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It is currently 20 Jul 2018, 02:07

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 Post subject: 12J8 Headphone Amplifier
PostPosted: 09 Jul 2018, 22:45 
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Joined: 28 Dec 2010, 22:07
Posts: 316
Hi all,

I was playing with a 12J8 tube before and found that it did okay. I was using those little 1K:8 audio transformers that Radio Shack used to sell, which are capable of a few hundred mW into a speaker. Not ground-breaking, but decent for headphones. It's altogether possible that a larger transformer could provide better fidelity and soforth.

https://frank.pocnet.net/sheets/127/1/12J8.pdf

Anywho, I am proposing a headphone amplifier that will use these as output tubes. I would like to implement some kind of gain stage prior to the tetrode so I can effect some negative feedback. That might make the tiny transformers more willing to perform well. I do remember that the sound was kind of rich in the highs and I could mellow it out by placing a 100nF capacitor from the plate to ground. I might look for a 12U7 (which I'm convinced is a 12AU7 proven for low-voltage) or I might use one of my clear-top RCAs.

Either way, I will provide more information when there is more information. I'd like to make this one exceptionally hum and noise free, something I haven't achieved with other tube amps I've made. The last headphone amplifier I built using tubes was a 12AU7/IRF510 model. IIRC, the gain of a 12AU7 is something like 5 at 12 volts if the next stage is a high-impedance load like a MOSFET or even an LM386.

I'd consider getting better transformers, but I also have no interest in getting Edcor transformers that will cost at least $50-60 for two models (when shipping is factored in) and will take months. Maybe some feedback from the experts here could be helpful.

Ed

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PostPosted: 10 Jul 2018, 06:55 
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Joined: 19 May 2011, 05:38
Posts: 26
Those transformers bottomed out at about 300cps. they wee made for transistor radios. you might try anything else. Stay away from communication transformers (telephone duty)


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PostPosted: 10 Jul 2018, 09:15 
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Joined: 28 Dec 2010, 22:07
Posts: 316
When I get back to my work bench, it might be good to run a test and plot out a frequency response curve for the amplifier. What you're saying makes sense, because the sound was very bright and airy when I wired it up. It would be okay for a radio receiver (especially with some audio filtering) but for a "hi-fi headphone amplifier" I'd want to use something else. It's also possible that I could drive a FET or a transistor with 12J8, as that is what they were made to do in the first place.

I also have one or two pieces of 70v, 10w PA speaker transformers. I could wire one of those to see what the frequency response curve would be. I have used one of those in the past with 12AU7 and 24 volts and it worked well. What I don't want to do is corner myself into buying $25-30 worth of transformers for an amplifier that will put out 20 mW. I still like the idea of using no semiconductor parts in the amplifier. If I decide to use the extra diodes in the 12J8 to make a level indicator, I may not be able to get around that though.

Ed

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