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In Reply to: RE: Direct Drive posted by AJ on May 03, 2016 at 16:22:11
I got quite an unpleasant jolt from the speaker wires of my NYAL OTL3 while it was playing music once. I can't remember why I touched them, but I was sorry I had.
But perhaps a DD amplifier could have been fatal.
To work on my Beveridge DD amplifiers, I bought a special probe rated for 6kV. It looks like a short sword with an insulated "hilt", so that voltages at the sensor end cannot jump across to the handle end. On one occasion, the tips of my fingers were overhanging the hilt, just on top of its bulge, not really on the danger side. Nevertheless, while measuring (only) 1600V, I got zapped on my exposed fingertips, which later split open and required quite a while to heal. Needless to say, I was very careful thereafter about how I gripped that probe. Bill Thalmann showed me his scars from working on my unit, too; he got it much worse but also only in the fingers, fortunately. Across the chest and you're a goner for sure. The Bev amps use +/-1600V, or 3200V from + to - poles. The Sound Lab ESLs require much higher voltage, although I don't know, and SL won't say, exactly what that voltage is, probably at least 6kV.
Obviously Direct Drive amps are the correct way to drive and ESL. The Model 3 requires 750 Volt-Amps (VA) to play Miles Davis at high levels. The problems with transformers are many and the load presented to a conventional amplifier is about 1 ohm at high frequencies. The transformer I designed for the Model 3 added quite a bit to that load due to winding capacitance. Since the load is capacitive it causes transistor amps with load line limiting to clip at low volumes, this is why ESLs are best driven with transistors with no limiters, but those are rather rare. No wonder tube amps tend to drive ELS as they have no limiters but now we are going through two transformers and still not enough current. Low voltage OTLs, though some like them, are not the best choice either as they cannot supply any where near the high currents required by a 1 ohm load.
The virtue of an DD amp is its high operating voltage. At 1500 volts one needs only 1/2 amp to get the 750 VA required to play trumpet music. I only make these amplifiers for sophisticated users who are comfortable with high voltage.
On the safety factor let me remind all the high voltage DC is not the same as AC. Its not the voltage but the current that matters and it's path through the body. AC causes the heart to fibrillate and DC defibrillates (as seen on TV with the paddles). I have unintentionally taken a shock across the chest several times. Most recently with 3 KV on a Bev 2SW amp from arm to arm and more recently while packing the amp being discussed here by a similar voltage. All got was a burn the size of a pin head on my finger and a jolt. I said one bad word and moved on. If used normally the warm tubes will discharge the B+. The polarizing has to be discharged manually but it is DC in a capacitor.
One should be careful while playing music however as that is AC, but only the bass is dangerous around 60 HZ. High frequencies are not as dangerous.
There are far more dangerous things around us every day. Of course we dont want a child or pet to stick there nose in these things. You probably dont want them in near your turntable for obvious reasons.
AC has a higher propensity to interrupt conduction in the heart muscle, resulting in cardiac arrest. I am guessing that's the basis for the difference in lethality between AC and DC.
Are you saying that it is an intrinsic characteristic of ESLs to present a one-ohm load? I bow to your far superior knowledge, but it does not seem to apply to my Sound Lab 845PXs. I measure impedances in the range of 20-30 ohms, from 50Hz to 2000Hz, about 8 ohms at 5kHz, and reaching as low as 2 ohms only above 10kHz, where the energy demands are minimal. This is using a non-factory full-range audio step-up transformer with a 1:90 turns ratio. (Below about 500Hz, this transformer is in parallel with the Sound Lab bass transformer, which has a much higher turns ratio, probably 1:250, but there are no other passive parts between the amp and the paralleled primaries of the transformers except an inductor in series with the bass transformer.) Since the impedance seen on the primary side will vary as the square of the turns ratio, I concluded that the step-up is a major factor in determining what the amplifier sees and that the intrinsic Z at the panel is much higher. In my view, many modern ESLs are "ruined" for tube amplifiers by added circuitry in front of the step-up audio transformer, such as low value resistances in parallel to effect a crossover, as SL does, all of which tends to bring down Z to values that indeed are very difficult at times for tube amplifiers. (In stock form, Z was around 4-5 ohms at mid-range frequencies for my 845 PXs. There may have been a 2-ohm dip in there too, at 1-2kHz.) With a solid state amplifier, these added circuits do not present much of a problem, but I have yet to meet an SS amplifier that can come close to the musicality of an OTL driving an old school ESL, such as a KLH9 or Quad 57 or my modified Sound Labs.
But I sure do not disagree that direct-drive is Nirvana. I bought my Bev 2SWs just on a lark, to have something to listen to in my basement far from the madding crowd; now I am addicted. Did you sign every Bev DD amplifier? You signed the ones in my 2SWs and dated "1979". Thanks for your contribution to this great speaker.
I didn't mean to start a storm. The transparency of the amp speaks for its self. It really is the way to drive these speakers. Others like de Paravinci have flirted with versions of DD for Quads for example.
I have experience in electronics as an engineer in another discipline. I can handle the risk. I have built 211 amps and I'm ok.
Roger was good enough to take it on and he has provided a great opportunity to hear what was a really good speaker turn into one that is quite extraordinary. I am glad that I stumbled on to Audio Circle and realized that Roger was able to make something up Great memories of the Dayton Wrights I had and the Acoustat X Servo amps.
The Beveridges still are able to do things that are out of scope for even the modern speakers. With the DD even better.
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