This is a very quiet preamplifier, but is only suited to low impedance inputs - the noise figure degrades rapidly as the input impedance is increased. The design - in particular the collector current for Q1 - was based on the noise / current / impedance graphs for the Philips version of the BC549 - minor variations are likely with different transistors, or BC549 devices from other manufacturers.
The entire circuit is naturally Class-A, and with a gain of 32dB, has an output impedance of less than 100 Ohms. The recommended load impedance is 22k or greater, so it is quite capable of driving a set of tone controls or a fader. Buffering with a good quality opamp will naturally reduce output impedance (and also increases output drive capability and open-loop gain) as shown in the example in the figure.
The exact same design has also been used as a virtual earth mixer for the mix bus in mixers from 6 to 24 channels. The only change is to remove the 1k2 resistor at the input, and connect the mix bus directly. The optimum impedance must be retained for low noise, so for a 10 channel mixer, each channel should have an output resistance of 12k to 20k to the bus. Fewer channels require lower resistance and vice versa.
This is the figure;
The circuit seems to discourage people because of the single +30V supply. The up figure shows the resistor values needed to run the preamp from a 12V supply, but naturally the output voltage is dramatically reduced before clipping. I have included this version to demonstrate that lower voltage operation is possible, as this seems to be something that people want.
