t but modified to take the ECC99 tube.

[TMoney]

It might also be helpful to have Kevin's voltage and current measurements in this thread as well.

Reposted from the other site:

Voltage

I get asked about this fairly often, there are a number of different ways of measuring the voltages

so here in one place is all the amps i know about, and all are rated the same. The voltage is

peak to peak voltage measured from one stator to the other stator.

For solid state amplifiers, the output voltage is usually very close to the power supply rails.

For OTL tube amplifiers, the Rp of the output tube(s) usually takes about 60 volts off of the

maximum voltage swing, for Stax made amplifiers the negative power supply is increased to

compensate for this. These are theoretical maximums, in practice the voltage swing will be less.

There is of course more to these amplifiers than just the voltage ratings, Slew rate into a

capactive load is also very important, i will be measuring some of these amplifiers and will

add slew rate at a later time.

2400 volts peak to peak (+/-600 power supplies)

koss esp950

2300 volts peak to peak (+/-575 power supplies)

KG’s new Silicon Carbide jfet based space heater.

2000 volts peak to peak (+/-500 power supplies)

Stax T2, DIY T2, KGSSHV (ixys parts)

1800 volts peak to peak (+/-450 power supplies)

KGSSHV (sanyo parts)

craig has said his new electra amp does 1800 volts, power supply unknown

Ergo Jecklin

1600 volts peak to peak (+/-400 power supplies)

BH, BHSE, singlepower ES1,ESX (after being modified to remove the 100V offset)

1400 volts peak to peak (+/-350 power supplies)

KGSS,Stax T1,T1s,T1W,313,323,717,727,sra-14s,srm-1 mk2,srm-monitor

woo audio wes

1300 volts peak to peak

sra-10s/12s

1200 volts peak to peak (+/-300 power supplies, and +600 capacitively coupled)

KG tube #1,aristaeus,HEV70,HEV90,RSA A10, Stax srm212,srmXh,srm300,Exstata

Egmont,Cirolan,SRM-1,srm-3,woo audio ges,srm310

1100 volts peak to peak

sra-3s

srm-001 (when the batterys are at full power)

More added later

Current

Here are some of the output stage currents. Remember that every single amplifier has balanced outputs, so

there are actually 4 output stages. The new eddiecurrent thing probably has output transformers, so its different.

kgss 8.3 ma x 4

kgsshv 5.5ma x 4 (you can turn it up if you have lots of heatsink)

T2 14ma x 4

ES1/ESX 8.5ma x 4 (when its modified to work correctly) (if you change out the plate resistors with current sources, up to 20ma x 4)

kg tube amp #1, aristaeus and hev90 6.3 ma x 4

kg silicon carbide space heater 50ma x 4

koss es950 1ma x 4

BH,BHSE 18ma x 4

most of the other stax tube amps are about 7.5 ma x 4

srm313,srm323 5.5ma x 4

srm717,srm727 6.6ma x 4

The rest i will have to look up later.

Edited February 26, 2012 by TMoney