Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shaun on December 15, 2021, 06:26:40 pm
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Hi All.
So, I built an amp. SE stereo, designed for audio. It came out surprisingly well, considering I'm still a relative novice. While I'm sure there is a lot that could be improved, it sounds better than I had any right to expect. 6V6's powering full-range neodymium Dayton Audio spkrs. I've done a lot of research regarding the addition of a headphone output jack, with mixed results.
I'm assuming I would use a stereo switching jack. For the jack to be located on the front control panel, I'd like to run from the balance pot (or other handy preamp location) to the headphone jack (obviously some added circuitry will be needed).
I know there is impedance balancing to be taken into account; I want to create something that would work with "standard" headphones, which I know covers quite a range. I guess I could add an impedance switch to cover options from 32-300 ohms. But I believe a range somewhere around 32-100ohms should be possible without an impedance switch (I think).
Apparently, according to the wizard who lives in my computer, "typical" headphone amplifiers output 1-5mW into 16 ohms. I have 12AT7's as stereo drivers, with the balance pot right after them. 12AT7s have good current output (as far as I understand it, that's a good thing in this instance). I'm also fairly sure the 12AT7s are putting out considerably more than a few mWs, so I may be barking up the wrong tree in wanting to take a line off the preamp section. I also seem to think that a 12AT7 has an output impedance of around 10k, which means a transformer might be needed to reduce that impedance down to approx. 1/8th of what the HP impedance would be - like 16ohms. Still, I can dream, cain't I?
The preamp seems like a good place to divert the signal connection, rather than running a line from the speaker outputs, because it would mean not having to use a dummy load and/or disconnecting the speaker outputs at the OT transformers. It also means that I would not have to run an output line all the way from the back of the amp to the front. (I guess I could put the HP jack on the rear panel, but that means removing the back panel for access. I could simply get rid of the back panel, but it still means accessing the back panel each time I want to use headphones.)
I suspect I might get a better headphone sound straight off the preamp (based solely on Spidey-sense).
It would seem possible to employ a simple voltage divider and and/or added resistance on the signal-out in order to control voltage at the jack. But I am inexperienced in matching the correct impedances.
Anyway, enough said. Have a good laugh at my folly if you must. I will probably still learn more here than anywhere else online. As always, this is the best forum ever, and thanks for any help.
Shaun