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kevin gilmore

current feedback electrostatic amp

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Thank you. I am very satisfied with this amplifier although I'm thinking of modifying some detail. It´s not a Carbon but I think it is a very, very interesting option if you are looking for an SS amplifier. (Thanks Dr. Gilmore)

For control the offset I recommend the Joamat trimmer mod. (100R resistors negative rails) 😉

 

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Done. I miscalculated the height needed for all the stuff on the amp boards, so the build is super tight. As the first one made as a prototype, I think it's good enough. I will increase the width and depth by a little bit next time. Now for some serious listening and comparison with Carbon and GG.

image.thumb.png.c24ce0633e648462248a61b7b7335943.png

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20 hours ago, joehpj said:

Done. I miscalculated the height needed for all the stuff on the amp boards, so the build is super tight. As the first one made as a prototype, I think it's good enough. I will increase the width and depth by a little bit next time. Now for some serious listening and comparison with Carbon and GG.

image.thumb.png.c24ce0633e648462248a61b7b7335943.png

Impressions please! 

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The amp was getting  really hot at 70°C.  It was running at 22.5 mA.  I have to reduce the bias current. Should I change the 2k ohm resistor or the 500k one? I am a bit confused.

 

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Edited by samsie
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Here Joachin explains how to do this in the simplest way.

 

On ‎2‎/‎15‎/‎2019 at 6:04 PM, JoaMat said:

OK, now I’ve reached further. Replaced all eight 2K resistors with 2K trimmers. Makes it easier to change the current. Set at 1.6K current start at 2.5 mA and climbs to 4.5 mA. IMO it’s still an interesting amplifier and I believe it’s possible to build an all SMD version.

IMG_0065.thumb.JPG.002a52d78b3385b9bc7848693d6597cb.JPG

 

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14 hours ago, samsie said:

The amp was getting  really hot at 70°C.  It was running at 22.5 mA.  I have to reduce the bias current. Should I change the 2k ohm resistor or the 500k one? I am a bit confused.

 

22.5mA current is a lot for a push-pull amp so you can reduce it quite a bit to manage the heat.

Changing either the 2K or 500K accomplish the same thing -  IIRC, they form a voltage divider. 

Curious to hear what you think of its sound compared to your other amps.

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We haven´t talked much about this amplifier. 🙄

I usually need some time to adjust to the sound of a push-pull amplifier. Once time passes (hours) I feel very comfortable with him.

Honestly, I don´t see any flaw: it has a fairly defined and balanced sound, enough power... to put something negative, the only problem I see is that not all adapt to the sound of a push-pull amplifier (paradoxical) ... But that's what Carbon is for.

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6 hours ago, jose said:

We haven´t talked much about this amplifier. 🙄

I usually need some time to adjust to the sound of a push-pull amplifier. Once time passes (hours) I feel very comfortable with him.

Honestly, I don´t see any flaw: it has a fairly defined and balanced sound, enough power... to put something negative, the only problem I see is that not all adapt to the sound of a push-pull amplifier (paradoxical) ... But that's what Carbon is for.

That answers a lot for me. I have a breadboarded CFEA up and running and was not warming up to the sound of it. I set it aside and went back to my GG, much prefering it as my daily amp. Maybe I need to play the CFEA for hours as you suggest to really retrain my ears. 

 

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I think you're not being "fair". I means:

My favorite amplifiers are the BH/GG but comparing them with the CFAe is like comparing apples with oranges.

I always try to make a comparison with something that has a certain similarity (in this case, solid state amps, not hybrids) so I think of the Carbon more than the BH / GG.

Finally, I think that this amplifier is new and it has had less "development time" than other designs by Kevin, (although Joachin has done an excellent job with his modifications) I don´t know exactly why . Perhaps it is not very showy ot too exotic...


I hope not to offend anyone.

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My experience and logic with the CFA amp echo what Jose said.

Both the BH and Megatron cost lots more and are much more complicated to build than the CFA and I do prefer my BH and Megatron to the CFA on an absolute scale.  

Yet the CFA is a very nice sounding amp in its own right. To my ears it has a very organic and musical sound signature. If I state my personal preference between the three KG-designed all solid state amps I have built (CFA, Carbon and KGSSHV), some people will probably be choking their beer through their noses. 

Kevin can correct me if I am wrong but I think the electrostatic CFA amp is meant to be an easy and low-cost to build amp and it just ends up punching well above its class.

The dynamic CFA on the other hand is a different story!

Just my own 2 cents worth.

 

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Indeed and the basis for one of the most expensive electrostatic amps...the MSB ;)

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130K ... nice paperweight. 😎

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I only listened between the CFA and GG with my SR-009. I like the sound of the GG quite a bit better. Can’t say where everything sits on an absolute scale. So I am fair, but not as experienced as others here. That’s all. 

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5 hours ago, mwl168 said:

Yet the CFA is a very nice sounding amp in its own right. To my ears it has a very organic and musical sound signature. If I state my personal preference between the three KG-designed all solid state amps I have built (CFA, Carbon and KGSSHV), some people will probably be choking their beer through their noses. 

I’m drinking a beer ATM. So witch is your favorite KG all Solid State Amp?😊

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10 hours ago, Laowei said:

I only listened between the CFA and GG with my SR-009. I like the sound of the GG quite a bit better. Can’t say where everything sits on an absolute scale. So I am fair, but not as experienced as others here. That’s all.  

Laowei, sorry I didn´t mean "fair" in an offensive sense. It has nothing to do with experience. I didn´t pretend to be pedantic or presumptuous.

I was referring that both amplifiers were designed with different things so, for me, it´s not correct to compare them.

 

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 No problem, Jose. Just having a very bad day on my end, and seems like I took it wrong.

Edited by Laowei

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2 hours ago, vivolo said:

Hello

I have put together a BOM for the CFA on mouser

The KSC2752OSTU is available at digikey. 

Does anyone know of a suitable 3K resistor? I am having trouble finding one. 

You can substitute Dale Vishay RN60 series for KOA Speer MF1 resistors. 

__________________________________________________________________

Have had my CFAe amp powered up for over a week now and things have settled in.

Tried out my IEM SR-003mk2 with new super comfy ear pod attachment and covers. These ear pods and amp together are a surprisingly sweet sounding combo. 

8FDF4382-3F73-48E3-AF00-0303DAFE702D.thumb.jpeg.f7ad6ba1389b523c5d1cecb6aedfff5a.jpeg

Edited by Laowei
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Would it be alright if I were to use all RN60s? They are only 1/4 watt, and are rated for 300V.

Are there any spots in this circuit that require higher rated resistors?

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Some members have built their amplifiers with RN60 and have had no problems. If you only need one value to complete the amplifier, use the RN60 there. Once mounted check that everything is correct.

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7 hours ago, vivolo said:

They are only 1/4 watt, and are rated for 300V.

They are mil spec and thus derated, so can actually handle higher loads. If you look at Vishay’s CMF line, it’’s the same resistors as RN60D but with about twice as high ratings.

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