12AX7 Phono Preamp Example

Shown above is a complete 12AX7-based phono preamp that uses passive equalization. ( 12AX7 Passive Equalization Preamp.ckt ver 4.2) ( 12AX7 Passive Equalization Preamp.ckt ver 2000)The gain at 1kHz is +49dB and the output is close to flat from 20 to 20kHz, as shown below.

The topmost graph above shows +20dB divisions; the bottom graph, 0.1dB divisions. The plotted “Gain” line is custom defined in the “Edit Signal” dialog box. Here we specify that Gain equals the signal leaving the pre-emphasis transfer function divided by the input signal’s magnitude and then this result has 20dB subtracted from it to reference Gain to 1kHz.

Why not just plot and display the raw output signal instead? We could, of course, but then the output would not be flat, as it follows the RIAA equalization curve, which brings back to the problem of frequency/amplitude lookup tables. Furthermore, the graph displays the Y-axis in relation to a 1-volt output signal, and as the input signal was only 0.001Vac, the graphing would fool many into believing that the preamp had no gain. For example, in the graph below (taken from the second voltage meter’s output), the gain at 1kHz appears to be –11dB down, which it is relative to 1Vac, but not the actual 1mV input signal.  (The old pros would quickly work out in their heads that 0.001 volts is –60dB relative to 1 volt and they would subtract –60dB from the displayed –11dBs of gain at 1kHz to get the final gain of +49dB; for the rest of us, the defined plot method is best.)