KG CFA3 headphone amp
Jul 22, 2020 at 12:17 PM Thread Starter Post #1 of 137

RudeWolf

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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.

SQX0wRd.jpg


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...

6J4uh6V.jpg


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.

t02pLMT.jpg


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.
 
Jul 25, 2020 at 8:04 PM Post #2 of 137

paradoxper

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This amp is way too good for this dolt platform. :smile:

As you know, I admire your build which inspired mine.

All things possible through Kevin Gilmore. :grin:
 
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Aug 1, 2020 at 8:44 PM Post #4 of 137

chillysalsa

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Fantastic build, nice work!

It looks like you have some seriously good thermal management there.

I wonder how it compares to the Dynahi. Have you compared to other amps?

I've compared my Dynahi to a clone of the Krell KSA-5 which is supposedly loosely based on, and I definitely feel the Dynahi is super in terms of detail and refinement.
 
Aug 2, 2020 at 6:36 AM Post #5 of 137

RudeWolf

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Fantastic build, nice work!

It looks like you have some seriously good thermal management there.

I wonder how it compares to the Dynahi. Have you compared to other amps?

I've compared my Dynahi to a clone of the Krell KSA-5 which is supposedly loosely based on, and I definitely feel the Dynahi is super in terms of detail and refinement.

Thanks! This was basically the smallest case I could fit everything in. Quiescent current is 200mA per output stage which keeps the temps a good 10C above ambient. For the GRLV psu I could've got away with just bolting the pass transistors to the bottom plate, but hey, I got some heatsink length left, so I used that.

And yeah, the Dynahi seems to be the closest thing to this. There are some folks on the other forum who might have both the Dynahi and CFA2 built. At least one of them preferred the CFA2. For some time I pondered whether to go for the SuSy Dynahi build, but having had the Dynalo, I thought it would be more interesting to be the first to tackle the CFA3.

A really interesting option to me seems building a tube input board Kevin published a few years ago and tack two CFA2 outputs to it. You'd have to forego dual mono powering, as you'd need two GRLV's and one +100VDC PSU per channel, but the result might be even more interesting. Especially with tube rolling.
 
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Aug 11, 2020 at 3:23 AM Post #6 of 137

paradoxper

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Thanks! This was basically the smallest case I could fit everything in. Quiescent current is 200mA per output stage which keeps the temps a good 10C above ambient. For the GRLV psu I could've got away with just bolting the pass transistors to the bottom plate, but hey, I got some heatsink length left, so I used that.

And yeah, the Dynahi seems to be the closest thing to this. There are some folks on the other forum who might have both the Dynahi and CFA2 built. At least one of them preferred the CFA2. For some time I pondered whether to go for the SuSy Dynahi build, but having had the Dynalo, I thought it would be more interesting to be the first to tackle the CFA3.

A really interesting option to me seems building a tube input board Kevin published a few years ago and tack two CFA2 outputs to it. You'd have to forego dual mono powering, as you'd need two GRLV's and one +100VDC PSU per channel, but the result might be even more interesting. Especially with tube rolling.
Stop giving more ideas!

I'm interested to hear how the SS topology sounds directly compared to SuSy Dynahi. Although I do plan on running only from CF. But, you know, who knows!
 
Aug 23, 2020 at 9:16 AM Post #7 of 137

paradoxper

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Aug 23, 2020 at 11:26 AM Post #9 of 137

paradoxper

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Oh yeah! How is it treating you? Bias stays stable with ss/zf switching?
I'll provide some impressions after I bring in a few more phones. The SS/ZF gave us some trouble. Haha.
 
Aug 27, 2020 at 4:40 PM Post #10 of 137

Dukei

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It ain't diy if there are no problems along the way:relaxed:. Luckily it got resolved, detailed explanation of the fix is posted over at headcase.
Regarding the amp, I still can't make up my mind which mode I like the most. I guess it depends on the headphones in use...
 
Sep 1, 2020 at 11:22 PM Post #11 of 137

Ethereal Sound

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I'll provide some impressions after I bring in a few more phones. The SS/ZF gave us some trouble. Haha.

Very interested in hearing what you think of it! Especially how you think it compares to some of the others amps you've tried as of late.
 
Sep 2, 2020 at 4:43 PM Post #12 of 137

paradoxper

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Very interested in hearing what you think of it! Especially how you think it compares to some of the others amps you've tried as of late.
I'll start comparisons today. Was planned with Verite and TC in-mind, however, it seems my recently purchased Verite may have been lost by UPS. We'll see.

The CFA is very, very good. Builds upon the Dynahi strengths with more, hm, finesse and less warmth perhaps.
 

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