DIY headpone amp with ~300mA output - diyAudio
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Old 1st June 2012, 04:22 PM   #1
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Default DIY headpone amp with ~300mA output

I made my new amp with good parameters. The amplifier is able to provide up to 300 mA output current. The amplifier is based on the idea of ​​increasing the output current by paralleling the buffer amplifiers. I used the following components and parameters:
IC1 - Analog Devices OP27 - precision opamp gives low output offset
IC2/3/4 - Philips NE5532 - good opamp as a buffer - gives ~ 50mA(and low distortion).
Schematic:
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Photo of my sample:
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Audio interface - E-MU 0202 USB
Thd of amp:
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IMD:
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Old 1st June 2012, 06:42 PM   #2
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I guess these were unloaded measurements? Looks very clean in any case.

50 mA per 5532 seems a little optimistic. According to tests performed by Sijosae, about 35-40 mA is more realistic. Anyway, 6 of them give plenty of output for a headphone amp.

The only choice I'd diagree with is the OP27. It is an old precision op-amp design with bias current cancellation that is optimized for DC rather than AC performance. While the low effective bias current keeps your volume pot crackle-free and saves on a few (cheap) components, Douglas Self notes high levels of common-mode distortion at high frequencies (which is consistent with measurements I've seen lately), along with real-life voltage noise rather being worse than for a 5534. I'd try an LME49710.
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Old 1st June 2012, 07:17 PM   #3
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Good replace for AD OP27 - TI OPA227.
LME49710/LME49720/LME49740/LM4562 can give bad clicking noise.(in some designs i got this noises)
NE5532 - that optimised NE5534 for sound. For A=1 difference is minimal.
I think, no need in NE5534..

Last edited by stridervvv; 1st June 2012 at 07:20 PM.
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Old 2nd June 2012, 09:36 PM   #4
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Quote:
Originally Posted by stridervvv View Post
LME49710/LME49720/LME49740/LM4562 can give bad clicking noise.(in some designs i got this noises)
That makes me wonder whether the very low specified input bias currents may not actually be real.

This lab outlines a simple way of measuring Ib. Should be quick to build. Maybe try it some time?
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Old 2nd June 2012, 10:21 PM   #5
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may be i got fake opamps... (fake lm4562)

Last edited by stridervvv; 2nd June 2012 at 10:48 PM.
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Old 2nd June 2012, 10:57 PM   #6
jcx is offline jcx  United States
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the 5532 are slow for output buffers, their phase shift prevents using faster input/global feedbck op amps - you need to add a local compensation Cap to the input op amp if you use significantly faster parts
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Old 3rd June 2012, 07:25 AM   #7
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No need in super fast buffers for sound band frequences.
9v/us can give 1,8Mhz(without distortion) on 5v of out(super ears that hears distorsion over 22000hz? I don't think so).
500V/us;1000V/us;2000V/us - video opamps(need in big output swings and high frequences).
In top audio chips used 3~20V/us(3-20V/us )
"Very high speed opamp needed in for sound " - delusion.
High speed opamp started from 9V/us
High speed opamps have high offset voltage - very bad for sound. - low power headphones can be burned.
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Old 3rd June 2012, 10:35 AM   #8
rjm is offline rjm  Japan
Richard Murdey
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Quote:
Originally Posted by sgrossklass View Post
I guess these were unloaded measurements? Looks very clean in any case.
Indeed. Show me 16 ohms on the output, then we can discuss.

As an aside, funny how people just take as fact this notion that the problem with op amps re driving headphones is the lack of current output, and the solution is therefore more op amps.

I've never seen a credible justification for that, theoretical or practical.

Why stop at 300 mA after all. If you want output current, a complimentary pair of transistors can do AMPS...
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Old 3rd June 2012, 02:38 PM   #9
jcx is offline jcx  United States
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do you understand feedback theory, loop stability?

"speed" can also mean GBW product - critical for Nyquist stability - gets "interesting" when you add amps in series in a loop and ignore added the phase shift

with a loop gain of 3 you have some margin with a unity gain internal compensated input op amp of ~ the same GBW as the 5532, but if you put a 5534 in the input op amp position you would have a marginally stable system without adding compensation components

if someone wanted to use the amp with sensitive IEM then it would be "reasonable" for them to change the amp' gain to unity - then you are courting stability trouble with input op amp even just the same GBW as the 5532

without a warning on acceptable types less informed diyers will see the input op amp as a opportunity for "op amp rolling" - many of the "popular" (but often inappropriate) substitute op amps are too fast for your circuit - have >20 MHz GBW - will likely outright oscillate with the "slow" 5532 "buffer" you show

there is a reason the popular IC buffers have ~100 Mhz corner frequency - so you can mostly ignore the added loop phase shift when using these buffers inside feedback loops of most (earlier op amps anyway - everything is getting faster today) unity gain compensated op amp circuits
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Old 3rd June 2012, 03:05 PM   #10
rjm is offline rjm  Japan
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Quote:
Originally Posted by stridervvv View Post
"Very high speed opamp needed in for sound " - delusion.
True in general, but his point was that if you make a compound amplifier, with the buffer in the feedback loop, the buffer circuit itself has to have an open loop bandwidth significantly higher than the op amp itself, or risk compromising the stability of the feedback loop.

You built this amp with an 8 MHz op amp as the buffer wrapped up in the feedback loop of a another 8 MHz op amp. Maybe I'm wrong, but that seems like asking for trouble.

Each of those NE5534 chips is running as a unity gain buffer, well over 60 dB feedback ... wrapped up in another 60 dB or thereabouts from the OPA27. That circuit is pretty tightly cranked. Wouldn't take much to send it spinning.

By the way, for unity gain operation you are supposed to use a 22 pF cap on the COMP pins of the NE5534...

(edit: While I was writing this jcx posted his response above - so, yeah, what he said. )

(and another thing: why not just keep the voltage gain stage independent? Like the circuit attached. Good night!)
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Last edited by rjm; 3rd June 2012 at 03:19 PM.
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