KG CFA3 headphone amp

If you frequent the DIY forum here, then there’s a good chance you know about Kevin Gilmore and his contributions to the DIY design library. From Dynalo to Dynahi, to top of the line electrostatic amps like the KGSS and it’s fancier cousins. Less known are his current feedback designs for dynamic headphones. There’s even one for e-stats! Having built Dynalo I wasn’t interested in it’s bigger brother, the Dynahi, so I went for the CFA3.

What is the CFA3? It’s a discrete omnivorous input stage with two CFA2 amps tacked on. This means that it can take and output BAL and SE signals without any kind of extra conversion. So, if you have a DAC that can really spread its wings with BAL outputs, but you have a SE headphone (like me), then you’re in luck. This amp has one of the earlier versions of the input stage which can be built separately, but here one can use it with zero negative feedback. It can also work in super-symmetry mode, but for that I’d rather get one of the later versions of KG’s input stages and build the two CFA2 for it separately. This way one can even use a tube input stage for interesting results. Maybe someday…

As you can see, I tried to do the build dual mono with separate toroids and Golden Reference clone PSU’s powering their respective channels. The small PCB on the front panel is a headphone protector which does three things:

  1. Connects the headphones a good 5 seconds after powering on, so they don’t experience no DC transients when the amp turns on
  2. Disconnects the headphones immediately after powering off, so they don’t see any of the DC transients due to asymmetrical power rail discharge
  3. Measures DC between each channels positive and negative phases and between them and ground. The CFA3 is DC coupled, so it will cheerfully amplify any DC found in faulty DAC output stages, thus cooking your headphones

Initially I was using a THAT340 monolithic bipolar transistor for the input stage, but after a while I decided to swap them out for Toshiba 2sk389/2sj109 low noise jfets. It’s hard to describe the sound difference, but even with THAT’s the amp was terrific. Toshibas made biasing much easier and DC drift was way less. And it seemed to sound more right.

The switch on the back is used to short each negative input to GND when SE inputs are used.

In conclusion I’m very happy with the build. The unique circuit topology makes it a curious standout, but it also sound absolutely fabulous. Previously I was using a maxed out ECP Torpedo 3 and the CFA3 seems to build upon its qualities. For one bass seems more extended and controlled and soundstage appears to be wider and less fuzzy. In comparison the T3 seems a bit mid heavy as well and maybe too gentle. If pushed the CFA3 will tear your head off with metal music, something the T3 was more reluctant to do. It doesn’t seem to prefer any genre of music and is completely free of any glare often present in transistor amps.

I hope you liked the writeup and may it inspire you to build something! Feel free to ask, if you have any questions.

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Love to see a comparison between this amp and the new ECP Audio’s T4 from Tom at Beezar audio…

Please post a link to the build inforamtion, I love DIY and this looks like a great project!!

Thanks for posting!!
Alex

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Outstanding looking amp. Congratulations on your success.

I’ve a Pure Bipolar from Birgir @ Mjolnir Audio. I think these types of amps provide the best this hobby has to offer.

Sorry, no T4 in sight for me. I expect the CFA3 to give it a good run for its money. My experience shows that no cap sounds like no cap and the same goes for transformers. The CFA3 has none of those in the signal path, so who knows… You must also keep in mind that the BOM for the CFA3 is around 1000EUR, so for retail that translates to 3 - 5k monies, depending on the sales model.

As for the build, there isn’t much info. You can comb this thread for what there is.

Thanks!

I’ve met Birgir a few times when I was studying in Reykjavik, a really swell guy! The Pure Bipolar looks like a Supersymmetry Dynalo with a slightly modified Golden Reference PSU. The first amp I built was the original Dynalo with AMB’s Sigma22 PSU. Tricked it out with Twisted Pear’s Joshua Tree 128 step relay attenuator. It was a good amp, but I ended up moving to Sjostrom’s QVR08. Sold the Dynalo to a Head-Case forum mod.

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