Current amp vs Voltage amp
I think that you'll find this thread and this one to be informative. Also, if you haven't already, see Atmasphere's paper on Paradigms in Amplifier Design. Regards, -- Al |
Al, Ralph and OP. As Al and Ralph know, I have been acutely, maybe obsessively, interested in the OP's Q. I drive a pair of Paradigm S8s (v3) with an ARC VS-115 tube amp. I can't think of a worse match because the S8s have one of the most wacko impedance curve and phase angle plots I have ever seen. Indeed, when I called Paradigm, I was told the S8s were designed and voiced to be driven by a high current, high power SS amp. OTOH, ARC and Paradigm said I could still get away with my VS-115 -- whatever that means. I was feeling a bit down until I read the review and bench test reports on two ARC tube amps. Stereophile's report on the Ref 150 and Soundstage's report on the VS-115 (my amp). Both use about 12 to 14 dbs of negative feedback, which I will touch on below. Now here's the interesting point that I always overlooked. Take a look at the graphs that report the FR output results of both amps when driving a standard dummy load. I copied the URL sites of both reports. They dummy loads are pretty similar and may be standardized. Does power output as a function of frequency vary -- yes!. But not as much as I would have thought: +/- 1 db, or so with both amps driving the dummy load off the 8 ohm taps. Could that variance "flavor" the presentation? I think so. John Atkinson's comments (re the Ref 150) about this phenomenom (sp?) are interesting: "All three taps offer quite a low source impedance for a transformer-coupled design; as a result, the modulation of the amplifier's frequency response, due to the Ohm's Law action between that impedance and that of our standard simulated loudspeaker, was relatively mild. From the 8 ohm tap (fig.1, gray trace), it was ±0.8dB; the 4 ohm tap offered ±0.4dB, the 16 ohm tap ±1dB." In other words, driving a speaker off the 4 ohm tap resulted in a lower variance of FR as a function of frequency. Up until now, I had been driving my speakers off the 8 ohm tap. I always thought the presentation was a bit "forward." I think I now know why. The S8's 28 ohm impedance peak at the 2.2K Hz crossover point should have resulted in an exaggerated FR output and I think it did. Even at 1 or 2 db, I think it changed the acoustic presentation. I recently switched off to the 4 ohm tap. At times I think the presentation is less defined, maybe flatter. But I turned up the gain a bit and I think the detail is all there. Even the bass is flatter, more extended, maybe more honest. I've been using the 8 ohm tap for so long, I need to re-educate my ears. Time will tell. Perhaps Al and Ralph will chime in here. But I think the reason for my experience may be that the use of negative feedback lowered my amp's output impedance. This may have the effect of making my tube amp perform somewhat "SS-like" when presented with varying impedance curves. Dunno. Well that's all I have to say, which has been a lot. I look forward to reading Al's and Ralph's responses. The URLs I referred to are posted below. http://www.soundstagenetwork.com/measurements/amplifiers/arc_vs115/ http://www.soundstagemagazine.com/measurements/test_amplifiers.htm http://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements http://www.stereophile.com/reference/60/index.html |
I always tell owners of ARC to try the 4 ohm tap. Some like it better some not so much. Note that there aren't many true 8 ohm impedance speakers especially just past the woofer impedance peak. Woofers usually dip to around 3-6 ohms. You might even get better bass control and definition off that tap. |
