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I returned to DIY electronics in 2009 after a 20 year pause by building a few kits to get in shape. This blog is for me to keep track of my progress.
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Removing hum from Aikido Cathode Follower (ACF-2 9-Pin)

Posted 7th March 2017 at 04:00 PM by alexcp
Updated 7th June 2017 at 09:41 PM by alexcp (Edited for brevity and, hopefully, clarity)

I purchased from Glass-Ware.com an Aikido Cathode Follower 2 (ACF-2) All-in-One 9-Pin PCB, designed by John Broskie, the editor of Tube Cad Journal (tubecad.com).

The PCB holds a pair of cathode followers, each loaded by a triode current source, and a respective power supply. In my build, a JJ E88CC with 220ohm cathode resistors shows 0.003% THD @1kHz 1Vrms with pure 2nd harmonic distortion.

As I tested my ACF, I noticed audible buzz at its outputs @0.15% THD+N - that's 1.5mVrms(!), well above the THD, and it was not a grounding problem.

A careful look at the schematic (attached), confirmed by some Spice simulation (an LTSpice model is also attached) revealed that the "Aikido" in the ACF-2 relies on the exact match of the voltages across the capacitors C18 and C19, including the hum component. These capacitors are in series for the rectifier (e.g. hum) current and, together with R4/R7, are in parallel for the signal current. In the ideal Aikido world, the hum voltage across C18 would cancel that across C19, providing hum-free signal output.

In reality, ACF-2 provides no rejection of the hum caused by the mismatch of the AC components of B+ vs. B-. A mismatch is easily created by C18 and C19 having slightly different capacitance, by the two halves of the power transformer's secondary winding having slightly different number of turns, etc. My SPICE model shows that 0.1% mismatch in the secondary winding voltages alone produces a 1.5mVpp 60Hz square wave at the output, while 10% capacitance mismatch between C18 and C19 produces a 1.5mVpp 120Hz sawtooth. In the real circuit, I observe that the difference between the voltages across C18 and C19 is a 120Hz sawtooth @1Vpp, which translates into several mVpp at the output.

One solution would be to split C5 into two capacitors each, one connected between B+ and the ground, the other between B- and ground - see the attached schematic. That wouldn't eliminate any mismatch but would make it less relevant, as there would be a lower impedance path for the signal current to ground from the anode of U1a (U2a) and the bottom end of R5. The PCB is not designed for this, unfortunately, but one can place two radial capacitors in the space allocated for C4, connecting them to C4 pads and to the ground pad nearby. I ended up installing 2x 470uF 200V caps in each channel and leaving in place C5. This reduced the hum from 0.15% to ~0.001%.

Another solution, untested by me, would be to disconnect the center tap of the power transformer's secondary winding and match C18 and C19.

Yet another solution, also untested by me, would be to build a tracking B+/B- regulator and connect it where R4/R7 are connected now.

Note: the attached schematic was designed by John Broskie and published in his Tube CAD Journal. John is the author; I just built his design. I reproduced the original schematic to illustrate the post above.
Attached Files
File Type: asc acf-2.asc (8.0 KB, 50 views)
File Type: pdf acf-2.pdf (51.1 KB, 14 views)
File Type: pdf acf-2 one solution.pdf (51.8 KB, 17 views)
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