REF75 is the newest power amplifier offered by U.S. manufacturer Audio Research Corporation, introduced as a starting model in the prestigious Reference line. During the time of this review it was not yet listed on the manufacturer’s website. Moving up the price list, there is the REF150 and the two top models – the REF250 and the REF750. The “REF” stands for the ‘reference’ line while the numbers denote power output. Since these are all tube amplifiers, not solid state, such high power outputs denote that they cannot be SET devices.
Indeed, all ARC power amps are push-pull, fully balanced designs, i.e. all stages – input, gain/drive and output circuits – are fully balanced. The manufacturer’s propensity for balanced designs, inherently resistant to any symmetrical distortion, is manifested in the fact that the amplifiers feature only XLR balanced input connectors. Another characteristic is that speaker binding posts only accept spade lugs and bare wire. The normally open ends for banana plugs are filled with golden pins that cannot be removed. And one more thing – the mains socket is designed for 20 A rated plugs, larger than the standard 16 A plugs, and as such is probably a bit of an excess.

According to the manufacturer, the REF75 is housed in an enclosure reminiscent of ARC older, classic designs. The front panel sports two illuminated analog power/bias meters, one for each channel. The meters are rectangular shaped, reminding the 70s designs. Previously, round meters were used. Below each meter there is a knob to choose between power output and bias monitoring mode. The outer dimensions of the REF75 are the same as the of the REF150 amp. The front panel, handles and top cover are available in black or silver (natural).
As it turns out, most of the circuit design is derived from the larger REF150 sibling, the main difference being the number of KT120 output tubes used – here we have a pair of matched KT120s per channel. The REF75 employs the latest “Wunderwaffe” in the tube world, an oversize variant in the 6550 / KT88/KT90 family of beam tetrodes, called KT120. At this time it is only offered by New Sensor Corporation, an American company with manufacturing plant based in Saratov, Russia, and offering tubes under Sovtek, Svetlana, Genalex and Electro-Harmonix brands. We met KT120 earlier in the Leben CS-1000P amplifier (see the review HERE).
The input stage is a hybrid, solid state/tube circuit. That design was earlier used in ARC preamplifiers, such as the REF2 and the REF3. Each channel features a JFET at the input, followed by a single 6H30 dual triode as a driver tube.

SOUND

A selection of recordings used during auditions:

CDs and SACDs

• Abba, Gold - Complete Edition, Polar/Universal Music Japan, UICY-91318/9, 2 x SHM-CD (2008).
• Ash Ra Tempel, Ash Ra Tempel, MG Art/Belle, 101780, SHM-CD (1971/2010).
• Assemblage 23, Bruise. Limited Edition, Accession Records, A 128, 2 x CD (2012).
• Carol Sloane, Little Girl Blue, Sinatra Society of Japan, XQAM-1036, HQCD (2010).
• Dead Can Dance, Anastasis, [PIAS] Entertainment Group, PIASR311CDX, Special Edition Hardbound Box Set, CD+USB drive 24/44,1 WAV (2012).
• Depeche Mode, Enjoy The Music....04, Mute, XLCDBONG34, maxi-SP (2004).
• Depeche Mode, John The Revelator, Mute, LCDBONG38, maxi-SP (2006).
• Frank Sinatra, Sinatra Sings Gershwin, Columbia/Legacy/Sony Music Entertainment, 507878 2, CD (2003).
• Hilary Hann, Bach
• Concertos, dyr. Jeffrey Kahane, Los Angeles Chamber Orchestra, Deutsche Grammophon, Universal Music LLC [Japan], UCCG-50058, SHM-CD (2003/2011).
• Hilary Hann, Hilary Hann Plays Bach, Sony Classical, SK 62793, Super Bit Mapping, 2 x CD (1997).
• J. S. Bach, Sonatas & Partitas, wiol. Henryk Szeryng, Sony Classical/Sony Music Japan, SICC 840-1, 2 x CD (1965/2007).
• Jean-Michel Jarre, Magnetic Fields, Dreyfus Disques/Epic, EPC 488138 2, CD (1981/1997).
• Miles Davis, Kind of Blue, Columbia/Legacy/Sony Music Entertainment, COL 480410 2, "Master Sound", Collector's Edition, Super Bit Mapping, gold-CD (1959/2005).
• Simone Kermes, Viva!, Archiv Production, 477 9843, CD (2007-2008).
• Tadeusz Woźniak, Tadeusz Woźniak, MUZA Polskie Nagrania/Polskie Nagrania, PNCD 1289, CD (1974/2010).
• Vangelis, Spiral, RCA/BMG Japan, 176 63561, K2, SHM-CD (1977/2008).
• Yo-Yo MA & Bobby McFerrin, Hush, Sony Music/Sony Music Hong Kong Ltd., 543282, No. 0441, K2HD Mastering, CD (1992/2012).

Audio files

• Assemblage 23, Rain Falls Down WAV 16/44,1 [z:] Bruise, Accession Records, A 128, Limited Edition, 2 x CD (2012).
• Audiofeels, Sounds of Silence, WAV 16/44,1 [z:] Audiofeels, Uncovered, Penguin Records, 5865033, CD.
• Bob Dylan, Blowin' in the Wind, WAV 16/44,1 [z:] Bob Dylan, The Freewheelin', Columbia/Mobile Fidelity, UDSACD 2081, SACD/CD (2012).
• David Sylvian, World Citizen (I Won't Be Disappointed) + Angels, WAV 16/44,1 [z:] David Sylvian, Sleepwalkers, P-Vine Records, PVCP-8790, CD.
• Dead Can Dance, Anabasis, WAV 24/44,1 kHz [z:] Dead Can Dance, Anastasis, [PIAS] Entertainment Group, PIASR311CDX, Special Edition Hardbound Box Set, CD+USB drive 24/44,1 WAV (2012).
• Kankawa, Dear Myself, 24/192 WAV [z:] Kankawa, Organist, T-TOC Records, UMVD-0001-0004, Ultimate Master Vinyl, 24/192 WAV; reviewed HERE.
• Keith Jarrett, January 24 1975. Part I, WAV 24/96 [z:] Keith Jarrett, Köln Concert, WAV 24/96, HDTracks..
• Me Myself And I, Elytom Encon, WAV 16/44,1 [z:] Me Myself And I, Do Not Cover, Creative Music, 005, CD (2011).
• Pieter Nooten & Michael Brook, Searching, WAV 16/44,1 [z:] Pieter Nooten & Michael Brook, Sleeps With The Fishes, 4AD, GAD 710 CD.
• Sonny Rollins Tenor Madness, WAV 24/96 [z:] Sonny Rollins, Tenor Madness, WAV 24/96, HDTracks.
• Stan Getz & Joao Gilberto, Corcovado (Quiet Nights Of Quiet Stars), WAV 24/96 [z:] Stan Getz & Joao Gilberto, Getz/Gilberto, HDTracks.
• The Alan Parsons Project, Sirius + Eye In The Sky, WAV 24/192 [z:] The Alan Parsons Project, Eye In The Sky, DVD-A.

Japanese editions available from

Audio Research REF75 is a chunk of good sound, good music. But what else can be expected from a manufacturer that has quality products in its blood and it seems that even a change of ownership – now it’s owned by Quadrivio SGR from Italy – and the death of its founder and “good spirit”, William Johnson, hasn’t changed that. What hasn’t changed either is the ARC “signature sound” that I hear in all their products. And what a sound that is!
The Reference 75 sounds big and massive. Bass is saturated, full and strong; slightly emphasized in the 100-200 Hz range. This gives the sound the extra oomph and builds really large virtual sources. I can’t imagine a situation where one would call a system featuring the REF75 to sound dry, thin or light. There is no such option. There would have to be something seriously wrong with it for that to happen (e.g. forgetting to switch on active subwoofers in the Avantgarde Acoustics – which I once witnessed ...).
The sound is thick and malleable. Bass is stronger than usual (in the range I mentioned) but that doesn’t affect midrange, doesn’t add weight to it. And such emphasis is easy to achieve – adding some midbass to get dark, thick, unhurried midrange. Here the pace is slightly restrained but not by said coloring. I will get back to that later.

I was particularly interested in this aspect of presentation since there are many warm, slow tube amps out there and despite having their supporters, in the longer run they tend to sound tedious, tiring and boring. Of course I know that they are often paired with over-bright speakers to correct overall tonal balance, being treated as a kind of tone control. I would be hard-pressed to butt in and come down on that since it’s only natural to look for synergy in one’s audio system and that includes tonal balance. However, it seems there are some limits one shouldn’t venture beyond and forcibly warming the system with a slow, warm amp is exactly something like that.
The ARC Reference 75 can help affect the tonal balance. As I said, it offers sound with strong, clearly emphasized bass. Yet it’s not senseless thumping nor is it dipping everything in amber to solidify it forever. It’s much more subtle. When we pair the ARC with the type of speakers such as Wilson Audio (which makes sense in that both manufacturers use each other’s products for their new designs, and they even have a common Polish distributor, audiofast), the somewhat dry, short bass of the latter will be fantastically complemented by the slight bass excess of the former.
I was very interested to find out how that kind of sound affects treble. Strong, fleshy bass usually colors midrange. Here it is different.
Bass, by its nature, sets the tone of this presentation regardless of chosen repertoire. But perhaps most important is what the REF75 does with midrange. It is big, not particularly selective but has good resolution. Both density and depth are at a good level. That creates a very atmospheric, immersive sound. I was extremely pleased to listen to some records that hadn’t sounded so well in my system for a long time (excluding my reference amplifier).
Small ensembles sounded incredibly well. That is another interesting trait because such a large amplifier with such high power output is usually associated with high volume levels, with the kind of repertoire from Vader to Megadeth or from a concert album by Queen to imposing works of Mike Oldfield. And we would be right – the American amp can do that all really well. But what captivated me is the way it presents what’s small, what’s individual.

The solo violin sounded beautiful – whether on an old recording of Henryk Szeryng or on a later one of Hilary Hann. Both albums are wonderful but have been recorded in a completely different way. The amplifier made them sound somewhat similar by providing a strong bass base yet did not disturb midrange. The presentation was dense, very emotional. I could get immersed in or wrapped around by it. It might have seemed dominated by midrange saturated with harmonics but it was not. Midrange was in a right proportion to upper range. There were just so many sounds and so much going on but it happened through presence, palpability, proper saturation and decay, not by over detailness.

Equally interesting were albums featuring vocals as the dominant instrument – from Me Myself and I through Audiofeels to the fantastic young Sinatra from the 40s on the album Sinatra Sings Gershwin. The latter features a track recorded in 1947 and originally issued on shellac titled I’ve Got A Crush On You that sounded simply spectacular with the American amp. Despite its age it perfectly captures the character of Sinatra’s vocals. The REF75 showed it very well adding some weight to the sound, which actually turned out for the better.
It was the same or similar with other albums, such as the Japanese release of Gold – Abba’s hits or the Yo-Yo Ma and Bobby McFerrin duet album etc. I could go on with many more albums because the amp treated them the same way – elevated them regardless of how they originally sounded and in a sense revived them. It was as though it had their sound pass through some kind of conditioner, giving them all its own imprimatur and that of its designers.

All those who know Audio Research products, however, know that you must sacrifice something in return. You could get nearly everything but you’d have to pay three or even four times more. At 42,000 PLN the Reference 75 offers a whole lot of “sound” yet it also has its limitations.
The first one concerns improvements in tonal balance. I think they are improvements, not problems, because they are inherent to and almost synonymous with ARC gear. First of all, bass is stronger than in the reference. It will be a blessing to lots of systems and many an album, but you have to keep it in mind assembling your own audio system. It can go down quite low and is fleshy but there is no such unrivaled, such well-controlled bass extension as that exhibited by the Soulution 710, the Accuphase A-200, etc. Except that these are solid state amps; a tube amp simply can’t overcome some limitations. Thus, unless we have such refined and expensive amps for a direct comparison, we will not even know what we are missing. All the more so as the REF75 bass isn’t sloppy by any means and is better controlled than e.g. in the Tenor Audio 175S I once reviewed, although the latter has solid state output stage, high power output and mighty price (see HERE). One just needs to carefully pair the ARC with the system and the speakers. For example, the Tabla USB-S/PDIF converter from Human Audio reviewed for the same “High Fidelity” issue, not the best example of bass control, added too much “goodness” to the REF75 sound. With fast transients decay was too long.
Midrange is f…ing good, reminding a small, low powered SET amp, maybe just without its ultimate delicateness and ethereality. Everything else, presence, density, harmonies, etc. is present. Treble is most difficult to define because it simply IS. Seemingly slightly rounded, like midrange, but without unpleasant consequences. Its extent is dictated by a given recording and nothing else. One thing that can be pointed out as superior in more expensive ARC amplifiers is treble resolution and selectivity. Since in the REF75 these are slightly inferior to midrange, treble can be perceived to be quieter and darker. That was true e.g. with the quite warm in itself recording of Miles Davis’s So What from the gold edition of Kind of Blue released in the Master Works series. The cymbals seemed to be warmer than the rest of range and slightly hidden in the background. That is until they hit harder, until something struck in the background, until the recording opened up. In reality, treble is not at all withdrawn, nor is it warm or lacking. It can be full and resonant, clearly exemplified by the WAV 24/44.1 recording of the new Dead Can Dance album Anastasis issued on a USB in a special box set edition. Percussion instruments featured prominently on this album sound strong, vibrant, sometimes even piercing – just as (I assume) they should.

Conclusion

I already reached some conclusions above. I didn’t mean, however, my discussion of some inferior traits of the REF75, which ultimately is only a product of human hands, to obscure the main message of this review. That is a fantastic amp. If I were to mention them again it’d probably just to remind you to be aware of the fact that nothing is for free and that you need to carefully rethink the whole system the ARC would be paired with. I would see it in the company of well saturated, natural sounding components with well-controlled bass. A potential customer would be a fan of chamber, vocal or experimental music, even electronics; generally rather instrumental music, not bands from the stable of Accession Records – Assemblage 23, Diorama, [: SITD:], Diary of Dreams – to name a few. With them, the amplifier will be too much involved in tonality and not enough in rhythm. It may seem paradoxical because it’s quite a big amplifier with high power output, equipped with KT120 power tubes to secure a better speakers control. And that’s probably true. It’s just that said power and control translate into exceptional ease in midrange and provide the amplifier with something more than just “good” presentation of vocals, violins, etc. They gave it the kind of magic that is just as difficult to get for that money as it is in more expensive products. It just happens or not, and money has nothing to do with it.

DESIGN

Although the REF75 is a tube amplifier, its design is different than 90% of devices of that type available on the market. From the outside it looks like a classic solid state amp and only numerous vent cutouts in the top and side panels indicate the need for efficient cooling. It is, however, convection cooling with (thankfully) no fan forcing air movement.

Front and rear

The REF75 is a tube power amplifier with solid state power supply. The unit is very large and looks very solid. Enclosure is not particularly rigid, but attempts to do so – what sets the tone is a thick, solid front panel with large, analog power/bias meters looking like those used in measuring equipment in the 70s. That’s also how studio amplifiers looked back then. The meters have been manufactured by Hoyt Electrical Instrumentation, an old (1904) American company whose founders manufactured meters as early as 1890! They are located symmetrically with a large mechanical power switch in the center, accompanied by a green LED. Under each meter is a knob to switch placed on the manufacturer's typical shape. The knobs are used to switch between power output and bias monitoring for each output tube. Bias calibration must be done manually using a plastic rod (included). On both sides of the front panel are large handles, the same as those in classic studio and laboratory equipment.
The rear is sparsely populated. It sports two balanced XLR inputs at the top, located quite far from each other. Below are gold plated speaker terminals, with separate taps for 4 Ω and 8 Ω. They are rather small and only accept spades or bare wire, not banana plugs. In the center is a small toggle switch to turn off meter illumination, two 12V trigger sockets and a 20 A IEC connector.

Interior

The interior is spacious, offering lots of free space around the tubes. The electronic circuit is mounted on a single PCB, located near the front. The audio signal from the XLR inputs via long cables gets to the front. The cables are three twisted lengths of stranded copper wire. As I said, they are long and vibrate hard. Moreover, they run right between the two power tubes – I’d rather they were not there.

The input buffer and gain stage is built on JFETs. It is followed by a single Sovtek 6H30 dual triode per channel with rubber vibration damping rings to minimize microphonics. The 6H30 is a driver for output tubes.
Output stage consists of two Tung-Sol KT120 beam tetrodes per channel working in push-pull AB class. Output stage coupling is a combination of “ultralinear” and Audio Research’s patented “partially cathode-coupled” topology, which is superior to conventional pentode or triode operation. Driver tubes have gold plated pins while the output triodes come with standard pins. The tubes arrive in a separate foam-lined box. To install them one needs to open the amp by removing the bent top and sides’ plate. The bias needs to be adjusted manually; adjustment points are conveniently accessed from the front panel. Bias adjustment range is shown on the front meters.
The input JFETs are direct-coupled. Thus, the only coupling capacitors in the signal path are between the driver tube and the output tubes. They look very solid. The manufacturer claims that the capacitors are brand new design developed specifically for 40th Anniversary preamplifier.

Solid state power supply is located in the center, between gain circuits. It’s divided into three sections with separate secondary windings for JFETs, anode and filament voltage and another for bias voltage (-75 V). What primarily draws our attention is a large capacitor bank for anode voltage filtering. It consists of 12 Nichicon capacitors, arranged in neat equal rows. Before them is a very complex voltage regulator system for the input JFETs, with many capacitors of different types and from different manufacturers. It’s worth noting that all rectifier diodes are bypassed with polypropylene capacitors to minimize diode switching noise.
The transformers are mounted at the rear panel; the mains transformer in the center, with speaker transformers on both sides. There are not mounted directly on the bottom plate of the enclosure but instead are bolted to a large channel bar additionally stiffening rear enclosure section. The output transformers are small but – as it was once explained by Eva Manley – what matters most in push-pull configuration is the quality of transformer steel sheet and the precision of the windings, not the size.
The enclosure is made of bent sheet metal and its top corners reinforced with additional angle bars forming a sort of “cage”. The top panel forms a whole with the sides – it’s bent sheet metal with lots of small interior cooling slots. The unit sits on five rubber feet, four in the corners and the fifth in the center.