Fig.1: the frequency response features a slight rise in the low bass region and is just -1dB down at 180kHz.
The 12AX7 valve audio pre-amplifier in the November 2003 issue
was "the project that we swore we would never do". This may have been a tad
embarrassing but the project has proved to be surprisingly popular. It looks
like quite a few more people than we expected did want to try out "valve sound"
The November 2003 design was intended for use mainly with
electric guitars and musical instruments, which is why we gave it a gain of
about 60 times. But not long after the November issue appeared, we started to
get letters and emails from people wanting to use two or more of the preamps
with their hifi sound systems. They wanted to know how to adapt the basic preamp
design for this kind of application.
As it stands, the original design has far more gain than is
necessary and would be seriously overloaded by the signals from a CD player,
tuner, cassette deck or whatever. To make it suitable for these "line level"
signals, we need to lower the overall gain to about four times. As well, we
needed to show how to fit a volume control, as the original preamp didn't