Potentiometer output impedance vs headphone impedance

[Matt Syson]

Hi
There are many things 'wrong' with this scenario but essentially you need to ensure that the output circuit will be happy 'driving' the 55 Ohms of your headphones AND the pot.
Now, if you want the pot to have a useable 'law' then it should only be a few times greater resistance than the headphones (say use a 100 or 200 Ohm pot).
Using a pot with much higher resistance may appear a good idea in terms of 'loading' the amplifier output (where less is good) BUT when using it you will find that you won't hear much until the pot is turned up to 9 or 10 (if you imagine a scale of 0 - 10).
The important figure to find is the rated MINIMUM load for the headphone socket. With the values suggested, if it is OK with 30 Ohms or greater then you will be fine.
If you think of it like ratings on a HiFi amp, it will say something like '8 Ohms nominal, safe (or minimum) to 4 Ohms'.
Matt S

[JeremyS]

So I guess I'm on the right track with the one I have.
I've got a 250 ohm / 1 watt logarithmic pot

If I understand correctly, you're saying is that there won't be a loss of sound quality because of impedance as long as (headphone + potentiometer) are in 'acceptable' range for the driving device.
I.e
you don't really measure the output impedance of the pot to the phones separately, they kind of just act as one entity ?

[Matt Syson]

Hi
You could reasonably argue that having ANY pot is 'harming sound quality' as ideally the output of the headphone amp would go direct to the headphones so providing a 'constant' loading situation (it is not altered by adjustment of the volume).
However, with a pot of around 250 Ohms that you have, it is the 'next best thing' so without a complete redesign of your setup it is the way to go.
The impedance 'seen' by the headphones will alter with level setting and this is unavoidable. It is not as grave as attempting to do this with a loudspeaker where you would definately hear changes to response.
There is no such thing as 'sonic purity' as it is ALL a matter of subjectivity and personal taste so just get over that and stop worrying about it.
Matt S

[dualflip]

Why not just using a buffer?

[JeremyS]

Ern... I don't know much about them.
Can I get one that's small enough (almost in-line) How much do they cost?