JimL

If they are using 6922 tubes in the front end, they may not need a low voltage PS, since the LV PS in the solid state amps is just for the front end. OTOH, it appears there are a couple heatsinks in the PS, so maybe they do have a low voltage PS. If they are using the 727 for the rest, hopefully they will reconnect the global feedback. The output heat sinks do appear larger than the 727, and the power consumption is up to 95 watts, so it appears there is quite a bit more current in the output stage.

Just saw a brief video intro on HF. SRM-T8000 hybrid with 6922 inputs and solid state output stage, about the same max output as the SRM727

Congratulations from a fellow retiree!

By the way, I agree with you that a single MOSFET current source doesn't have a very good input capacitance. The real question is, is the BJT capacitance less than, equal to, or greater than the MOSFET capacitance. As a single device I would bet the BJT is better, but when cascoded, I don't know. The cascoding definitely minimizes the effective input capacitance.

Corrected comment above. However, note that in Pimm's measurements as posted by laowei, the 2 gigohm resistor impedance also deteriorates above 1 kHz. Actually, if you figure out the effective capacitance where the deterioration occurs, it is on the order of a few pf, which is more than acceptable, considering that the capacitance of the headphone is about 100 pf. The added load on the active device is negligible. The BJT may appear to be more linear because its impedance is significantly lower. Remember, if the overall impedance is 10-fold lower, but the effective capacitance is the same, you won't see the deterioration until 10-fold higher frequency. For electrostatic headphone amps, I prefer the higher impedance because the capacitative part of the headphone impedance is so high. This pushes more of the signal current to the headphones.

You're correct, I was away from home when I wrote that. Pimm's measurements also show some deterioration above 1 kHz, but so does his measurement of a 2G resistor. In any case, Pimm did have some more complex current sources which exhibited even better test results than a cascoded MOSFET current source. I never claimed that a cascode MOSFET was the ultimate, but it does give excellent performance for a minimum in parts cost. The downside is the need to adjust individually, whereas a BJT current source that uses the on-voltage of a transistor or LED is pretty much set and forget. Pros and cons.

You should be able to use it with any electrostatic amp. Dr. Gilmore has suggested that you keep the level down in electret phones to below 200 volts, but that is still pretty loud. The jack is wired exactly the same as with the standard Stax phones, but the bias pin is not connected to anything.

I'm not sure that is the case. On the Gary Pimm website (which seems to have disappeared), he gave measurements of cascoded FET CCS which showed performance flat over and above the audio range - in fact, as good as a very high impedance resistor. Same for Walt Jung's articles, which showed very high, flat impedance to well above 20 kHz. And that makes sense, because although the disadvantage of MOSFETs is high capacitance, in the cascode CCS the "lower" MOSFET sets the current, and the "upper" MOSFET largely shields the lower MOSFET from voltage variations, so that it sees a nearly constant voltage over a large voltage swing, and therefore produces a constant current regardless of voltage swings, which translates to a high and consistent impedance even to very high frequencies, equivalent to a very low capacitance.

Apparently they demonstrated the latest prototypes with a Blue Hawaii SE so presumably they are using the Stax plug.

One way around that is to pull them out when you're listening and put them back when you're done.

Nice, but they look a bit close to the back wall.

One suggestion, if you're setting up the current for the output current sources, you can do this using a low voltage source, e.g. 2 9V batteries in series across the current source for preliminary set-up. Because of their high impedance, the current will change very little between low and high voltage. Less chance of arcy sparky.

Actually, should have gotten to oxygen. What we were investigating was the assembly of elements within stars, which was an active area of investigation back then at Caltech, which had a very strong astrophysics section - Willy Fowler was there and he won a Nobel prize in physics some years after I left. Helium is two protons and two neutrons. In theory the next step would be to fuse two He nuclei together to make beryllium, but Be8 is unstable (the stable atom is Be9 - 4 protons and 5 neutrons). So Fred Hoyle proposed that 3 He atoms would make C12, but in order for that to happen, there had to be a resonance in the cross-section just above the ground state that would allow that to happen with sufficient frequency to form C12 to any extent, and in fact, that was found. My experiment was designed to look at the next step, which is C12 + He going to O16, so I had a very pure C12 target and relatively low energy alpha particles bombarding it, and measuring the cross-section to see if there was a low energy nuclear resonance that would allow that next step to occur. The resonance itself was too low in energy to measure directly but the idea was to measure the tail of the resonance which should have resulted in a higher cross-section than expected without it. Actually I am surprised that I remember much about it considering how long ago that was. Also, somewhat off topic.

I remember seeing/using one of those as a physics grad student using an accelerator for alpha particles (He nuclei) looking for a low energy resonance in C12 to O16.

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Do a search earlier in this thread around 12/02/16 to 12/11/16 to see what reliable people here think of Lampizator. It ain't pretty.

Wow! A Stax SRM-T1S went for $810 and a T1W for $896 on ePay in the past week!

Cool, but...a normal bias output? Isn't that a little like killing a fly with a small nuclear device? Not that there's anything wrong with that.

In terms of price, they are aimed at the same market. In terms of quality, well.... If you do a search for electrostatic headphone amplifier schematics, both commercial and DIY, you'll find a lot of resistor loaded output stages, you'll find several constant current loaded output stages, and you'll find some transformer output stages, but you won't find any choke loaded output stages. The Woo WES uses choke loaded output stages. Now, there are two possible reasons why they are unique in this respect. The first is, they have a patent and nobody else can use it - however choke loaded output stages have been around for a long time, so that's not it. The second reason is, they don't know what they are doing and are using an expensive topology that is demonstrably inferior for this particular purpose. See my thread on current requirements for electrostatic headphone output stages for infinite gory detail on why current loads are the optimum way of loading output stage active devices from a strictly engineering perspective. Second, the WES has a passive power supply rather than a regulated power supply. Now, Stax also use a passive power supply, however they are much cheaper and much smaller so there's a good engineering reason for that. In an allegedly state-of-the-art design, there is no reason NOT to use a regulated power supply since the ideal power supply provides a perfectly stable, noiseless and unvarying voltage regardless of the demand on it, something no passive supply can do, and something a regulated supply is much closer to doing. In other words, IMHO the WES is state-of-the-wallet, but not state-of-the-art.

Another WES plus 009 just went for $5100, which makes it about $2600 for the WES alone. Not so great on the resale value compared to a BHSE.

Interesting. Last time I studied physics was in the 1970s.

That must be the Louis XIV edition.

FIFY. Being massless, they are the ultimate in speed.

I remember years ago when Monster cable was promoting their original multi strand interconnect with thick wires for the bass and thin ones for the treble, Frank van Alstine was speaking to a Monster rep and suggested an experiment where he would take the small wires from the interconnect, solder them to an AC plug and have the rep hold on to the wires while he plugged it in. Much to his surprise, the rep declined.

I have to admit I'm a bit surprised that the KGSS DX didn't get more bids - the going price was less than some used SRM727s go for. Also, as the Woo auction also included a pair of SR009s, which are probably around $2500 for the phones alone, makes the WES about $3100, vs $4990 for a new amp w/o any upgrades.