listening with headphones?

[K-MAN]

I know allot of people listen to there tube amps with headphones and I do to now that I added a phone jack to the back of my amp and rigged a switch to turn of the speakers when I use the phones. My question is since headphones aren't the same impedance of loudspeakers does this put a strain on the amp? I have no idea what the average impedance is for headphones but I'm sure it's far from 8ohm's. So do any of you see this being a problem and if so how can I fix it?

[erichayes]

Hi K-man,

You're running more risk of damaging your hearing than damaging the amplifier with your set up.

Get a couple of 8Ω 20 watt resistors (Radio Shack #271-120) and a couple of 150Ω ½ watt ewsistors (RS #271-1109). Connect the 20Ω resistors across the 0 and 8Ω speaker terminals on both channels of the amp. Take a 150Ω resistor and connect it from the left 8Ω terminal to the tip terminal of the headphone jack. Take the other 150Ω resistor and connect it from the right 8Ω terminal to the ring terminal of the jack. Finally, connect a wire from either 0Ω speaker terminals to the shield or sleeve terminal of the jack.

If you want to get fancy, you can wire the network (resistors and jack) along with a DPDT switch (275-172) and binding posts (274-661), and mount the whole thing in a project box (270-1805). Use the plastic lid so you don't have to worry about insulating the red binding posts. The center terminals of the switch would go to the amp speaker terminals, one set of outside terminals would go to the hot leads of the network and the remaining outside terminals would go to the red binding posts. The black bining posts and the headphone ground would be wired together and connected to either of the 0Ω speaker terminals

[K-MAN]

Ok I understand the idea of the two 150ohm 1/2watt resistors, I put those in as the manual said to lower the voltage going to the phone jack. But what's the purpose of the two massive 20watt resistors? Is it to put a load on the speaker outputs for when I switch to headphones? and if so wont they cause a problem when I'm normally listening to the speakers? Sorry for all the questions but I'm still learning and this area of electronics has a steep learning curve Yellow_Light_Colorz_PDT_14

[erichayes]

You're learning faster than you give yourself credit for. Yes, the large resistors are to supply a load to the amp when you're using headphones. If you wire the network/switch circuit as I suggested, the load resistors will take the place of the speakers when the switch is in the "headphone" position.

If you want to forego the switching circuit, you can connect the load resistors in parallel with your speakers to the lowest impedance tap you have on your transformers--if you're using original Dynaco Z-565s, that would be the 8Ω tap; Triode and Hammond would be the 4Ω tap. The only problem with this latter arrangement is that the speakers will be working regardless of whether you're listening on 'phones or not.