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Old 19th March 2012, 12:28 AM   #21491
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OK, to leave out too much more confusion. Over 4 decades, I have tried: UA741, HA911,NE5534, and finally AD825. I'm going with the AD825.
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Old 19th March 2012, 12:33 AM   #21492
wahab is offline wahab  Algeria
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Quote:
Originally Posted by john curl View Post
For the record, the humble uA741 works pretty good in this amplifier. Amazing isn't it? Why?
Because the output transistors are connected in common emitter mode ,
thus providing effective voltage gain , hence , if they have let s say
a 20dB voltage gain , the poor 0.5V/uS slew rate of the 741 will be increased
accordingly , as well as GBW product , of course.

Last edited by wahab; 19th March 2012 at 12:36 AM.
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Old 19th March 2012, 12:37 AM   #21493
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Bingo, wahab!
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Old 19th March 2012, 01:52 AM   #21494
wahab is offline wahab  Algeria
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Originally Posted by Bonsai View Post


John, why are you proposing a common emitter output and not common collector for this design?

I'd think with rails at +- 15 you'd have no output swing issues. 3V peak into almost any headphone you care to name would blow your ears out anyway.
Surely the best option price/complexity/perfs wise.

A 5534 + 2 output power devices should allow to push down
10Khz THD contents below -100dB at 4V rms/30R , the targeted output level,
while retaining the 100mA Iq.
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Old 19th March 2012, 02:11 AM   #21495
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What's the lowest closed-loop gain that you can use with the schematic shown? I've assumed that it's so high (26x) to avoid additional stabilisation components.
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Old 19th March 2012, 04:07 AM   #21496
1audio is offline 1audio  United States
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What about this: http://www.ti.com/lit/ds/symlink/tpa6120a2.pdf 100 MHz and .0002% THD? The 7th harmonic isn't much less than the 2nd, but they are both in the -110 dB range with no load. Almost no external parts. The plots below I just measured.

For what its worth except on the most insensitive headphones (Audeze? which is what I used as a load) even 100 mV is ear splitting loud. With a load the distortion increases but still really low.

This may not be the last word in headphone amps but you can't ignore it now. At least you get a lot for $2.00.

I have also found that a series resistor never seems to help. A voltage drive always seems to sound better, unless the bass depends on major resonance, which will usually sound pretty woolly in a headphone.
Attached Images
File Type: jpg TPA6120A2 distortion 1K no load.JPG (224.8 KB, 206 views)
File Type: jpg TPA6120A2 distortion 1K 1V 41 Ohms.JPG (216.2 KB, 200 views)
File Type: jpg TPA6120A2 distortion 1K 100 mV 41 Ohms.JPG (215.2 KB, 197 views)
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Old 19th March 2012, 04:23 AM   #21497
jcx is online now jcx  United States
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interesting choice John, you point out that the input op amp in your circuit doesn't require high slew rate, nor high output current

then you select an op amp which has both - and at a high price in performance specs that might matter

the AD825 12 nV/rtHz noise is speced at 10 kHz - datasheet writers only do that when the 1/f corner is over 1 kHz - and 12 nV is poor, expecting several times that at 1 kHz is worse for audio

noise is consequential in headphone amplifiers with higher sensitivity headphones
I use a 10 kOhm volume pot just to keep Johnson noise low, wouldn't use an op amp that gives noise figure even 3 dB worse


Samuel Groner's Op Amp Distortion measurement paper ends his AD825 section with
Quote:
...Not particularly well suited to low distortion applications, and relatively expensive
http://www.sg-acoustics.ch/analogue_...mps/index.html

lots lower noise fet input op amps exist, not every product can afford the OPA627 but even at the AD825 price level there are options

look at the OPA1641 - 5.1 nV/rtHz, 20 V/us, 11 MHz GBW

normally I wouldn't consider 1 nV/rtHz class low noise bjt input op amps in this position but even they can still beat the 825 on worst case noise with their ~2 pA/rt/Hz current noise and 20 kOhm or less volume pot - OPA1611


there are even CMOS op amps that beat on noise vs AD825 and cost less than $1


Demian, I have been proposing a composite circuit with "good, "audio" input op amp wrapping a outer feedback loop around the TPA6120 - doesn't seem to float John's boat - even mentioning that his buddy Walt Jung likes the topology for audio doesn't seem to help

Last edited by jcx; 19th March 2012 at 04:43 AM.
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Old 19th March 2012, 04:35 AM   #21498
Bonsai is offline Bonsai  Taiwan
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Quote:
Originally Posted by john curl View Post
Actually, the alternative suggested by JCX, has just as much peak output. It just does not have as much CLASS A output . . . very high rev'ing one.
I know that. That's why I cannot understand why you're going for a common emitter output stage.

Use a decent opamp (5532/34 or a 49710 for example) and wrap it around a simple class A push pull follower and you have single digit ppm at 20KHz up to 3V out and no nasty poles to have to worry about in the output stage.

The TI TPSxxxx device is also very good, but I can understand the reluctance for an all IC solution when there are more difficult ways to achieve the objective (joke!)
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Old 19th March 2012, 04:39 AM   #21499
bcarso is offline bcarso  United States
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Originally Posted by 1audio View Post
[snip]

I have also found that a series resistor never seems to help. A voltage drive always seems to sound better, unless the bass depends on major resonance, which will usually sound pretty woolly in a headphone.
I'd conjecture that the series resistor is an artifact of the earliest headphone outputs on the front panel of "integrated" amps/receivers, where the speaker connections were routed through closed-circuit jacks and the simple goal was to limit the power to the phones when plugged in. Bad news all around --- the contacts go south and interfere with the speaker drive, the cans are driven from a high impedance and, unless the makers have carefully taken this highish drive impedance into account, suffer from frequency response anomalies and nonlinear distortions.
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Old 19th March 2012, 04:52 AM   #21500
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Actually the 100 Ohm source impedance is enshrined in that place where all useless and progress defying standards are: the EU IEC standards. There are also maximum output standards and sensitivity standards and everything needed to make a bureaucrats heart glow.
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