Audio Research REF 75 vs VAC Phi 200

Actually, based on correspondence I have had with another member about his VAC tube amp, I understand that his amp has a variable negative feedback (NF) control. As is usually the case, NF often times reduces an amp's output impedance and correlatively increases the amp's damping factor (DF).

Ok ... where I am going with this point is amp/speaker compatibility. As I wrote in my post above, I am not familiar with VAC amps. However, if they use little or zero NF, then an inference is permitted that they also have a "high'ish" output impedance and a "low'ish" DF. These attributes could result in acoustic colorations if the speaker's impedance characteristics vary considerably as a function of frequency.

In the case of the member mentioned in the first paragraph, his speaker choice was quite deliberate. Namely, one having "gentle" impedance characteristics that make for an easy to drive load with a minimum of acoustic coloration.

By contrast, most ARC amps use more NF and consequently have "low'ish" output impedance and "high'ish" DF for tube amps. As a result, this class of tube amps performs somewhat "SS-like" and can couple with a broader range of speakers.

That is the longer version of what I had in mind. I try not to be a name dropper. I'm sure the member I referred to above knows who I had in mind. If he wants to chime in, he should of course feel free.

Bruce (Bifwynne), your discretion is appreciated :-)

As the aforementioned unnamed member, I'll second all of the excellent comments offered by Bruce and by Russ (RCPrince). With a slight modification to Bruce's statement that "these attributes could result in acoustic colorations if the speaker's impedance characteristics vary considerably as a function of frequency." At the end of that sentence I would add the words "unless the particular speaker is known to work well when paired with an amplifier having highish output impedance." Some speakers whose impedance varies widely as a function of frequency will still work well when driven from a high output impedance, and in some cases better than when driven from a low output impedance. It depends on the intention of the designer, as Ralph (Atmasphere) has said a number of times.

Specs don't appear to be provided for the Phi 200's output impedance or damping factor, or for the amount of feedback it uses, but it's probably safe to assume that the amount of feedback is considerably less than the 15 db used in the Ref 75, and its output impedance is somewhat higher than the 1 ohm (damping factor 8) specified for the Ref 75's 8 ohm tap (or the 0.5 ohm output impedance the Ref 75 can be presumed to have on its 4 ohm tap).

Also, consistent with the comment by Russ, VAC amps seem to generally have gains that are significantly higher than average, and the Phi 200 is no exception (30/36 db or 31/37 db, depending on what document is looked at, for the balanced and single-ended inputs respectively; and, no, I don't have that reversed). The gain of the Ref 75 is spec'd at 25 db balanced.

So if your preamp has relatively high gain, say upwards of 10 db, and/or if you are using primarily digital sources, and/or if your speakers have relatively high sensitivity, that would work in the direction of favoring the ARC.

And if your speakers have wide variations of impedance as a function of frequency, and they are known to work well with solid state amps (meaning that they match up well with amplifiers having low output impedances/high damping factors), that would also work in the direction of favoring the ARC.

Otherwise, my personal instinct would be to go with the VAC, although I suspect you would probably do well either way, as Alan and Russ indicated.

Best regards,
-- Al

Thanks Al. Your point underscores the importance of knowing what a speaker designer had in mind when voicing the speaker.

Of course, one may trip into a great sounding speaker that can do triple duty, i.e., mate with: (i) a very low output impedance SS amp, or (ii) a high output impedance tube amp, and/or (iii) a "low'ish" output impedance tube amp. Presumably, such an all-star speaker would have: (i) a ruler flat 8 ohm impedance function across the acoustic spectrum; (ii) a zero phase angle function across the acoustic spectrum; and (iii) high sensitivity.

And we should all believe in the Tooth Fairy too. :)

Being within driving distance of the company for service means a lot. Therefore I'd audition the ARC and if you love the sound, go for it.

I recently read a thread where people were having issues with their ARC amps, and it was mentioned that some preamps do not play well with ARC amps and could cause problems with them. I have no idea about the details of this or if it's correct, but I'd at least look into this and see if your preamp is a good match if you ultimately decide on an ARC amp. I'm sure others know a lot more about this, but just wanted to bring it up just in case it's a legitimate issue. Best of luck.