Phono stage based
on Transcendent RIAA
Installing the SILK MC trafos for my Dynavector XX2 mkII cartridge made an external power supply necessary and the PSU section was placed in a separate cabinet making the whole thing quite bulky. Re-building the TRANSCENDENT phono stage has be on the to-do list for a long time and besides getting a smaller unit next to the grammophone, improving sound from hardwired amplifications stage, full PP capacitor power supply and better caps in riaa-equalisation step wouldn't hurt either.
While waiting for the right time to start building, a number of RIAAs have been auditioned and nothing came up that convinced me to go elsewhere. For the separate power supply I decided to make a circuit board as the number of components and overall complexity would make a hard-wired solution look like a bird's nest. No tubed regulation was decided either as I don't think it pays off. Re-building may not be the right word as all parts will be new including power supply, thus a semi-hard-wired copy.
Finishing the phonostage and setting it up for the first time is nerve wrecking! Will it be silent or take you into a new hum-war? I was exactly minded for another round of hum fighting, but this proved an unnecessary worry. Hooking it up and turning the line stage volume control max only left the usual sssshhhh of the universe. Yes, I do think it sounds a tad better but I don't blame you for thinking me biased from all the working hours.
Most parts were bought locally (ELFA, not cheap but around the corner), except for: Teflon sockets from Hong Kong (search eBay). MC trafos from SAC Thailand (read below). Tubes bought on eBay from Russia (search eBay). Hammond choke came from Finland (hard to find Hammond + decent shipping rates for a single unit).
|The Power Supply Board (BACK TO TOP)|
a special transformer made with all secondary
windings would be rather costly - and having a range of
spare transformers - the solution below was decided.
Included is a small 9V transformer allowing the power
supply to be turned on/off from the RIAA box via a
low-voltage DC line. Hunting for expensive Black Gate
electrolytic caps is fortunately no longer needed and
even for (valve) power amps, film caps seems a much
better solution - if space allows, because film caps
certainly are bigger than the old electrolytic caps. Here
22 uF/400V PP Cross Caps from Jantzen is used throughout.
The amplifying stage is going to be hardwired as circuit simplicity makes this easy. Tube teflon sockets have been bought as these are now available from Hong Kong at reasonable price. The quality of these teflon sockets is second to none.
|Chassis (BACK TO TOP)|
|Schematics (BACK TO TOP)|
|SILK MC trafos (BACK TO TOP)|
SILK Supermalloy MC-step up transformer from SAC Thailand: http://www.sacthailand.com/. These are MC220 allowing 1:10 with 100 ohm loading impedance, or 1:20 with 25 ohm loading impedance. Download wiring diagram here.
|Aftermath - and RIAA tweaks|
|Being a novice in tube technology,
my tube adventures usually
create some response from readers more experienced in the
art of building amplifiers. This RIAA was no exception.
Opinions may be diverse when it comes to loudspeakers, but when it comes to tube amplifieres, people have indeed very strong opinions on what's good and bad. Every designer seems to hold on to certain principles and regard other designers' circuitry with great suspicion, sometimes even contempt based on (anonymous...) statements found on the web.
I build what others have done
and I listen. Over the years I have built amps based on a
wide range of principles and I have borrowed amplifiers
to gain experience on sonic signatures from different
circuitry. The choice of valves, capacitors and resistors
and their impact on the final sound makes an unendless
number of variables.
almost choked on his morning cereals when seeing
ICEL polyprops for the passive network. Ideally I would
have used 1% Philips polystyrene, but my local supplier
told me these weren't made anymore, thus the 1% 10n ICEL
PPs. I had some Russian 10n silver micas, but wasn't sure
they had the necessary precision for this delicate part
of the phono stage. Eventually I found 0.3% 10n silver
micas from another supplier and changed all 10n caps for
these including the grid cap on upper 6C45pi tube.
The 2200 uF/10V electrolytic cap on the lower 6C45 cathode was removed and despite lower gain, this did not to any noticeable degree improve the sound, but if less is more, I leave it out. Later, after having changed the ICEL caps with silver micas, I tried inserting cathode caps again, this time 100 uF/16V black gate type N, and realised I missed the loss in gain. Never say never....
Resistors: From what I have been able to pick up from the web, MOX resistors for power (e.g. 1W Beyslag for anode and cathode resitors) and carbon for signal, e.g. grid resistors and resistors in series with the signal - or generally where current is low. I'll see if I can find values suitable for this circuitry.
Some people will reach for PiO for coupling caps - I've
tried - and to my ears they're not the best. Mundorf
silver/golds have been used extensively but were finally
replaced by tin foil caps. Whether wound on teflon or
polypropylene seems irrelevant from my experience. I have
bought a few tin foil caps wound on teflon, but they
don't sound any better than their cheaper brothers wound
on polyprop. I guess the sheer weight of the tin foil is
a major contributor to the sonic signature of these caps.
Angela has these up to 1 uF but doesn't ship to Europe.
Audyn and Mundorf tin foil caps do the same thing and
Mundorf even have 2.2 uF/250V. Audyn e.g. 1.8 uF/250V -
and a uF range interesting even for loudspeakers (6.8 uF
Reviewing the whole project - and listening to my favourite vinyl recordings - these last months of work seems mission accomplished. I may be building more phono stages in the future to experience other kinds of circuitry, but this phono stage makes a perfect match to my beloved Dynavector XX2 mkII cartridge.