Pre-Amplifier Pre-Amplifier for Phono & Mic

Pre-Amplifier  Pre-Amplifier for Phono & Mic

“Why can’t I just use a good mic pre?” you might ask. Well, a little history is in order.

In the golden era of recording, grooves were cut into a disc by a lathe. The grooves physically represented the waveform that was recorded.

Playback is achieved by having a needle (connected to a magnetic pickup) riding in the groove. The side-to-side variations of the groove move the needle and magnetic pickup. The electrical output from the pickup is our audio signal, just waiting to be amplified.

To get a certain amount of time on a 12" disc that is spinning at 33-1/3 RPM, the grooves could only be cut so big. If you varied the groove too much it would cut over into an adjacent groove, destroying it. This is where mastering engineers really earned their pay!

There was still one problem to overcome. As frequency increases, the amplitude of the waveform required to reproduce the same sound level drops. Therefore, low frequencies need a bigger groove than high frequencies. That also means that high frequencies would get lost in the noise floor as their groove size got smaller.

To overcome this effect, pre-emphasis, an equalization curve, was applied to the signal. This artificially increased the size of the higher frequency wiggles in the record to keep them detectable above the noise floor. To recover the audio properly, the preamp used on the magnetic pickup must “de-emphasize” the boost in the highs.

The specification for the de-emphasis is that the frequency response should start to fall off between 50 Hz and 500 Hz and then again around 2.1 kHz. The actual specificatio ns are in time constants (obtained by multiplying the appropriate resistance and capacitance) and are 3180, 318, and 75 microseconds.