The Claret Preamplifier
Shindo Claret - a copy of the world reknowned Kit Preamplifier by Thorsten Loesch
I build this Preamplifier recently. A friend in Germany was saying just how good this one was, I had to try. My friend got me a schematic and I was cracking. I used a spare "HiFi" style box I had around as case, most of the other needed components where also in my "junkbox".
The Claret copy, while not able to match my "reference", the modified Arthur Loesch Preamp (called Toccata) for transparency and detail it comes close enough for me not feel any major loss.
On the other hand I really like the "tone" of this preamp. The sound is smooth, silky with a nice strong and gutsy bass. Overall the sound is what a Friend once described as "sounding expensive". For me the pre sounds perhaps ever so slightly too nice, but I have more then enough records and CD's that benefit tremendously from the "nice" treatment.
For those interested in the Design, there is very little unusual about this Pre, however the basic design seems exceedingly well thought out.
The Phonostage is MM only, mostly the classic dual ECC83 with negative Feedback Equaliser. The main difference here is that both ECC83's operate with much lower Anode Load Resistors than common and the second stage operates at much more current than common with only 22kOhm Anode load, the second cathode Resistor is 500 ohm and bypassed.
This gives much more current to drive the RIAA Network which in addition is very high Impedance, thus avoiding the "gray" colorations so often found with the ECC83 feedback circuits, which are due to the second stage slewing into the Capacitance of the RIAA Network. The RIAA Network is not strictly according to the RIAA Timeconstants at LF, this is explained by the much lower gain of the two stages (due to the low value Anode Resistors) that neccesitates a different EQ to get the LF range right.
The Linestage uses a 500k LINEAR LAW Volume Control which together with the 680k Feedback resistor from the Output of the Linestage gives an almost logarithmic law behaviour of the Linestage as the VC also changes the linestage Gain.
The Linestage itself uses dual ECC81 or 12AY7/6072 (I used the latter) with one Valve as simple Amplifier, cathode bypassed and with 100k Anode Load. The second Valve operates as RC Coupled Cathode Follower, or more precisely "bootstrap" Cathode follower, normally at 6.6mA and with a Load of 7k5 in the Cathode.
The PSU uses a EZ80/80 Rectifier with an old Parmeko oilfilled Mains Transformer.
The Transformer delivers 230V-0V-230V at 100mA DC and 6.3C 4A) It operates into a 22uF Reservoir Capacitor which is followed by three 1k/22uF RC Filter Cells, one more 1k/22uF Filter splits the PSU off to each Linestage where it is about 250V. More Filters (22k/22uF and 150k/22uF respectively in cascade for the 2nd and 1st PhonoStage per channel) feed the Phonostage. The HT is absolutely clean, nothing I can see of mains noise comes though.
The Capacitors used where (in keeping with Ken Shindos approach) not my usual Ansar Polypropylen but cheap and cheerfull Nichicon VX Series Axial Units (the same Sakuma San uses routinely). The 1K Resistors where generic, low magnetic vitrious enamel wirewounds, the others Metal Film.
The LT is generated from the 6.3V 4A Heaterwinding which also feeds the Rectifier Valve with AC. I used a Voltage Doubler with 4,700uF Capacitors and then a 3.3 Ohm 10W Resistor as Filter into another 4,700uF Capacitor.
The Resistor is in the negative leg of the Circuit and the direct negative Voltage is hard grounded, thus elevating the Heaters 4V above Signalground without resorting to additional Voltage Dividers and at a very low Impedance. This is needed to reverse-bias the Heater/Cathode Diode which otherwise can result in additional distortion. A 0.33 Ohm Resistor (5W) in the connection from Transformer to the Rectifier is used to adjust the final heatervoltage and as "currentbrake" to reduce the peak current in the heaterwinding.
In my realisation I used a Carbon Pot for the Volume Control, fitted two switched Phono-Inputs and switched for all but one line Input both Ground and Signal, these Inputs are mostly for less critical AV Gear.
The Coupling Capacitors are all Arcotronics "KP72" Series Foil & Film types (I rate these about Level with Hoveland Musicaps) in 1000V and all cathode Bypass Capacitors are Elna Cerafine.
Most resistors are a specific type of very good sounding no-name Metalfilm Resistors (from Maplin UK - only their "M" Code Resistors need to be used - other generic Metalfilms sound very different and much worse) which sound to my ears and those of many of friends much like Holco's but with slightly more "detail" and "bite".
Fianlly, I currently use Mullard EZ81 as Rectifier, Mullard CV4004 Single Seam Anode in the Phonostage and Mullard CV4004 Box Anode in the second stage. The Linestage uses an GE 6072 ***** (Black Anode - five star special quality Valve).
All in all I have been very surprised by this Preamp as it uses several of my favourite Gripes (ECC83's in the Phonostage and with NFB, this very rarely sounds anything but very foggy) and Cathode Followers in the Linestage (even though I reduce my objections in the context of the CF being within a Negative Feedback loop - still CF's tend to generalise while at the same time sounding rather agressive and grainy).
Indeed, this preamp has all of the features I consider usually to be sonically WRONG with Vintage Equipment and yet manages to sound great. It has just enough of the Tubyness left to make for slightly "rosetinted" view of things, but not enough to cause major colorations.
Basically, it retains all of what I have often liked about better Vintage Preamps (this "golden glow", this smoothness just like expensive Swiss or Belgian chocolate - not the Cadburies crap they push on the people here in the UK) without the compromised transparency and detail, the less that ideal noiselevels, the weaknesses at the frequency extremes so common with Vintage Pre's.
This Pre has some of the best bass I ever heard from a Pre, better than my "reference" preamp. The treble is not as open and airy as the modified Arthur Loesch Pre, but it comes quite close. I have on order a 500K linear law tandem cermet pot from Vishay/Sfernice which might be able to solve this too.
So, I'll eat some humble pie here, I could never get such good sound out of ECC83's and Cathode Followers and all, but Ken Shindo did it. As usual, it's not what you, it's how you do it that matters.
This Preamp will stick around my place, if not as the main Preamp certainly as backup Pre. At the Moment I have no urge to put the "Toccata" back into the system.
BTW, as MC Stepups I use at the moment a pair of antique (1950's) Ribbon Microphone Stepup Transformers from an old Grampian Console Amp I bought supposedly broken for a tenner. It also had basically unused GEC KT66 (pair) in it as well as the Box Anode ECC83.... Nice buy I must say.
Below some more pictures:
Topview showing the internal layout
Closeup view of the Powersupply Circuit
Closeup view of the Circuit of the Preamp
