May 10, 2022

LCR Phono preamplifier revisited

A few month ago I sold my E88C/E86C LCR Phono preamp, playing the recently build C3g/E182CC RC one. I soon realized I couldn’t live without. Not to say the C3g is inferior, it is the most enjoyable preamp I have today but I miss the absolute delicacy and sweetness the LCR had. My wish would be a preamp combining all qualities, even if everyone knows that the search for the ultimate phono, the one that brings pure enjoyment on any kind of music, is a never ending story.

Whatsoever, it didn’t take a long time before I decide to undertake a new one. New project, new interrogation. Is the final result a matter of network or tube choice, what has the most significant impact on sound reproduction ?

I personally think that active components, the way they are used, the dynamic behavior and the distortion figure under use conditions, have a much bigger influence upon sound than the EQ itself and decided to keep the same approach than in the RC phono, a first stage pentode instead of a triode.

Fortunately, when cost was in par with my wallet, I stocked a few transformers and can build a complete unit with the same irons previously used in the E88C LCR phono. Just had to buy new RIAA EQ sets.

Temptation was great to use the same tubes with the benefit of an A/B comparison, but the C3g gain would have been too high especially on a 20K load. I had to find a substitute with equal qualities and not too exotic. The search for the (almost) perfect equivalent took some time and I finally stopped on the E83F. A high Gm pentode, easy to source, apparently low distortion, low noise and primarily developed for class A audio applications in telephone repeaters. Surfing the Web I discovered it has been used in the last stage of the iconic TAB Telefunken V76 microphone amplifier. Choke loaded and capacitor coupled to the output transformer (full shematic here). This preamp has a reputation of excellence. A serious reference and a great encouragement for my project.

First stage

Beyond the basic DC study from datasheets, my old HP dynamic analyzer was of great help to set the best operating point and get a similar harmonics distribution than the C3g. Once done, gain is close to 160 with very, very low distortion.

Second stage

RIAA network plus transformers (A8713 20K/600 and TKS50 600/50K) has a 0.158 @ 1KHz overall gain which commands an amplification of 15 to get +45/+50dB. What immediately comes to mind is the 6SN7/12AU7 family of tubes. Having a good supply of miniature double triodes, it was obvious to give them a try.

From the dozen references tested I only kept six. All have outstanding performance, with a personal preference for Mullard M8136/CV4003 and Tungsram E80CC. These two fine tubes are just followed by Siemens 5814A triple mica and RTC 5814A. The others (Sylvania 6072A & GE 12BH7A) being slightly less involving while keeping all other qualities like great tonal balance, wide soundstage and lifelike restitution. Distortion was a bit higher with the12BH7A and 6072A had too much gain and a forward presentation.

DC point is set according to manufacturer recommendations. Below the E80CC setup, identical for M8136 and 5814. The 12BH7A needs a lower bias resistor



Schematic is very similar to the full triodes preamp

Note: LCR network can be either feed or loaded by 600 ohm, in any case you have to remove one 600 ohm resistor ( R4 or R12 ), this ensures network to work properly. Thanks to Bruce B. who pointed out this issue in my schematic.

Supply, parts and making of

Wishing to keep this phono as close as possible to the first one, at least in design, I calculated a double choke C input supply. For basics refer to the EC88 LCR phono supply post.

R7 serves two purposes: draws the necessary current to set DC voltage and acts like a crude shunt regulator.

Just one capacitor in signal path and another in G2, its choice will greatly influence final balance. Not really fan of mellow or colored restitution, I prefer a neutral sound, less appealing but more realistic. A few, very few caps have such ability and they are not super exotic, fancy, expensive…. ITT PMT/2R and relatives like LMT, Aerovox V161 (polyester, used in original Pultech equalizer) are close to that ideal. Finally choose the PMT/2R, a long time favorite. Thanks to Jean Hiraga.

During the first listening test I noticed some hum, about 2mV, a bit annoying. This is a recurrent problem with pentodes (partition noise & HV supply ripple) and to fix this issue I replaced Rg2 carbon resistor by a metal film one and added a hum cancellation capacitor in parallel. Calculation is tedious (RDH 4th Ed. p539) for precise cancellation and usually not a standard value. I used 0.15uF for 0.192 calculated and got 0.3mV residual noise. Inaudible at listening distance and barely discernible close to the speaker, I can now run intensive listening tests. Also added a 22K grid stopper.

inside PSU

Mullard 5Z4GY rectifier is the best performer in my system, great presence with deep bass.

Phono completed

E83F soft glow

Measurements

First observation, the preamp have a great dynamic potential and huge headroom. With 80mV input signal I got more than 30V peak/peak with no sign of clipping. Couldn’t go beyond because my scope was on the lowest selectable sensitivity.

Next step was a THD test. Just have a look at the pictures to see how good this preamp is. Considering signal generator + dynamic analyzer total harmonic distortion (~ 0,014%) this phono exhibit 0,021%@1V rms, mainly H2 (H3 being almost at background noise level).

At 5 Vrms distortion is only 0,18% with the same harmonics distribution. Not that bad…

Listenning report

Even if the E83F deserves an A+ whatever the brand, I found the Tungsram more attractive. As for the E80CC used in the SRPP line preamp, just two manufacturers for this fine pentode : Dutch group Philips ( Mullard, Valvo, RTC, Amperex …) and the Hungarian one. Not as glamorous as Mullard or Telefunken, Tungsram tubes are of excellent quality and completely underated.

Sound is well focused, tonally correct and ample. Less punchy than the C3g but still offering a very detailed and vivid sound. Most obvious qualities are clear (could be dry depending upon second stage tube - Sylvania 6072A) and clean, different of the E88C/E86C combo which, in my memory, was softer and less analytical. This is probably due to a more euphonic sound in a full triode setup. Pentodes are by far more neutral and I understand Telefunken choice for the V76 mic preamp when it is crucial to catch the sound during a performance with a total neutrality. The E83F is an uncompromising tube of exceptional transparency. When I compare to the C3g, the E83F appears slightly less seductive but certainly the most truthful.

After several hours of attentive listening this preamp is an excellent performer on philharmonic music, big band, human voice, cello solo and organ, but not only. It is the only phono that makes me hear or feel the recording studio acoustic. The 2nd stage tube that remains the best (balanced, detailed, neutral) on any kind of music is the Tungsram E80CC.

One drawback, you need top-notch drivers otherwise listening tends to be boring or tiring. Had the not so good idea to set the JBL 2470 in place of the Altec 288C in my homebrewed speakers. Will never do it again. Every improvement highlights mismatches and weaknesses.

vinyls, vinyls forever.....









5 comments:

  1. Hello Fred,

    Greetings from the U.S.!

    It is with great interest that I read your post about your LCR phono preamp.

    A few questions for you, if I could, about this unit;

    1) Was it necessary to terminate TR1 in a 600 ohm resistance? I ask this, as LCR networks typically require that the filter network only be terminated in the required termination impedance at the output of the network. The source or driving impedance is typically not critical, as long as the network is terminated in the correct output impedance.

    2) Does the combination of the 600 ohm resistor and the input impedance of transformer TR2 as presented to the output of the LCR network really result in a true terminating impedance of 600 ohms?

    3) What kind of RIAA equalization accuracy (+/- dB) from the ideal RIAA playback curve were you able to measure and obtain with this circuit?

    The build and overall aesthetic of this preamplifier are absolutely stunning. Beautiful work, indeed!

    I look forward to your comments, and thank you in advance.

    All the Best,

    Bruce Berman

    ReplyDelete
  2. Hi Bruce and thanks a lot for your interest and questions.
    1- you need to load the transformer to get the desired gain from the E83F. If no load, gain is more than halved because impedance seen from the network input is lower than 600 ohm
    2- Good question. I did not measure the impedance seen from TR2 but I guess it's fairly constant along the audio bandwidth. I'll try to see what's really going on and will let you know
    3- Acoustic Dimension provided this network with a guaranteed accuracy of +/- 0.5 dB.
    Hope this helps
    Best regards
    Fred

    ReplyDelete
  3. Hi Fred!

    Thanks very much for your very quick reply.

    With regard to the 600 ohm resistor (R4); as the LCR network is terminated by the impedance presented by the combination of R12 and TR2, wouldn't R12 and TR2 provide the required load for V1 to work into? I would try to remove R4, and measure the overall gain of the circuit at 1 Khz, and see what the result would be, unless you had done this during the initial testing of the design, and found that R4 is indeed required.

    But it is the combination of the impedance presented by R12 and TR2 that I find to be the real issue here, in what is obviously a very elegant and well-thought design. The impedance presented by R12 by itself is of course 600 ohms, which is exactly the value the LCR network wants to see, so as to maintain it's specified RIAA playback accuracy. But with TR2 in parallel with R12, the total impedance obviously is less than 600 ohms, and the impedance presented by TR2 is also going to vary as a function of frequency. So we have a terminating impedance that may be less than 600 ohms, and will possibly vary with frequency.

    Although the Acoustic Dimension LCR network may have an accuracy of +/-0.5 dB, that accuracy is only valid when the network is either driven by a true 600 ohm source, or is terminated by a true 600 ohm load. This is true of any LCR filter network.

    That said, I think the most important test would be to sweep this preamp with a 20 to 20 Khz source, fed through an inverse RIAA network, so you could see the true accuracy of it, in terms of the deviation from the ideal RIAA curve. If there is any substantial variance, my guess is that it would be due to the combined impedance of R12 and TR2 being less than 600 ohms. Perhaps you have already done this; if so, I would be very curious to see the playback error accuracy expressed in dB!

    Thanks & Best Regards,

    Bruce

    ReplyDelete
  4. Hi again, when I remove R4 the first stage gain is lowered. I made some measurements and it's obvious on my Oscope. That said I didn't performed a full bandwidth test with an inverse RIAA network because from my point of view TR2 is used to feed the last stage with a reflected load as close to 50K as possible. It will be interesting to see if there's any loading effect of the transformer itself vs frequency and I will perform some measurements that way. Thanks for insight about some issue in my circuit.
    BTW are you the one who published decades ago in Sound Practices ?

    ReplyDelete
  5. Hi Fred,

    Yes, that's me, Bruce Berman, of "Sound Practices" fame. I had written two articles for that great magazine many years ago. It's really a shame that SP no longer exists.

    Yes, if you could provide the +/- variance from the ideal RIAA curve, that would be very much appreciated, and the results would be very interesting and revealing. If there is any substantial variance from the ideal curve, and I suspect there may be, it would most likely be due to the effect of TR2 on R12. If you were closer to me, you would be welcome to borrow my inverse RIAA network, so you could easily perform this test.

    Another possibly would be to remove R12, and to add a resistor to the secondary of TR2, with the value of that resistor reflected through TR2 so that it appears as 600 ohms at the primary.

    Again, the importance of correctly terminating the LCR network in a 600 ohm load is most critical, so the accuracy of the network can be achieved.

    I look forward to seeing your test results once you perform them!

    Thanks & Best Regards,

    Bruce Berman

    ReplyDelete