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Using the Valve Preamp in a hifi system

Here's how to add a volume control and modify the November 2003 Valve Audio Preamplifier for use with line level signals.

By Jim Rowe

Click for larger image
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" for themselves!

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 provide one.

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