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