Aikido Cathode Follower preamplifier with remote

[ClefChef]

As an owner of Hypex NCore amplifiers I wanted a tube buffer in front of them to "soften" the sound and serve as an input switcher with volume control. Problem is that all tube amplifiers have gain - excessive gain that is. Most of them have no remote controlled volume and input switching, no trigger control, and no output muting on start-up to protect SS power amps from wild DC thumps on warm-up.

Having built dozens of Aikdo preamps and some ACF and other passives I decided to try and build a modern ACF version that would fit right into modern rack. It means NO GAIN, everything controlled via remote, output muting, trigger output.

Here are couple of pics of work in project: ACF board, input switching board, power relay with power delay sandwiched on top, 128-step relay attenuator board. Lots of work, lots of machining, drilling, tapping, painting, and soldering.

If I like the result I will build the second one exactly like the first one - all parts are already here.

 

[Einric]

I am VERY interested in your outcome.
I have an ACF-2 PCB sitting next to me that will eventually get paired to a Mini Aleph with all the bells and whistles you are talking about.

 

[SY]

You'll find that it won't "soften" the sound. In fact, it won't affect the sound much at all, since it's a good, transparent, low distortion design.

 

[ClefChef]

I am not after second order distortion - I don't need it. My goal is a passive preamplifier with low output impedance, with all the comfort features. What I REALLY want is a balanced version of ACF. John promised it in one of his blogs on TubeCAD.

 

[ClefChef]

Here's the inside and pic of how it would look in general. It turned out to be a complicated project.

 

[ClefChef]

And...it's finished.
It took me a while to optimize internal ground connections and fish out ground leaks through brass standoffs. The biggest surprise was the problem with the 12V trigger - the 1/8" plug is secured to the chassis and its negative contact makes direct contact with it. As usual I ran ground wire from the 12V trigger plug to the star ground inside chassis unwittingly creating a small ground loop. Add long external trigger wire and we have some additional HUMM. Solution - cut the ground wire leaving only positive connection and negative derived from contact with chassis.