Friday, February 16, 2018

GM70, Silver and Nanocrystalline

Just finished these up for a GM70 amp Jeffrey is building for a customer.  They are wound with 0.9999 pure silver wire on big nanocrystalline cores.

The silver winding wire is brought out for direct connection to the circuit.  While I am a firm believer that it is the amp designers responsibility for the ultimate packaging aesthetic, I do have some 125mm cubed aluminum covers that will cover them if needed.

After many years of insisting that doing multiple taps only adds to the compromises I want to avoid in a design, I have finally come up with results that I deem acceptable.  Below are the plots of a similar transformer wound with the ability for 4, 8 and 16 ohm taps.

For behavior below 100hz I look at the inductance plots to make the best choice of inductance vs. current for the intended application and set the air gap accordingly.  The plots below were taken with the equivalent of a 16W excitation voltage @ 50hz.  Pink would be suitable for a parallel 300B,  Brown an 845 or GM-70 and something inbetween green and blue could be chosen for a 211.



  1. Dave, you call it acceptable?? it is -3dB @ 70kHz as minimum! I call it excellent!

  2. Replies
    1. Jim,

      Agreed. Makes you wonder how a pair of Intact silver transformers would sound on my Yamamoto A08 that you have had on "extended loan" for so many years.

  3. This comment has been removed by the author.

    1. Inductance is very low for good low frequency response

    2. I prefer the direction taken by many of the Japanese designers where they will sacrifice a bit of low end for extended top end. The best approach to get both is using an offset winding. I recently did a pair of 304TL outputs with the same top end behavior that had a 80hy primary inductance with 200ma of current and a +- 30ma operating window.

    3. different strokes for different folks...

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  5. I thought you didnt do high voltage OPT’s? I remeber reading this somewhere on your forum. I just finished a GM70 last year. The 300b OPTs you built for are the best ive ever had, cant imagine how good these will be!


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