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Old 9th March 2009, 06:10 AM   #1
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Default single tube to drive 300ohm headphones

Hi,
a few questions- could a single 12B4 (per channel) in anode follower connection, drive 300ohm headphones?
how low the output resistance of the headphone amplifier needs to be to be able to drive the headphones?

i am thinking of using 2 pieces of 12b4 and connect them in a classical anode follower connection (one per channel) and maybe also use a ccs in the plate of the tube.
i think that 12b4 will have arround 150-250 ohm output resistance when used that way... is this enough (with the plate current of arround 10-15mA) to drive the headphones like Sennheiser's?
thanks guys
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Old 9th March 2009, 08:16 AM   #2
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It's a good idea....If You use Ia=20-25 ma, Plate resistance decrease.
Go ahead!
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Old 9th March 2009, 08:18 AM   #3
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so - you think that 150ohm output resistance is not to much if i need to drive 300ohm headphones....????
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Old 9th March 2009, 08:38 AM   #4
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Just had a quick look at the datasheet curves which suggest that the internal anode resistance will be too high; in the region of 700 to 800 Ohms at best.

You could (parallel feed) transformer couple it through something like a Hammond 119DA, but then you might not have enough gain because the mu of a 12b4 is only x6.5.

I suggest that you try the 6H30 (mu about 14), ccs load and parrallel feed transformer couple it.

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Old 9th March 2009, 08:58 AM   #5
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Gordy - thanks for the reply,
transformer is not an option. I would have to order it from outside the "delta quadrant" and it is, than, to expensive.
...
i tried srpp calculation with tubeCad software and it turned out that Zout is in between the 140 and 200 ohms. so if i use ccs in the anode and classic anode follower - the output impedance should be a bit lower than srpp i think - right
the qustion is if 150ohm is low enough to drive 300ohm headphones??
any opinions about that??!
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Old 9th March 2009, 09:16 AM   #6
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btw. - i do not have 6n30 but i have some 6n6p instead. they might be an option here - if single stage 12b4a doesn't make the job done???!
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Old 9th March 2009, 09:22 AM   #7
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200 ohm - very good imp, for OTL.....it's OK!
But sound will be better if You use pl. current 25 ma!
/I prefer You - 6S45P, - my OTL ampl. for Zenheizer HD580. Amazing tube!/
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Old 9th March 2009, 09:52 AM   #8
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Oooow!
You need apr. -20 V for 1 grid Your tube....You must put and preampl...
/6S45P needs only -1-2 v...like CD output /.
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Old 9th March 2009, 10:23 AM   #9
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i do not have 6s45p. only 6n6p, 12b4, 5687 that are something like usable (regarding low output impedance).....
so i have to do it with using them.

but i think if i use line level signal of arround 0db and using 12B4A with ccs in the anode - i might get something like 3-6 times gain of the complete stage. maybe i am wrong - but this could be enough together with bellow 200ohms of output impedance?!

i am not sure...

can somebody tell me - how much gain do i really need for driving the 300ohm headphones from line level voltage as input of the headphone amplifier?! and also - what is the output impedance that i should try to get at least (do not tell me - the lower, the better - i allready know that ).
is it 10 times lower than impedance of the headphones or more or less... what to look here ???!
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Old 9th March 2009, 11:16 AM   #10
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2 parallel triodes 6N6P is a good idea . U=apr.140-150 v, Rk=100-300
ohm, You must try, to adj. Ia=20-25 ma., and Ck=1000mkf ELNA,
Ca=100 mkf/450v, Ruotput-ground=15 kohm/for saving Headphones/.
Ra=apr.3-6 kohm, You must try.
/sorry for my bad English/.
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