simple preamp

I have a pretty simple soundsystem. On the one end is a notebook with external soundcard and on the other end is a pair of hefty active speakers (not the kind of toys you buy at the computershop...). In between is a simple preamp mainly used for volume and tone control.

The preamp is designed with simple opamps.

filter section



The design of the filter is based on an article I found in a 'studio sound' of 1981.
The high filter is of the shelving type, filter response is controled by C3 and R13. adjust C3 for a different characteristic. R13 determines the amount of boost/cut, with the given values the boost is some 12dB at 10kHz.

The low filter is of the bandpass type. Boost is 17dB at 35Hz. Lowering R12 to 100K will bring the resonance frequency to 60Hz. R11 determines the amount of boost/cut but also affects the bandwith. The C in the formula for the frequency is C6.

To produce the graphs I set up a spice model of the ciruit, when I put the circuit together I had to actually measure the whole thing..

treble control cut

treble control boost
bass control cut
bass control boost

to gain or not to gain

When dealing with computer soundcards there is little need for amplification, the standard output level being 2V. Replace R5 by a log pot and you're all set.

In the original design I gave U1A 10 dB of gain, 300mV output level then being the norm.
If you are a noise freak you could put a low value (say 2k2) volume pot at the output.

balanced in

The balanced input section from the studio sound article


output section

Originally this was meant as a distribution amp. Alternatively there is an output for my 400 Ohm DT100. I paralleled two 5532 opamps to achieve that.


preamp with iboy on top.