C3g Phono preamplifier Part 2

Supply and more 

Classic C input supply with two LC cells for excellent ripple attenuation.  

Two other RC cells will floor the rectifier residual to negligible value. Especially important for the first stage where signal is in the mV range.

Great care has been taken in time constants calculation to ensure good transient restitution. For basic calculation refer to LCR phono preamp part 3.

Construction with high quality parts as always.

I used both electrolytics and paper capacitors. I found the combination well balanced and use it with satisfaction for years now. Paper and aluminum foil are very reliable with a super fast discharge time. These old caps are a must in power supplies along with paper in oil. Unfortunately these are usually too big to fit the chassis I use

I built the preamp on a soldering board, old-fashioned, but very practical and compact. Therefore, it is possible to change a component very easily if necessary.

RIAA capacitors are Philips KS 1% polystyrene and Stéafix silvered mica. These are in parallel to give C5 a correct value and soldered on the opposite side. Value chosen taking in account the E182CC Miller capacitance. About 100pF.

Resistors 1% MRS 25. I use these for a long time and are perfectly suited for this task.

Pay attention to solder load resistors away from the board, they become hot in operation. I chose Kiwame 5W ones.

Grounding needs special care. To avoid hum and ground loops I used a single wire for each stage connected to a central point on the ground bus. Heaters are well filtered and polarized to prevent cathode leaks from V3.

C3g are Siemens, the only ones I stock. Other makers are TFK, Valvo, Lorenz, ITT.

The E182CC is a Philips design and sold under many brand names. Most come from Heerlen plant. Mine are RTC . These professional tubes were sorted at the factory to ensure a long time use and excellent reliability.

The making of

Listening report

Untill today I thought RC network was inferior in all respects to LCR one. Nothing is more untrue.

I am playing this preamp daily for more than a month now and it is just amazing. The most obvious quality is its dynamic capability, but it is not the only one.

Music is vivid, colourful and alive with excellent imaging. Soundstage is wide, not as deep as the LCR. Low end is very tight, ample, well controlled, but slightly rearward. The most remarquable is the medium / high that deserve an A+ note. Clear, precise, well focused and detailed to the extreme. No harsh or aggressivity even on loud music playing trumpet, guitar or drums.

On the very demanding Isao Suzuki trio record, snare & cymbals are rendered with incredible realism and I can feel cello's attacks like never before.

If I had to give an overall appreciation I would say the LCR is sensual and lush while the RC is more Rock n Roll and appealing. These are great qualities considering the music I listen to and it is very addictive. Today my LCR preamp is mainly used for human voice and orchestral music.

Of course this is very subjective and depends greatly upon room and overall setup. For this listening tets I used my old Garrard fitted with a FR24 arm, a Entré EC 30 cart (a rarity by Matsudaira-san also renowned for some of the best carts ever: Kiseki, Acuphase, ZYX and now MySonicLab) and its dedicated ET 100 transformer. The C3g phono is coupled to the 6J5 line preamp driving my MC1/60 SE amplifier.

Speakers are heavily modified Klipsch La Scala with upgraded Alk Engineering network (all original plastic capacitors replaced by waxed paper & foil, teflon and some excellent sounding Aerovox V161), Martinelli's sanded wood horn + JBL 2470 and Beyma CP25 tweeter.

Great jazz vinyls for this report

C3g Phono preamplifier Part 1

A while ago browsing the web to enrich my knowledge, I stumbled on a phono schematic around the C3g. Well known in the hobbyist world and a must try for many, this tube just average in triode mode is a real champion as a pentode.

I discovered these great qualities while seeking a first stage tube for my LCR phono stage. Sound was really involving, but gain was too high for comfortable listening in my setup and I did not persevere in that direction until today.

 

The scheme I found from Intact audio forum, is very simple, but I noticed a few design mistakes that will surely affect the final result. A weird RIAA network and a rather high output impedance. Both easily fixed. 

Network is passive and when properly calculated can give a very accurate correction.   In that way the best is to follow Lipschitz 1979 publication. To make it short we have to keep in mind that network is dependent upon first stage Z_out, second stage input resistance Z_in and Miller’s input capacitance C_in. This last point is often neglected and a reason why you cannot change tubes without modifying network time constants.

Now, components calculation is easier. We just have to write down all we need.

 

 1 – Time constants

                                            R’₁C₁ = 2187 µS 

                                            R’₁C’₂ = 750 µS 

                                            R₂C₁ = 318 µS 

                                            C₁ // C’₂ = 2,916 

 2 – Components value 

                                            R’₁ = ( Z_out + R₁ ) // Z_in 

                                            Z_in ~ 50 Z_out 

                                           C’₂ = C₂ + C_in 

First stage gain, Zout and bandwidth

A pentode gain is merely equal to tube Gm by load. If we want to keep second stage input resistance in a reasonable value (~ 500K), C3g load have to be something like 10/15K to respect Lipschitz recommendations.

A graphical study will help find the best operating point and I finally stopped on 12K@10mA. Gain will be very close to 150 and Z_out 11,6K assuming a 400K internal resistance. Rg₂ (Z_in) can either be 510K or 560K depending upon RIAA resistors availability. These have to be 1% or closer tolerance.

To keep gain high G2 is fully decoupled to the lowest frequencies. As a bonus, decoupling dramatically reduces the grid-anode capacitance. Effect on bandwidth is a huge improvement, with high frequencies well above 10Mhz. To prevent any unwanted oscillation it is necessary to use a grid stopper close to G1 pin.

Second stage gain and Z_out

I did not go with a 5687 like in the original design. Amplification factor is too low and this tube needs a lot of mA to sound right.

Because gain of a standard phono preamp is about 300/350 or +50dB, I have to find a tube with at least a gain of 20 to compensate RIAA network insertion loss (-20dB @ 1KHz). After much thought to decide if I should use a high µ triode with degenerative feedback or a classic medium µ anode follower, I stopped on the great sounding E182CC fully bypassed.

With a µ of 24, a low internal resistance and quite a high Gm (15mA/V), it is a good G3g companion. Moreover, the second triode will make a nice shared current coupled stage. I used this topology in the E88C/E86CC LCR phono preamp with great satisfaction.

Output impedance will be about ρ/µ+1 (~65Ω). This is far better than the 4K of the original design and will allow good frequencies transmission even with long cables. 

 

preamp schematic 

 

more to come...

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......