Bass compensation is therefore needed during playback. Secondly, records produce a certain amount of hiss which is covered up in post-production by boosting the gain of the high frequencies before pressing. Counter-EQing during playback compensates for this. Finally, magnetic cartridges produce a weak signal which must be boosted to match the rest of the amplification and this too is done during playback. In the mid-1950s, compensation standards were established by the Recording Industry Association of America.
The resulting RIAA preamp has been built into every hi-fi and stereo amplifier with phono or turntable inputs since then. A separate RIAA preamp is necessary when you are connecting a turntable to a mixer which does not have one built in.
The phono preamplifier is the key component needed to match the output of the turntable to the line input of the amplifier or audio mixer. If you’ve tried connecting a turntable to a modern amp or hi-fi unit and you’re not happy with the results, it’s probably because you don’t have a suitable turntable input.
Manufacturers tend not to include a turntable input anymore because the vast majority of customers don’t play vinyl. However, you can still obtain turntables and magnetic pick-up cartridges. The signal from these pick-ups is much smaller than from a CD, tape deck, radio tuner or auxiliary source – so if you connect to any of those inputs you’ll not have much volume. Also the tone will be totally wrong – all treble and no bass. What you need then is a phono preamplifier, it connects between turntable and amplifier or hi-fi unit. It lifts the signal to the required level and restores the tonal balance.
Retrieving the signal from a record groove takes carefully balanced engineering to restore the sound back to the original so it can be applied to the input of an amplifier or audio mixer.
All gramophone records are cut with falling bass (LF) and rising treble (HF) below and above some mid-range frequency (MF). It’s this trick that makes it possible to keep the groove a constant width.
It is a phono preamplifier’s job (otherwise known as gramophone amplifiers, turntable pre-amps or vinyl disc equalisers) to precisely perform the opposite effect to that of the record cutting machine. This is called equalisation (EQ for short) which boosts the bass and cuts the treble by just the right amount. It also has to amplify the tiny signal from the pick-up cartridge, with minimal noise and distortion, so it’s big enough to drive an amplifier.
