On Wed, 02 Mar 2011 16:50:46 -0500, Bitrex
wrote:
On 3/2/2011 4:32 PM, John Larkin wrote:
On Wed, 02 Mar 2011 15:11:37 -0500, Bitrex
wrote:
On 3/2/2011 11:40 AM, John Larkin wrote:
I've always sort of liked the classic "GE" tape head/mic preamp
circuit:
ftp://jjlarkin.lmi.net/GEcircuit.jpg
but it occurred to me that it might also make a nice headphone amp...
ftp://jjlarkin.lmi.net/GE_headphone_amp.JPG
Neat, like a parafeed amplifier. The choke will have to be pretty big
for audio frequencies...
You could just use a power resistor and tweak the drain voltage
operating point. It's just not as elegant.
John
The circuit I'm working on now as my "exercise in simplicity" from the
thread a while back uses another transistor for the load on the output
stage, simulating an inductor. This also has the advantage in Class A
of being able to set the idle current to half of the desired maximum
output current, instead of the usual need to set the idle at the full
output current.
If I remember correctly I think if one uses a resistive load the
quiescent idle current will have to be _more_ than the maximum output
current to get the desired output current into the load when the
transistor shuts down, because of the drop across the resistor.
Yes, you can swing almost all the way to the rails with a
constant-current load, as opposed to a resistor, so it's more
efficient... like an inductor. The inductor allows close to 2x Vcc p-p
voltage swing, if that matters.
But the constant-current load bumps the active parts count by 50% !!!
I think you could do a nice headphone amp with one mosfet. Or go nuts
and use a GaN fet.
John