Adding headphone out to 10WPC EL84 PP amp

[kahlfin1k]

So I've done some reading on this and think I have it right but wanted to get a second opinion from all you experts!

It seems a safe way to do this would be to use an L-pad to maintain an 8 ohm load on the OPT and a voltage divider in series with the phones for attenuation. The only thing that confuses me is if I should tailor it exactly to my headphones nominal impedance (Grado SR225i at 32ohms) or a standard 120 ohm output impedance. Advantages/disadvantages? I used this calculator http://www.nu9n.com/tpad-calculator.html. to come to the values in the attached diagram which would give me 20 dB attenuation. I might end up trying different impedances/attenuation but is this concept the right idea? Any reason this is a bad idea or other parts of my amps design to consider such as NFB which comes off the 8 ohm tap also (included in diagram)? Obviously there's a switch in there too. Was thinking a 6 amp mini dual toggle. I know noise can be a large consideration for headphone amps too but this amp has no audible hum or noise so I'm not too worried about that!

Thanks!

 

[Tom Bavis]

That will work fine - and values aren't critical (amp should see around 8 Ohms - but +/-2 Ohms is close enough) . Many headphones don't respond much to source impedance, but some will give a better frequency response and lower distortion with a low value (higher damping factor). There might still be some hum and noise if your headphones are very sensitive - if so, a shunt resistor (across the headphone output will lower the level and raise damping factor.

 

[kahlfin1k]

So I could completely eliminate the 5W input resistor and just change the 3ohm to 8 and then have the output resistor like 200 or so correct? Would the output resistor in series with the phones need to be a high watt resistor too?

 

[BmWr75]

I've built a of of these and they work fine.

http://sound.westhost.com/project100.htm

 

[kahlfin1k]

So impatience got the better of me and I got what they had at Radio Shack which is 2 8ohm 20W non-inductive resistors. so for now I'm going to go with a 200ohm output impedance. Any reason this is a bad idea? Basically eliminated the 5ohm input resistor and made the resistor across the outputs 8 ohm and raised the value of the output resistor to accommodate the desired 20dB attenuation. Is there any reason I need the input resistor?

The attached diagram is how it stands now (well in my head).

 

[kahlfin1k]

So I got it all finished up and by dumb luck the 230 ohm attenuation I used was exactly matched to speaker output level! Dead quiet too! Man, IMO this blows the headphone amp in my AU-517 out of the water with my Grado SR225i's especially the detail and love the sound of the EL84s in the phones now!