Audio Research Vs115

My VS115 is one of the earlier ones with the 6550's. At some point I will upgrade the tubes to KT120's. I've also blown one of the resistors in the amp. I ended up replacing the resistor myself--it's a 10 minute job if you know how to solder. Apparently this is a common problem.

Other than that, I love this amp. When I put it in my system, it totally changed the sound of my system in the area of midrange accuracy, imaging, and 3-dimensional soundstaging. I was using Classe solid state electronics before.

Compared to some tube amps, this is a very wideband amp. With laboratory test equipment I have measured its frequency response into an actual loudspeaker load as flat out to 20 kHz and -3 dB at 50 kHz. This probably helps account for why this amp does not sound rolled off in the highs and has very good detail.

I also have an LS-27 and Reference CD-7, so you can see I am sold on ARC.

Bifwynne - The speakers under test were Thiel CS2.3 driven from the 4 ohm tap. The CS2.3 are a notoriously difficult load for amplifiers, with large swings in impedance over frequency and a minimum impedance of 2 ohms. However, the Thiels and the VS115 match up extremely well, far better than with solid state amps. I currently have the Sonus Faber Cremona M and get pretty much the same results with the VS115.

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.

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.

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?