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Old 5th May 2014, 05:48 PM   #1
dadod is offline dadod  Croatia
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Default GainWire-CLASS-A-poweramp NGFB

This is my first attempt to design a power amp without global NFB, 30W/8ohm and 60W/4ohm. It works in Class A up to full power on 8 ohm load and goes in Class AB on 4 ohm. Distortion does not change with frequency.
The gain block is a kind of my GainWire preamplifier and output is a novel output buffer working in class A. It uses separate power supplies for the input and output.
Here are two schematics, one with the volume control at the input, and other with the volume control after input gain block(current conveyor). The second one has advantage to have the signal/noise ratio constant over whole volume regulation and the nose is lower then the first schematic. The second one has to use DC servo to keep output near to zero voltage, this is needed because change the volume resistor value change output voltage a bit. The first schematic uses simple trimpot(P2) to set zero output voltage.
Thermal stabilization is obtained with two diodes(D1, D2) and it's quite stable. Here is ideal place for ThermalTrak output transistors from ONSEMI.
Damir
Attached Images
File Type: jpg GainWire-ClassA-POWERamp-sch.jpg (157.6 KB, 990 views)
File Type: jpg GainWire-ClassA-POWERamp-DCservo-sch.jpg (145.1 KB, 942 views)
File Type: jpg GainWire-ClassA-POWERamp-Gain.jpg (279.7 KB, 880 views)
File Type: jpg GainWire-ClassA-POWERamp-FFT20k1W.jpg (128.1 KB, 775 views)
File Type: jpg GainWire-ClassA-POWERamp-FFT20k30W.jpg (149.6 KB, 739 views)
File Type: jpg GainWire-ClassA-POWERamp-square.jpg (100.2 KB, 196 views)
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Old 5th May 2014, 07:53 PM   #2
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Very good
Excellent
Like last preamp.
Your designs are better and better.
Like teen prodigy;-)
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Old 6th May 2014, 11:58 AM   #3
dadod is offline dadod  Croatia
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Quote:
Originally Posted by padamiecki View Post
Very good
Excellent
Like last preamp.
Your designs are better and better.
Like teen prodigy;-)
Thanks Pawel, you are to kind.
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Old 6th May 2014, 12:08 PM   #4
dadod is offline dadod  Croatia
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Clipping, and output impedance!
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File Type: jpg GainWire-ClassA-POWERamp-clipping.jpg (95.1 KB, 153 views)
File Type: jpg GainWire-ClassA-POWERamp-outimp.jpg (263.8 KB, 154 views)

Last edited by dadod; 6th May 2014 at 12:15 PM.
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Old 6th May 2014, 12:22 PM   #5
bimo is offline bimo  Indonesia
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Harmonics profile is very good.
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Old 6th May 2014, 12:25 PM   #6
AKSA is offline AKSA  Australia
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Damir,

Pawel is correct........ clever circuit, very good performance, love the phase linearity in the audio band. I make it 1.35 degrees at 10KHz?

Hugh
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Old 6th May 2014, 12:40 PM   #7
dadod is offline dadod  Croatia
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Quote:
Originally Posted by bimo View Post
Harmonics profile is very good.
And it's even better at 1 kHz.
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File Type: jpg GainWire-ClassA-POWERamp-FFT1k30W.jpg (152.4 KB, 186 views)
File Type: jpg GainWire-ClassA-POWERamp-FFT1k1W.jpg (149.0 KB, 144 views)
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Old 6th May 2014, 12:46 PM   #8
dadod is offline dadod  Croatia
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Quote:
Originally Posted by AKSA View Post
Damir,

Pawel is correct........ clever circuit, very good performance, love the phase linearity in the audio band. I make it 1.35 degrees at 10KHz?

Hugh
Thank you Hugh for your kind words.
Yes the phase at 10 kHz is 1.35 degrees and 3 degrees at 20 kHz, could be a bit lower with the input shunt cap lower value. Maybe not to good idea to lover that cap to much?
Damir
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Old 7th May 2014, 09:01 AM   #9
dadod is offline dadod  Croatia
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Picking in the gain response(at 15 MHz) worried me, so here is improved schematic. A cascode was added and cascode compensation, even if it was not needed to shield BC5xx transistors from to much voltage(as 30 V is not to much) but in that way behave of the input stage is tamed better. Surprisingly how an amplifier with no GNFB can easily oscillate.
All others data stayed the same, some phase shift was improved.

I think that this amp would sound similar to an excellent tube amp, but with more grip in low frequencies as the output impedance is quite low(54 mohm).
Damir
Attached Images
File Type: jpg GainWire-ClassA-POWERamp-improved-sch.jpg (163.4 KB, 297 views)
File Type: jpg GainWire-ClassA-POWERamp-Gain-improved.jpg (277.9 KB, 256 views)
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Old 8th May 2014, 01:34 AM   #10
bimo is offline bimo  Indonesia
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Quote:
Originally Posted by dadod View Post
And it's even better at 1 kHz.
Can you explain why the harmonic profile is very good?
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