Any Members Tried the Audio Interface CST-80 SUT?

You matched it well roberjerman:)

The Cotter P Transformer was for very low impedance cartridges (2 to 30 Ohms) which would have been ideal for the FR1 MK3F which has a 10 Ohm internal impedance.
  

As an update for anyone interested I just discovered that I could reduce the reflective load from 470 Ohms to 40 Ohmn by installing a pair of 6600 Ohm Parallel Resistors.

The 40 Ohms would then be 20x the internal impedance of the Spectral cartridge and much more acceptable.

OK so the Audio Interface SUT arrived last night and first observation is that the unit is very small and very well made. A high quality product indeed.

Trouble is I’ve gone and put myself in a self inflicted dilemma which you will read about a the end of this very long post.

Firstly I love my existing Fidelity Research FRT-4 SUT. Not just because of the quality but also because of the versatility. It allows me to run up to three turntables, (I currently run only two), and switch between any cartridge I like with a simple turn of the knobs. It cannot be faulted in any way.

That all said I sat down last night with my copy of Melody Gardot "My One and Only Thrill" to do some A/B testing. The track I used was "Somewhere Over the Rainbow" as this has a variety of instruments, a beautiful vocal performance and lots of fine and subtle detail. I also know the track inside out and back to front.

I started with the FRT-4 and as usual it sounded excellent. I listened for many different things such as the tone and clarity in Melody’s voice, the timbre of the instruments, the sparkle in the higher frequencies, the extension of the bass with the lower frequencies etc. I also listened and observed the "air" around the instruments. For example how much space appeared between each instrument and the singer.

In this very critical analysis and without yet installing the Audio Interface SUT I picked up on a very very slight opaqueness to Melody’s voice that I thought could be improved upon. Apart from this minor observation I thought everything sounded sublime.

Switching to the Audio Interface and being totally cognizant of all of the small nuances of the track I just listened to, I observed the following.

The slight veil I had noticed over Melody’s voice was lifted and it became clearer. This slight improvement in clarity also extended to all of the other instruments and was apparent throughout the frequency spectrum and not just the higher frequencies.

Also the air around instruments grew a little and it felt that the subtle background instruments now had their own little place in the room instead of being slightly overpowered by the main instruments or singer. In other words the music appeared to come alive that little bit more.

From a tonal perspective, whilst there was a very slight increase in the resolution of higher frequencies it was in no way a "brighter" performance than the FRT-4. They both share a very similar tone and I think it simply comes down to the improvements in clarity.

I performed this A/B test on this particular track multiple times before switching to a couple of other tracks on the same album and in each case I think I can sum up that there is a slight improvement on the overall clarity and sound stage when using the Audio Interface SUT.

If a can use a very simple analogy it would be like when you are at the optometrist having your eyes tested and you think everything is totally in focus and then he switches to that last lens and your vision becomes that little bit clearer. That is what it seemed like last night.

However with that all said, there could be a possibility that the interconnect cable used had something to do with this improvement in clarity. For example the FRT-4 uses it own built in interconnect that is now some 30 years old, whereas the Audio Interface does not have a built in interconnect so I used my newer "Straightwire" interconnects. Not sure on that one, but it could be a contributing factor.

Now for the dilemma. I think I like the Audio Interface over the FRT-4 but it is early days and I need to do more comparisons at the weekend. It is also going to be a hassle when it comes to using my other TT or when switching cartridges as there will be no more simple turning of a knob. It will be a case of pulling out cables and ground wires and reinserting other cables and ground wires.

Also, as it is only a 1:10 SUT it will not work well with my much loved Spectral Reference without the use of some parallel resistors. In fact it is only compatible with my Shinon Boron Red as my other carts are either very LOMC or MM.

I think I will have to keep both for now and see how things go.

Sorry for the long post.

You promised a review, and delivered on that promise — a thoughtful and thorough analysis. May I ask what information appears on the CST’s case? Mine only says “CST-80 / II H” — and a serial number “40 1003”. Not much to go on. Maybe the spaced-off “40” in the S/N means 40ohms, which would mean 20dB gain.

But it handles my FR1Mk3F’s 0.14mV easily — in fact, with the volume on my preamp at only 8 o’clock, it’s blasting. Your 0.2mV Spectral should be fine — though its impedance is wickedly low.

Your dilemma is superior SQ (though not by much) vs Versatility. If you’re like me, I’d go with versatility... but always feel something’s missing, now that I know what’s ‘not there’. I’d always miss that extra ‘something’ and inevitably go back to the CST. But you have two great SUTs, and that’s not a bad dilemma to have. Our pursuit of the divine in music is fraught with dilemmas.

You can buy, or make, a switch-box to handle more turntables — but will that degrade SQ? A bit, probably — the FR’s switching capabilities may account for it’s slightly lesser clarity.

Even though its captive leads are old I’m sure they’re good (it’s FR after all; they used silver coils in their MCs at the time). I’m not sure replacing them with RCAs would help. At least the old leads are a direct signal path — yes, you could use “better” ICs, but chopping the signal up with additional solder-joints and plug connections might nullify any benefits a “better” cable could give you.

You’ve probably seen this review/lab-test of the CST when it was produced; it contains thorough data:

http://www.gammaelectronics.xyz/audio_12-1982_audio-interface.html

The site is also an archive of old reviews of serious equipment, fun to leaf through...

PS — No need to apologize for the long post — it wasn’t long enough! I'll be very keen to read your further ruminations and conclusions...

Hi bimasta, thanks for your kind words. Mine did not come with any box or literature but the sticker on the bottom states CST 80 40 Z which is the same as yours, 40 Ohms / 20 dB gain.

It’s funny that I too have the FR1 MK3F which is currently fitted to my other turntable so I will definitely give it a try this weekend and see how it sounds. For this cart I typically use the 3 Ohm tap on the FRT-4 which has 31 dB of gain and a 1:36 turns ratio.

I will also probably provide a further update once I put in some hours and do some more A/B comparisons with more diverse music genres.

Watch this space:)