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Tuesday, August 12, 2014

D3a LCR Phono Stage Cost Down - Phase 1

Hi!

So far the D3a LCR phono stage is the best I have to offer for vinyl playback, except for the similar preamp equipped with Telefunken EC8020 tubes or the exotic DHT phono stages. The circuit is built without compromises an this comes at considerable cost and requires also a lot of space. Since that price is not affordable for many who inquire about such a phono stage, I developed a cost down version of it. At the same time also the size has been reduced which is also an issue for many.




The goal of this phase 1 of the cost down was to down size the circuit with no or only minimal impact on the sound quality. Phase 1 indicates that this version will be followed by yet another further cost optimised version. More about that later.

An obvious possibility for cost and size optimisation are the many chokes used in the original circuit and the oil capacitors. The chokes have been minimised to just 2 in the power supply. All oil caps which are not directly in the signal path got replaced by electrolytics. The rest is kept the same. 2 stage circuit, both stages transformer coupled for low output impedance and 600 Ohm LCR RIAA EQ.




The reduction of number of chokes and oil caps allowed to mount everything inside the chassis which halves the volume. Everything is mounted on sub modules in sandwich fashion. The vibration damped mounting of the tube sockets as seen in the original design is kept, although now hidden inside the chassis:





The inside of the finished signal section:





The power supply is constructed in a similar fashion with everything inside the chassis. Due to the restricted size, the all tube rectifier bridge had to be replaced by a single rectifier, a 6AX5 augmented by silicon diodes to form a hybrid rectifier bridge. Both the high voltage and heater voltage maintain the choke input supply theme. All the caps in the PSU are electrolytics now. Power transformers for high voltage and heater supply are kept separately:




The finished prototype of phase 1 of the cost down in operation:




Although the chassis look nice arranged like that or stacked, they are supposed to be kept further apart from each other. Although usable as above a slight hum got induced into one channel (the one closer to the PSU). But separating the units by just 10-15cm already made the hum disappear.




How big a toll did the down sizing take on the sound? Actually surprisingly little. This is due to the fact that only parts not directly in the signal path got replaced. All the strengths of 600 Ohm LCR equalisation are maintained: Very natural and detailed representation wish beautiful tone colours and very clear articulation of voices. Maybe resolution is a tad lower compared to the big preamp.




A down side for some might be that the tubes are not visible anymore. Therefore these units are much easier to dust off.




If you had contacted me in the past for a LCR phono stage and the price was too high, check again. These might fit the bill now. The phono stage can be built for MM or MC sensitivity with built in MC step up transformers. MM gain is 42dB at a very low output impedance. Gain can be increased by 6dB with a 4 fold increase in output impedance. Gain could also be reduced down to 36dB if needed. MC gain depends on the step up ratio of the MC transformer whig best selected.




The first listening results were so encouraging that I decided to follow this one by a phase 2 of a yet further cost reduced version. That version will let go of the transformer coupling in the output stage and will do without chokes in the power supply. Stay tuned for an update when phase 2 gets finished.

Best regards

Thomas

2 comments:

  1. Hi Thomas,

    Cool build, as usual.. When the Tango LCR unit stocks expire, are you going to sub them with Hashimoto LCR's in the future? Could the PSU chassis accomodate large value polyprop axials (e.g., claritycap, etc.) for the HT section instead of lytics if the customer wants them? Thank you.

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    Replies
    1. Hi!
      Thanks :-)
      This unit does not use Tango LCR modules. There are plenty choices available. The top version of the D3a phono uses oil caps throughout. If a customer want to avoid electrolytics, this is the better choice rather than modifying this concept. But in general I am quite flexible and can accommodate special wishes.

      Best regards

      Thomas

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