headphone amplifier = tube + mosfet

[sparkle]

i was thinking ......
i have 300ohm headphones and i would like to use 12B4A tube at the input of the amplifier together with mosfet follower at the output of the amplifier to drive them....
something like this in attachement.....
ccs is borrowed only for diy purposses from Mr. Pass... ccs in the tube section is plane and simple - did not want to use cascode ccs (it would be probably better) but i don't want to complicate things there to much...
opinions are welcome.....
thanks.
daniel

p.s.
0.22uF coupling cap will be bigger - something like 4.7uF and input coupling cap will be arround 1uF....

[Zen Mod]

Originally posted by sparkle
i was thinking ......
i have 300ohm headphones and i would like to use 12B4A tube at the input of the amplifier together with mosfet follower at the output of the amplifier to drive them....
something like this in attachement.....
ccs is borrowed only for diy purposses from Mr. Pass... ccs in the tube section is plane and simple - did not want to use cascode ccs (it would be probably better) but i don't want to complicate things there to much...
opinions are welcome.....
thanks.
daniel

p.s.
0.22uF coupling cap will be bigger - something like 4.7uF and input coupling cap will be arround 1uF....

input cap ?

why?

[JimT]

It may be that I'm just being overly cautious, but as this is a headphone amp, I think you have to make an extra effort to reduce power supply hum. You might want to consider not connecting the 27k/47k bias network directly to the signal line. I would put a capacitor at the 27k/47k juncture and then join this now very stable DC bias voltage to the signal line through a 100k resistor.

Jim

[sparkle]

Zen Mod - you are right - i really don't need that input cap - i thought to put it there only "just in case".... but i really do not need it...

JimT - thanks for the idea.... nice.... you have something like this in Your mind - right

[destroyer X]

Congratulations...when ready post images for us.

regards,

Carlos

[sparkle]

i will do that - it is a slow process of building it (not much time to do that) but it is going forward....

i am thinking if i need that 10uF cap... it seems to me it is big and that i can got with 1uF with no problem - but also i have seen some not so rectangular outputs when trying the schematic with 20Hz step signal at the input.... with 10uF - everything is fine - with 1uF the signal is not very good when using 20Hz... so i am a bit worried to use 1uF only ...

what do you think guys ... should i stick to 1uF between tube and mosfet stage, or should i go for 10uF which is looking far better when using 20Hz step input into the tube???

[sparkle]

i've decided to go with 1uF coupling cap between tube and mosfet stage .... this will be enough.....

[JimT]

Originally posted by sparkle
Zen Mod - you are right - i really don't need that input cap - i thought to put it there only "just in case".... but i really do not need it...

JimT - thanks for the idea.... nice.... you have something like this in Your mind - right

Yes thats it.

I think the 12B4 has a gain of about 6. If thats the case, you will need more volts on the mosfet section to be able to handle all that the tube sends it. On the other hand, unless your headphones are very inefficient, you will not need all of the gain of the tube so things should be fine. My 130 ohm Sennheiser 595's are almost painfully loud with a zero gain headphone amp.

ZenMod can set you right if I have made a mistake.

Jim

[sparkle]

the voltage on the follower is not fixed - i will decide about it the moment i find some suitable xformer in my junkbox....

my intention is to build everything out of the box first and see if i need the tube or not... and than to decide what to do... when i see how much gain i will need and if the amplifier is sounding good, than i will put it inside the box.....

[sonidos]

Weird. Yesterday I was looking at that tube yesterday and 6sn7gt/12sn7gt for a hybrid headphone amp. Lo and behold...