Review: VTL Stereo Deluxe 90/90 power amplifier

 

A year or two into my audio journey, I was in college when an friend replaced his heavily modified McIntosh 240 amplifier with a set of VTL Compact 100 Monoblocks.  Our little audio community were a little disappointed since this change also meant the removal of his Quad ESL-63s.  These electrostatics were replaced by a big ol' pair of UREI 813 dynamic speakers, using the Alnico version of the Altec 604.  But, to be fair, the Quads - without a subwoofer, weren't the best for rocking out.  Understandable to us 20-somethings.

Along with the amplifiers came the VTL book, something that he loaned to me.  At the time I didn't know too much about circuits so I read David Manley's words with much interest.  I was a poor college student at the time and was firmly stuck in the (then) more affordable world of vintage gear.  Dynaco was more my speed with the PAS-3X and the Mark IVs being in my system at the time.  Owning new tube gear was just out of my means.  

I'll hit the fast forward button on my life.  So many years later my interest in modifying or building my own tube or solid-state gear has declined.  Some of it is cost, why make a tube amp when there are so many reasonably priced units out there?  And a lot of it is patience and my close up vision isn't what it used to be.  The idea of staring at a circuit board or a snake pit of wires hour after hour no longer holds the same interest.  It is so much easier to buy something, and at such a reasonable price, this used VTL Stereo Deluxe 90/90 power amplifier was a must buy when I saw it on Audiogon,

 

Okay, what is the VTL 90/90?  It is a 90WPC - their claim, not mine - tube amplifier using a pair of 6550s (or KT88s or KT90s) and a pair of 12AT7s per channel.  No tube rectification, no B+ regulation, and no power supply choke.  Instead they use a large amount of capacitance to get the B+ ripple down.  The output tubes are also connected in Ultralinear, which means the screen grids of the output tubes are connected to taps on the output transformer primary.  Bias is relatively low, only 35mA per tube, meaning heavy class B operation.   The output iron is of note - being made by Mercury Magnetics - which are known for high quality.

This is not really the way I would build an amplifier.  I prefer, at least philosophically, pentode output stage and some type of regulation for the screens and the input stage.

Physically speaking the VTL is not as imposing as my Sonic Frontiers SFS-50.  The SFS just looks better built with the meter, larger chassis, and bigger output transformers.  It also has a trick cascoded differential 6922 input stage, and well, lots of regulation going on.  The VTL, on the other hand, has a Mullard 5-20 type circuit with a parallel 12AT7 input tube and a long-tail phase splitter (with no current sink) afterwards.

So, you are already asking after that long introduction, how does the VTL sound?  I'll try to keep this short. I will be comparing directly against the Sonic Frontiers SFS-50.

A quick note on tubes:  I used vintage three-hole Tung Sol 6550s and GE labeled (made by Lansdale) 12AT7s for the VTL.  Not the pictured tubes which came with the amplifier.  The SFS-50  amplifier used the same output tubes and Tungsram ECC88s. 

The VTL has a big bold and dynamic sound.  This is a fun amp to listen to!  Bass, something the company was known for, has a deftness that a lot of solid-state amplifiers could only dream about.  How is this possible?  I don't know given the lower output impedance of silicon based amplification.  The Sonic Frontiers, however, seems to best the VTL in the very lowest bass notes.  I suppose that makes sense with the bigger output iron and all that regulation. 

 

What surprised me the most about the VTL 90/90 is how natural it sounds.  The character of the amplifier is the same from top-to-bottom.  It's just an easy amplifier to listen to.  The SFS-50, on the other hand, is more neutral.  When it comes to detail the Sonic Frontiers is the clear winner, but that, as I will mention later, this comes at a cost.

Soundstaging?  The VTL is w-i-d-e and deep.  Vocals, like in The Ubiquitous Mr. Lovegrove by The Dead Can Dance has real presence and body.  This is a real treat, as was my American copy of Pink Floyd's Dark Side of the Moon.

So where does this amplifier fail?  This is a thorny question and one that drives into the pros and cons of this hobby.. It is a matter of detail.  I love detail but I can also be actively annoyed by excessive amounts of it because lesser recordings sound, well, bad.

The Sonic Frontiers, with its 6922 front end, has so more detail.  Like the sound of the hands striking the aforementioned Dead Can Dance song.  It really breaks down the recording into more component parts that the VTL amplifier.  The VTL blurs this detail - only by a bit - but is also a touch more engaging.

What amplifier do I prefer?  It's a matter of taste.  The VTL is just enjoyable, sounding more like a powerful Dynaco amplifier.  The Sonic Frontiers is better for those days I want to eek out more detail and hear the best that my system can do in a purely audio geek sense.

  

System:

Rega P8 turntable with Rega Ania Pro MC cartridge 

to a Denon AU-S1 step-up transformer

to Cardas Neutral Reference interconnects

to an Audio Research SP-8 (with a mix of vintage Mullurd and Amperex tubes)

to Cardas Iridium interconnects

to the amplifier(s) being tested here

to Cardas Iridium speaker cables

to KEF R500 loudspeakers 

 

Review: Sonic Frontiers SFS-50 power amplifier

 

Introduction: It has been a long time since I've wanted to change power amplifiers. The latest iteration of the Franken-Eico ST70 - once completely modded and using some very nice vintage tubes - became my favorite vintage power amplifier ever.  It's only limitation, or so I thought, was the relatively small power output.  Not a big issue with my 88dB efficient KEF R5s in a mid-sized room.

However I have been considering getting a different pair of KEF speakers, the much newer (and improved) KEF R500 Metas.  These are 4-ohm speakers with only 86dB of efficiency.   This would be harder drive for the Eico amplifier.  With that in mind I started looking getting some more power.  Since I'm a vintage tube guy at heart, a pair of Dynaco Mark III was one of the first things that came to mind.  These can make a solid bit of power and have different driver and power supply boards available.  This would make the project a whole lot easier than hardwiring (like my Eico ST70 has).  

However prices have certainly gone up on vintage gear.  The days of getting a $150 Dynaco ST70, or a $500 pair of Mark III have long disappeared.  With the price of the amplifiers, new input and PS boards, tubes, I was looking at a total cost of a more modern amplifier.  A hunt through Audiogon and saw two possibilities. 

 The first was an Audio Research VS55 that would have been a great visual match for my (much older!) SP-8 preamplifier.  However the use of 6N1Ps - and no mention of being able to use 6DJ8s - made me reconsider.  Also looking at the schematic showed a rather low B+ along with no regulation for the (pentode output) screens.  Within a few days it was sold, leaving the next amplifier on my list as the only affordable choice.

The Sonic Frontiers SFS-50 is an early 1990s design with a Joe Curcio circuit design.  The input/driver is a cascode differential with a current sink on the cathodes and power supply regulation.  This circuit provides very accurate phase splitting and wide bandwidth even before loop feedback is applied.  The output circuit is pure pentode in push pull class A with regulated screens, allowing the use of the 6550 or EL34.  The visual look, at least to my eyes, looks upscale with a touch of Heath W-6 mixed with classic Jadis.  The output transformers are also enormous, usually meaning a good bass response. The interior circuit photo shows some nice parts quality - like Holco resistors, WonderCap and RelCap coupling capacitors, Solen power supply bypass caps, and shielded cabling for both the input and feedback. There is a lot here to like and follow some of my more recent DIY experiments.

 

Upon receiving the amplifier I found myself with one heavy box.  Removed from the packing the Sonic Frontiers amp is also much bigger than I expected, taking some muscle to slide it into place next to my right speaker.  The tubes that came with the unit aren't exactly high on my list - Chinese 6N11 and the Sovtek 6550s.  The cold sound that came out of the speakers was indicative of both the  winter temperatures here in Michigan and the "solid state in a bottle" effect of less-than exemplar tubes.  An hour of listening led to one heck of a headache.

I had some older Shuguang 6550s and a pair of tested & section matched 1970s Amperex 6DJ8s on hand.  That really improved the sound, but there was still an unpleasant forward nature.  I could hear some great imaging but I was not satisfied with the overall sound.  The idea of putting the Eico ST70 back into the system was starting to become a real possibility.

 

Luckily I have an old friend with a deep knowledge of tubes.  After a phone conversation he came over the next day with a box of different makes of 6DJ8s.  From the get go - and the nature of the differential input circuit, we decided on using a quad of 1970s Mullard 7308s - the tested and spec'd military version of the 6DJ8. Even with the system just starting to warm up, we decided to start with some well-known records from my collection.

 

Listening: 

Casino Royale is a 1967 soundtrack for a James Bond spoof.  The well-known track, at least to audiophiles of yore, is the The Look of Love sung by Dusty Springfield.  It's extremely well recorded with a clean and wide soundstage.  The sensual vocals and saxophone solo really stand out with plenty of detail with natural decay.

 

 

 

I'm lucky to have 1993 pressing of Dead Can Dance's Into the Labyrinth.  The song The Ubiquitous Mr. Lovegrove is a favorite of mine - for the sonics and the music.  It's a track I've used for several reviews so I know it well.  With the SFS-50 the bass had tremendous extension and depth.  The sound of the fingers on the drums had more detail than the Eico too.  Also the left and right soundstaging width was mesmerizing.  A truly audiophile experience in all the right ways.

This album sound quality improves with every upgrade.  The second song of Lonnie Liston Smith's Visions of a New World is titled Love Beams.  The mix is dense with a lot of small percussive sounds.  With the SFS-50, the vocals sounded deeper than ever.  The small bells rang and shimmered, standing apart from the rest of the instruments.  

 

 

 

 

The Who's Tommy double LP has several different pressings of varying quality.  My favorite - at least available at a sane price! - is the German Tracks version.  This album is not exactly well-recorded but has moments that shine through - like the hard dynamic guitar playing of Pete Townshend and, of course, the drumming of Keith Moon.  With the Sonic Frontiers I heard touches of more detail and a deeper soundstage than I remember before.

 

 

 

Conclusion:   Much to my surprise the Sonic Frontiers turned out to be extremely neutral and could separate instruments better than any amplifier that I've owned.  The bass was phenomenal, both deep and precise, lacking tube bloat or smear.  The treble - at lease with the Mullard 7308s - was not etched, just natural and extended with plenty of air and detail.  The forwardness, which I thought was part of the character of the amplifier, was also gone.

Detail retrieval was phenomenal - I was hearing little bits of information that I had never heard before, making my Rega P8 sound even better than I thought.  Soundstaging was deep and wide, vocals and instruments far back (if mixed so), giving an effect that added real breath and realism to the music.

This is not a "tubey" amplifier by any stretch, nor does it sound like solid-state either.  Very fast but no etched upper mids, or tizzy treble.  My friend and I were both actually blown away how good this sounds, especially considering the price.  This is no expensive Audio Research D-250 or EAR 509s but still the Sonic Frontiers delivers extremely high fidelity, more so than any amplifier I have ever owned.

 Next up is an output tube upgrade.  Stay tuned!

 

System:

Rega P8 turntable with Rega Ania Pro MC cartridge 

to a Denon AU-S1 step-up transformer

to Cardas Neutral Reference interconnects

to an Audio Research SP-8 (with a mix of vintage Mullurd and Amperex tubes)

to Cardas Iridium interconnects

to the Sonic Frontiers SFS-50 amplifier

to Cardas Iridium speaker cables

to KEF R500 loudspeakers