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Wednesday, February 20, 2013

Making of a D3a LCR RIAA Phono Preamp - Part 2

Hi!

The initial construction steps of the D3a LCR phonostage have already been shown in part 1. In the meantime the wooden frames for the chassis arrived and the phono stage got completed.





The different sub modules get assembled into a stack which mounts behind the front plate:





Starting from the left, two B+ decoupling chokes and the transformers of the first stage which drive the LCR RIAA which is mounted on two tiers in the middle followed by the transformers of the second stage.

The inside of the power supply unit:






The preamp section completely assembled and wired up, before the bottom lid is attached:




The power supply:




Finished preamp section:






The power supply unit:




Some people prefer red LEDs as power on indicator.

Phonostage and PSU together:





The back sides showing the input and output connectors:




How does it sound? I am very pleased with the result. The phono stage has a very detailed and transparent presentation with especially accurate voice articulation as I am used to from LCR phono stages. This implementation has a very solid and colourful bass which reaches very low. Measurements also confirm the good bass response with the -3dB point at about 10Hz which is excellent for a transformer coupled design like this. The top end is also reaching quite high with the -3dB point around 40kHz.



As expected gain is quite high. It measures a bit above 40dB (this preamp is MM sensitivity, meant to be used with an external MC step up). While this does not seem extremely high, it is combined with an output impedance of less than 200 Ohms. Which means the phonostage can drive either a transformer volume control directly with ease. Or, if more gain is needed it can feed a linestage with an input transformer, which can yield 12dB additional gain if a 1:4 step up is used. Alternatively the output transformer of the phono stage could be rewired for close to 1:1 ratio which would also yield about 12dB more gain at a still reasonable output impedance of about 2 kOhms.




The D3a proves in this phono stage that it is a worthy alternative to the very rare EC8020 which remains my favourite small signal tube. After completing this phono stage and listening to it, I increased my stock of D3a tubes.


Stay tuned for updates of the matching linestage and 45 mono blocks as they get finished

Best regards

Thomas




4 comments:

  1. Thanks Charlie,

    the matching line stage and 45 monos are almost finished. Just waiting for some chassis parts to complete them

    Best regards

    Thomas

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  2. Hello Thomas!

    This is a really outstanding craftsmanship. The chassis are really beautiful!

    I hope you explain the implementation of the LCR RIAA equalization in an seperate article and maybe also the inclusion of the Neumann constant?

    Also I think that the D3a is one of the best compromises for an Phono amplifier as it has quite low plate resistance, a really low equivalent noise resistance and a high amplification factor. The only disadvantege is the quite high Miller capacitance in triode mode which have to be taken into account for an MM cartridge but not neccessarily for an MC.

    Best regards,
    Bernd

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  3. Hi Bernd!

    Thanks!

    I plan an article on LCR network calculation but that will take some more time. I agree that the D3a is an excellent choice for phono stages, especially when a bit more gain than usual is required. Although it has a highish miller capacitance, it is still well below 200pF. This should not be a big problem for a decent MM cartridge and no issue for MCs

    Best regards

    Thomas

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