Buffer headphone amp

[Dxvideo]

Thats the Damian's circuit for LT1210;

But I am confused that whats OPA604 for? Its a kind of POSITIVE FEEDBACK or what? If so he has a big trouble!
Does anybody have any idea?

[endia]

Hi Ozgur,

i have some cmoy, a47, meta42 and sijosae design zen headamp pcbs, plus a few but can't remember their names now.. they all single sided and etched in a small pcb house. i'm not planning to use most of them and of course free for you..

also, i have not an excuse and as soon as joining the forum.

naim

[gfiandy]

The OP604 is eing used as a DC servo so that the design does not need any coupling caps and won't have any bias DC offset on its output.

Regards,
Andy

[Dxvideo]

Hi Naim,
What a nice proposal! Thank you. I will contact you about that matter in next week.
Looking forward for your membership to our club.

And for GFiandy,

Its interesting. I have never had any experience with DC servo circuits. Op-amp choosing is important on this design? I mean does this feedback carry any kind of signal? If its important for just its precision then I can use another more available alternative like TL071?

Thx.

[ybserge]

Theres another way to get rid of DC at the output when working with 1210. One has to change the feedback resistor value from
2K to 1K. However the gain is reduced. In this case some nice sounding previous stage should be added before 1210, possibly an opamp, that OPA604 might happily do.

[Dxvideo]

Is it a rule to put a buffer before headamp? I can run a 80W power LM3886 without any buffer, and it works fine! But I did not see any kind of "nude" and good sounding headamp.
Whats that? Somebody like to make things more complex?

[gfiandy]

The servo opamp carries a very heavly low pass version of the signal. This means that you get minimal interaction from the DC opamp on the main path. However for best sound quality a good opamp here will out perform a cheap one.

The most inportant characterisitic of the DC servo opamp is the input offset. However it will carry signal upto 1K at very low levels this can interact with the main path.

I have also noticed in listening trials that the quality of the low pass capacitor (C4) significantly affects the sound quality of the main path.

Also note that this circuit has significant noise gain if the input to the main opamp is not driven from a low impedance source.

Regards,
Andrew

[Dxvideo]

So if we use the DC servo to cancel input cap. because of the input cap is important on sound quality and the C4 is still critical on sound quality even while DC servo exists:
The DC servo idea is not effective as guessed.
Am I true?

[gfiandy]

I think this is rather over stating the case. When AC coupling the cap is directly in the audio path and can introduce measurable distortion.

The DC servo works to much lower frequency using a much higher quality dielectric than can be used for AC coupling. All I am saying is that the type of capacitor is still significant to the sound quality.

If cost is not a problem I would go with a DC servo as you will get a better result.

If you want a simpler or cheaper solution then AC coupling makes sense.

Regards,
Andrew

[steppler]

I have a small question.
What is better quality please:
Damians LT1210 or SDS Labs HeadAmp ?

Thank You very much.