Jun 21, 2018

LCR Phono preamp Part 3

Power supply

A good sounding preamp or amp always means a well calculated power supply, this is the circuit heart. Too often neglected or subject to approximations because calculation is time consuming and most hobbyist do not evaluate the real impact on music restitution. A supply is NOT an oversized transformer plus a huge amount of filtering capacitors.

Tech corner with useful reminders about time constant and ripple attenuation.

1- Time constant
Picture from the Net. Courtesy of Landr Journal
To understand the great importance of time constant we first need basic information about audio signals. These are totally different of the sin waves we use to measure power, distortion or phase in our amp and preamp.
An audio signal is a very complex association of a fundamental and many harmonics of different amplitude and duration. For our supply it is a succession of non-symmetrical transients that form the signal envelope or ADSR of music. To make it short we must deal with attack transients that are in the 15 to 20 milliseconds for the fastest instruments (piano, guitar, drums), which means the supply must recover in that interval of time before the next attack.
The first LC or RC cell time constant should not exceed that value.
In the real life it can't be that precise due to the components tolerance, especially electrolytic capacitors, thus it is a safe practice to calculate a first TC not more than 10 milliseconds (but not under 5 milliseconds or sound becomes harsh).
Note: For proper transient response, TC should increase after each decoupling cell.

2- Ripple
For this preamp I choose a CLC filter.
Ripple with an input capacitor depends upon C, total current in circuit and mains frequency. It can be appreciated by the relation

                                                          Vr = Itot / C x f

Itot Total current in circuit (Ampere), C input capacitor (Farad), f ripple frequency (twice the mains with a full wave rectifier)

3- Smoothing cells
LC and RC networks are voltage dividers for the AC current that comes from the rectifier. They are necessary to smooth out the ripple.

Let's calculate the preamp supply step by step.

I have a 240V / 320V_0_320V power transformer followed by a GZ32 rectifier and a 24µF C1 capacitor in a circuit drawing 60mA @ 375V.

                                Vr1 will be 60 10-3 / 24 10-6 x 100 = 25V or 6,6%

Time constant depends upon C and Rs the total series resistance. To calculate Rs we must:
1- measure primary and half secondary (for a full wave rectifier) transformer winding resistance (respectively 12Ω and 120Ω) to determine the total equivalent resistance Req
2- calculate transformer voltage ratio n = Vs/Ve = 320/240 = 1,333
3- calculate Req = Rsec + n2 Rprim = 120 + (1,777 x 12) = 140Ω
4- determine rectifier internal resistance Ri (~260Ω)            O.H. Schade method
5- calculate Rs = Req + Ri = 400Ω
6- calculate TC = Rs x C1 = 400 x 24 10-6 = 10 milliseconds

Note: a GZ34 rectifier with its lower internal resistance (~100Ω) will give a
6 milliseconds TC, on the other hand a GZ37 (~500Ω) something around 18 milliseconds. This to say that changing the rectifier will affect both TC ratio and DC voltage, reason why sound also changes (dry and bright with a GZ34, warm and mellow with a GZ37). Too many people just make a plug, play and listen test without considering this factor and recalculating the whole supply as it should be done.

a first cell (L = 10H/160Ω and C2 = 100µF) makes a voltage divider where

                                             Vr2 = Vr1 ZC2 / ZL + ZC2

      ZC2 = 1 / 2 π f C2 (~ 16Ω @ 100Hz)             ZL = 2 π f L (~ 6,3KΩ @ 100Hz)

      Vr2 = 25 x 16 / 6300 = 63,5mV / -75dB and TC = 16 milliseconds

and so on... until I reach at least -150dB ripple rejection and proper TC to feed my preamp.

PSU schematic



Parts and construction

Quality parts are chosen for their reliability and sound qualities. In addition to the MP/MKV and F&T electrolytic I usually favor, the filtering bank will be fitted with excellent vintage SAFCO-TREVOUX 15µF PIO.

These military caps are incredibly rugged, well made and heavy. They are new and fresh as if they just came out of the factory, still in their original 1959 packing. These caps have one of the lowest loss tangent I measured on paper in oil capacitor and give a smooth and clear sound. This upgrade is at the price of a cumbersome supply weighing about 20 lbs (the hermetically sealed transformer could power a 6V6 push pull), hence the two handles to ease manipulation.
Be very careful during wiring, especially pins selection, ground returns and heaters. Remember that input signal is in the same magnitude than heater/cathode leakage noise and it can be a nightmare to get rid of a low frequency buzz.


most of the wiring is solid



Power supply with sub chassis to ease construction




Tubes I used

EC88
I have Siemens (Germany) and TFK (Philips Heerlen plant) on hands. See part 2
Tests were conducted with Philips E86C as output tubes.
To say the truth both 88's are great sounding tubes with very detailed music, lot of life and great sound stage. Maybe I will keep the Philips Heerlen that appear to be the quietest tubes... by a very slight margin.

EC86
Replaced the Philips E86C (Mullard made) by TFK EC806S and I entered in a different world. The Telefunken's are audibly better by any means. Deep, wide, alive. They are THE tubes to go! Incredible punch with sweetness. Accuracy is astounding. With the Philips the overall sound is very pleasant but by comparison music appears slightly out of focus, like behind a curtain. Amazingly had the same feeling with the E80CC Philips vs Tungsram in the SRPP preamp.
So, I made another test with the 88's and the best association is E88C Heerlen + EC806S TFK, no contest. Perfect balance, articulation and clarity.





Some vinyls for this test, out of my 5000+ LP's collection, mostly jazz















37 comments:

  1. Hi Fred,
    I'm sorry to hear that you'll stop writting in your blog. I very much enjoyed your posts, always very well explained and documented. Let me also praise the astonishing quality of your builds, judging by the pictures. Wish I could have heard one of your builds.
    Thank you very much,
    Best regards,
    Maxime

    ReplyDelete
    Replies
    1. Hi Maxime and thanks for your comment.
      If by any chance you travel to my country I will greatly appreciate to make you listen to some of my constructions

      Fred

      Delete
  2. I'm sorry to hear the end is upon us. I've greatly enjoyed your posts and builds over the years.

    ReplyDelete
  3. Thanks for a marvelous posting! I definitely enjoyed reading it,
    you will be a great author. I will remember to bookmark your blog aand will come back
    someday. I want to encourage you to definitely continue your great writing, have a
    nice afternoon!

    ReplyDelete
  4. Excellent, what a weblog it is! This web site presents helpful
    facts to us, keep iit up.

    ReplyDelete
  5. Bonjour Fred,
    quel dommage ! mais je comprends vu la somme de travail demandé... Tu nous as livré de magnifiques réalisations, à l'ancienne, avec un sens de la pédagogie pointu. J'ai pu constater que tes conseils étaient excellents, sur les capas de liaison par exemple que j'ai adoptées. J'espère pouvoir passer un jour pour écouter ces magnifiques montages. Longue vie au DIY et au chauffage direct.
    Bien cordialement.
    Bruno Castelluzzo

    ReplyDelete
    Replies
    1. Bonjour Bruno et merci pour le commentaire.
      Si d'aventure tu passes près de chez moi ce sera avec plaisir pour un écoute.
      Cordialement
      Fred

      Delete
  6. First, I would like to 'thank you' for posting your many blogs

    I have picked up more tips in your writings here than anywhere else. Your explination for why component values need to be, and the relative maths has been easy to follow.

    The beautiful build quality of your projects is to aspire to!

    Just maybe you might continue your blog writing at some point in the future, I know I am not on my own when I say this. I will keep returning... Thanks again. Red.

    ReplyDelete
    Replies
    1. Hi Red and thanks for your positive comment.
      May be I will be back to my blog in the future but for the moment I am busy moving in another house with a room dedicated to music.
      Fred

      Delete
  7. I enjoy your great articles as well...this one in particular around the time constant of PSU etc...how would you calculate a choke input ? It sounds on a regular basis faster, tighter to me than CLC...so TC is smaller ?

    ReplyDelete
    Replies
    1. Hi, TC and ripple calculations for LC is the same as for CLC ones except two points:
      1- Rs must include the choke resistance. In my example an input choke of 100 ohm will give Rs = 500 ohm and with a 24µF C1 capacitor TC will be 12 milliseconds.
      2- Ripple is appreciated by relation Vr = Vdc / 6 π²f² LC√2

      Hope this help.
      Fred

      Delete
  8. Undeniably believe that which you said. Your favourite reason seemed to be
    at the web the simplest thing to be mindful of.
    I say to you, I definitely get irked whilst folks think
    about concerns that they just don't understand about.
    You controlled to hit the nail upon the highest and defined
    out the entire thing without having side-effects , people could take a signal.
    Will likely be back to get more. Thank you

    ReplyDelete
  9. Hi there mates, how is all, and what you want to say regarding this paragraph, in my view its in fact remarkable for me.

    ReplyDelete
  10. I recently discovered your blog, I love it!!!
    I really like your methodology and use of high quality vintage parts.
    I understand all the hard work you have placed into your blog, it is very much appreciated.
    I hope you will continue to post and future projects you create.

    Frank

    ReplyDelete
    Replies
    1. Hi Frank thanks for your positive comment. I have a few projects on the bench and certainly will undertake writing new posts in a near future . I am actually busy moving in an other house.
      Stay tuned.
      Fred

      Delete
  11. Hello there, just became alert to your blog through Google, and
    found that it's really informative. I am gonna
    watch out for brussels. I will appreciate if you continue this in future.
    Numerous people will be benefited from your writing.
    Cheers!

    ReplyDelete
  12. Right here is the right site for everyone who wants to understand this topic.
    You know a whole lot its almost hard to argue with you (not
    that I actually will need to…HaHa). You definitely put a new spin on a topic that's been written about for many years.

    Excellent stuff, just excellent!

    ReplyDelete
  13. fantastic post, very informative. I wonder why the opposite specialists of this sector do not understand
    this. You should continue your writing. I am confident, you have a huge readers' base already!

    ReplyDelete
  14. Hi there, this weekend is good for me, for the reason that this point in time
    i am reading this great educational piece of writing here at my
    home.

    ReplyDelete
  15. Superb, what a webpage it is! This blog gives useful information to
    us, keep it up.

    ReplyDelete
  16. You have mawde some really good points there.

    I looked on the internet for more info about the issue and found most people will go along
    with your views on this website.

    ReplyDelete
  17. That is a good tip particularly to those fresh to the blogosphere.
    Short but very accurate info… Thank you for sharing this one.
    A must read article!

    ReplyDelete
  18. When some one searches for his vital thing, so he/she wishes to
    be available that in detail, therefore that thing is maintained over here.

    ReplyDelete
  19. If some one wants to be updated with hottest technologies after that he must be pay a visit this web site and be up to
    date daily.

    ReplyDelete
  20. Hello, I desire to subscribe for this webpage to obtain latest
    updates, thus where can i do it please help out.

    ReplyDelete
    Replies
    1. Hi and thanks for your interest.
      Just go to "Members" and hit follow button.
      Fred

      Delete
  21. Thanks for ones marvelous posting! I certainly enjoyed reading it,
    you might be a great author.I will always bookmark your blog and will
    often come back later in life. I want to encourage you to ultimately continue your great posts,
    have a nice weekend!

    ReplyDelete
  22. Greetings! Very useful advice in this particular
    article! It is the little changes which will make
    the largest changes. Thanks for sharing!

    ReplyDelete
  23. What's Taking place i'm new to this, I stumbled upon this I've discovered It positively useful and it has aided
    me out loads. I am hoping to give a contribution & help different users like its helped me.

    Great job.

    ReplyDelete
  24. Attractive part of content. I simply stumbled upon your blog and in accession capital
    to claim that I acquire in fact loved account your weblog posts.
    Any way I'll be subscribing on your feeds and even I achievement you get admission to consistently rapidly.

    ReplyDelete
  25. Hi there mates, pleasant post and pleasant urging commented here, I
    am genuinely enjoying by these.

    ReplyDelete
  26. Wow, that's what I was exploring for, what a stuff!

    existing here at this web site, thanks admin of this site.

    ReplyDelete
  27. Its such as you learn my mind! You seem to understand so much approximately this, such as you wrote the guide
    in it or something. I feel that you just could do with
    some p.c. to force the message home a bit, but other than that, that is great blog.
    A great read. I'll certainly be back.

    ReplyDelete
  28. Excellent site you have got here.. It's hard to find
    high-quality writing like yours nowadays. I truly appreciate individuals like you!

    Take care!!

    ReplyDelete
  29. Excellent post however , I was wondering if you could write a
    litte more on this subject? I'd be very thankful if you could elaborate a
    little bit further. Bless you!

    ReplyDelete
  30. Thank you for the auspicious writeup. It in fact was a amusement
    account it. Look advanced to more added agreeable from you!
    However, how could we communicate?

    ReplyDelete
    Replies
    1. Thanks for your comment.
      The easiest is to communicate through blogger contact form.
      Fred

      Delete