May 24, 2018

LCR Phono preamp Part 2

In the last post I described a constant coupled stage with a gain of 53 and low output impedance.
For my project I need a gain of 2800 to compensate the LCR setup insertion loss (0,158_see part 1), means about 52 (2800/53) for the first stage.

Only a high Gm triode or pentode will give such an amplification on the low Tamura load with a wide frequency response. Not so many candidates when you must find tubes with inherent low noise & distortion. Among usable triodes my choice went to the E88C. complete data here.

Almost unknown, this tube is a dream when you have to deal with very small signals. Primarily developed for UHF use where high gain and low noise are mandatory, they share these qualities with a few others tubes like the EC80, the EC8010, but have one of the lowest distortion & noise figure you can expect from a triode. This emission noise is hard to measure. With a differential amplifier coupled to my HP 3561A dynamic analyzer I got a -107dB (4,5µV) noise on an open grid tube.

Means the micro details from the vinyls won't be hidden by thermionic emission. Moreover, the very low Miller capacitance (1,2pF, about half the EC86 one) will insure extended highs.

Telefunken (Philips made) & Siemens

Its medium ρ makes this tube an excellent candidate for transformer coupling. With a 20K load @ 10mA it works in a very linear region and gain is high (~51).
It is a super quality frame grid tube with outstanding construction whatever the manufacturer.

Preamp overall gain will be 51 x 53 x 0,158 = 427 or +52,5dB.
The 5mV out of my MC transformer will produce 2,10 volts, just as expected.

Full schematic

Alternate driver

For those interested I performed some tests with a C3g pentode and got good results, way better than in triode mode, the E86C balancing its natural dry and analytical sound.
I finally preferred the E88C because final gain was too high but this can be favorable with very low output carts. The G3g will even give lower distortion than the E88C with no Miller's effect. To get it usable, cathode is unbypassed and gain is reduced by a factor of ϒ where

                                                ϒ = 1 / 1+Gm x Rk          (~ 0,29)

the first stage gain will be       Gϒ = Gm x RL x ϒ          (~ 82)

and the preamp overall one 82 x 53 x 0,158 = 686 or +56,5dB.

A 5787 voltage reference tube makes an unusual feature to insure steady Vg2 in place of the decoupling capacitor. It is a minimalist shunt regulator that draws 3 time the grid current. The gas tube return must be connected to V1 cathode and not to ground. In that way, with an unbypassed resistor, any cathode voltage change due to grid input signal won't affect voltage setup and Vg2 will remain constant.
I set the current to 6,5mA, but any value between 5-25ma may be used taking in consideration that regulator noise is proportional to current flow and may vary from one maker to another. Had best results with Cifté (Mazda) and Raytheon WA series.

Next time, power supply and tests