Unity gain buffer for Sennheiser HD600

[Gopher]

I read the "white paper" describing the performance advantages of their 0-ohm headphone amplifier. Hmmmm.....this is the first measurement I've ever seen that proves that a 30 ohm resistor generates so much distortion!! I guess you have to take it at face value---their headphone amplifier does not like to drive resistive loads in series with headphones. I don't know why that is. I'm also not sure if that is true with every power amplifier extant.

It's not the 30 ohm resistor or the amplifier that's generating the distortion, it's the headphone driver itself. Note that this distortion reduces when the headphone impedance goes up.

 

[pilau]

Note that this distortion reduces when the headphone impedance goes up.

Precisely!

*note: also, amps can clip if they don't have enough juice to drive a speaker

 

[dotneck335]

"The output resistance of the BUF634 is about 10Ω. Therefore, series output resistors for
decoupling the individual buffers are no longer necessary."
Ah! I see--thanks for the link. So the 10 ohm resistor is not really needed.

 

[theAnonymous1]

Can you say which solution would yield the flattest frequency response, or color the signal the least?

One has an output impedance set by resistors and the other is effectively 0-Ohm (or close enough). Does that matter to you?

 

[pilau]

One has an output impedance set by resistors and the other is effectively 0-Ohm (or close enough). Does that matter to you?

Which is which?

 

[theAnonymous1]

Which is which?

If you look in the schematic Mooly gave, there are resistors outside of the feedback loop. Output impedance will de determined by the value of those resistors.

EDIT: I failed to see the resistor values before. I now see they are 1R, so the output will be 0.5-Ohm. Will that resistance outside of the feedback loop make any noticeable difference? You'll have to ask an expert that question.

There's also the question, is the 3V/µs slew rate of the NJM4556 enough? Ugly topic.

 

[pilau]

How did you make that out? What is the formula? Thanks

My goal is < 0.05 Ohm output impedance. Is it possible to change the values on those resistors to achieve that?

 

[theAnonymous1]

There are two buffers in parallel per channel with 1R on the output. 1R in parallel with 1R = 0.5R.

They are required for current sharing of the outputs.

 

[pilau]

So if I replace all four resistors with 0.1Ohm or lower, would I get 0.05Ohm output impedance then?

 

[theAnonymous1]

Too low a value and the op-amps will fight each other and start to oscillate.

 

[pilau]

How do I find out what value is too low? Or are you implying that < 0.05 ohm is not achievable with this circuit?

 

[theAnonymous1]

You need an o-scope to experiment with lower values. I assume if the designer chose 1R resistors, he did so for good reason.

Nesting the buffers in the feedback loop of another op-amp will get you what your looking for, but at that point you might just be better off with a BUF634/LME49600 board.

 

[pilau]

An output impedance greater than 0.05ohm defeats the purpose of this project...

Would you say it is achievable with that BUF634 buffer module from eBay?

 

[TNT]

I think you are chasing ghosts...

if you are "just" mixing for yourself in your home studio why not make the sound as you want it - it will be possible with the quad capture as is.

//

 

[pilau]

Because plain and simple the Quad Capture is not capable of driving these headphones accurately, which are the best I can afford at this price. I figured with a small investment I can make the interface suitable for these cans. Also it's an interesting project and an opportunity to learn!

 

[pilau]

Also, please, don't drag me into this argument again... I know what I want to do, and that is to to lower the headphone output my of interface from 47ohm to < 0.05ohm. Can you assist with the project? Thanks.

 

[theAnonymous1]

An output impedance greater than 0.05ohm defeats the purpose of this project...

Would you say it is achievable with that BUF634 buffer module from eBay?

I already mentioned that they are running open-loop on that board, so no. They need to be nested in the feedback loop of an op-amp.

Not very many options on ebay. Personally, I would build the board I linked to earlier. There's a kit on ebay that comes with parts, but I won't even link to it because it's missing things critical for stability.

 

[pilau]

I would build the board I linked to earlier. There's a kit on ebay that comes with parts, but I won't even link to it because it's missing things critical for stability.

You got me confused–it's missing critical things but you still recommend it?

 

[theAnonymous1]

Sorry, the one I linked to is just a PCB (you supply the parts). It's basically the reference design fromt he LME49600 datasheet.

There is a different BUF634 headphone amp KIT (parts included) on ebay as well, but I don't recommend it.

 

[pilau]

You're talking about this one, right?

LME49720HA LME49600 Headphone Amp Pre Amp Audio Signal Amplifier Buffer Board | eBay

And where exactly do I source all the parts for this circuit?