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...

Visseaux A710 SE amplifier Part 2

Power supply and parts

CLC filter along with RTC Philips 5R4GYS rectifier. From my humble experience this is one of the best choices for the money when you have to deal with medium voltage/current.
One of my favorite tubes for its sweetness with a very involving midrange and overall detailed sound. Very good treble with a great delicacy, tight low end, this rectifier is very dynamic and fast compared to the RCA or Brimar which tends to be harsh for the first and somewhat dark for the second. Good imaging and depth and wide sound-stage are a bonus.

I used two chokes for hum free DC and calculated the cells for the following time constants 12,18,26 and 40 milliseconds assuming it will provide good recover on transients.



Parts were chosen for reliability and " une fois n'est pas coutume " I did not use electrolytic except heaters and 6463 cathode decoupling (Philips LL, nasty good and reliable). Over the usual F&T caps I preferred top quality Siemens paper foil and very low ESR ICAR polypropylene.

Resistors are Sprague Koolohm and AB carbon composition. Heaters are LC filtered for hum free on my 104dB speakers.
Special Yamamoto sockets are used to hold the A710 B4 base along with any other UX4 type 10 triodes.
Point to point wiring, star grounding and short leads. Once finished amp is compact but heavy, weighing about 45 pounds (ca. 20 kg).

What a thrill when I listened to this amp for the first time. Like in my memory...gold!
The A710 is a superlative triode, a tube for hedonists. I guess the magic is in the very small details I usually do not hear with other amps. Main drawback, poor records are even poorer.

Amplifier fed with regular thoriated type 10 by Visseaux too. Second best choice when you can source a pair.

and some good sounding Philco VT25A

Needless to say that driver tube choice is part of the final sound. Gave a try to various 6463 brands and the winner is Telefunken without contest. Clear, clean, neutral and very dynamic.
The men at Ulm plant knew how to make top quality tubes.

it would be nothing without good vinyl's or CD's









tiny listening room in my house attic

Visseaux A710 SE amplifier Part 1

New post, new tube. Time to discover one of the finest directly heated triodes ever.
One that favorably compares to the very best and sought-after tubes like PX4, AD1, RE604 and RS242.
One that belongs to a glorious and vast family of triodes but so unique it can be considered a family by it’s own. One so rare that just a few aficionados know its existence but probably never held one in hand. One with a beautiful shape and sound, maybe THE most beautiful sound (at least to my ears…)


I could use all the superlatives to describe the magnificent sense of music this triode hides in its glass envelope I would be far from the emotion I fell each time I listened to this beauty. Let me just say this tube always brought gold to my ears. I built an amp decades ago, kept it for a while and sold it because I did not have the speakers to go with. Fortunately, I kept a few on hand and now it’s time for a rebirth.

No more suspense, this triode is a 10 but not anyone. Among the numerous varieties produced for more than half a century by almost all actors of the tube industry, the Visseaux A710 holds a place apart because it outperforms IMHO all the other ones by his liveliness and his musicality and is only disputed by the RS242_E130. It is probably the fact of its oxide coated cathode that produces a soft glow when lights up. This tube is also fitted with and unusually large bulb, large plate and can be confused with a 50 at first glance. Despite these characteristics it is a true 10, and we can’t expect more than 1W/1.5W when properly loaded and biased. Don’t be fooled by this flea power, it plays really loud. This watt drives my La Scala effortless whatever the music.

To extract the quintessence, I will use a very simple circuit. The best would have been a simple step-up transformer to directly drive the tube. An impossible quest regarding the gain ratio and impedance. A second-best choice is a very linear and neutral driver combined with a high quality inter-stage. I have a very positive opinion about Japanese transformers an acquired a nice pair Tamura B5002 for this project.
It can handle 10mA unbalance current with a good bandwidth. This helps a lot to find a suitable companion.

Driver

I need a triode with a relatively high Gm, a low ρ and a µ ~ 20.
Good candidates are E182CC, 12BH7A, 5687 excellent in the VT25/VT25A push-pull, 6350, 6463 and 6840.
The two last are the most interesting but the GE 6840 is hard to source (I just have one in stock). No regret, the 6463 is an excellent performer - full data here -
This double triode primarily intended for computer use is not stated for audio. Believe me it is absolutely untrue.
Punchy, clear, linear (this could have been first) the 6463 is very close to a 6SN7. In fact, one unit is exactly a paralleled 6SN7 and what goes in is what comes out, not more not less.
Loaded 10K@10mA / -8V, I have my power horse that will drive the Visseaux with high gain and low distortion.

6463 load and bias



and A710 working point



Amp is almost drawn, just have to decide the A710 best load, 10K or 14K, remembering that theoretical optimum operating conditions for a perfect triode are :

                                    Load                   R_Load = 2 ρ
                                    Grid bias         Eg = 0.75 Eb / µ
                                    Plate current      Ib = Eb / 4 ρ
                                    Max power     Po_max = Eb / 16 ρ

In the real life power is lower and can be graphically determined by relation

                               Po = 1/8 ( Emax – Emin) (Imax – Imin)

1.4W with 10K load and 1.1W with 14K
Guess my ears will judge as a last resort



more to come, stay tuned...