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Old 13th February 2014, 08:41 PM   #1
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Lightbulb My CFA project

In one of the recent posts I mentioned that my next design will be CFA.
Here it comes. First diagram shows the basic concept (well, pretty well-known architecture). Devil in in details.

Some time ago I got an idea of using an OTA (operational transconductance amplifier) topology for an input buffer. OTA is based on current mirrors and acts as a highly linear voltage-to-current convertor. Which is very suitable for CFA. This design is also very suitable for symmetric differential input (no global NFB at this stage makes it very convenient), though it can be used as non-differential as well - just use one of two inputs. Looks pretty elegant to me.

This design shows rather good performance in simulator - minimal compensation, high frequency responce, low phase shift at 20kHz, low THD-frequency dependence.

I started prototyping - already developed PCBs for the prototype - waiting for them to be produced. OTA buffer and the rest of the amp will be built on separate PCBs, allowing to experiment with different designs in the future.

If somebody is interested, I will publish the news on this project here.

Cheers,
Valery
Attached Images
File Type: jpg OTA-CFA.JPG (35.4 KB, 440 views)
File Type: jpg OTA-CFA-01-Balanced.jpg (357.3 KB, 459 views)
File Type: jpg OTA-CFA-02-Balanced.jpg (250.9 KB, 431 views)
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Last edited by vzaichenko; 13th February 2014 at 08:44 PM.
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Old 15th February 2014, 11:26 PM   #2
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Default Ultimate performer

Slightly improved version (OP). 60W output @ 20k - THD is less than multisim can measure with a standard meter.

34db gain is because of the balanced input (28db + 6 db).

Comments? Ideas?
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File Type: jpg OTA-CFA-03-Balanced.jpg (438.6 KB, 337 views)
File Type: jpg OTA-CFA-04-Balanced.jpg (279.3 KB, 320 views)
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Old 16th February 2014, 01:13 AM   #3
RNMarsh is offline RNMarsh  United States
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Hi,

Circuit looks interesting. Do you have test equipment to measure the prototype?

THx-RNMarsh
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Old 16th February 2014, 01:40 AM   #4
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Hi, at the moment I use a sound-card based solution for examining the spectrums and measuring THD. It gives me resolution up to roughly -110db (good pro-grade card e-mu 1616m). I can only measure at 1kHz at the moment because of the software license limitation, though I am going to solve this issue in the nearest future. I used it a lot with different designs, so if I see harmonics somewhere at the noise level (like at the spectrum attached - my previous VFA design) - it will be ok

I've got a pretty good "hardware" generator and digital oscilloscope with FFT as well, but the picture there is less precise, so I use the card so far...

Kind regards,
Valery
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File Type: jpg THD 1k-01.jpg (68.5 KB, 108 views)
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Old 24th March 2014, 07:54 AM   #5
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Thumbs up Prototyped. Awesome amp!

Hi All,

Yesterday finished my CFA prototype.
Final schematic is attached. Also see the Bode plot (phase shift @ 20kHz is only 5.3 degrees) and 50kHz square wave form (also pretty small phase shift).
Tested with non-balanced input.

Extremely natural sound, especially audible at fast fronts (drums, bells) and acoustic piano (Steely Dan's "Two against nature" sounds better than with any other amp I listened to in my environment so far).

More than satisfied. Next step - will try it with the input buffer from Damir.

Cheers,
Valery
Attached Images
File Type: jpg CFA_00002.jpg (268.2 KB, 140 views)
File Type: jpg CFA_00001.jpg (299.3 KB, 135 views)
File Type: jpg OTA-CFA-07.jpg (195.7 KB, 162 views)
File Type: jpg Bode-01.jpg (94.0 KB, 151 views)
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Old 24th March 2014, 08:43 AM   #6
dadod is offline dadod  Croatia
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Quote:
Originally Posted by vzaichenko View Post
Hi All,

Yesterday finished my CFA prototype.
Final schematic is attached. Also see the Bode plot (phase shift @ 20kHz is only 5.3 degrees) and 50kHz square wave form (also pretty small phase shift).
Tested with non-balanced input.

Extremely natural sound, especially audible at fast fronts (drums, bells) and acoustic piano (Steely Dan's "Two against nature" sounds better than with any other amp I listened to in my environment so far).

More than satisfied. Next step - will try it with the input buffer from Damir.

Cheers,
Valery
Valery,
For me it looks as typical VFA, unusual but with symmetrical LTP. Why do you call it CFA, please explain?
BR Damir
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Old 24th March 2014, 11:09 AM   #7
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Quote:
Originally Posted by dadod View Post
Valery,
For me it looks as typical VFA, unusual but with symmetrical LTP. Why do you call it CFA, please explain?
BR Damir
The right side of it is actually a voltage amplifier (VA), driven by current. The input buffer is a v-to-i converter (output is current). The point where the buffer output, VA input and left side of R39 come together is the current summing point (as outlined at the attached diagram).

What I plan to test - is to exchange my input buffer with your gainwire (without your output buffer). In this case your output will also be current. Simulation showed even higher linearity of the whole system.

I thought about it from the very beginning, so I made the buffer as a separate module, so I can change components easily now
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File Type: jpg OTA-CFA.JPG (35.4 KB, 74 views)
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Old 24th March 2014, 11:48 AM   #8
dadod is offline dadod  Croatia
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Quote:
Originally Posted by vzaichenko View Post
The right side of it is actually a voltage amplifier (VA), driven by current. The input buffer is a v-to-i converter (output is current). The point where the buffer output, VA input and left side of R39 come together is the current summing point (as outlined at the attached diagram).

What I plan to test - is to exchange my input buffer with your gainwire (without your output buffer). In this case your output will also be current. Simulation showed even higher linearity of the whole system.

I thought about it from the very beginning, so I made the buffer as a separate module, so I can change components easily now
I haven't build CFA power amp yet, except that line amp, and I suppose that both type VFA or CFA could sound equally good. I very interested to see how you are going to implement the GainWire gain part(it is really current amplifier with v-to-i input convert and current conveyor as the gain stage).
I don't understand use of R39 in your schematic, could you explain?
Thank you in your interest in the GainWire.
best regards
Damir

Last edited by dadod; 24th March 2014 at 11:51 AM.
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Old 24th March 2014, 12:54 PM   #9
Waly is offline Waly  United Kingdom
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Quote:
Originally Posted by dadod View Post
I haven't build CFA power amp yet, except that line amp, and I suppose that both type VFA or CFA could sound equally good. I very interested to see how you are going to implement the GainWire gain part(it is really current amplifier with v-to-i input convert and current conveyor as the gain stage).
I don't understand use of R39 in your schematic, could you explain?
Thank you in your interest in the GainWire.
best regards
Damir
R39 is shunt-shunt feedback across the output stage half. It reduces both the output and the input impedance, while setting the the transconductance gain of the output stage to R39 (at LF). The total voltage gain is the output stage input impedance (R39=47k), divided by the input stage degeneration (R4=1.8k), since the input stage is a transimpedance stage.

But then by any accepted definition, I don't think this is anywhere close to a CFA. By the same logic used by the author, a classic VFA is also a CFA, since the input stage is transconductance and the "VAS" is in fact a transimpedance stage.
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Old 24th March 2014, 01:04 PM   #10
dadod is offline dadod  Croatia
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Quote:
Originally Posted by Waly View Post
R39 is shunt-shunt feedback across the output stage half. It reduces both the output and the input impedance, while setting the the transconductance gain of the output stage to R39 (at LF). The total voltage gain is the output stage input impedance (R39=47k), divided by the input stage degeneration (R4=1.8k), since the input stage is a transimpedance stage.

But then by any accepted definition, I don't think this is anywhere close to a CFA. By the same logic used by the author, a classic VFA is also a CFA, since the input stage is transconductance and the "VAS" is in fact a transimpedance stage.
Thanks Waly, but isn't it a kind of positive feedback as it is connected between output and non inverting LTP input?

Last edited by dadod; 24th March 2014 at 01:07 PM.
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