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I finally completed a proof of concept for a really snarky-looking headphone amplifier tonight. This idea has been rattling around in my head for more than a year, but other projects have delayed it until now. I'm using the 6SL7/6SN7 combo in the "Frequency Compensation" thread below to drive a pre-WWII dual triode. The triode includes two "mouse-ear" anode connections on top, and they're both grounded to the chassis behind the tube. That means you can walk over to the amplifier while it's playing music and actually grab hold of the anodes. No shock, no buzz, nothing. Really, it's crazy. Output power is about 0.6W, plenty for phones. Can't wait to get both channels done and rebuild it in a real chassis. Wow, this is just too much fun! I gotta go touch it again...
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Buy Chinese. Bury freedom.
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I did a PP amp with these many years ago
Al
I don't want to identify the exact tube type until I'm sure I have enough. I've seen prices for this or that tube really jump after someone on a forum like this one started talking them up. Anyway, I don't know yet if this design will prove entirely successful. Frequency response isn't what I was hoping for, and I need to determine the cause. My first thought has to do with filament-cathode capacitance at the output stage (it's a follower), but it could be something else entirely. I'll know more after a little more work this evening.
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Buy Chinese. Bury freedom.
This turned out to be the output transformer. It's an inexpensive 5K:8 that I've frequently used for prototyping small single ended pentode designs. Its shortcomings are more apparent in an all triode amplifier like this with no NFB. I've minimized the response problem now by loading it differently. Originally, with an 8 ohm load, response was down about 3.5 dB at 20 kHz. With a 16 ohm load, it's only down about 1.5 dB. Output power doesn't suffer drastically with this change, about 0.65W VS 0.50W. Next step is to test with music, then check harmonics. :)
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Buy Chinese. Bury freedom.
> > My first thought has to do with filament-cathode capacitance at the output stage (it's a follower) < <
Is it really a follower? If the anode is at ground potential while the cathode at a negative one, it would not be a follower if the input is referenced to ground. For all intents and purposes this would be a grounded cathode stage, only turned upside down - or so I think.
" If the anode is at ground potential while the cathode at a negative one, it would not be a follower if the input is referenced to ground."
The primary of the output transformer is in series with the cathode. The grid is referenced to the bottom of that primary. The anode is grounded for AC. So, it's a follower, but with a twist - the anode is also grounded for DC. That renders the exposed anodes safe for my livingroom. :)
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Buy Chinese. Bury freedom.
> > The grid is referenced to the bottom of that primary < <
Then yes, it's a follower. I thought that you had actually referenced the grid to ground, hence my comment.
Wow perfect, mouse eared tube for a pair of mouse ears.
twystd
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