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CFA Headphones amplifier
CFA Headphones amplifier
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Old 22nd March 2015, 02:11 PM   #11
arpagon is offline arpagon  Bulgaria
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Well, real schematic's THDs can be lowered even more by lowering the values of NFB resistors R23, R24 from 2.1k/300 Ohm to 620 and 142 Ohm respectively but amplifier will become a little bit unstable. So one can trade THD for stability.

Of course, I've already started to make the PCB but I suppose it will be ready at least in a week.

Regards
Alex
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Old 12th April 2015, 05:26 PM   #12
arpagon is offline arpagon  Bulgaria
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As promised to Richard, PCB is ready:
- bottom layer:
[IMG][/IMG]

- top layer:
[IMG][/IMG]

- Silk Screen:
Click the image to open in full size.

Will make some boards for testing purposes.
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Old 13th March 2016, 11:16 AM   #13
arpagon is offline arpagon  Bulgaria
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After almost an year delay and the populated PCBs from first version sent to a friend who "forgot" to send them back to me, I decided it's time for the second turn. I changed schematic a bit - the output is now with paralleled transistors (not driven cascode as in the previous version), the voltage sources for feeding the servo are different. A new PCB was designed and produced and here's results:


Click the image to open in full size.


Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Power supply is +/- 24V regulated, the output transistors are biased at 70mA each.

There weren't any problems aliving schematic (nothing unexpected). After setting the DC modes I checked schematic's stability with 100kHz square wave signal. Simulated slew-rate was approx. 317 V/uS (20V rising edge for 63nS), the measured result is approx. 266 V/uS (20V rising edge for 75nS), which I think is not so bad at all . I didn't take the pictures from o'scope screen, but I'll do that soon. I haven't time for THD measurements yet - probably the next week.
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Old 13th March 2016, 02:35 PM   #14
mlloyd1 is offline mlloyd1  United States
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CFA Headphones amplifier
looks nice.
thanks for sharing.
what drove the changes you made?
do you plan to share the final schematic?

mlloyd1
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Old 13th March 2016, 03:27 PM   #15
arpagon is offline arpagon  Bulgaria
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Oh, schematic, here it is:

Click the image to open in full size.

I had a problems with stability and driven cascodes at the output. I tried various approaches and finally gave up - cut some traces and put transistors in parallel. For feeding the servo - this way it is independent of the PSU voltages and beside that we already have the zeners needed .
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Old 27th March 2016, 12:17 AM   #16
arpagon is offline arpagon  Bulgaria
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And here's the promised picture of slew-rate measurement:

Click the image to open in full size.

It's strange that cursors on the picture are shifted while I clearly remember that they were properly aligned on the scope screen . Nevermind it's obvious that the rising time is 66 nS which yields 242 V/uS for 16V swing.
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Old 13th August 2016, 12:35 AM   #17
RNMarsh is offline RNMarsh  United States
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There are some very good HP amps around.... this one is amazing. A CFA, with high slew rate and ultra low distortion at any frequency and output level and load impedance.

I had a chance to measure it and here is an example --- 5KHz, 22 Ohm load, 3 v rms;

CFA HP Amp.JPG




At a higher Freq and output voltage -- 10KHz, 5v rms and 8 Ohm load, the THD was just .01%


THx-RNMarsh

Last edited by RNMarsh; 13th August 2016 at 12:53 AM.
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Old 13th August 2016, 03:01 AM   #18
ammel68 is offline ammel68  United States
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CFA Headphones amplifier
Which amp is that Mr. Marsh?
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Old 13th August 2016, 03:20 AM   #19
mlloyd1 is offline mlloyd1  United States
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looks like the one just a few posts up in this thread.


mlloyd1
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Old 13th August 2016, 05:05 AM   #20
ammel68 is offline ammel68  United States
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CFA Headphones amplifier
I do believe you're correct mlloyd1...didn't notice the tall caps and sinks.

Certainly not a simple design with a low parts count, but appears to be a well engineered design.

Arpagon, will boards be available or can you share the Gerber files for your board?
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