Help with headphone amp gain

[saurus]

Hello all,

I am building a non-portable headphone amplifier, and I am uncertain about the best way to handle the gain between the stages. My current design is as follows:

1) Input stage (OPA2134, 2X gain)
2) 50K volume pot
3) Gain stage (OPA2134, 3X gain)
4) Crossfeed circuit
5) Output stage (Pavel Macura's OPA2134 + BUF634 design, 1.5X gain)

Power supply is +/-15vdc, it will be hooked up to basic home sources (i.e., CD, tuner, etc),and I want it to drive both 30 and 300 Ohm headphones.

Should all the gain be in one stage, or distributed between them? How is the best way to do this?

Thanks for the help!

 

[lineup]

couldnt you skip the first 2X OP
and use PMA OPA2134 at 3X gain instead of 1.5X

Will also give a toal gain of 9.

How much total voltage gain you need, if used with CD-player as source,
depends on what headphones you will use.

Higher impedance 300/600 Ohms headphones like Sennheiser need a bit more, say 7-9X.
While 32/100 Ohms headphones could do with half, like 4X.
Also depends if sensitivity of phones is very low or high.
Sensitivity is: dB/ mW

Is there any voltage level lost in crossfed, or does it have gain 1X?

 

[saurus]

Eliminate the input stage?

I thought that without the input buffer was needed, because without it the volume control has an effect on the input impedence, and it colors the sound. Is this not true?

 

[lineup]

Re: Eliminate the input stage?

I thought that without the input buffer was needed, because without it the volume control has an effect on the input impedence, and it colors the sound. Is this not true?

Yes, it can effect input impedance and what the OPamp see, but I dont think it would 'colour' the sound.

 

[saurus]

I see. So, should I split the gain between the first opamp stage and the PMA opamp/BUFf634 output stage with 3X each? Or is it better to have more gain in the output stage - is there a difference in distortion/sound quality?

 

[amplifierguru]

Hi Saurus,

A typical 32R headphone has an efficiency of 100 dB/mW. 100 mW will test your eardrums at 120 dB. A simple chip driven amp with gain of 2-3 from a CD drive can test this.

A quality op-amp driving an EF stage can do this easily. If you want to drive a 300-600R headphone to the same level you will need an extra 4 x . The output stage could run at 40mA Class A.

Cheers,
greg