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Friday, February 8, 2013

Making of a D3a LCR RIAA Phono Preamp - Part 1

Hi!

I have been working on a complete amplification  chain containing phono stage, line stage and mono amps during most of the last month. They will all share the same chassis style as my 10Y line stage. Each with external PSU, so 8 chassis total. I will show all of them in dedicated posts. This first post shows the current state of the D3a phono stage.




I already announced this phono stage in the D3a Tube of the Month article. The circuit topology is described there. The reason for the choice of the D3a is the higher amplification this tube can yield in an LCR phono stage compared to an E55L for example. The person who will use these electronics has a very low output cartridge. So every bit of gain is needed.

As already shown in previous posts about preamps, each socket is mounted on a small sub plate which is vibration damped through rubber pieces. Here a top view of the main mounting plate which will later be exposed on the top of the chassis:




The underside of the plate showing the signal and heater wiring:




Two of the 4 B+ decoupling chokes used in the circuit are hidden under the two black covers in the middle. The remaining chokes and transformers will be placed inside the chassis:




Both stages of the circuit use Lundahl LL1692A transformers. The 600 Ohm LCR RIAA is built discretely, using intactaudio RIAA coils:





The reason for the use of discrete LCR networks was the wish for the inclusion of the Neumann time constant. The second half of the network got modified such that the attenuation of the high frequencies stops around 50kHz.

Another requirement was the use of XLR in and outputs wired in parallel with RCA connectors:




The backside also carries separate binding posts for signal ground and chassis, the power supply connector and a ground lift switch. The back plate is prewired as far as possible before it is mounted to the preamp:





The PSU is constructed in a similar style, the top plate:




The power transformer will late be hidden under a black cover. The wiring underneath:




The separate power transformer for the heater supply and the B+ smoothing chokes will be placed inside the chassis on sub modules:




The front and backside panels of the PSU:




Now only the wooden chassis are missing. As soon as these arrive, the phono stage will be completed. This will be covered in part 2. 


Best regards

Thomas

14 comments:

  1. Great!
    Can we see schematic?
    I tried to find everywhere but with no success :(

    ReplyDelete
  2. Hi!

    This phono stage is available as a kit or partial kit. Schematics will be provided with the purchase of parts. Gotta keep at least some secrets ;-)

    Thomas

    ReplyDelete
  3. dear Thomas,
    could you please let us know the strategic parts you can supply for this phono ?

    best regards,
    Pedro
    alicante(costa blanca

    ReplyDelete
  4. Hi Pedro,

    I can supply everything needed for this preamp.
    The important parts are the signal transformers and LCR coils.
    Also the power supply chokes and I believe the power transformers are very important.
    I can also provide the capacitors.

    You can also get a subset of parts for example if you have a specific preference for caps and want to use others or if you have a LCR network already.

    Schematics will be provided also with a subset of the parts. I will also be available for consulting during the construction in case of any questions.

    Contact me be email: thomas -at- vinylsavor -dot- de

    Best regards

    Thomas

    ReplyDelete
  5. Thomas,

    Have you tried the Lundahl LCR Inductors?

    ReplyDelete
    Replies
    1. Yes I did, but prefer the intactaudio coils

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  6. There is a beauty coming up :-)

    ReplyDelete
  7. Very nice, Thomas!

    I wonder what rubber parts are you using to decouple the small tube plates? I noticed you use thicker solid core wires for ground for the tubes. Will these thick wires decrease isolation of the tubes?

    I like very much that no screws are visible from the top due to use of the blind tapped holes. What thickness of the top plates are you able to get away with? 5mm?

    Regards,

    Tony

    ReplyDelete
  8. Hi Tony,

    The rubber elements are from a local supplier. The impact of the ground wire on damping is minimal. If the sub plates damping is too soft, it is not effective in absorbing vibrations.

    The plates are 4mm thick

    Best regards

    Thomas

    ReplyDelete
  9. Thanks, Thomas.

    At first I thought you are using O-Rings but I noticed it is a single rubber element. It is pretty good machining of blind holes if you can only use a 4mm plate.

    Regards,

    Tony

    ReplyDelete
  10. Hi Thomas, I'm building something similar, :-) Did you find any need to screen the inductors?

    Thanks,
    Pete

    ReplyDelete
  11. Hi Pete,

    no screening necessary. This phono is very quiet even if used with a MC cartridge without MC step up transformer. Just keep enough distance to the PSU. half a meter is enough.

    Thomas

    ReplyDelete
    Replies
    1. Thanks Thomas, much appreciated, that makes me more confident that mine should be ok!

      Beautiful build, BTW, I love your style.

      Cheers,
      Pete

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