Adding a headphone jack to a valve amp.

[Diabolical Artificer]

I built a simple SE 3W 6V6 valve amp & want to add a headphone jack as I intend to sell it & it adds a feature. The OPT's have a 600r tap, so the plan is to use a stereo jack that has two DPST SW's internal to cut OP to the speaker jacks and re-route to the jack. In front of the jack will go a little DPST toggle SW to SW between low & 600r/hi ohm phones.

To make sure there's always a load present I am going to solder two 1k R's across the speaker terminals but what about dropping level? Would two 100r per channel in series with the phone jack be about right? Would these same 100r dropping R's drop too much signal with 600r/hi ohm phones? Problem is I have no phones of whatever description.

The amp can also be SW'd between pentode and triode mode, enough features for anyone. Pentode max OP is about 2.5W, triode max OP is less than a watt.

Andy.

[Lucien Nunes]

You only need a few milliwatts to drive a pair of phones! And it's not just about level - the source impedance affects the frequency response.

AFAIR IEC 61938 recommends a source impedance of 120Ω to drive any phones regardless of their impedance and historically many phones were designed to give a correct frequency response with this source. But more recently I believe a low impedance source has become the accepted norm for typical consumer 32Ω phones and some people prefer the response that way, with higher damping. There is no correct answer these days; impedance, sensitivity and Q all vary widely amongst popular designs.

To deliver say 5mW into 32Ω phones with a 120Ω source resistor (and that is enough even for lower-senstivity phones) would take just 1.9V swing at the amplifier output. 1W into 8Ω from your low impedance secondary is 2.8V so not a million miles off. You might want to load the amplifier more not just to protect the output transformer but to move the load impedance nearer to the optimum point for distortion.

Conversely, the 600Ω winding would be delivering up to 39 volts at 2.5W which would require an L pad not just a series resistor. I would stick with the Lo-Z output particularly if you want to go for the lower-than-120Ω-source sound.

[snowman_al]

You do not say if this is mono or stereo, hifi or or workshop amp.

Any way, I think you are making it f a r t o o complicated. You can satisfy some of the people some of the time, the more you 'offer' the more likely you are going to fall into a gap.

I agree with Lucien, just focus on the standard low impedance phones (8 - 32Ω). There are too many variables even then...

[paulsherwin]

Are you sure there's any real demand for this? Anyone keen enough to use a valve amp for HiFi is likely to want a separate dedicated headphone amp if they like to listen that way.

[GrimJosef]

As far as demand goes, there is a 'bottom end' to the valve hi-fi market (i.e. the amp costs a few hundred pounds, not closer to a thousand) where the owner might well want it to drive headphones. They're commonly used for late-night listening or for listening loud when you've got party walls/floors/celings with your neighbours. I can see a headphone socket being popular among those customers. Some more expensive amps offer them too but I suspect they make less of a difference to buyers as the price rises.

Cheers,

GJ

[Diabolical Artificer]

Thanks Lucian, I forgot about the impedance/frequency issue.

Any way, I think you are making it f a r t o o complicated

Your right,fair point.

Are you sure there's any real demand for this?

No, just a mad (in retrospect) idea my over imaginative brain had the other day : )

As far as demand goes, there is a 'bottom end' to the valve hi-fi market

Yep, the amp isn't worth much and I wouldn't be daft enough to ask a lot for it.

I can see I'm going to have to get some head phones and test this out, winging it is no good.

Thanks all, Andy.

[joebog1]

I have been caught with this Andy. I built a dedicated headphone amp with enough grunt to drive speakers quietly. The person who ordered it had wooden headphones.
( in BOTH senses of the word ).
I used a fat double triode in pushpull with cathode feed to the specially designed and made output transformers, with 15 watts worth of iron in them. Amp made about 3 watts into 32 ohms.
WELL. the amp was a failure !! I didnt have any wooden headphones laying about, and didnt have the money to buy some ( audiphool pricing ). If you want to drive headphones, as already suggested, make a dedicated headphone amp. AND !!!! dont forget the KISS priciple.

Joe

[Diabolical Artificer]

Yes, I think I'll knock the idea on the head Joe. Wooden headphones? May as well strap a couple of bookshelf speakers to one's head.

[joebog1]

https://duckduckgo.com/?q=Hi-Fi+qual...ages&ia=images

Just to tease you a bit Andy!!!. NOTE the 22 lbs weight !!!! of a couple of them.

Joe