Audio Research Vs115

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.

Thanks Jake. I think the impedance/phased angle plots are demanding as reported in Stereophile mainly because of the 2 ohm dip and general 4-5 ohm range. But other than that, the impedance is quite flat between 4 and 5 ohms across the freq. band. The low bass peaks are typical. Nevertheless, it does demonstrate the VS-115's impressive performance capability. How many amps out there can push power into such low impedance loads??

Bifwynne - I should mention that a network analyzer (sometimes called a "vector network analyzer, or VNA) can also be used to measure the frequency response of an amp into any load. The same instrument will measure the output impedance of the amp as well. My home lab includes a VNA for RF but unfortunately it does not go down to audio frequencies.

Out of curiosity, what speakers are you using with your VS115?

Paradigm Sig. 8 v3. I've had a devil of a time getting reliable and timely info about the specs, but finally got through. The S8s have a 21-22 max ohm peak between 1K and 2K Hz; and a 4 ohm min. in the bass region. I've been in private contact with one of our tech members and he did some math calcs that indicate the VS-115 should be able to drive the S8s with a relatively flat output response because the amp's output impedance is low for a tube amp. Soundstage and Stereophile bench tested the VS-115 and Ref 150 and they share similar electrical attributes re: NF (about 12-13), DF (about 7 or 8) and output impedance (approx. 1.1 ohms off 8 ohm tap; about half that off the 4 ohm tap). Based on his calcs, FR may be off +/- .5 db at the peak, which ain't so bad.

VS-115 maintenance tip. Just replaced the four 6H30 cathode follower tubes. May have run them out to almost 4000 hours. BIG, BIG difference. The amp definitely sounds better: tighter bass, more transparent, and so forth. I think in the future I will switch out the 6H30s before they have 3000 hours of use.