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Old 23rd August 2011, 10:32 PM   #1
1001 is offline 1001  Italy
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Question Yet another simple class A headphone amp

hello everybody,
I'm going to test this simple amplifier for my 32 ohm sennheiser hd 448, the voltage gain is 2.5, the rms output power is a little bit more then 100 mW (32 ohm load and my ipod nano), all discete bipolar never switch and the output stage of AD797 too. what do you think about this circuit? maybe there are some errors...
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Old 24th August 2011, 06:22 AM   #2
jaycee is offline jaycee  United Kingdom
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It should work quite well, I would probably make C7/C8 larger. I'd probably make R2/R11 4.7 ohms as well
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Old 24th August 2011, 09:53 AM   #3
1001 is offline 1001  Italy
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well, good tips, but what do you think about sound quality?
this net would be free of crossover distortion (AD797 too) due to class A biasing, it would has very low output offset (absolute and thermal drift) and very low noise.
I'm going to do this circuit and test for noise, stability, dc offset, stability of biasing current, offset thermal drift, quality of sound and reliability.
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Old 24th August 2011, 08:08 PM   #4
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I will read Your comments so please keep posting. I like the low part number high quality designs. I'm also interested in how the final amplifier will look like (PCB mania or something...).
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Old 24th August 2011, 08:41 PM   #5
jcx is online now jcx  United States
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if you learned how to use this free schematic drawing tool you could gain some more insight about how the circuit works

Linear Technology - Design Simulation and Device Models

I've simmed simialr topologies Cmoy headphone amp?
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Old 24th August 2011, 10:34 PM   #6
1001 is offline 1001  Italy
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Quote:
Originally Posted by jcx View Post
if you learned how to use this free schematic drawing tool you could gain some more insight about how the circuit works...
I did a preliminary sim of my circuit before a future testing to check the feasibility, I need to know if it is enough stable, how the thermal drift can change the biasing current of the output stage and possibly fix it.
if the circuit pass this test it 'would' be better for instance of any cmoy or O2 amp, with lower output impedance, lower distortion, lower noise but it needs more current because of its full class A design.
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Old 25th August 2011, 03:24 PM   #7
1001 is offline 1001  Italy
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while I am waiting to receive toshiba bipolar from an ebayer seller to test my circuit, I am simulating another all class A topology, just a little bit stranger: this is the open loop gain of the whole amp, with 207 dB of gain @ 20Hz and 102 dB @ 20 Khz
hypothetically, if it is real what do I expect from this amp? does it sound better than conventional amp?
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Old 26th August 2011, 06:00 AM   #8
jcx is online now jcx  United States
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everything has its limits - feedback factor grossly exceeding your S/N is fairly useless, "costs" you in needing higher loop gain roll off rate, actually reducing available feedback factor at higher frequencies

http://trs-new.jpl.nasa.gov/dspace/b.../1/98-0905.pdf doesn't explain the noise limit but does show the basic "conservation law" of feedback

for headphones heavily biased output buffers can have few 0.1% "open loop" distortion, adding 80-120 dB loop gain likely gives as much performance as can be distinguished from system noise at audio frequencies - ears do not do 10s or 100s of seconds of averaging to reach down into the noise floor like we can with fft
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Old 26th August 2011, 12:49 PM   #9
1001 is offline 1001  Italy
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Quote:
Originally Posted by jcx View Post
...actually reducing available feedback factor at higher frequencies...
my design has 102 dB @ 20 Khz open loop gain, I think no one opamp have the same gain. of course the stabilization network decrease available feedback factor but at higher frequencies, over 70-80 khz (in my design).
Quote:
for headphones heavily biased output buffers can have few 0.1% "open loop" distortion, adding 80-120 dB loop gain likely gives as much performance as can be distinguished from system noise at audio frequencies - ears do not do 10s or 100s of seconds of averaging to reach down into the noise floor like we can with fft
I agree, but since I had made and tested this network but without output stage many years ago, I think it could interesting to test the circuit by AP equipment and for sonic performance. don't you think it's a good idea?
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Old 26th August 2011, 12:57 PM   #10
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Default Headamp

Please see here:
http://www.ac-vogel.de/TransBuffer_A...fer_APN_1.html

This amp is tested by Audio Precision 2 and works with several Loads.
Don't forget heatsink :-)
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