LIE Belin Line preamplifier revisited

...SOLD...

Back on the air.

In an older post, I detailed a line preamplifier using outstanding LIE Belin transformers. The study was made around the C3g tube, triode wired, but despite the good results in terms of bandwidth, gain, distortion, the music restitution was disappointing. Sound was dry, thin and lifeless. I was really disappointed.

I let it apart, being involved with the LCR phono calculation and construction.
Now it is done I have time to put the LIE back to the bench.
Obviously, the tube used was the problem. The G3g prove to be an excellent tube as a pentode but just average when triode strapped, at least in this design.
Once again confronted to the transformer voltage ratio (0,1) I was on a very narrow path to find a suitable candidate. My previous tests showed that if the EC8010, E86C, 6C3Pi, 6C45Pi could be good contenders in terms of µ and ρ, they mostly bias around -1,2/-1,5V which is incompatible with the usual 2Vrms (± 2,8Vp) out of my DAC or my phono.

Is there karma in DIYer life? During a speleological journey in my stock I accidentally dropped a box of weird and bizarre miniature tubes. The kind of 9 pins ones I got for cheap and intended to stay unused whatever could happen. Among these obscure and useless tubes, one caught my attention. Not saying that I had immediately the solution to my problem. At that moment I was just curious and took time to read datasheet because the construction was really uncommon, two different units in a single envelope. This is how I discovered the 6DE7. Full data here

For sure not a champion at first glance, but a careful study of the Ip/Up curves revealed that one part of this tube was very similar to half a 12BH7A and the other one resembled a scaled down 12B4A.
The great linearity seemed to be very promising.
It encouraged me to quickly calculate gain and distortion. After a few trial and error to determine the best load for the two triodes, I found it could make a nice two stages amplifier. Of course, I was primarily looking for a single tube to match my transformers but it was a purely arbitrary choice.
Do a preamp with a single triode sound better than one with two?
I had to verify that.


Several drawings later left on paper a two-stage direct coupling preamp with a gain of 60. This is pretty high but the use of an attenuator behind transformer, like in the E80CC line preamplifier, will halve gain and for that purpose I have a Tech Lab 150/150 on hand. Very good looking and mint.
This kind of balanced attenuator, alike Daven and Langevin, was common in professional gears. Extremely sturdy, made with cost no object parts, it was intended to last a lifetime if not more. Resistors where sorted to insure less than 2% imbalance.

Contacts are pure silver, coated with a special grease to prevent oxidation.
These were the days of superior construction.

Overall gain will be 3 and this is interesting to drive the rather low sensitivity VT25/VT25A push pull amplifier.

Calculating a direct coupled stage is easy.
I begun by the medium µ stage operating point. 100K in anode, 3.5K in cathode and 350V HV are best suited for low distortion.
Gain is lowered by un-bypassing the cathode. This increases internal resistance but helps to achieve a even lower distortion figure. Idle current is 2 mA, anode voltage about 150V.



Second stage operating point was chosen from Ip/Up curves too. Anode load 15K (transformer reflected impedance with 150 ohm load), Ia 22mA, Vg -20V.
Directly coupled to the preceding stage, grid is at the anode voltage which imposes a cathode 20V higher at +170V.



The tube drawing 22mA, cathode resistor for proper biasing will be 170V/22mA or 7.7Kohm. A 325V HV supply will set anode to cathode voltage at 150v.
This resistor must be oversized to dissipate more than 3W. I used paralleled 15K/5W Beyschlag carbon film ones. Long time discontinued but worth the effort to source some NOS. Clear and clean sound.

New circuit...



and new construction and wiring

This update is powered by my LCR phono supply till I build a dedicated one.

Listening report... subjective and personal

I listened to the GE's (Canadian made) I have in stock and they sound much better than the C3g in this setup. Vivid, airy, wide, natural. Like the 6J5 line preamp, very involving and alive but smoother and faster at the same time. Great on vocals. Excellent instruments separation on orchestral or complex music giving a crisp restitution. Maybe the best on Cello (what a thrill listening Fournier or Starker playing Bach suites) with a rock solid low end. IMHO the 6DE7's are underrated and should be seriously considered by the tube lovers community as a premium choice for line or driver stages.

LIE Belin preamp, MC1/60 amp, Klipsch La Scala playing Coltrane

and some other records for this test

C3g Line preamplifier Part 2

...SOLD...

Power supply

From the beginning this preamp was intended to be an exercise in the 6J5 way of simplicity and I calculated bias such a way they could both share the same power supply. The two units are very similar in terms of HV and current requirements and it helps me to make a quick switch for comparisons.

Parts and construction

Even if the circuit is as simple as the 6J5 line preamp, implementation is different. All parts take place on a thick CNC aluminum sub chassis that fit in a much larger cabinet mainly due to transformers size.
High quality L pad attenuator (Siemens silver contacts rotary switch, Dale RL7R resistors) and other selected components to match these superb transformers. Loctal sockets are original Siemens ones and input rotary switch a Grayhill 44H.

Bias is set by Dralowid vintage German carbon resistors, they have a slight grain and a laid-back sound I like very much.

During my tests I noticed better dynamics and lower distortion increasing RL to 160 Ω. Also the C3g works better without feedback.
Be aware of possible oscillations with high Gm tubes. I used grid and anode ferrite beads soldered next to the socket lugs. Wires should be as short as possible and close to chassis (connected to ground and acting like a shield).

Construction details

pictures are worth a long story

almost done, still waiting for the silk screened front plate

On the bench for tests

Low distortion (0.08%@1Vrms) and good squares (100Hz and 10KHz) for a transformer coupled tube without feedback.



Listening report

The rectifier plays a considerable role in the result and among the many ones I stock I prefer, for this preamp, the Mazda GZ32. This is a great sounding tube and still a bargain for the price. My all-around favorite one is the above mentioned plus Philips 5R4GYS, Belvu 5U4G, RFT EYY13, Mullard GZ30, STC 5Z4GY/S depending upon use.

This preamp needs some burn in time to reveal its own character.
Sounded thin and obscure at the beginning, with “thump” bass, but after 2 weeks listening to different kind of music it gained in transparency and revealed a surprising analytical capability with lot of dynamics as said before. It is now evident that this preamp has a completely different character and appears to be detailed to the extreme (it makes me feel the 6J5 sound coarse, which is not). This restitution, like under a microscope, can be an inconvenient on poor or just average sources. I have some CD's, mainly AAD's, I can't listen to because I hear the surface noise of the original material! The C3g is an incredible information catcher but with very little soul.

This, to my ears, puts the preamp on the cold side of music reproduction and I will reserve it for string music, piano or small baroque formations. For me it is not an all-around preamp, but I have to say that my trials were made with the 801A amp/La Scala speakers, a very “clear” combo and probably not the best suited for this tube. I would certainly have a different perception using my Wharfedale Super 12 RS/DD and MC1/60 amp but with a very small listening room it is not possible to have the two systems at the same time.

Another interesting point is a great 3-dimensional restitution, almost like the E80CC SRPP. This peamp has a wider / deeper sound-stage that gives the sensation of being surrounded by music. The 6J5 is flat by comparison. If you like that kind of sound the C3g is the tube to go, taking in consideration that it can be the fact of the tube, the transformer or both...
However, ambiance is not rendered accordingly, and I miss the very living like 6J5 restitution, its "rough" sound gives a more human and pleasantly textured music.
Once again it is just a matter of taste and not a definitive judgment.

some CD's for this test

C3g Line preamplifier Part 1

...SOLD...

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......

E80CC SRPP Line preamplifier Part 3

Power supply

Unlike the 6J5 line preamp I used a CLC filtered supply. I usually prefer the LC for its very smooth ripple behind choke but listening tests demonstrated better dynamic with SRPP. A good calculation of ripple rejection and time constant helps to achieve a fast recovery supply with a minimum of cells. To minimize inter modulation the supply is splitted in two after the second capacitor



Ripple calculation

On C₁ the ripple is estimated by formula     V_C1~ = 10 I/C     where I is the current through circuit (in mA, here 7mA) and C the filtering capacitor (in µF).
V_C1~ about 2.2V_pp or 0.77V_RMS with a time constant of about 7 millisecond.

On C₂ the ripple is calculated by the voltage divider formula    Vout = Vin Z₂/Z₂+Z₁    where Z_L1 is 37.6K @ 100 hz and Z_C2 50 ohm at the same frequency.
V_C2~ is about 1.05 mV_RMS with a time constant of 24 millisecond. Thanks to the 60H choke that smooths out the ripple with great efficiency. A 20H one would have been enough but I had this one on hand.

The same applies on C₃ with R = 2.7K and Z_C3 133 ohm. It leaves a 0.05mV_RMS ripple which represent a -130dB attenuation at a time constant of 33 millisecond.

Parts

Good sounding Philips/RTC EZ81 rectifier along with old Tango Hirata choke and some Siemens MP/JS and F&T capacitors are the parts of choice for this power supply.

Tube choice and tests

Not a long or tedious quest for different brands of tubes to be tested, just two makers for this fine tube; Philips (can bear Valvo, TFK, Siemens and so on names) and Tungsram. I attentively auditioned these two competitors and the result is disconcerting. In this setup the Philips that bears a reputation of excellence is not the winner of the test. The musical rendering appears fuzzy despite some great qualities in terms of tone and speed. It gives a blurry image that leaves the observer perplexed. On the other hand, the Tungsram brings music to a level of clarity and enjoyment seldom heard. It has incredible precision and gives a density to the sound that makes it perfect on any kind of material. Very detailed, not to say clinical sound but on the good side of absolute neutrality. You like it or not but it never leaves indifferent and if you are after audio perfection you get very close to it. Furthermore this tube gives an almost holographic image of the sound stage. Great, great tube !
As expected distortion is very low. I get 0.9% @ 1KHz and 2V_RMS output, mainly second order. Noise floor is also very low and hard to measure on my FFT analyzer, thanks to the very good CMRR (or PSRR, it's the same) of this circuit. In facts we have the same power supply ripple rejection qualities than in parafeed setup and some will consider the SRPP a parafeed cousin with an active load in place of the cumbersome anode choke.

SRPP line preamp completed

...in its industrial blue gray and black look. All front and rear plates anodized aluminium, CNC machined by Schaeffer AG. Outstanding workmanship for a clean, professional appearance.

same connecting organization than the 6J5 line preamp. Outputs are doubled by Lemo 0 coax outlets and this feature will certainly disapear in future works. The little male plug is a nightmare to solder considering the 0.6 mm central pin !

Last minute update...

I made a transformer connection mod. Although I did not notice any sound change, it is intellectually more interesting not to have the C₁ electrolytic decoupling cap in the signal path.



some CD's and LP's I appreciated a lot, and there are many others ...