Computer music is very interesting- I think this is one of the most exciting periods in hi fi in a very long time, specifically because we are seeing a new playback medium which has the means to be as good as, if not better than the reference for audiophile sound quality- the vinyl record.
That's why high resolution digital is exciting- we will soon be able to get all new music released with the same sampling rates as the original recording, with bit perfect transition to the playback device we use at home.
At the moment most music is recorded at 192Khz/96Khz, then has to get dumbed down to 44.1Khz to suit CD playback.
Initially analogue tape was the master reference- the vinyl masters were/are made from tape masters , but not everyone is equipped with a suitable reel to reel tape deck , and the record companies are certainly not going to release their precious tape masters to the public.
And making a vinyl record involves many steps , from recording the tape, which can take many processes , transferring it to a cutting machine to make the master , which is then used to press the vinyl. Many preamps, power amps and recording techniques used in a long series of chains to finally get music from a miniscule diamond point in a equally miniscule groove.
But there is no denying that this "black magic' works - a well recorded , well made vinyl record can sound superb- even now 42 years after hearing my first record I'm still amazed at the quality of sound that can emanate from the grooves.
The 3D imaging and sound-staging of a good vinyl record can leave even the best digital struggling- digital is getting close, and certainly is more accurate than vinyl, but I doubt that I will ever give up my 1000+ vinyl records as eagerly as I did with my CD collection.
Now we get on to the subject of this post, phono preamp design, but firstly let me say I'm only interested in moving coil cartridges and I dont build moving magnet compatible phono stages.
All the best cartridges in my view are MC, sure there are some good MM cartridges, but I only use the best so MC it is. MM suits others- your choice.
A typical MC cartridge has an output voltage of around 0.2mv , that's 2 ten thousandts of a volt, so to get it up to a useable voltage it has to be amplified around 10,000 times. Add to that the fact that vinyl records are made with a frequency response that cuts out most of the bass, which has to be accurately restored by the phono preamp, and it adds up to some serious engineering.
Starting at the input, MC phonos usually use a step up transformer to get the initial gain up to something suitable for the second stage. A simple tube gain stage cant be used because tubes are too noisy with this much gain- the noise floor gets amplified to the point it is audible. The usual step up trans is quiet and can do a good job, but I've always prefered an active device using a transistor or j-fet. The j-fet is really like a miniature tube, is very linear and very quite. It has no transistor glare and doesn't suffer from the phase problems that step up trans can. It also presents an easier and more compatible load for the cartridge.
Speaking about loading some people get real fussy about loading of MC cartridges. In simple terms, a MC cartridge is really another step up transformer, and the loading is there to primarily damp the ringing of the cartridge, which is really high freq oscillations. IMO a really good MC cartridge needs minimal loading, around 1K being plenty. If you need to go down below 100 or even to 10 ohms loading, then imo , that cartridge is poorly designed and is not worthy of a good MC phono. Typical signs of a badly designed cartridge is a strident glare and nasty treble. Heavily loading it might fix it but heavy loading also robs dynamics and vividness.
Supratek has been using a cascode input stage using a j-fet and the E180F (6688) pentode tube, wired as a triode, since the late 90's. The E180F is a frame grid tube , one of the last of the great tube designs with outstanding characteristics. Wired as a triode it is extremely linear , and in conjunction with the j-fet is a very high gain, low noise first stage.
Between the first and second gain stages we implement part of the RIAA filter (which compensates for the bass roll off on vinyl records) We've always used a simple passive RC filter here and it can be capacitor coupled or directly coupled to the grid of the second stage tube. Both methods have advantages and disadvantages, depending on how the second stage is set up.
Between the 2nd and 3rd stage we implement another part of the RIAA filter, and finally after the 3rd stage we have the output to the line stage. Lately we've been using grid chokes on the output as it allows us to implement a steep subsonic filter that doesn't detract from bass tightness and solidity, and grid chokes sound great.
On the Chenin and Cortese phonos the ECC88 tube or it's derivatives the 6922, CCa, 7308, or close substitute the 6N1P is used for the 2nd and 3rd gain stages. The Grange uses the ECC88 and the 6E6P triode wired. Loading of the plates is done by either resistor or CCS(constant current source), cathode biasing is both resistor and what we call lowR bias which is a form of CCS that allows us to eliminate capacitor bypasses. In fact eliminating as many capacitors as possible from the entire preamp has always been a major aim. There is less than half a dozen electrolytic caps in a Supratek preamp, none used as coupling caps. Compare that to a typical phono pre which could have as many as 50 electrolytic caps in circuit.
Over the years we have tried every conceivable variation of this basic configuration. The following circuit is perhaps the most technically interesting Kevin Covi and myself developed- a fully DC coupled phono stage with no coupling capacitors between each stage, the only caps used were for RC filters.
To keep the circuit at the correct operating points a servo circuit is employed and although tricky to set up, it worked very well. However although a technically advanced circuit it actually didn't sound any better than a simpler circuit with far less complexity.
The phono circuit we use in 2011 is quite different and is the result of 25 years of phono design and is perhaps the pinnacle of moving coil cartridge pre-amplification.
