Headphone stage & preamp output

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carlmart
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Re: Headphone stage & preamp output

Post by carlmart » Sun Jun 29, 2014 8:10 am

So it looks you would prefer an NJM4556 for the headphone amp, as I don't want to use driver transistors.

Supply will be 12v and single.

What about the pre-driver before the THAT 1606 output? Demands are not that great there, though.

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JR.
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Re: Headphone stage & preamp output

Post by JR. » Mon Jun 30, 2014 11:11 am

If you know what cans you plan to use you may get away with a strong opamp. You can also parallel multiple opamp outputs together to get more drive capability, or use a dedicated headphone driver IC.

The last dedicated commercial headphone amp I made at Peavey/AMR used external power transistors and would put a few watts into speakers. 8-)

Wayne has a Hifi headphone amp here somewhere...

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carlmart
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Re: Headphone stage & preamp output

Post by carlmart » Mon Jun 30, 2014 11:36 am

This head stage will go inside a portable unit, powered by 12v single supply.

What I may try, if in field tests can't handle well 50-60 ohms cans, that are the usual in location video.film recording, then I may try some driver output transistors.

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JR.
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Re: Headphone stage & preamp output

Post by JR. » Mon Jun 30, 2014 12:48 pm

+/- 6V peak at 50 ohms is 120 mA peak.. That is too much current for most opamps while some new stuff is stronger output (look at the data sheets)... Again a dedicated phone amp chip will drive the cans.

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carlmart
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Re: Headphone stage & preamp output

Post by carlmart » Mon Jun 30, 2014 1:16 pm

Most dedicated amp chips I know (I don't know them all) are designed for +/- supplies. Not the case here.

Any single supply chip you would suggest?

Edit: I've seen some chips for portable stuff, but they are designed to drive ear-phones, not proper headphones.

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JR.
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Re: Headphone stage & preamp output

Post by JR. » Mon Jun 30, 2014 2:40 pm

carlmart wrote:Most dedicated amp chips I know (I don't know them all) are designed for +/- supplies. Not the case here.

Any single supply chip you would suggest?

Edit: I've seen some chips for portable stuff, but they are designed to drive ear-phones, not proper headphones.
The last time I used a standard chip for headphones was back in the '70s and back then I used a National Semi LM380 (?).. It was in fact designed for single supply. I make no claims for sound quality while 0.2% THD at 8 Ohm, should do a little better at 50-60 ohms.

I do not really recommend any one part. Back in the '70s I was more inclined to use canned solutions than I am now. For several decades I routinely throw a pair of transistors on the output of a decent opamp and move on...

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carlmart
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Re: Headphone stage & preamp output

Post by carlmart » Mon Jun 30, 2014 3:59 pm

I agree, the first head amp I made I think was single supply, using a TL072 and two BC small current transistors as drivers.

It could drive my 50 ohm AKGs quite well, and the supply were a few batteries.

I might save time and do the same here again.

carlmart
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Re: Headphone stage & preamp output

Post by carlmart » Tue Jul 01, 2014 7:23 am

I've drawn two op-amp based boosted options for the headphone amp.

Version 1, if I remember well, is the one I build more than 20 years ago. I believe mine was powered with 9v or 12v supply.

Gain arrange was fixed back then, with a pot a the input. This version would be closer to the one I need now.

Version 2 is based on what a reputed mixer did use, but you can consider it a generic boosted version, as I could google a lot of them all alike.

It's dual supply, so if I use it I think I should ground the -V supply and half the 6K8 resistors.

Some put two diodes in series, some put LEDs instead of the diodes. Output transistors could be medium power types too, like BD139/BD140, but I'm not sure it's necessary.

What I wonder is if any of you has compared or listened to any of them, or should recommend anyone.

The LM380 and LM386 chips are available too, and would probably have less distortion than the 0.2 specified on the data sheet. Once again, I never compared them to the ones I'm proposing, and I know no mixer design that ever used them.

If I remember well, the boosted IC was always the preferred option, so there should be valid reasons for that.
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JR.
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Re: Headphone stage & preamp output

Post by JR. » Tue Jul 01, 2014 8:14 am

For 50-60 ohm cans either approach should be OK (I've used both), I have never used 33178 but ASSuming it is decent performance either output topology should do well. For the low voltage PS (12V) you could get a little more output swing by adding bootstrap caps to version #2. (Split 6.8K resistors into pairs of 3.3k Rs, with caps from opamp output to junction of 3.3k Rs).

Version #1 as is will keep it simple.

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carlmart
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Re: Headphone stage & preamp output

Post by carlmart » Tue Jul 01, 2014 9:15 am

Version 1 would have to be 12v too. I just kept the original voltages on each one, just to show what they originally were.

I am not sure yet about the non-THAT chips being MC33178 or NJM4556. Perhaps I will have to listen to both on the circuit and see if there's any difference. The MC specs are slightly better, but not too much to anticipate much.

Let me see if this is what you meant. I put just 10uF for the bootstrap caps, as I don't know how to compute that.

There shouldn't be great demands from the cans that will be used with this amp, probably Sony, AKG or Sennheiser, and I'm not sure it will drive a 600ohm can.
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