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Friday, August 8, 2014

10Y Differential Line Preamp, Part 2


In the previous post about this differential line stage I showed some of the initial chassis construction. In the meantime the preamplifier is finished and the concept is proven.

After the subassemblies with the tube sockets had been mounted, part of the many chokes used in the circuit got attached on the top side.

These will later be hidden under some beautiful metallic bordeaux red covers. The bottom side of the plate with some wiring:

The main part of the circuit is in the inside of the chassis:

The transformer volume controls can be seen on the left side. Each input goes through an input transformer which splits the signal to drive the two tubes per channel. These are mounted in a vertical row behind the volume controls. Each set of input transformers is wired for different step up, 0dB, +6dB and +12dB. Behind the input transformers are line outputs which drive the volume control autoformers and many more chokes.

As can be seen in the photo no capacitor whatsoever in the signal section. A truly capacitor less signal path!

The finished preamp:

Here with it's external power supply:

The power supply is basically the same as that of the single ended version, just adapted for the increased current.

The backside shows the three inputs and pair of outputs:

Side view:

The four 10Y tubes which amplify the signal:

The ground configuration needed a bit of tweaking. Now the prototype is playing and is very quiet. First listening tests immediately showed the qualities of this concept. Eliminating the last capacitor from the signal path is certainly worth the effort. While the single ended 10Y line stage is a brilliant performer with gobs of resolution and tone, the differential line stage adds even more naturalness and body.

Stay tuned for more detailed listening comparisons between the differential and single ended line stages.

Best regards



  1. Hi Thomas,

    Thank you for sharing this beautiful design. May I ask the kind of source you tested the preamp with? Do you have a differential phono stage to feed this preamp?
    Is it relevant to use a differential setup if interconnects are short and the source is not differential in essence? What are then the theoretical benefits against a SE preamp?
    Please forgive the potential lack of knowledge my question may reflect.

    Best regard,

  2. Hi Maxime,

    I tested this preamp with a D3a LCE phono (SE) and SE 45 amps. This design is an exercise in eliminating the last capacitor from the signal path. Many claim their transformer coupled design as capacitor free, but neglect the last cap in the power supply which still is in the signal path. This design truly is without any cap in the way of the signal. By comparing it to the single ended 10Y line stage, the sonic impact of that cap is audible.

    I will also develop a differential phono stage to be used with this design

    Best regards


  3. Hi Thomas,

    great job! How did you manage to implement the autoformer volume control on the output of this differential design?


  4. Hi! I purposely call the design differential and not symmetric. I don't see any benefit in a simply symmetric design which just doubles everything. The output is earth free and can drive a balanced input

    Best regards


  5. Hi Thomas,

    How do you (forced) balance the currents for each differential pair? The linestage Looks great... Thanks.

  6. Hi!
    I don't want to reveal to0 many of the technical details as this is my top line stage. Just that much: Currents don't need 100% balanced. The transformers used work well with up to 5mA if unbalanced currents