[Brent Hutto]

Is there a back-of-envelope way to ballpark the proper gain for a headphone amp?

I know it depends on input voltage, is "line level" always pretty much that same? By that I mean will a home CD player and a portable CD player and a portable MP3 player line-out generally be in the same ballpark of voltage and output impedence?

Assuming you know the input, I guess the other factors are headphone impedence and sensitivity. So if you have 50-ohm a headphone that yields 90dB for 1mW, what would you set the overall voltage gain on the amp in order to avoid having to run it in the bottom 1/4 of the volume pot range all the time?

 

[Garbz]

I think there are possibly too many variables. The line levels are not all the same, output impednaces also are not all the same (especially for tube based sources).

There's not only the sensitivity and impednace of the headphones, but also how loud you like listening, and the environment you listen in. I.e. train listeners would listen at louder volumes then late night listeners.

Ballpark most people start with 11 gain and work from there.

 

[Dougigs]

...unless you're using Grados (or other very low-impedance cans), in which case you start with gain of 4 and work from there. Sometimes downward.

 

[Brent Hutto]

Yes, that's about what I figured. Too many unknowns.

It just seems in reading anecdotal comments from people who buy amplifiers it's a common problem to have too much gain and only use the low end (with dodgy channel level matching) of the volume pot. I guess the ideal off the shelf amp would have a dip switch or jumper giving at least two possible gains from the same amp.

 

[Dougigs]

Sometimes the solution is to change the volume pot and leave the gain as it is. 50K is not always an ideal value. If it's low-scaling, try paralleling a 50K resistor across the outer legs (on each side), thus a) turning it into a 25K pot that may be better suited to your needs, and b) often actually improving both the scaling of the pot and giving it a more stable impedance.

(Conversely, a 100K pot is not to be sneezed at -- it's the standard value for vacuum-state circuits, and I use 250Ks a lot too).

I don't think this needs to seem like rocket surgery. Choose a gain that's high-ish for high-impedance loads, or lower for low-impedance loads. If you're blasting your head off at the bottom of the dial, you can either tweak your pot or change your main gain-setting resistor (halving it is good), or both.

Or, hell, stick a larger resistor in series. Same effect.

 

[nikongod]

i personally like a gain of 5 or 6 as a starting point.

after you get the first amp or 2 belted out, its just a mater of transfering the gain you already have.

an EXCELENT reason to start with a c-moy, with only 2 resistors to swap, it takes only a miniute to make gain changes. then you can build a (insert whatever you like) at a good gain for you and your headphones and source(s) on the first try.