A different Class A Power Amp - Current Feedback

stoc005 · 16th April 2009, 02:28 PM

Here is the beginnings of a current feedback Class A power amp. It is very simple but does simulate well. Currently it can deliver 90 V P-P (1v rms input) into 8 ohms with one pair of output devices as shown/drawn. No signal power in the output devices is very high. No signal output current is 5.5 Amps. With 50 volt supplies....over 500 watts in those 2 device. Will have to add more devices to spread out the static power. It's flat from DC to over 3 Mhz and the phase response is near 0 degrees to over 1 Mhz. I'd bet the measured frequency response is better than many audio amps out there. No signal DC out is 350 millivolts. A bit high, I think. Need better matched input devices. The emitter resistors in the outputs are not needed. I have them there just to measure output current. The input is low impedance and will be difficult to drive. Maybe needs a buffer/op-amp. Since it is Class A, there is no crossover distortion possible. Simulates clean but doesn't like lower load resistances. This design was stolen from an Analog Devices current feedback op-amp and scaled up using discrete SMT devices as shown. No, I will not be building it soon. Want to work with it in simulation and really understand it.

Comments??
What is a reasonable target for the no signal DC output voltage on a power amp??
Bill

stoc005 · 16th April 2009, 02:30 PM

Here??

Nelson Pass · 16th April 2009, 08:25 PM

Quote:

Originally posted by stoc005
What is a reasonable target for the no signal DC output voltage on a power amp??

50 mV is a reasonable target.

Wavebourn · 16th April 2009, 08:41 PM

I'm not syn08, but I would use his favorite sign for such a term as "Current feedback" when it is a feedback by voltage actually...

Also, when people go for class A why do they use complementary emitter followers that are good only when power efficiency is desired?

brig001 · 16th April 2009, 08:53 PM

I suspect a mismatch between Q3 and Q2. Is there any way you can measure hfe's and match them? Alternatively, reduce the impedance of the feedback loop. The feedback is on the emitters of Q3 & 2, which could be carrying 100mA. Any mismatch here will put on offset on R1.

Alternatively, you could make I1 or I2 adjustable and trim it out.

I must admit to not being familiar with this circuit, so may be off beam here, but either idea is worth a try.

nelsonvandal · 17th April 2009, 09:20 AM

Why not start with a more typical CFA like Hiragas le Classe A or this http://www.eecg.utoronto.ca/~kphang/ece1371/cfa.pdf

ontoaba · 25th February 2011, 05:40 AM

Looks very nice one, another simple CFA. Lets say output bumping feedback if "current feedback" harder to be noticed, the outputs bumps then the current flow.

16th April 2009, 02:28 PM	#1
stoc005 diyAudio Member Join Date: Feb 2007 Location: Midwest in the USA	A different Class A Power Amp - Current Feedback Here is the beginnings of a current feedback Class A power amp. It is very simple but does simulate well. Currently it can deliver 90 V P-P (1v rms input) into 8 ohms with one pair of output devices as shown/drawn. No signal power in the output devices is very high. No signal output current is 5.5 Amps. With 50 volt supplies....over 500 watts in those 2 device. Will have to add more devices to spread out the static power. It's flat from DC to over 3 Mhz and the phase response is near 0 degrees to over 1 Mhz. I'd bet the measured frequency response is better than many audio amps out there. No signal DC out is 350 millivolts. A bit high, I think. Need better matched input devices. The emitter resistors in the outputs are not needed. I have them there just to measure output current. The input is low impedance and will be difficult to drive. Maybe needs a buffer/op-amp. Since it is Class A, there is no crossover distortion possible. Simulates clean but doesn't like lower load resistances. This design was stolen from an Analog Devices current feedback op-amp and scaled up using discrete SMT devices as shown. No, I will not be building it soon. Want to work with it in simulation and really understand it. Comments?? What is a reasonable target for the no signal DC output voltage on a power amp?? Bill

16th April 2009, 08:41 PM	#4
Wavebourn diyAudio Member Join Date: Sep 2006 Location: Pleasant Hill, CA	I'm not syn08, but I would use his favorite sign for such a term as "Current feedback" when it is a feedback by voltage actually... Also, when people go for class A why do they use complementary emitter followers that are good only when power efficiency is desired? __________________ The devil is not so terrible as his mathematical model! Wavebourn: We Create Creativity!

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16th April 2009, 08:53 PM	#5
brig001 diyAudio Member Join Date: Jan 2009	I suspect a mismatch between Q3 and Q2. Is there any way you can measure hfe's and match them? Alternatively, reduce the impedance of the feedback loop. The feedback is on the emitters of Q3 & 2, which could be carrying 100mA. Any mismatch here will put on offset on R1. Alternatively, you could make I1 or I2 adjustable and trim it out. I must admit to not being familiar with this circuit, so may be off beam here, but either idea is worth a try.

17th April 2009, 09:20 AM	#6
nelsonvandal diyAudio Member Join Date: Nov 2006	Why not start with a more typical CFA like Hiragas le Classe A or this http://www.eecg.utoronto.ca/~kphang/ece1371/cfa.pdf

25th February 2011, 05:40 AM	#7
ontoaba diyAudio Member Join Date: Nov 2009 Location: Kudus, & Malang	Looks very nice one, another simple CFA. Lets say output bumping feedback if "current feedback" harder to be noticed, the outputs bumps then the current flow.

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