JimL

Just for perspective, every time you walk across a carpet on a dry day, and touch your hand to a doorknob getting a little electric shock you are discharging a kilovolt or two of static electricity. .

The original SRX schematic shows a 250k to 500k attenuator, but that is most likely due to it being designed to be driven by a tube source. As long as your source can drive 10k, which almost everything these days can, a lower impedance attenuator should be fine.

Nice to get away from cones in boxes.?

Sure looks like it - see top photo where there is a split volume control. Also second photo shows four sections to the volume control, which is what you would expect for a balanced amp.

Hmm, I recognize the Quads of course, and some McIntosh amps and what looks like a Fisher 500C, and perhaps Dahlquist DQ10s behind? Don't know about the rest.

IIRC, HiFi+ gave a rave review to the Trilogy H1, a grossly overpriced version of the 4 tube Egmont/TubeCAD circuit, swallowing hook, line and sinker the manufacturer's claim that it was a totally clean paper rethinking of electrostatic.headphone amps.

Really looking forward to this.

Appreciate the offer but I'm going to be away from home for a month or so visiting friends so, I'm out.

So, the HFM "rebuttal" does have some interesting points. 1) 1k resistor outputs vs 5.1k for the Stax - no reason given for the change, presumably the cost for either resistor is the same 2) 2M resistor bias outputs vs 5M for Stax - again, no reason given, should be identical cost 3) op amp input vs discrete MOSFET input. Presumably for cost reasons. 4) output MOSFETS supposedly to increase voltage output capability, claimed very low capacitance, but part number not identified. Are we talking a cascode transistor (bottom) MOSFET (top) combo? Don't know. 5) the text says they are comparing two schematics, however they post the SRA-12S schematic but use photos of the circuit board instead of an actual schematic.

For an inexpensive alternative, you could get an old SRM-T1, then for around $100, replace all the old electrolytic capacitors and put in my CCS mod. For another $100 substitute a TKD pot for the Alps, although you lose the balance function. Won't sound as good as the KG alternatives, but will sound better than any standard Stax amp IMHO.

WOW! Nice!

A method I use to determine average prices is to go to eBay, look up an item, click on "Completed Items" and look at how much people actually paid (those are the auctions and Buy It Now items where the price is green - if the price is in black it didn't sell). I also add in the shipping cost if that is listed.

Well, I can imagine some types of music where "raw" might be preferred.

Both the SRM-1/Mk2 and T1 are old enough that replacing all the electrolytic caps would be recommended routine maintenance. I have no experience with a SRM-1, but a couple friends and I compared my modded T1 with an SRM-727 with global feedback mod. We all felt the T1 was more refined, and we all preferred it to the modded 727.

Service? Repair? What's that? They probably just toss it in the garbage and send out a new one.

Sounds like it was built just barely good enough to get it past the warranty period without failing.

Pretty hot! In more ways than one.?

One other detail is how the push-pull output stage is biased. This is basically what determines whether an amp is Class B, AB or A. For class B, the output stage is biased at zero, meaning with no signal there is no current running through the output devices. This is basically only used in radio frequency transmitters. In class AB, there is some current running through the output devices with no signal, but when a signal is sent through the amp, each of the two output devices only reproduce half plus a bit more of the output, and the combination of both devices is needed to produce the whole signal. In reality the output devices are turning on and off during the output, which can produce crossover distortion, which is a transient that occurs when the device turns on or off. The problem is that this distortion actually gets worse as a proportion of the output was you go to lower power, until you get down to the point where the signal is small enough that the turn on and turn off does not occur. Now with most music other than the compressed crap that some modern producers are putting out (i.e. loud all the time), the fact is that much of music is at a volume that is 10-20 dB below maximum, so that an amp is usually running a 1-10% of max power. For example, take a typical speaker with a sensitivity of 86 dB/watt/1 meter. Now, most people listen a few meters from the speakers, so the sound level there is several dB lower, but this gives you a rough idea. Typical conversation is about 60 dB, so 86 dB level is like standing a few feet apart and yelling at each other. What you can see is that with most music, the amount of power an amp is putting out is usually on the order of a watt or less. In early transistor amps crossover distortion was a problem, because they were run at such a low bias so that there was lots of crossover distortion being produced a lot of the time. The solution is to increase the bias so that most of the time the amp is running in class A (ie. both output devices reproducing the whole music cycle), but that means more current and bigger heatsinks. Tube amps, OTOH, generally ran with enough bias current that they were basically class A up to a couple watts, which for most music meant they were actually class A most of the time. . Now for true class A, there is enough current running that both devices output some signal current at all times. In theory this should only matter for music peaks. In reality, since active devices (tubes, transistors) are more linear in the middle of their operating range rather compared to their limits, this decrease distortion at all levels.

Well, I think things are a bit more complicated than that. For speaker amps, you generally need a lot of power unless you have very efficient speakers, e.g. horns, which is another topic altogether. We're talking about 50-100 watts per channel or more. So the amp needs to produce 40 volts peak for a 100 watt amp, and on the order of 10 amps if we have a 4 ohm speaker. Now, a class A push-pull amp has a maximum theoretical efficiency of 50% (single ended class A has a max efficiency of 25%), so that means it runs at 100-200 watts/channel all the time. This means lots of tubes (old days) or lots of heatsink for solid state. All this means lots of heat and lots of money (and that doesn't even include the cost of running the damn things). So basically, until Mark Levinson and Krell came along, pretty much all amplifiers were class AB because nobody thought that there was a big enough market to support a class A commercial amp. Class AB was considered the best compromise between cost, efficiency and sound quality. It was never considered the best, just that Class A was considered too expensive. The first Mark Levinson Class A amp was rated at 25 watts (but was reportedly class A down to 2 ohms, at which point it was a 100 watt amp), was mono, and was big, heavy and cost a few thousand dollars - I think you could have bought over 100 NAD3020 integrated amps for the same money, and the 3020 was stereo, had roughly the same power rating, and included a phono section. But, it turned out that there was a market for it, and Krell came out with stereo class A amps that had more power and generated more heat. And it's gone on from there. The limiting factors for speaker amps is basically how much power do you need and how much money (and size, and heat) do you want to spend getting it. For non-electrostatic headphone amps it is significantly different. Here, you only need a few watts at most, into higher impedances than speakers have. So low to moderate voltages (say 15 volts at most)and relatively low currents (an amp or so at most). Based on specs, the least efficient non-electrostatic headphone is the HFM Susvara, and 5 watts into that should produce 120 dB, which is enough to give you hearing damage in minutes if sustained. So in theory, a class A headphone rated at 5 watts into the Susvara's impedance should do the trick. Which means that an amp that draws 25-30 watts from the wall should be able get you close to the threshold of pain In that case, there isn't a lot to be gained by dropping back to class AB. That said, some cheap headphone amps ARE run in class AB because the manufacturer doesn't want to lay out money for heatsinks. Every penny counts. For electrostatic headphones, it is different again, stat headphones have very high impedances, so you need very high voltages but relatively minuscule currents - a few mA. because you need to have high voltage - the Stax amps generally have +/-350V supplies, the KG amps generally run +/-350V to +/-450 volts, the Stax and DIY T2 run +/-500V supplies. Now the Stax amps run around 5-7 mA/output device, the KG amps up to 20 mA/output device, and there are 2 output devices per channel. So given that, the Stax amps draw about 40-55 watts from the wall, the KG amps run about 140 watts to more than 200 watts (the latter from a DIY T2, which also has 8 tube heaters using energy).

Oh, I agree, just commenting on the fact that it took him a year to respond. I just hope it didn't take him an entire year to figure out a response..

Yeah, well, in addition he managed to convert the Shure plug to a Stax 5 pin (hence the comment that he made the headphones compatible with Stax pro bias), so if some poor buyer plugs them into a Stax amp, KABLOOIE!

No biggie, as apparently the M03 is a POS anyway. And I'm guessing that if someone should accidentally plug that modded KSE1500 into third party amp (AFAIK they are all Pro bias only these days) that didn't have the appropriate bias safety resistor, the current dump would zap the diaphragms. Maybe I'm missing something, but I don't think so.

So, wait a minute. Looks like he changed the Shure plug to a 5-pin Stax pro plug, but how does that make the KSE1500, designed for a 200V bias supply, compatible with Stax pro bias (i.e. 580 volts). Magic?

A bit late to the party, aren't we? That post was ONE YEAR ago!

1. Opinions vary but to me the SR-007 Mk1 is a bit soft, the SR-007 Mk2 with port mod is close to neutral and the SR-009 is on the bright side. 3. The advantage of Blu-Tank over a fitted piece of plastic is that the Blu-Tak can mold to any small irregularities and form an air-tight seal, whereas there may be some small gaps with a printed plastic piece. As the whole idea of the port mod is to seal off the port completely, Blu-Tak should do a better job.