Graham's Blog Archive
A Different Approach to Record EQ: Part 2
Part 1 might have ruffled a few feathers . . .
I explained we’d ended up with RIAA record equalisation to marry-up the front end and the back end. The front end being the record cutter, with its linearization problems and the driver amplifier’s dynamic range limits. The back end being the playback cartridge, a piezoelectric device whose otherwise flat response was marred by top-end resonance and low-end bass droop.
You see, most explanations are completely different to mine. They show you a graph demonstrating that the recording curve is rising in frequency – with a kink in the curve in the mid frequencies; and a graph demonstrating a falling curve – the inverse of the recording curve – to show how the playback cartridge needs to be equalised for it to sound right.
You get all sorts of explanations as why this has to be: to keep the bass modulations within the groove width; to apply pre-emphasis so that on playback noise is reduced – and so on. Although, in a way, these seem plausible, they are not the reason, so how did they come about?
The answer is the magnetic cartridge. Moving magnet, moving coil or moving iron; each one is magnetic and has one main common characteristic. That characteristic is in the magnetic cartridge’s frequency response. It works like a car’s alternator: without any regulation, the output of an alternator rises the faster it turns. In a magnetic cartridge the output voltage increases with frequency. A magnetic cartridge is a generator just like the alternator. As frequency rises the speed of the interaction between coil and magnet increases, and therefore the output voltage increases.
The great thing about the magnetic cartridge is that its output can be considered to be otherwise “flat”. OK, yes, the output is rising so can’t be flat, but it’s at a constant rate – it is therefore LINEAR. It does not suffer the flagging bass and the peaking in the treble you get from a piezoelectric crystal.
Because of this linearity the output of a magnetic cartridge is given a name, and that name is “constant velocity”. That simply means that volts rise in proportion to frequency. On a V/F plot you would see a straight line graph starting at zero and rising at an angle of 45 degrees.
Now for the interesting bit! Let’s superimpose the RIAA recording curve onto the above imaginary V/F plot, and we see this…
And that curve is what people mistakenly believe to be the RIAA playback curve. It isn’t! What it is is the combination of the record’s curve and the magnetic cartridge’s constant velocity response.
Because of that “belief” the two become homogenous (become as one), and therefore are then treated as homogenous in playback. All playback “technology” – the understanding of how to EQ the playback signal so that it produces a “flat” output having the right tonal balance – is homogenous! All phono preamps – because everybody now uses magnetic cartridges* – treat the signal as being homogenous.
See you over at part three.
(* crystal cartridges are still used on low-cost “retro” record players, but this article is intended for the high fidelity user and as such only magnetic cartridges would be used)
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