Single Ended CFA

[r_jik45]

Got it.

 

[AKSA]

Thanks Bogdan, very good graphs.

HD

 

[manniraj]

I went through the thread but could not find exact DC rails min/max that the amp can be powered with. Please suggest the AC ratings as I am planning to use a simple single rail CRC psu having pair of caps for each channel and use most of the parts from my stash

Thanks

 

[chat72]

The topology is very flexible. I'm experimenting with scaled down version of the same circuit for a preamp / headamp on a proto board.
It's DC coupled and it doesn't need a servo!

Look interesting, any update?

 

[Vunce]

The weather is getting cooler now in the Northeast USA and Class A amps are starting to wake up from hibernation. I decided to give Borko’s S.E. CFA a little makeover by swapping out the linear PSU for an SMPS. I had a MicroAudio SMPS630-S-44 just sitting so I contacted Sami to confirm this unit can handle two channels of 1.5A bias current.
I made a couple of changes prompted by Borko’s latest schematic to the amp boards by removing R15 and replaced with jumper and lowering C10 to 47pF.
Once powered up and cooking bias was set to 1.5A, during this time I was monitoring the temperature of the SMPS. Ultimately, I wasn’t completely comfortable with the transformer reaching 60°C. I installed a small fan and temperature is 42°C now.
Output ripple was a bit high (140mV) so a parallel pair of 22,000uF capacitors were installed between the SMPS and amp boards, now 15mV measured with Fluke DMM.
This amplifier has been back in the main system for the past week and it sounds wonderful! No thumps/pops at power on or off, and it’s very quiet at idle. With 91dB speakers I can’t tell its on with my ear right up to the speakers.
This is one of those amplifier gems that flies under the radar, more of these SE CFA amps should be built and enjoyed!! I know I am….😁

 

[fazildiken]

Hi Thimios,
When I separated the secondaries and fed each an SLB psu the hum was still there, not possible!! I investigated further and found the problem, one of the RCA adapters had a faulty ground connection. Problem solved, this amplifier is dead quiet at idle now. Impossible to tell it’s on
Now I wonder if the single SLB would have been fine, probably. But, I like to use dual PSU’s with my builds anyway.

I moved the SECFA to the main system for long term break-in, it’s driving my 10F/8424 & RS225-8 FAST TL floorstanders, 8ohm and efficiency is in the low 80’s dB. The combo sounds very good!
Sweet job Borko, I believe you have a keeper in the SECFA

Hi Vunce,
The circuit you made is perfect. I want to make the same circuit. Can you share the schematic and gerber files of the "SLB Single rail PSU" circuit you use in the circuit?

 

[gary s]

Nice Vunce, I was only just looking at this project here a few days ago, working out my build order for the first 3 months or so for next year, it is on the list with 3 other amps. I will have to look through the thread again to catch up with any changes or mods.

 

[fazildiken]

The weather is getting cooler now in the Northeast USA and Class A amps are starting to wake up from hibernation. I decided to give Borko’s S.E. CFA a little makeover by swapping out the linear PSU for an SMPS. I had a MicroAudio SMPS630-S-44 just sitting so I contacted Sami to confirm this unit can handle two channels of 1.5A bias current.
I made a couple of changes prompted by Borko’s latest schematic to the amp boards by removing R15 and replaced with jumper and lowering C10 to 47pF.
Once powered up and cooking bias was set to 1.5A, during this time I was monitoring the temperature of the SMPS. Ultimately, I wasn’t completely comfortable with the transformer reaching 60°C. I installed a small fan and temperature is 42°C now.
Output ripple was a bit high (140mV) so a parallel pair of 22,000uF capacitors were installed between the SMPS and amp boards, now 15mV measured with Fluke DMM.
This amplifier has been back in the main system for the past week and it sounds wonderful! No thumps/pops at power on or off, and it’s very quiet at idle. With 91dB speakers I can’t tell its on with my ear right up to the speakers.
This is one of those amplifier gems that flies under the radar, more of these SE CFA amps should be built and enjoyed!! I know I am….😁

Hi Vunce
My PCB Cards have arrived. Soldering is about to be completed. I have a few questions I want to ask. Can I test it with 27v dc and 27000mf capacitors first? How many watts of power can I get with 27v dc and 1 amp bias current?

 

[fazildiken]

Hi,
Assembly is finished. Tests have started. 28vdc 1amp bias current. There is a small problem. At the first startup, the sound was distorted for about 10 seconds. Then the sound improved. I ran it for about half an hour. The temperature was about 50 C.

 

[UR5FFR]

This is an update of the circuit, now it has about 9dB less feedback, I think it sounds better and transient behaviour is lot better, no overshoot on a square wave... The corrections are in red on the schematic, Rs added and C10 and R14 have different values (other end of the R14 is now conected to the output node, not gnd like before).
Cheers, Borko.

Try replacing R16R17 with power Schottky diodes - this will increase the maximum output power and reduce distortion. I compared different options in my amplifier with similar circuitry https://www.diyaudio.com/community/...fet-design-in-class-a-a-b.404961/post-7501281

 

[bogdan_borko]

Hi,
Assembly is finished. Tests have started. 28vdc 1amp bias current. There is a small problem. At the first startup, the sound was distorted for about 10 seconds. Then the sound improved. I ran it for about half an hour. The temperature was about 50 C.

Nice. It is normal, at the startup amp needs 5-10sec to stabilize, but just in case, check the DC voltage at output it should be about half of the supply voltage.

 

[bogdan_borko]

Try replacing R16R17 with power Schottky diodes - this will increase the maximum output power and reduce distortion. I compared different options in my amplifier with similar circuitry https://www.diyaudio.com/community/...fet-design-in-class-a-a-b.404961/post-7501281

Thanks, I`ll try that when I have the time..

 

[fazildiken]

Hi everyone,
Boxing is finished. Bias current is 1.25Amp. Coolers are sufficient for this current. Since there is not enough space in the box, I mounted the toroid transformer on the ceiling. The sound is good. Thanks to everyone who worked on this issue.

 

[chermann]

looks nice. well done!!!

chris