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Old 5th June 2011, 04:11 PM   #1
mltube is offline mltube  United States
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Join Date: Feb 2010
Location: Dark side of the Moon
Lightbulb Nice OPT's for headphone amp...

I have recently aquired these for cheap cheap... Looking for design ideas, Thinking... Thinking... Thinking....
These are 32k pri. ptp @ 35ma with 12ohm sec. 4watt

I have built many tube audio amps with great success, however this will be a first for headphones.

The idea that's stewing in my mind is a 6sn7, 6bh7, or 6350 pp output, but this is a new chapter of learning and was hoping for suggestions for the front stage.

I do have a matched pair of NOS mullard 7247's, (pre and phase inverter in one envelope) but concerned if it would be to much gain for this application. I will probably try them anyway... Or would self-split be better here?
((and no I haven't a circuit drawn up yet, it's all in my head still.) Most of the time I build off the top of my head, get things in spec then adjust for sound preference.)

My primary input devices for this will be Reel to Reel, and turntable, but may try with cdp in the future.

I can build and tweak this untill its DIY perfect to my ears.

All that I ask is other possible tube selection or circuit ideas and or constructive criticizm.

Thank you for any helpful thoughts, (this should be fun )

Michael
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Last edited by mltube; 5th June 2011 at 04:39 PM.
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Old 7th June 2011, 12:35 AM   #2
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Join Date: Aug 2009
Location: Sacramento
Hello,
Those are cute little transformers. Building headphone amplifiers is fun!
I think of transformers as impedance matching devices that are turn ratio dependent. Like a transmission matches the engine RPM and torque to the wheel speed of the car an output transformer matches the plate voltage and current at the plate of the tube to the requirements of the headphones. The impedance ratio of the transformer perhaps is on the steep side. The load line will be very flat.
If you connect 36 ohm headphones to the transformer secondary the primary impedance reflected to the plate of the tubes will be 96K. A resistor in parallel with the headphones may help.
I am thinking that you may be happier with a tube with a lower plate resistance like a 6BX7 and a transformer more in the range of 6K to 30 ohms. The load line will match the headphones to the plate resistance and the extension high frequencies will be much improved.
DT
All just for fun!
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Old 7th June 2011, 04:15 PM   #3
mltube is offline mltube  United States
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Join Date: Feb 2010
Location: Dark side of the Moon
Default The cute little xfmrs....

Quote:
Originally Posted by DualTriode View Post
Hello,
Those are cute little transformers. Building headphone amplifiers is fun!
I think of transformers as impedance matching devices that are turn ratio dependent. Like a transmission matches the engine RPM and torque to the wheel speed of the car an output transformer matches the plate voltage and current at the plate of the tube to the requirements of the headphones. The impedance ratio of the transformer perhaps is on the steep side. The load line will be very flat.
If you connect 36 ohm headphones to the transformer secondary the primary impedance reflected to the plate of the tubes will be 96K. A resistor in parallel with the headphones may help.
I am thinking that you may be happier with a tube with a lower plate resistance like a 6BX7 and a transformer more in the range of 6K to 30 ohms. The load line will match the headphones to the plate resistance and the extension high frequencies will be much improved.
DT
All just for fun!
Thank you for responding!
All the points that you have mentioned make good sense.

The only reason that I have started with these is that they were only $1.50 and they test good, so what the heck.... right?

Just for fun, and not "hi-end" use... I have been looking through the tube pile, there are hundreds of nos old tubes that can be run with batteries, and then couple to a mediocre set of low inpedence headphones.

No matter what it will sound better than the sand state portable junk produced today.

I will put somthing togather here in the next few days and post the results with pics.

Thank you again
Michael
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