Monday, August 15, 2016

Octal EAR 834P phono preamp


During my visit to NYC earlier this summer, my buddy Ding loaned me his home-brewed octal version of the EAR 834P phono preamp using 3 x 6SL7 octal tubes. Powering the preamp is a General Radio 1201-B tube regulated power supply.


Ding is not into boutique parts. Instead, he used carbon film resistors, mylar coupling capacitors and electrolytic caps gathered from Hamfests and surplus stores. Proper grounding and parts placement ensured a very quiet phono stage.


Traditional tube phono stages like the classic Dyna PAS 3 and Marantz 7 employed active RIAA EQ networks inserted in the negative feedback loop from the final stage to the input stage. In the 834P, Tim de Paravicini used active RIAA EQ with a clever twist - the EQ network is fed from the cathode follower output back to the second stage only. IMO, the benefits of this topology are: a 1st stage unimpeded by negative feedback, low noise and low output impedance.

I've been enjoying this phono stage so much that I'm inspired to build my own version. Good job, Ding!
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6 comments:

  1. ErnstSeptember 9, 2017 at 9:40 AM

    Hi Joseph,

    Your report sounds rather interesting. Has Ding exactly copied the plain 834p schematic and just dropped in the 6SL7 to replace the 12AX7's? Or did he amend specific values?

    Kind Regards,

    Ernst

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    1. JESeptember 10, 2017 at 7:11 PM

      Good question! He told me it was based on the original circuit but upon closer scrutiny the 2nd stage is direct coupled to the cathode follower and since he used a 6SL7 there, the cathode load is 120k.

      Delete
  2. ErnstSeptember 13, 2017 at 12:53 PM

    Hi Joseph,

    Maybe Ding would like to share the cirquit diagram with the DIY community?

    All the best,
    Ernst

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    Replies
    1. JESeptember 22, 2017 at 9:42 AM

      The circuit Ding used looks very similar to this -http://usr.audioasylum.com/images/3/33813/834p_variation_2.jpg - instead of 12AX7 + 12AT7, he used 6SL7s in all positions.

      I will post a loktal version, stay tuned ;)

      JE

      Delete
  3. martinJanuary 5, 2018 at 2:34 PM

    Hi Joseph,

    Ik have a lot of 6c8g tubes in stock so I also like to see schematic.

    Best regards
    Martin
    Netherlands

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    Replies
    1. JEJanuary 8, 2018 at 12:22 AM

      Sorry Martin, AFAIK, the 6C8G will not work on this circuit. Designing a phono stage around that tube is beyond my abilities and not within the scope of this article.

      JE

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