current feedback electrostatic amp

[spritzer]

I like that one a lot.    I'll dig out the new boards tomorrow and take a picture. 

[congo5]

No rush, as you said no parts available, all I have are the ksa1156

a combined  board would simplify the wiring a bit

Its been 10 months and I would like to hear other opinions about this amp.

possibly its a great low budget amp, ( or spend 37k and you know its the Best Ever)

 

 

 

 

 

[MLA]

It is a great amp, fairly independent of price range I think (though 37K obviously is the Best Ever...)

When I build mine, I only had it for a couple of weeks and really liked it a lot. Then a friend asked to borrow an e-stat setup so it went to him for further opinion. A month later he declared he’s not giving it back unless I force him to do so. Have to build another one I guess  

[congo5]

13 minutes ago, MLA said:

It is a great amp, fairly independent of price range

Thank You MLA

It seems way better than expected, different but very very good.

[MLA]

I agree. Built a 360-ish volt version before, so will do a 400V next time. 

For the record, as good as Dynahi and Dynalo are, the CFA2 is where I plug my dynamic cans in most often. Guess I have a personal preference for current feedback.

Or perhaps more adequately, Kevin's current feedback designs are awesome

[congo5]

8 hours ago, MLA said:

Kevin's current feedback designs are awesome

Absolutely agree with everything above!

I often look up and admire my Dynahi, Dynalo and Kerry's mini as I plug into the CFA2

[spritzer]

Ok, found them...   

[congo5]

Here is what I drew up, no fancy ground plane like yours.......yet

and why did I double the caps? dumbo turbo?

 

 

 

[spritzer]

I like it and that ground planes are fancy...    I do spend a lot of time on my ground planes though. 

[congo5]

   Yes I consider artistic ground planes "Fancy"

This amp seems to have very good noise rejection as I can't hear anything.

but why not have a ground plane......

This is what I will make next, when the parts come in

Think this is better then my last, with ground plane, lower ESR and a bit smaller.

Edited October 20, 2017 by congo5

[mwl168]

37 minutes ago, congo5 said:

   Yes I consider artistic ground planes "Fancy"

This amp seems to have very good noise rejection as I can't hear anything.

but why not have a ground plane......

This is what I will make next, when the parts come in

Think this is better then my last, with ground plane, lower ESR and a bit smaller.

This looks very nice and intriguing. Where is the connector for outputs?

[congo5]

at bottom center of each side are two holes for a solder loop or I plan to use spade connectors

that is where Kevin put them and I  copied his work

Another benefit for the different caps are that they are shorter.

So the board assembly is smaller

Edited October 20, 2017 by congo5

[joehpj]

Does this amp need extra standalone input stage? And can this amp ran single ended input?

[congo5]

no its complete, SE in or BAL

[kevin gilmore]

fairly low input impedance though, use 10k pot

its remarkably simple and effective

[congo5]

Looks to me like these could sub for input transistors.

PBHV2160ZX

And PBHV3160ZX

data sheet is very much same except for lower collector and emitter capacitance

Yes/No?

[joehpj]

@kevin gilmore once mentioned JFET input. I have some J74 and K170. Any possibility to use them? Though I think I'll still stick to BJT anyway.

[kevin gilmore]

for this design the inputs need to be 600v parts. so no jfets. it would need to be cascaded inputs, and change in the input bias to get that to work

[cloudhead]

Is there any good subs for ksc2752?

[spritzer]

Nope...and it has been delayed for about a year now. 

[mwl168]

Another electrostat CFA lives. This one born on the 4th of July?

The PCB is congo5's layout. The amps is powered by my GRHV PSU with ~405VDC rails. 

Many thanks to Kevin for another wonderful design, to congo5 for the PCB boards and many advices along the way and to sorenb for his generosity of coming to my rescue and providing the missing, unobtanium sands. This build would not have been possible otherwise.

I did not match any of the transistors - barely had enough for this one build. The output balance of the two channels sit at 6.6VDC and 4.4VDC after one hour running. The offset is 10.7/4.2VDC and 9.7/5.3VDC. For both channels, the non-inverted output offset is the higher one. The heatsinks are at a toasty 52C after one hour running, about 28C rise from the cold start.  Will need to find a chassis with larger heatsinks.

 

[jose]

Well done Mwl. A single pcb with two channels is very practical.

I would like to add one of those amps to my collection but unfortunately, the KSC2752OSTU will be in BO until November. ?

 

[Pars]

Very nice Michael!

[mwl168]

Some observation/measurements.

Using 1M (500K + 500K) input bias resistors as shown on the schematic, with 400VDC rails, gives 18mA output current @ 52C (heatsink temperature) per phase per channel. Being a true push-pull circuit, this is equivalent of 36mA output current of a SE circuit (such as KGSSHV and Carbon)!!

Also, the bias is at least as much a function of output transistor temperature as of the bias resistor value. After consulting with Kevin and congo5, I experimented with a few different bias resistor values. With 1.05M bias resistors, I got 14.5mA @ 45C and 17.2mA @ 55C. With 1.1M bias resistors, I got  13mA @ 46C and 14mA @ 49C. 

I settled on 1.05M bias resistors which gives 14.5mA output current and the heatsink, when standing upright, remains at 45C after fully warmed up.

This is a great sounding amp. For the parts cost (when you can get them), this ES CFA probably takes the crown of the ROI of all the ES amps I have built.

I would also go on the record that I concur with MLA and congo5; as much as I like my Dynalo and Multiamp, it's the dynamic CFA amp I plug my dynamic headphones in when I crave for some music!

[JimL]

Sounds like it's even less expensive than the SRX Plus! Which means there are now two good budget amps with different sonic "flavors."