Graham's Blog Archive
A Different Approach to Record EQ: Part 3
Homogenous must be the least-used word in hifi, but as we found in part 2; it describes the type of RIAA EQ used in phono preamps to a tee.
Stanley P. Lipshitz and Walt Jung (both awarded Audio Engineering Society fellowships) have been prolific authors and recognised experts on the subject of RIAA equalisation since the 1970s, and in every article I’ve read from them, I’ve never known them split the cartridge and record responses to deal with them separately. They always treat them as being homogenous.
Lipshitz proposed several complex equations to solve the interaction between different stages of the necessary filtration, and found any number of problems caused by the amplifier stages used with those filters – and suggested a number of solutions. Other authors such as John Linsley-Hood have suggested that splitting the equalisation tasks into separate stages improves the sound, and there has been much debate about the use of active and passive EQ techniques – BUT ALL look at the homogenous curve and not its constituent parts.
I therefore set out a couple of years ago to try and EQ these constituent parts separately. Which to do first – the record or the cartridge? I can say in hindsight that the cartridge is best “EQ’d” first, but that only came about through empirical means (trial and error).
As the points of inflection – the turnover frequencies (sometimes referred to as the time-constants) are at 50Hz, 500Hz and 2122Hz, the cartridge EQ should ideally have its slope such that it does not interfere with them and that it is constant well beyond the audible spectrum. As the cartridge output rises with frequency, we need its EQ to fall with frequency, and at the same but inverted rate.
The easiest way to achieve this is with integrator negative feedback around an amplifier stage so the slope starts from a very low frequency, and preferably at least a decade below the first point of inflection of 50Hz (3180uS).
Theoretically this can be done, but in practice it would lead to a requirement of about 80dB closed loop gain at 5Hz. This does not consider any loop gain essential to reduce distortion, and in my opinion some 40dB of loop gain would be essential.
It would therefore require an amplifier with 120dB gain, and although some op-amps can do this and maybe up to 130dB, they are few and far between, and there are other factors to be considered. The integrator capacitor value is determined by the amplifier’s output current at its slew-rate. The number of op-amps available that can do it all falls to less than a handful. We also have to consider their noise performance, and then we find . . . Well I found none.
A discrete amplifier stage is out of the question. If an op-amp with all the investment a multinational semiconductor manufacturer can afford can’t do it, then I doubt any individual could.
As it often is in the imperfect world of practical electronics, one sometimes has to accept a compromise to see the job through. The compromise I chose was out of necessity, and as the “integrator stage” could not be made to “roll-off” from a decade below the first point of inflection, I actually chose the first point of inflection. So in-effect this compromise does not fully separate the cartridge response from the record response – it can be argued that it is still homogenous, but only including the bass turnover frequency – the one at 50Hz.
As it has been proven, our ears are most sensitive to the middle frequencies, and that’s where the two curves are dealt with separately, the “different approach” is non-homogenous where it matters most. Luckily we’re not as sensitive to what happens around the low frequency turnover. I therefore set the EQ to “flatten” the cartridge EQ and provide the record EQ at 50Hz.
Part 4 will discuss how this proved to be beneficial in other ways.
Other posts on this topic: