How to Connect a RIAA Phono Preamplifier
Your stereo amplifier or preamp doesn't have a phono input and you want to connect a turntable to listen to your vinyl records?
Well, you need to add a dedicated RIAA phono preamplifier, which interposes between the turntable and one of the line inputs of your Hi Fi amplifier.
Just connect the turntable to the phono preamp using a pair of RCA signal cables, not forgetting to connect the ground (GND) as well.
The phono preamp will then be connected to the line input of your audio amplifier, using an additional pair of RCA signal cables.
No matter which input of the amplifier you connect the phono preamplifier to (CD, DVD, AUX-IN, TUNER) they are all identical, with the same sonic result.
Some particular models of turntables, it should be noted, do not even need a phono preamp, as they already integrate it inside (this is the case of USB turntables).
Why use a RIAA phono preamplifier?
At first, when 78 rpm vinyl was introduced, recordings were very limited in audio frequency, as the bass range required an excursion in the groove of the record, but this was immediately incompatible with the surface of the record.
At the time, therefore, we were content to obtain a range of frequencies ranging from 150 Hz to 3000 Hz.
Later, between 1925 and 1956, all the major record companies developed a special equalization curve capable of strengthening the higher frequencies and attenuating the lower ones during the recording phase.
An inverse curve is applied to the output of the audio signal from the turntable, to allow correct listening.
Since it was unthinkable that each record company would use its own equalization curve, since each turntable would then have to have different settings in order to correctly reproduce the specific recording, in 1956 the Recording Industry Association of America (RIAA) decided to develop a standard.
The so-called RIAA Equalisation Curve was born, based on the format used at the time by RCA and Victor since 1953 (see Wikipedia).
The quality of a phono preamplifier is assessed precisely by its ability to follow, with maximum precision, the dictates of the RIAA standard.
All in all, the scheme of a RIAA phono preamplifier is quite simple: it is an audio filter built using carefully selected capacitors and resistors, so as to meet the specifications of the standard.
The curve of 6dB per octave corresponds to a first-class filtering circuit (in essence, it is achievable with a single capacitor).
The skill of the manufacturer, therefore, lies in knowing how to carefully select the components using precision resistors and capacitors (i.e. with very low tolerances).
It should be noted that, between 1930 and 1950, the old piezoelectric cartridges used in gramophones already had a natural curve of 6dB (550 to 2134 Hertz).
They only needed a linear preamp (like a microphone) to gain the audio signal.
These piezoelectric cells, however, were anything but Hi Fi and used a pressure between 5g and 10g.
Back to the present day, the turntable stylus has an audio signal output level that is very low: 3mV for the moving magnet (MM) models and 0.3mV for the moving coil (MC) models.
It is therefore necessary to increase the level of the audio signal and bring it to 0.6V, which is the typical value accepted by a standard line input on an audio amplifier or preamplifier.
A further criterion for assessing the quality of a phono preamplifier is its intrinsic noise level.
Working with such low level audio signals requires the obligation to use electronic components also characterized by a very low noise level.
Care must also be taken with the power supply section of the phono preamplifier.
The latter, in fact, must be extremely silent, that is, it must produce a very low level of electrical interference because, inevitably beyond a certain threshold, they would alter the quality of the original audio signal.
In the case that your audio amplifier does not have a phono input
Newer audio amplifiers rarely have a phono input and, if they do, the phono input is of modest quality.
Precision in following the RIAA curve is of fundamental importance and, as explained above, requires the use of components of the highest quality and selection. Apparently, "only" few resistors and few capacitors are needed, but their scrupulous combination determines the goodness of the final result that can be obtained.
The same attention must be paid to the selection of all the active components (transistors, valves, integrated circuits, power supply): they must have a very low noise combined with the ability to maintain a constant gain of the audio signal.
It should be remembered that a good RIAA phono preamplifier must be an excellent preamplifier of audio microphonic signals, characterized by an extremely precise equalization and by a very low noise level.
This explains why often, in the most valuable phono preamplifier models, the power supply is external, just not to create electrical interference to these very weak audio signals.
The chassis, moreover, is often built with particular tricks that allow to isolate as much as possible the microphone effects on the condensers.
There are, however, cheap phono preamps, which are good for listening to a turntable and get good results. These models are suitable if you have an entry-level audio amplifier and the only goal is to be able to hear vinyl records without too much pretension.
Obviously, the most demanding and experienced audiophiles need specific solutions and much more performing from a sonic point of view, which allow uncompromising performance.
Only a Hi Fi phono preamplifier of this type can correctly handle the best turntable cartridges on the Hi Fi market and allows true high fidelity listening.