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Old 23rd June 2007, 12:00 PM   #11
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Hi Conrad,
I was just curious.
Currently I'm working on a low noise instrumentation amp. Parallel opamps would help me a lot, but I beleaved this was more an academic approach.
Regards
Jürgen
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Old 30th August 2011, 06:51 PM   #12
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I know it's an old topic, but it was my inspiration. Simple design and buffered output. I was however limited by the application to single-sided power supply. After reading a paper by Charles Kitchin from Analog Devices Inc. I was able to successfully implement a solution using zener diode. Also, I decided to implement a ground channel just to get rid of the output capacitor (which would otherwise be mandatory). You can see the final design in the attached picture.

I'm satisfied with sound. Crispy clear with plenty lows. Although I'm not an audiophile and using rather basic Sennheiser HD201 headphones.

Notes:

1) I used NE5532 for IC1 trough IC4 but any opamp will do of course.

2) Clipping occurs at signal levels below 3 V because of such a low supply voltage and further reduced by biasing. Not a limiting factor since I found that this was plenty headroom.

3) Since most headphones use shared ground it's possible to remove 2nd ground channel and just add additional buffer(s) the one remaining ground channel. But this configuration works, too.

4) IC3 and IC4 can be removed without affecting the circuit. But more power is always good.

5) 100K resistors can be swapped by lower values (as long as they are all the same). This will result in higher consumption and better response at ultra-high frequencies (which is totally unnecessary).

6) I used 47R stabilization resistors per original design, but I reckon smaller values will do just as good. Optimal value probably depends on the actual opamp used and is a bit above my knowledge level to be honest. 47R works good anyway.

7) Gain is 2 per original design. It's more than sufficient for headphones.

8) I've noticed adding a ground channel reduces PSRR by a factor of 2. Tested using a very noisy switching PSU before (with output cap) and after adding ground channel.

Click the image to open in full size.

Last edited by hwfanatic; 30th August 2011 at 07:00 PM.
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Old 30th August 2011, 09:48 PM   #13
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Originally Posted by hwfanatic View Post
8) I've noticed adding a ground channel reduces PSRR by a factor of 2. Tested using a very noisy switching PSU before (with output cap) and after adding ground channel.
Typo. Adding a ground channel increases PSRR (reduces noise).
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Old 30th August 2011, 11:19 PM   #14
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Originally Posted by Juergen Knoop View Post
so you have tried it Conrad?
If you're talking the noise issue, Ampex did that in the RF amplifiers (4 parallel) in the AVR-3 2" quadruplex video tape recorder. The theory is really simple. The noise is random so adds as root mean square but the signal is in phase so adds linearly. Running 2 amps in parallel increases the noise 1.414 but the signal is 2.0 so the _ratio_ is better. All you need is more real estate and money but in some places it's a good investment.

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Old 31st August 2011, 05:43 AM   #15
qusp is offline qusp  Australia
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Quote:
Originally Posted by hwfanatic View Post
Typo. Adding a ground channel increases PSRR (reduces noise).
hmm, i wonder how it reduces noise by adding noise and doubling output impedance?
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Old 31st August 2011, 06:10 AM   #16
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All I can tell you is that to my ears the switching PSU noise was reduced to approximately half of what it used to be before I added the ground channel on my proto-board. Don't know how. On a side note, it's one noisy PSU, so it is very easy to notice.
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Old 1st September 2011, 04:49 AM   #17
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Last night I've unintentionally disconnected ground channel input from circuit ground. I was amazed that all power supply noise has thus disappeared. It's like PSR was barely active before this. Now it's completely silent with no input. I feel dumb I haven't thought of that before... But at the same time I'm content that for the first time since I've started working on this project it works flawlessly. It seems ground channel is essential in this application (PC on-board audio line out, PC power supply). I'm not sure why, though. I'm confident on-board audio uses common ground with the PSU. This is the reason voltage splitter don't work in this application and biasing is required. Insight, anyone?

Revised circuit attached.

Click the image to open in full size.
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Old 1st September 2011, 06:29 PM   #18
jcx is offline jcx  United States
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its best if you can use the "output gnd" for everything, signal input gnd, feedback gnd, output driver return - it should be treated as the "true" gnd for the circuit

if the single supply V is an isolated "wall wart" or battery that is DC isolated from the rest of the world then the reference for the active supply splitter gnd should be the midpoint of the single supply, use 2 equal R - you don't need the Zener reg

Click the image to open in full size.


also check out The Objective2 (O2) Headphone Amp DIY Project

the associated home page links to pretty complete design Blogs on parts selection, parallel op amps and lots more
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Old 1st September 2011, 08:42 PM   #19
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I want to use PC PSU. Line out and PSU share ground. If I'm not missing something I don't think I can use voltage splitter in this application.
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Old 1st September 2011, 09:16 PM   #20
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Hi all,

Last night in toilet, I had a lamp flashing in my mind!
Very catchy phrase to captivate the audience from the beginning !
If you don't mind, I'll use it sometimes.

Good luck with your project.
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