V-Twin wrote:

Hi all!
I'm playng around with the project of my phono preamp.
After many schematics, now i'm thinking about a EC8010 -- RIAA network --
RCA 76, both tubes CCS loaded.
Maybe this isn't a good design, but i'm a rookie in this field, so be
patient...
At the operating point the plate current for the EC8010 is 25 mA and 4mA for
the 76, both are about at 120V on plate.
Therefore the EC8010 drives more power than the 76. I wonder if this makes
sense. Maybe i'm wrong, but it seems to me that usually every stage drives a
following more powerful stage. It has to be so, or there is no relation
between the power dissipated in a stage vs. the power dissipated in the
following stage?
I' didn't found much information about this.

Some folks use a lot of current in the input stage of their phono
amp to allow the chosen triode to work where its Gm is the highest,
and its noise the lowest.
Hence the use of 6C45pi, 417A, and paralleled 6DJ8 tubes, etc,
and this supposedly allows the use of moving coil carts, without a step up
tranny.

I used to use the circuit shown at
http://www.turneraudio.com.au/htmlwe...tubepreamp.htm
but now I have a 2SK369 in cascode with a frame grid pentode, the 6EJ7,
running in triode, with 6 mA of Ia and this gives me
20 dB less noise when MC is used, and 10 dB less noise with MM.

Once you have got past the first stage, there is no need for high current
following stages,
since the SNR isn't a problem, and a gain stage with 3 mA is ample.

CCS loadings can be achieved with transistors, j-fets, or pentodes.
The CCS load is a vet high impedance load, and thus the gain of the triode
becomes cloas to its Mu, and the thd falls to the lowest it can be
without using a balanced circuit, which reduces the thd even further.
IN phono input stages, thd isn't a main concern,
and typical thd figures at 100 mV of of output is maybe 0.01% of 2H,
even when the load is a resistor of say 5 times the RA of the tube.
The noise of the stage is determined by the grid input noise, the shot noise,
and the flicker noise of the tube, which is worst at LF,
and spoils many tube phono amps, so tube choice and
careful selection is very important.
The CCS needs to be low noise, as would the R used instead,
but the Ra is in shunt with the anode R load, so R has little effect
if Ra is a small fraction of it.
Same goes for the following grid bias R, which is effectively
in parallel with the load R supplying DC to
triode.

Patrick Turner.