LCR Phono preamplifier Part 2

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In the last post I described a constant current 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 EC8020 and 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
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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

Note: LCR network can be either feed or loaded by 600 ohm, in any case you have to remove one 600 ohm resistor ( R4 or R12 ), this ensures network to work properly.

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. 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

LCR Phono preamplifier Part 1

...SOLD...

Building a phono preamp is a difficult exercise in the way we have to deal with very small signals that must be greatly amplified without any loss and/or unwanted noise.
If, like me, you enjoy the qualities of moving coil cartridges to extract the quintessential parts from your beloved vinyl’s the expected signal out of a quality transformer or other step up device is roughly in the 5mV range. This signal must be amplified to about 2V to accommodate the usual low gain line amplifier. This means an overall amplification of 360/400 or +50/52dB.

To reach that goal most of the schematics found in literature use 3 or 4 stages.
In my personal conviction ʺ the fewer, the better ʺ I expect this high gain with 2 stages to minimize sonic coloration from the tubes to be used. To add difficulties, I also want a low output impedance to drive any kind of load.

Before spending lots of time with math (load lines drawing, gain calculation, bandwidth and so on), I had to determine which kind of correction would be the best for an accurate and neutral transcription of signal.
The most popular and cost-less designs use active feedback with all the inherent difficulties and compromises due to its implementation like phase shift, distortion, output impedance and eventually oscillations. A better alternative is a passive RC network (split or not) but to work properly it imposes a low impedance source and a load at least 50 time greater than the first series resistor.
I prefer, by far, the LCR network despite its cost. Very precisely wounded chokes, close tolerance resistors and capacitors can give an equalization within ± 0,1dB.

However, there are two drawbacks using such a RIAA correction

1 – Needs to be driven with a low and constant impedance within the audio range. The most practical way is the use of two matching transformers, one on each side. Such a setup is cost no object but worth the investment.
For that purpose, I can use some good Tamura transformers I have on hand (A8713 20K/600 and TKS50 600/50K).

In that way the future preamp architecture will be:

                  Input tube II Transformer II LCR EQ II Transformer II Output tube

2 – The LCR EQ introduces a -20dB (X 0,1) signal attenuation, means these
two stages must have something like 70/72dB gain.
To simplify calculation of these two stages, it is convenient to consider the transformers & LCR units as a single device with a gain of 0,158 or -16dB.

Going backward in my design is also helpful as I exactly know what my final requirements are: 2V with low output impedance.

Final stage

The best way to get the low output impedance and wide bandwidth I need is a cathode follower but alone, with a gain of 0.95/0.98, it would be useless for my project. A nice solution is a constant current direct coupled stage. It consists of a classic anode follower DC coupled to a cathode follower and sharing the same supply. This setup was widely used in professional equipments to source a volume or a tone control.
The tubes I choose are the EC86 / EC806S, renowned for their warm, detailed and dynamic sound. The Russian 6C3Pi was another possible choice, cheap but very well made it is very linear too, unfortunately a different pin-out makes a quick comparison test impossible. Too bad, I previously used this tube in a DAC buffer stage and it was very rewarding. Gave a detailed and silky smooth sound.

Back to my E86C's. An 18K load will give a gain of 54 and a switch to un-bypass V1 cathode give me the possibility of a lower gain (~35) if necessary.
A 150 ohms bias resistor sets current at 10/11mA. These tubes sound better when drawing current and generate lesser noise.
The cathode follower with a gain of 0,98 will set the overall one at 53 and its AC load will be around 13K, considering the input resistance of the following line preamp. High enough to give very little distortion, characteristics being almost vertical (dashed lines).
Output impedance will quite low at 70 ohms.





The 1Nxxxx diode is necessary to protect V2 cathode from stripping when applying HV on a cold tube.

more to come...