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Old 13th July 2014, 12:43 PM   #1
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Default Portable headphone amp

Even if I raised a similar question in the Chip Amps section, and headphone amps looks like a VERY large field, perhaps someone can tell me a story on what made a remarkably good audio for them.

My requirements are:

1) 12 single supply.

2) Must drive 50-75 ohm headphones to good levels, 600ohms if possible. So capable of large current.

3) Using output boost is an option.

4) It must use chips, not transistors or valves.
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Old 13th July 2014, 01:29 PM   #2
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I made something like you are looking for a while ago. Although I made an emphasize on lover idle current, as it was going to be powered from a 9V battery. Should be no problem running it on 12V. Here how it looks like:
Click the image to open in full size.
If you are interested, I can try to dig up the schematic.

Also I would like to point to some miss-concept in your requirements. If you willing to drive ~600Ohm load, you will need higher voltage, but not currents swing. In this case single 12V might be a limiting factor, unless you will add an SMPS or charge pump to boost or invert supply voltage.
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Old 13th July 2014, 03:05 PM   #3
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Yes, please try to dig up your circuit. I'm very curious.

And yes, it should be unlikely I will be able to get anything from 600ohm loads.

And no, no SMPS for anything on this project. There will be 48v supply for the microphones, but very low current.
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Old 14th July 2014, 09:19 AM   #4
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The amplifier itself look pretty much like this. I've simplified it a little bit. There where a few extra things which I think will not make too much difference.
There is also a supply rail splitter on that board, based on LM7322. It generates mid supply of 9V. This mid supply also was used as a signal ground. Depending on supply arrangement in your case, it may not be possible to reuse such solution.
Also, if supply source allows, some different opamp may be chosen for the design. These low power high speed guys are quite touchy in terms of stray capacitance.
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Old 14th July 2014, 12:02 PM   #5
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Hi Sergey,

Your design is based on Walt Jung's composite opamp design, which might have some restrictions for me, as my supply is 12v analog, as I said.

I do have to create a V+/2 reference ground, so I don't see how I could apply it to that circuit, if at all possible.
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Old 14th July 2014, 12:14 PM   #6
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This design is perfectly usable with unipolar +12V supply with a half supply "virtual ground". You just need to change net names as per what you have on your board. +V goes to +12V, -V goes to 0 and ground goes to you virtual ground - 12V/2.
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Old 14th July 2014, 12:25 PM   #7
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Depending on how you are going to organize virtual ground, you may need if add DC decoupling capacitors. Although it is better if you can avoid to have one. At least at the output path.
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Old 14th July 2014, 01:42 PM   #8
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Yes, DC decoupling capacitors are needed in V+/2 arrangements. But that is not such a serious problem to me.

But two questions are on that design:

1) If I can replace the global feedback resistor with a pot, to set levels, as I can't use a pot a the input.

2) How to connect Vref in the inside chip.

It's not that easy to replace the grounds connections with Vref. Some you can't.
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Old 14th July 2014, 01:47 PM   #9
jcx is offline jcx  United States
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I like multiloop composite amps with CFA output - but sergey's circuit, gain/feedback values may push stability limits

I really wouldn't try that without a local feedback C on the input op amp to slow the global loop - with the LMH6723 at Av 11 you are dropping to a few 10s to MHz corner (CFA do run out of GBW at high gains), may add too much phase shift for the 22 MHz input op amp


wanting 100+ mA output and portable aren't a good combo for battery life

the OP should clarify his "12 V" supply details - the LMH6723 abs max 13.5 Vs may be exceeded if ever ran from a unregulated "12 VDC" wall wart with poor the regulation of a small xfmr being pushed to limits for cost

likewise some battery type nominal and peak fully charged V differ by enough to matter in chip choice


the AD8397 is a possible choice for the output current - but handling the power dissipation from its tiny power pad may be a challenge - the tiny pad is harder to solder by hand

I've used the TPA6120 - still surface mount, power pad - but much bigger - Peranders project pages shows it being soldered by hand

Last edited by jcx; 14th July 2014 at 01:59 PM.
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Old 14th July 2014, 01:54 PM   #10
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Well, the supply would never be unregulated wall wart, as it will always be battery powered.

There might be an external DC input, but it would be regulated inside.

Though I would prefer chips that can take up to 18v at least, because batteries may be 14.2v sometimes. We are still considering if we regulate to 12v or not.

The design I'm favoring for now is an NJM4556 with transistor boosting at the output. I'm looking for options that can improve on that, but they should be simple to implement and not expensive.
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