Hi!
I have now designed a headphones amplifier to be used as PA driver also.
The amplifier is as usual a hybrid. It uses one section of a twin triode (ECC82/12AU7) along with a BJT (BC639 preliminary) per channel.
It also uses a cossor (I almost said
) and AC-heating.
The use of a BJT as emitter follower is due to the fact of low(er) output impedance.
Actually for a small-signal BJT output transconductance may be written:
gm=Ic/25 S, where Ic is in milliamps.
With regard to available transformers in my concept I had to lower the Ic to some 18mA which then should give an output impedance of slightly over 1 Ohm.
You should be aware that tubes seldom have transconductances, gm, over 10 mS which means that their output impedance is in the order of 100 Ohm. At best.
And to build a successful tube-based HP-amplifier you would need some seriously special configurations.
So I am just simplifying stuff. This while I think a emitter-follower does not contribute to sound quality so much.
The intention of this amplifier is to boost line-level (what is line-level, actually?) to PA level (1,5V in my case). This while having the feature of being able to select one of two sources as well as heaphones drive when wanted.
Best regards, Roger
I have now designed a headphones amplifier to be used as PA driver also.
The amplifier is as usual a hybrid. It uses one section of a twin triode (ECC82/12AU7) along with a BJT (BC639 preliminary) per channel.
It also uses a cossor (I almost said
) and AC-heating.The use of a BJT as emitter follower is due to the fact of low(er) output impedance.
Actually for a small-signal BJT output transconductance may be written:
gm=Ic/25 S, where Ic is in milliamps.
With regard to available transformers in my concept I had to lower the Ic to some 18mA which then should give an output impedance of slightly over 1 Ohm.
You should be aware that tubes seldom have transconductances, gm, over 10 mS which means that their output impedance is in the order of 100 Ohm. At best.
And to build a successful tube-based HP-amplifier you would need some seriously special configurations.
So I am just simplifying stuff. This while I think a emitter-follower does not contribute to sound quality so much.
The intention of this amplifier is to boost line-level (what is line-level, actually?) to PA level (1,5V in my case). This while having the feature of being able to select one of two sources as well as heaphones drive when wanted.
Best regards, Roger
