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.

Hi Bruce (Bifwynne),

I've looked at the references you provided, and read your post, and I think that everything you've said is essentially correct.

Basically, your VS-115 has a lower output impedance than many or most other tube amps, on a given tap. The significant amount of negative feedback it uses is presumably a major reason for that. And it can be presumed that the output impedance of its 4 ohm tap is approximately half of the output impedance of its 8 ohm tap.

The lower the output impedance of a tube amp, the more closely the effects of the interaction of that output impedance with the impedance vs. frequency characteristics of the speaker will approach those of a solid state amplifier (which will have near zero output impedance, in most cases).

The one slight qualification I would make to your post (which you probably already realize) is to emphasize that JA's reference to obtaining a flatter frequency response with the 4 ohm tap than with the 8 ohm tap applies to his "standard simulated loudspeaker load," which has impedance characteristics that are quite different than those of your (and many other) speakers. As you probably already realize, for that and other reasons a generalization should not be drawn that 4 ohm taps will inevitably provide flatter frequency response than 8 ohm taps. It depends on the speaker.

In this particular case, though, given that your speakers were voiced for solid state amps, it can be expected that lower output impedance will result in a flatter response, everything else being equal.

Best regards,
-- Al

Thanks Al. One thing that the reports I cited answered for me was the magnitude of the FR variance. I didn't know how to convert the impedance and phase angle data into useable information.

As you can see from both reports, the ARC Ref 150 and VS-115 use roughly the same amounts of NF and have similar output impedance stats. Further, the amps were tested by different people at difference times which gives me some confidence that the data is reliable.

As I mentioned above, it appears that using the 4 ohm tap should yield a better FR result. I think it just might be doing that. Short of having an acoustician do precise measurements, I'll go by my ears. Just have to get use to a different acoustic presentation off the 4 ohm tap.

This post and others like it provides important information for folks who want to use tube amps. I think the take-a-ways are the White Paper Paradigms present the extreme cases. As we can now see, my tube amp doesn't neatly fit into the Power Paradigm category. The other take-a-way is that folks looking to match tube amps with speakers should be somewhat mindful about the speaker's impedance and phase angle swings and their tube amp's output impedance. Of course, there's no substituting for a serious listen.

My thanks to Al and Ralph for hanging in there with me.