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About JimL

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  1. Oh, cool! Cascoded pentodes would work really well. Lots of heat, lots of filament supplies.
  2. The prototypes have been demonstrated at several shows in the past couple years, not sure they are on the market yet.
  3. Thanks, spritzer. The article is already published but I can ask them to put in an addendum adding the 5M resistors to the bias supplies. I haven't seen the first gen T1 either (other than a photo on the internet) but the mods only concern the output stage so I think they should work equally well.
  4. To put it another way, compared to the Stax Mafia, this is like Stax juvenile delinquency.
  5. You're right about the LV supply (Stax did stick in some zener diodes to sort of regulate it), but this was meant to be a simple mod. It is intended to be bang for buck, and the most significant improvement is the constant current mod. Yes the power supply could be much better, but the constant current mod is a first order improvement, the power supply is a second order improvement. The intended audience wasn't the Stax Mafia, it was people who have a Stax amp and want to improve it without going to the expense of a KGST level amp. Or are just getting into Stax and are willing to do a little DIY (or have it done for them). I think more people would be interested in Stax if they could buy a $500 used amp, put $100 and a few hours work into it for some significant improvements, instead of spending a few thousand for the amp. Not everyone is interested in building from scratch. Actually, I didn't comment on the bias supply as I was not aware that was an issue, but now that I look at the schematic, I see that there should be a 5 megohm resistor after the last capacitor for protection. Damn!
  6. When you're going downhill, you pick up speed.
  7. That's precisely the idea. It doesn't quite get to the KGST level, but it's not far behind.
  8. I published a modification for the Stax SRM-T1/T1S/006 in AudioXpress July 2017, which is just out. The modification adds 5.1 kilohm output safety resistors and cascoded constant current output loads, which with their heatsink fits the space vacated by the plate resistors - see photo. Spritzer did something similar a few years ago with an SRM-007, adding constant current loads, but he also changed to 6S4A outputs. Unlike plate resistors, constant current loads burn up practically no signal current, so while the stock amp burns up 9.2 mA signal current in the plate and feedback resistors at its specified 300 VRMS maximum output, the modified amplifier only uses 2.8 mA driving the feedback resistors, leaving much more current available to drive the headphones. Since the output tubes are much less stressed, distortion is decreased and 2 dB of extra headroom gained. The modified amp circuit is very similar to a KGST or KGDT. This is no accident, as the KGST was designed to be “a Stax SRM-007t with no cost or retail considerations,” and the 007t is a higher power T1. The modification lacks the KGST’s regulated power supply, but larger power supply caps have been fitted. Since the amplifier is fully differential pure class A with current sources or loads at every stage, the current draw is pretty constant, minimizing power supply effects. The modified T1 can drive demanding headphones like the SR-007 Mk I, something that the stock design strains to do. The relatively flabby bass, somewhat dark tonality and soft treble are largely eliminated. Replacing all the old electrolytic power supply caps should also be done as routine maintenance. Parts cost for the modification is about $35, so total parts cost is around $100. With T1 amplifiers going for $400-$700, this is the best bang for the buck, a good starter project for someone wanting to get into electrostatic amp DIY without going to a full build.
  9. Just looked, actually both the Utopia and LCD-4 are on the Wall of Fame, which is what Tyll said he was going to do on his blog of June 9, "I will be leaving the Utopia on the WoF, and will add the LCD-4 to the Wall of Fame, though reservedly."
  10. Although it is true that tube amps tend to have higher measured distortion than solid state, IMHO a properly designed tube amp or preamp has low enough distortion that it is inaudible. As with all class A amps, distortion generally goes down as levels decrease, so a typical listening levels which is usually at least 20-30 dB below clipping for electrostatic headphone amps, measured distortion tends to be buried by noise. I agree that tubes sound different from solid state, I just don't buy that tube sound is due to "pleasant" distortion.
  11. So just to point out, the SRX Plus, based on the Stax SRX DIY design, has two differential amplification stages (first is cascode) and only one cap between input and output stages. The Plus refers to the use of MOSFET cascode constant current sources (CCS) replacing resistors to optimize circuit operation. The CCS set the circuit operating parameters but the tubes do all the signal amplification.
  12. As I recall when Walt Jung was doing his series on current regulators in AudioXpress he found some noise problems with the 317 regulator, don't know if that applies to the LT1117.
  13. Which is interesting because they claim it's a single-ended Class A, which is normally 25% efficiency at the most - should mean lots of heat. Supposedly the case is the heat sink, switching power supply (ugh - yeah, I know the SRM252 uses a switching PS but it's not in a $5000 system), "linear regulators" to the audio circuits, but I see no spec on their website about much power it draws from the AC line, which would allow a guess on how much voltage and current the amp draws. Stax solid state amps run about 30-55 watts from the wall (except for the 252) and they are push-pull so theoretically more efficient than single-ended. I doubt like hell that the Sonoma draws even that much or the case would be uncomfortably hot. Oh, yeah, and the max output is 145 VRMS, so a single ended supply of 0 and 450 volts should handle that. Plus MOSFET outputs, so easy to drive.
  14. I believe he is referring to Tyll putting the Sonomas on his Wall of Fame.
  15. Re: tube rolling, understood. For another tube flavor, you could try the SRX Plus. Can be built point-to-point, tubes handle all the signal chores, but with cascode MOSFET current sources, which greatly enhances the efficiency of the output tubes. Could probably drive dynamic headphones through a transformer, but I haven't really tried that.