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

E80CC SRPP Line preamplifier Part 1

This project was initiated by a request of a line preamp with selectable input sensitivity to match sources with different voltage outputs. This preamp should also have a low output impedance to drive long connecting cables without any loss.
With such goals in mind it became evident that a simple 6J5 preamp would not fit these requirements.

1- The use of two gain control stacked together would greatly impact music transcription.
2- The gain of this preamp would have been too low to accommodate an input voltage divider up to -6dB.

A high gain stage with a voltage divider at input and a volume attenuator at output (which is the best way to control the gain from my humble opinion) was the way to go.

For that purpose there are a lot of good circuits with good linearity and gain, cascode, µ follower, totem pole, push pull. Push pull circuit, seldom used in preamplifier, is very interesting because it cancels all even harmonics. With such a circuit you can expect very low distortion using triodes but a phase splitter (transformer or tube) add complexity to the circuit, increases final cost and is hard to implement in a standard chassis.

One circuit shares this low distortion, high gain capability without the necessity of a splitter stage, the SRPP.

I have been working with for a long time and I know how it works (well), its limitations and drawbacks (very few when correctly understood and used).
Some would argue that it have a special sonic signature, especially on voices, with a kind of emphasis that could be appealing but do not reflects reality.
Believe me, when correctly used and adjusted (a FFT analyzer is of great help) it can be very neutral without being clinical or cold with huge dynamic and stunning resolution and accuracy on complex music.

SRPP (very) short history and use

Originally intended for video (1943) it was widely used to drive low impedance capacitive loads with high gain and wide bandwidth.

SRPP as it should be set up.



As indicated by its name, the SRPP is a push pull even if not evident at first glance.
I won't bother you with any mathematical demonstration because it is not the main topic and there are very good publications that help to understand this circuit, Audio Xpress The Optimized SRPP and Tube Cad SRPP deconstructed.
Never forget it works like a push pull only when loaded and gain, distortion, bandwidth greatly depend upon this load. Moreover its linearity is a direct relation with the flowing current which itself depends upon load.
The good thing is if you have a well defined load that won't vary with the amplifier connected behind, you can fine tune the circuit to get the lowest distortion playing with only 3 resistors. Reason why I choose to load my SRPP with transformer.

Finding a good one was easy because I had some Tamura on hand.
The TKS20 used in this preamp is a 600/10K line input transformer but can be reversed used as there is no current flowing through primary.
Tamura are among the best transformers I had to work with. Unfortunately most are discontinued and prices skyrocket on the second hand market.

Previously I said that the best way to control gain was the use of an attenuator behind the transformer. I was lucky enough to have some Daven 600 ohm dual T pad, perfect for the purpose.
These pieces of engineering are unbelievably well made with plain silver contacts and sorted resistors for perfect tracking. They were built to last a lifetime. The only disadvantage is a 6dB insertion loss that have to be taken in account when calculating overall gain.
These attenuators are very hard to find today, especially NOS, and it is interesting to build its own T or bridged T attenuator. See Pots & Attenuators tutorial.

Tube choice

To reach my goals I needed a tube that gives a minimum overall gain of 4 (attenuator will divide it by 2). This means a SRPP gain of 16,30 (transformer voltage ratio seen by tubes is 4,08) and a tube with a µ of 20/30, a ρ of 7/12 K to correctly match the 10K dynamic (AC) load.
Double triodes like 12BH7A,13D3,E80CC would be good candidates. In this study it is stated that V1 tubes are identicals.
I finally used the E80CC because I had excellent results in the E140 amplifier and it is easy to source compared to the British 13D3. I made some simulations with the 12BH7A but gain was too low.



More to come...

Pots and attenuators

In every audio equipment there is a gain or volume control to set music reproduction to an enjoyable level . This control is usually devoted to a simple potentiometer mostly for cost reasons. On pricey materials you can find more elaborated and expensive devices like L pad attenuators but if there are just 2 or 3 resistors in signal path to minimize sonic alteration the reflected impedance constantly vary like in basic potentiometers.

In professional gears, like mixing consoles, pots are very seldom used and signal attenuation is provided by more elaborated units. T pad and bridged T pad are the most widely used.
If they essentially remain voltage dividers these particulars setups are intended to keep a constant impedance whatever the attenuation. Very important when used in 600 ohm lines they can be of real benefits to prevent bandwidth alteration in amateur constructions.

Such attenuators are by far the best way to control signal. Unfortunately good NOS units by Daven, Tech Lab or Langevin are scarce and expensive but it is possible to the average DIY'er to build its own one with the great advantage of choosing the impedance to meet particular loads (I have on hand some excellent transformers with uncommon 900 ohm loading impedance).

They are quite easy to build, you just need a good soldering iron, some quality pliers, a bunch of resistors and plenty of time. I personally use Dale RN60D, Dale RLR7C or Philips/Vishay MRS25 (excellent for the task, easy to find and affordable). I also prefer the bridged T attenuator over the more classic T pad because it is fully symmetric.

pic taken from allaboutcircuits.com

Bridged T calculator greatly eases resistors calculation.

A good rotary switch is essential. Must be of the shorting type (make before brake) and have at least 20 contacts. Some old Siemens are excellent but hard to source.
Fortunately very high quality equivalents have been made in the former East Germany by RFT and are easy to get. They have 24 contacts, silver or palladium on copper. With such a switch you can build a 0 to -40 dB & off / 2 dB by step attenuator.

Step by step construction

First edit a file with impedance load, desired attenuation and resistors values.
For a 600 ohm one with a 2dB step the resistors values are:



RFT rotary switches are the best available choice for the price. Cold war time material, they where intended to work under adverse conditions. Very well constructed they are easy to disassemble and reorganize to suit our purpose.

1- Fully disassemble switch (except rotary mechanism) to access the phenolic wafers.
The silvered ones need some gentle cleaning with a piece of fabric (Never ever use chemicals !)

2- From a 3 wafers unit, set 2 wafers back to back for the R1 series resistors using the small spacers.
These will be populated later. I choose to make the common loop first because it is the trickiest part to do.

3- Make a common (Ground) loop from a silvered copper wire and insert a short fiberglass (or any other insulation material) sleeve to prevent any unwanted contact

4- Common loop hold in place and soldered on contact 1, this is the off position

5- Shunt resistors R2 bent and cut to proper length prior to solder. It helps a lot to solder first the resistor on the opposite contact to the common loop. It makes this one stiffer.

6- Common loop fully populated

7- Now it is very easy to feed the R1 series resistors wafers. Be sure to properly align the wipers first.

All R1 resistors soldered...

8- Reassemble the shunt resistors wafer, add the two Z0 impedance matching resistors and you are done. Takes 8/10 hours for a complete unit, it's worth the effort for a good attenuator.

Any question, feel free to write