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Hum in Aikido Cathode Follower (ACF)

Posted 7th March 2017 at 03:00 PM by alexcp (My DIY projects)
Updated 13th March 2017 at 09:20 PM by alexcp

I purchased an Aikido Cathode Follower 2 (ACF-2) All-in-One 9-Pin PCB from The PCB is designed by John Broskie, the editor of Tube Cad Journal (, and the flexibility that he designed in is quite remarkable. The PCB allows to test various configurations, which is often not a feature of a design committed to a PCB. The quality of the PCB is outstanding; it withstands multiple soldering and desoldering without complaint. It is an outstanding product IMHO.

The schematic seems to be simple, with a cathode follower loaded by a triode current source. JJ E88CC with 221ohm cathode resistors shows 0.003% THD with pure 2nd harmonic, and select 6N1P-VI achieve 0.005%. (My 6N1Ps are hit or miss; some of them have much higher 2nd harmonic, and some of them have a mix of 2nd and 3rd at Ia=6ma and Ua=140V).

As I tested my ACF, I noticed excessive hum at its outputs - audible buzz in the speakers and 0.15% THD+N, well above the THD.
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