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Old 22nd June 2007, 01:18 PM   #1
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Lightbulb 2x Gain pure op-amp headphone amp idea

Hi all,

Last night in toilet, I had a lamp flashing in my mind! Why do I use two or more op-amps in parallel to drive headphone? Its an easy way to have a cheap and hi quality headphone amp! I plan to use TL074 for that project but OPA4134 is a better altenative.
Why I use four op parallel is; one TL can drive 30-35mA and four of them wil drive 120mA at least. This is more than enough I know but if I paralleled more than two op-amp then the final THD will become a olwer value as I know.

Click the image to open in full size.

This is my project, not tested yet but this weekend I will. What do you say about that?
Anyone have any idea?
Ozgur
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Old 22nd June 2007, 01:41 PM   #2
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And this is a power splitter for that head amp. To work with single 12v wall plug SMPS or just a single 12v battery.
Click the image to open in full size.
Also creates a dedicated ground channel.
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Old 22nd June 2007, 03:02 PM   #3
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allthough they are application notes from NS, BB (TI) and some stuff in the Walt Jung books about paralleling opamps, I have never seen real circuits doing that.
btw. check out the stability requirements for TDA2030!
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Old 22nd June 2007, 04:31 PM   #4
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http://headwize.com/projects/showfil...=opamp_prj.htm figure 12
http://www.diyfactory.com/projects/h..._schematic.pdf

and you can use higher output current op amps with a variation of this circuit for true Class A output with op amps scroll down to my (jcx) posts in: http://www.head-fi.org/forums/showthread.php?t=190991

use dual supplies where possible, active ground circuits are inefficient and can add distortion and have stability problems
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Old 22nd June 2007, 11:27 PM   #5
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Not a completely new idea, but very effective. Paralleling can reduce noise too. IMO, artificial grounds work far better on paper than they do in real life. I avoid 'em. Even though it requires more resistors, you should compare the advantages (and disadvantages) of inverting rather than non-inverting configurations.
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Old 22nd June 2007, 11:45 PM   #6
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so you have tried it Conrad?
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Old 23rd June 2007, 07:04 AM   #7
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I guessed that will not be a new idea. Somebody should think that before, because its very simple.

Anyway, I think I made a mistake on power splitting. In fact I was hopping to have a very cheap one. But TDA2030 works +24dB at least. So my power splitting idea cannot be realised with that IC. (Thanks juergenk)

Today I will try the circuit with a split supply. And will report you the results. I hope it works.

Do you know a power op-amp will work with unity gain? (a cheap and available one pls)

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Old 23rd June 2007, 08:42 AM   #8
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OPA551 200mA
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Old 23rd June 2007, 09:29 AM   #9
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Default It works!

I've tried the circuit on breadboard. With very **** connections.
The results;
It cannot work with 12v single supply (splitted with 4K7+220uF on bpth rails) Its works but with motorboat noise and a lot of distorted.
It also cannot work until 24v single supply. I've tried to split with LM317 (totally fail) with TDA2030 (oscillating) and with resistor divider. In all cases the gnd point always swinging between (+) and (-) rails. (Thats weird!)
It works perfect with +/- 12v real symmetric supply. In fact I made that supply with two seperated lab.PSU. And the rails are not exaclty matched (one 12,1v the other 11,7v). And the result is impressive. I didnt expect that result.
I've tried with TL084 op-amp and only in one channel. Two op-amp in parallel is enough but three or four has better sound. I didnot use an input cap and connected to my ipods headphone output and use with ipods headphones. I cannot hear anykind of noise while inputs open. Thats good! All I can say is;

That configuration has better distortion than the original output.

I'll make a PCB for two channel circuit. And will use it for my CD player.
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Old 23rd June 2007, 11:19 AM   #10
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Juergenk, I haven't built a headphone amp that way, but have used the configuration in instrumentation circuits for the improved noise performance. It may have been in one of LT's app notes by Jim Williams, but not sure. I've also used the artificial ground (TI makes (made?) a TO92 device for that), both the TI part, and my own circuit. It can be made to work ok, but it's also one more thing that can be unstable or insufficient to the task, and it's one more thing that can reduce reliability. I ultimately eliminated it from whatever I was working on because I was never comfortable with it. FWIW, I'm more of a fan of class A headphone amps, since the power requirements are modest- that's what I build for myself.
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