I wanted a warm, tube-like sound with as few stages as possible. I believe that part of the clarity and directness of a tube amp comes from the fact that there are so few phase invertions compared to a normal solid state (not to mention the different kinds of unpleasant distortions made by op-amps). So I came up with the absolute minimum setup: Two (inverting) gain stages and one source follower/impedance converter. JFETs, because they behave similar to tubes at overload conditions (of course, nothing but a tube really sounds like a tube…)
The EQ Section was originally planned to be like an Ampeg SVT filter. It sounded good, but I could not get rid of the hum (used a small transformer for the filter coil), also it was rather noisy compared to the rest of the preamp. So I had to make a compromise (op-amps!), but this circuit really works fine, and most important, it can be bypassed. In fact I rarely need it, just use the “Ultra High”- and “Ultra-Low”-switches.
The best sound I got with the “Sensitivity” trimpot turned about 3/4 way right. All the way up gave me a little too much dirt, because the first stage clipped and it was no use turning down the “Gain” pot. Of course, this depends on the instrument used.
Fig: Component Layout
Fig : PCB layout
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