May 10, 2018

LCR Phono preamp Part 1

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’s (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 costless 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 equipment’s 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...

2 comments:

  1. Hi Fred,
    I very much enjoy your post and blogs. Please continue. Always interesting thoughts and worked-through articles.
    May I ask if you thought about a mu-follower for the last stage? You then get the full mu of the tube and very low output impedance. (either ss or tube version).

    Also what happened with your previous serie on the C3g preamp? It was also very interesting.

    Best regards,
    Maxime

    ReplyDelete
    Replies
    1. Hi Maxime and thanks for your support to continue this blog.
      The mu follower was an interesting option with a high gain ( almost the tube's µ, the lower triode load being bootstrapped by the upper one) and the usual low output impedance of the cathode follower. I preferred to come with a circuit seldom used. Just a matter of choice.
      I will certainly put the C3g line back to the bench and try to find another solution to feed the LIE transformers. The sound I get is not my taste Have to take time for a better result...
      Best regards
      Fred

      Delete