As promised 2 days ago by someone wanting a line level preamp and buffer

with gain about 4 times, I have just posted 5 simple line level
schematics at
alt.binaries.schematics.electronic

1,2&3 are inverting with shunt FB applied to reduce the gain of a 6SN7
from about 15 to 4.

4&5 are non inverting preamps with gain nearly 4.

Note that values of all cathode bias resistors should be varied to
achieve the
DC voltages at the anodes and cathodes as shown and the DC currents.

I have not included a pair of cascaded common cathode
gain low µ triodes such as 12AU7 or 6H30 with series voltage NFB.

A pair of 12AU7 halves could be set up with unbypassed cathodes
to make two stages of gain of about 7 each for a total of 49,
then NFB can be applied via a resistor from the second triode anode to
the first triode
cathode, thus reducing gain from 49 to about 10, which is 14 dB of NFB,
and thd will be low and Ro will be lowish,
but perhaps at the expense of musicality etc and the subjective sound
quality.

The technical bother with this apprach is that the feedback resistor
from second anose to first cathode
has to be around 27k and it becomes a load which is too low
for the second triode.
The golden rule for signal preamps is that any NFB used must not
load down the output or any other tube.

The other alternative is a long tail pair of triodes driving a third SE
gain stage with
the NFB from the output anode back to the second of the grids of the
LTP, the first front one used for an input voltage.

But all the 3 triode line stages have slightly strayed from the
simple idea.

Ppl who like a the sound and technical function
of a lot of current in the output device may
find paralleling two halves of a 6SN7, 6CG7, 6DJ8, or using say an EL95,
EL84, EL86
will make a better output tube.
EL95 is a small power pentode used in european radios, and maybe a lot
are still around,
if not 6AR5 and other 7 pin small power pentodes make excellent
triode preamp tubes.

The EL84 or EL86 with 15 mA of current
and in triode gives gains of up to 18 and 12 respectively
with a bypassed Rk and Ra of 2.2k and 1.4k respectively.
So witjh a simple shunt feedback circuit the Ro from the anode
can be very low indeed.

I have 12AU7 CF output buffers in my own preamp and these
are single triode, about 4 mA of idle current with a CCS cathode sink
to each CF.
It can thus supply about 20vrms into 20k easily,
or up to 14 vrms into 5k RL.

I use maybe only 0.1 vrms into my power amps, and
the CF manages very well and sounds very well.

More versions of preamps are shown at my website
http://www.turneraudio.com.au.

I am about to upgrade the site to include more info.

Patrick Turner.

"Patrick Turner" wrote in message
...
As promised 2 days ago by someone wanting a line level preamp and buffer
with gain about 4 times, I have just posted 5 simple line level
schematics at
alt.binaries.schematics.electronic

[snip]

Thanks for the info.
I'm so obliged now I'm gonna send yoµ foc a dµtch pencil enabling yoµ to
scribble µ on paper.
Alt 230 keys inclµded :-)

Rgds,
Jan.