Project: BJT RIAA Pre-amplifier

To play back the signal recorded on a vinyl record, a RIAA preamp is needed. The low frequencies need to be amplified much more than the high frequencies, as shown in the frequency response plot in the bottom of the page.

The below circuit consists of

  • A two-transistor (NPN and PNP) DC-coupled high-gain amplifier which amplifies the input signal around 12000 times. The high gain is needed achieve the low-frequency gain of 1000 times (60 dB) through feedback.

  • A feedback circuit, tailoring the amplifier to have a gain of 60 dB at low frequencies, 40 dB at 1000 Hz, and 20 dB at high frequencies. The signal level coming from a record player is around 5 mV RMS at 1000 Hz, so this circuit brings the overall voltage level at the output to around 500 mV RMS, corresponding to line level. This is therefore easily played back on line level equipment.

  • A buffer/emitter follower to lower the output impedance, making so even equipment with lower input impedance can be driven. If the input impedance of the next stage is above, say, 47 kohm, the output may be taken directly from the BC557, leaving out the emitter follower.

A voltage regulator has been added to the design to keep a steady supply. This is very important as the amplification at around 50–100 Hz is in the order of 700 times, so millivolts of ripple on the input will be volts of ripple on the output.

The schematic is shown below.

 

I built the circuit into an Altoids tin, as shown below.

 

The construction is done Manhattan-style. The top circuit is the left channel and the bottom circuit is the right channel. Both, of course, follow the schematic above.

 
 

The frequency response of the RIAA preamp is shown below. The measurements show to be in very good agreement with both simulations and the ideal curve.