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#1 |
diyAudio Member
Join Date: May 2008
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Hello everyone
I have been wanting to make a fully discreet headphone amplifier for some low impedance headphones, 32ohms, for some time. Since headphone amps are relatively low power, why not make it class A. This amp is capable of over 70mW into 32 ohms and is heavily based on this amp here. http://headwize.com/projects/showfile.php?file=balkishan1_prj.htm comments welcome. ![]() |
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#2 |
Designer & Technologist
diyAudio Member
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Here is one more example (though, a practical design may be a bit improved against a dirty SIM version - I mean better arrangements of trimmers):
http://www.diyaudio.com/forums/showt...324&highlight= http://www.diyaudio.com/forums/attac...amp=1165512602 http://www.diyaudio.com/forums/attac...amp=1165512681 http://www.diyaudio.com/forums/attac...amp=1215726778 THD goes down with lowering of output level, especially higher harmonics disappear, it sounds very natural. Output follower is made of a pair MOSFET+BJT, for better transfer function approximation. It is loaded on a counter-modulated current source that greatly reduces current variations in an output voltage follower. Also, an input stage contains only 2 transistors, very short feedback path. And separate feedback loops in an input and output stages.
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#3 |
diyAudio Member
Join Date: Aug 2005
Location: K-town
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Hi ![]() A few minor things I see that may not be optimal. The degeneration of the input transistors is a bit large. I think 10 - 20 Ohms should be plenty, assuming you intend to match the input pair. I would suggest a matched array here if you can spare a few extra bucks. Quality is quality. ![]() ![]() Q5 and Q7 should have base resistors, ~1K or so, because if the VAS or the output swings to the negative rail, they may saturate and affect the current flow in Q3. Speaking of Q3 as the tail CCS, 300uA in each leg is a bit too low. At least 1mA is more respectable, I'd go for a couple. Likewise, 600uA VAS bias is too low. I would bet 100pf is way too large for the miller lag compensation cap. I would look into a two pole comp. scheme, MO. Missing Thiele network. ![]() I don’t quite understand the use of medium power devices in the output stage. Even if it is heavily bias with 20mA, at 5V, 100mW can easily be handled by a SOT-23. Seems a bit of sacrifice to me. ![]() |
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#4 | |
Account disabled at member's request
Join Date: Jan 2006
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Quote:
No way. Problem here is that the closed loop gain is rather low (~4). The LTP emitter degeneration has to be large, other wise a HUGE Cdom will be required for stability. |
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#5 | |||||||
diyAudio Member
Join Date: May 2008
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Hi CBS240,
Thanks for taking the time to look at this schematic. Quote:
The degeneration resistor values chosen are to reduce the open loop gain so as to bring the loop gain at 20Khz down to 30db. Using 20ohm resistors would set the 20Khz loop gain (or NFB) at around 44db... probably too high for stability? Quote:
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Increasing the Q3 tail source without any other change will increase open loop gain Quote:
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#6 |
Account disabled at member's request
Join Date: Jan 2006
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Hmmmm. Why starve the LTP and VAS if the output stage is going to be biased at 80mA?
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#7 |
Banned
Join Date: Apr 2005
Location: Stockholm
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Glen,
maybe to reduce global warming? |
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#8 |
diyAudio Member
Join Date: May 2008
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#9 |
diyAudio Member
Join Date: Sep 2006
Location: Graz, Austria
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May I ask you why you go for a downscaled standard opamp design?
When I see headphone-threads I somehow always expect to see something nicely simple, as the requirements for headphones are really low. Just a little current and let the preamp do the voltage gain. There were a few interesting designs around this, Grey's source follower and a nice jfet-amp (EUVLs DAO). Wavebourns design goes then in the more all-in-one solution, nice! Of course, if it suits you, go for it! Have fun, Hannes
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#10 |
Banned
Join Date: Apr 2005
Location: Stockholm
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solid,
consider a two-stage design. you don`t need that ugly VAS, all it does is provides a lot of distortion. Hannes, right? ![]() |
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