Mar 8, 2017

E80CC SRPP Line preamplifier Part 3

Power supply

Unlike the 6J5 line preamp I used a CLC filtered supply. I usually prefer the LC for its very smooth ripple behind choke but listening tests demonstrated better dynamic with SRPP. A good calculation of ripple rejection and time constant helps to achieve a fast recovery supply with a minimum of cells. To minimize inter modulation the supply is splitted in two after the second capacitor



Ripple calculation

On C1 the ripple is estimated by formula     VC1~ = 10 I/C     where I is the current through circuit (in mA, here 7mA) and C the filtering capacitor (in µF).
VC1~ about 2.2Vpp or 0.77VRMS with a time constant of about 7 millisecond.

On C2 the ripple is calculated by the voltage divider formula    Vout = Vin Z2/Z2+Z1    where ZL1 is 37.6K @ 100 hz and ZC2 50 ohm at the same frequency.
VC2~ is about 1.05 mVRMS with a time constant of 24 millisecond. Thanks to the 60H choke that smooths out the ripple with great efficiency. A 20H one would have been enough but I had this one on hand.

The same applies on C3 with R = 2.7K and ZC3 133 ohm. It leaves a 0.05mVRMS ripple which represent a -130dB attenuation at a time constant of 33 millisecond.

Parts

Good sounding Philips/RTC EZ81 rectifier along with old Tango Hirata choke and some Siemens MP/JS and F&T capacitors are the parts of choice for this power supply.






Tube choice and tests

Not a long or tedious quest for different brands of tubes to be tested, just two makers for this fine tube; Philips (can bear Valvo, TFK, Siemens and so on names) and Tungsram. I attentively auditioned these two competitors and the result is disconcerting. In this setup the Philips that bears a reputation of excellence is not the winner of the test. The musical rendering appears fuzzy despite some great qualities in terms of tone and speed. It gives a blurry image that leaves the observer perplexed. On the other hand, the Tungsram brings music to a level of clarity and enjoyment seldom heard. It has incredible precision and gives a density to the sound that makes it perfect on any kind of material. Very detailed, not to say clinical sound but on the good side of absolute neutrality. You like it or not but it never leaves indifferent and if you are after audio perfection you get very close to it. Furthermore this tube gives an almost holographic image of the sound stage. Great, great tube !
As expected distortion is very low. I get 0.9% @ 1KHz and 2VRMS output, mainly second order. Noise floor is also very low and hard to measure on my FFT analyzer, thanks to the very good CMRR (or PSRR, it's the same) of this circuit. In facts we have the same power supply ripple rejection qualities than in parafeed setup and some will consider the SRPP a parafeed cousin with an active load in place of the cumbersome anode choke.





SRPP line preamp completed

...in its industrial blue gray and black look. All front and rear plates anodized aluminium, CNC machined by Schaeffer AG. Outstanding workmanship for a clean, professional appearance.



same connecting organization than the 6J5 line preamp. Outputs are doubled by Lemo 0 coax outlets and this feature will certainly disapear in future works. The little male plug is a nightmare to solder considering the 0.6 mm central pin !



Last minute update...

I made a transformer connection mod. Although I did not notice any sound change, it is intellectually more interesting not to have the C1 electrolytic decoupling cap in the signal path.



some CD's and LP's I appreciated a lot, and there are many others ...









10 comments:

  1. Hello,

    Nice article and I really enjoy reading your blog.

    I have one question about this schematic.

    Does R7(600ohm) necessary and effect to sound?


    Thanks & best regards,

    ReplyDelete
    Replies
    1. Hi and thanks reading my blog.
      This resistor is essential. It is the transformer load, the attenuator have no specific impedance. Without the resistor the SRPP will see an infinite load. You can use any kind ( carbon comp, film, metal ) depending upon your taste.
      Fred

      Delete
    2. Hi again, spent some time visiting your blog. Very nice and informative too. Added to my blog list as a must read about one year ago. Keep feeding us with nice posts.
      Fred

      Delete
  2. Hi,

    Thanks a lot for your kind words.

    I also follow and read your blog for a long time too.

    In my understanding is T-type attenuator should acts like constant resistive for source and load sides.

    So output transformer already seen 600 ohm load from T-type attenuator without adding more resistor.

    Please let me know if I misunderstood something.

    Thanks again for sharing many great projects too!

    Nattawut

    ReplyDelete
  3. Hi Nattawut, a T pad attenuator acts like a constant impedance device, not a constantly resistive element. R7 is essential as the two pass resistors are 600 ohm too because in this case the three resistive elements are chosen to ensure that the input impedance and output impedance match the load impedance which forms part of the attenuator network. Means, if you connect a device with say 100K input resistance ( impedance ), the effective load seen by transformer will be 100K in parallel with 600 ohm, almost a 600 ohm load . If you forgot R7 the load seen will be close to 100K and the attenuator will not work properly because the attenuator series resistors are not chosen for this load.
    For a complete study of T pad attenuator you can read
    -
    https://www.electronics-tutorials.ws/attenuators/t-pad-attenuator.html
    -
    Very informative and clear.
    Will be nice to keep contact.
    Fred

    ReplyDelete
    Replies
    1. Hi Fred,

      Seem more clear for me! ^^

      So if I use input transformer like 600:25k after T-attenuator in your schematic.

      Can I omit this 600 ohm resistor or should leave it here?

      Thanks & best regards,
      Nattawut

      Delete
    2. Hi Nattawut, in that case you can omit R7 but must load you transformer secondary with 25K to get a reflected impedance of 600 ohm. For better common noise rejection you can use a symmetrical connection between the two transformers.
      Best regards
      Fred

      Delete
    3. Hi Fred,

      I will apply to my projects soon.


      Thanks again for your great information.

      Best regards,
      Nattawut

      Delete
  4. this preamp, is possible to be used as headphones amp? if yes, r7 isit necessary?

    ReplyDelete
    Replies
    1. Hi, I believe it is not suited as power amp even for headphone, the expected output will be very low.
      Anyway, R7 is necessary to have a reflected load for the SRPP.
      Hope this helps.
      Fred

      Delete