Audio Research Vs115

@Jake, your results line up with the test results from Stereophile and Soundstage I posted above. Did you also have a chance to near-field mic the actual SPL outputs of the Theils. That would seem to be the ultimate test.

I switched over to the 4 ohm tap and am getting used to it. Very different acoustic presentation, probably because the 8 ohm taps has a higher output impedance than the 4 ohm tap, ergo permitting a touch more output variation in the high impedance parts of my speakers, mainly the 2.2K to 4K Hz region of the freq. band. Per Atkinson at Stereophile, the SPL variation is almost half.

Jake, out of curiosity I googled to find bench tests of the Thiel CS2.3. May have looked at the wrong site, but the test results I found showed impedance remarkably holding at a near 4 ohms almost across its entire FR spectrum. As expected the impedance curve had the normal two spikes in the bass region, but that was it.

I'm not saying every amp could eat a constant 4 ohm load for lunch, but I was surprised that past 100 or 150 Hz, impedance hovered at 4 ohms. I must have looked at the wrong speaker. If you have a link to the bench test results you had in mine when you made your post, I'd be very grateful if you'd either e mail me or post it here.

Thanks.

Bruce

Bifwynne - I have a Radio Shack SPL meter but it's not really accurate enough to make good near field measurements.

The 4 ohm tap does have lower output impedance than the 8 ohm tap. However, if you are driving an 8 ohm load you'll get more power out of the 8 ohm tap. My Sonus Fabers are nominally 4 ohms so I use the 4 ohm tap.

Jake,

I re-checked the net to view the bench tests for the Thiel CS2.3 speakers. What I saw in fact was bench test results for the CS2.4s, which as I said had an amazingly flat 4 ohm impedance plot from 50 or 75 Hz through the audible acoustic spectrum.

Do you know whether the CS2.3 was configured differently? I ask because you posted that the 2.3s had modulating impedance plots? Do you have any idea what the SF Cremonas impedance and phase angle plots look like? Couldn't seem to find any hard data on the net.

The reason I am so curious is because I'm trying to gather some real life anecdotal information about how our amps perform when driving speakers with goofy impedance and phase angle plots.

Btw, how do you even measure the output performance of the amp and speaker combo? I have a digital meter that I use to bias the amp's tubes. But I have no idea how to check actual output with the amp hooked up to the speakers. Or how to check output impedance (with no load) off the 4 and 8 ohm taps. Can you advise?? I kinda would like not to damage my gear just to satisfy my curiosity.

Thanks and have a great Memorial Day weekend.

Bruce

Bifwynne - The CS2.4 has a somewhat smoother impedance plot than the CS2.3. Here are the measurements done on the CS2.3 in a Stereophile review: http://www.stereophile.com/content/thiel-cs23-loudspeaker-measurements. You can see the impedance dips to 2 ohms in the lower midrange and goes through big swings in the bass region. The phase angle varies all over the place, too.

I've seen impedance plots for the SF Cremonas but I have the newer Cremona M and I'm not sure if the data would be similar. The Cremona M is supposed to have "improved" drivers and a different crossover network.

To measure frequency response into an actual speaker load, I use a Tektronix audio signal generator driving one of the auxiliary inputs of my ARC preamp and I attach a Tektronix oscilloscope across the speaker terminals. I bought both Tektronix units on eBay quite cheap. I dial back the signal generator output levels so I see about 1V at the speakers and everything is happy.

Measuring the output impedance of the VS115, or any other amp, is a bit more involved. The easiest way is to get an impedance analyzer that works at audio frequencies. The analyzer is connected directly across the speaker terminals of the amp and it injects a low level signal back into the amp to make its measurements. I have an RF impedance analyzer but it doesn't go down to audio, so I've never done this myself.

Hope this info is helpful.