2x Gain pure op-amp headphone amp idea

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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.

An externally hosted image should be here but it was not working when we last tested it.


This is my project, not tested yet but this weekend I will. What do you say about that?
Anyone have any idea?
Ozgur
 
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)

Best regards,
 
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.
 
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.
 
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.

An externally hosted image should be here but it was not working when we last tested it.
 
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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.

 
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.

An externally hosted image should be here but it was not working when we last tested it.
 
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

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also check out http://www.diyaudio.com/forums/headphone-systems/193977-objective2-o2-headphone-amp-diy-project.html

the associated home page links to pretty complete design Blogs on parts selection, parallel op amps and lots more
 
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