Jan 1, 2018

C3g Line preamplifier Part 1

A rare find initiated this project. A pair of new and mint audio transformers by LIE Belin came into my hands at a moment I was thinking how I could put to good use some tubes I have in stock for a long time.

A short history about that French company will help to understand why I was so amazed by this discovery. During the 50's LIE was a manufacturer of top of the line professional audio equipment and turntables. The company was bought by Mr. Belin a French engineer inventor of the Belinograph the FAX ancestor. Under is clever direction the new company LIE-Belin sold cost no object equipment and parts to the former RTF (French Radio Television). These parts are now very sought after by French DIY'ers for their outstanding qualities especially turntables, but not only.

Belin turntables make the best of professional gears, like EMT's, look like toys. Their top of the line weighed about 250lbs and was intended to be used in the most severe conditions 24 hours a day. None of them had an issue. If you can source such player on the second-hand market you will understand what ʺ last forever ʺ means. Here a nice example of a TD32 from HIFI VINTAGE.

Back to my transformers. The ones I found are 10K/100 Ω split, capable of handling 60mA with a frequency response of 30/45 KHz ± 0,5 dB. Renowned for their transparency and accuracy, these beefy units weight about 4lbs and are as big as an output transformer. For some they are the best transformers ever and unfortunately some scrap merchants did not hesitate to dismantle complete working units to grab these gems. What a shame!

SN312 mixing console using LIE Belin transformers. Pictures from Audiovintage Forum

Tube choice

With such treasure in hands I was in need of a more than decent tube to go with, but just a very few can accommodate the steep voltage ratio (0,1) while providing a usable output voltage.
High trans-conductance and high µ triodes or pentodes (triode wired) are certainly the best choice. I stock EC806S, EC8010, 5842, D3a and C3g that can be good contenders. Drawing load lines revealed that the first ones did not have enough headroom, most biasing in the -1,5 to -1,8 V range. Means with a usual 2V DAC output, tubes will hardly clip even is not using the preamp full throttle. The best suitable was the C3g, triode wired (full data-sheet here).
NB any 10K/150Ω transformer from Hashimoto or Lundahl will certainly perform very well.

I am quite reluctant to remove the aluminum shield just for a “better” appearance. For those who want to know what's inside there is a pretty nice picture from Bartola Valves

Ip/Up curves show a very linear tube and the µ and ρ (40 and 2,3KΩ) will give a gain of about 32 or 3,2 on transformer secondary. In the case it would be necessary to flattens overall bandwidth I will provide a place for a switch to select a feedback loop (3 to 4dB).
The slightly higher gain than the E80CC SRPP will help me to drive with ease the rather low sensitivity VT25/VT25A push pull amplifier that I use today in my system.

Preamp schematic

more to come and happy new year......

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