Graham's Blog Archive

A Different Approach to Record EQ: Part 5 (final part)

Posted by Graham
August 17th, 2015

To recap: I’m equalising the cartridge and then equalising the record (with a very slight compromise). It would seem that all existing techniques of equalisation treat the cartridge response and record response as one and the same – homogenously!

Why bother? If you were able to ask the likes of the late John Linsley-Hood why he bothered to do the EQ his way, he would have told you “because it sounds better!” After all, that’s what he wrote in a number of his published projects. In my opinion I’m doing it the way I am because it sounds better too!

So what makes me think it sounds better? Firstly (and in my opinion) it achieves something I only ever found using a particularly good crystal cartridge. This cartridge used lead zirconate titanate crystals (sadly because of the lead content these can no longer be legally sold as new and replacements are sodium potassium tartrate tetrahydrate) and had a response almost as good as a moving magnet cartridge.

Record noise using that cartridge always seemed more distant and less intrusive, and the music seemed to flood out with great extension, authority and accuracy of tone at both frequency extremes. It also had a great feel of presence and able to convey musical emotion much better than I’ve been able to get from most magnetic cartridges. Anything able to draw the listener further into the music has to be good, and if you value my opinion at all, that cartridge did it! Furthermore, it was my “party-trick”. I have often demonstrated the old ceramic crystal cartridge as being a “new MC I’ve been trying”, and each time my subjects have been wowed.

I needed to replicate that for the many, but having no way to turn back the clock – it had to be done with a magnetic cartridge – any good magnetic cartridge.

Firstly the frequency characteristic of the magnetic cartridge had to be removed – the rising constant velocity output needed to be turned into constant amplitude to do the same as a “perfect” piezo-electric device. This was achieved by the “integrator” stage, and the integrator stage given the “slight compromise” has the benefit of constant loop-gain. It has just one capacitor stage to deal with the response, and it brings great stability to the integrator amplifier.

All that’s left is to EQ the record, and that can be as simple as the passive filter shown in the circuit of part 4. That made the circuit output-load dependant, but the addition of another amplifier stage took away that concern. Further prototypes were made and one featured at the National HiFi Show 2014 in the care of Origin Live. Others have been sent to “people with good ears”. The verbal feedback tends to cement my opinion regarding the sound.

If something is as good as claimed it needs to be protected by a patent, and to be granted one requires that the inventive principle is unique. Either that or you make absolutely sure its public so nobody else can patent it, and that’s partly the reason for these blog posts. Under patent application the principle is protected which enables me to tell you about it. If the patent application is rejected then it’s already public and puts a stop to anybody else patenting it.

So is it unique – is it a novel approach? That’s very important in succeeding with a patent. In exploring other registered patents the nearest I could find out of the five historically listed were US Patents 4470020, 4547819, 4644517 (this one uses a transformer) all seemingly describing the same thing.

But is it the same as mine? The answer is both yes and no. All three are “virtual earth” or “summing node”, an amplifier having the property of current to voltage conversion, but cartridge equalisation is by loading rather than the integrator stage! You’ll see what looks to be an integrator capacitor in the negative feedback but the claims indicate this is for stability – a compensation capacitor.

By operating the signal into a virtual zero volts (I say virtual because it’s “imaginary” mathematics – try dividing any number by zero…) the cartridge is loaded by such a small resistance/impedance that its response becomes flat (and its output just about non-existent in the process).

There isn’t any chance of loading the cartridge with the manufacturer’s recommended load or any chance of varying that load to suit the audiophile’s whims.

Furthermore, because of such loading taking the cartridge output incredibly low at all frequencies, the subsequent amplifier would have to be extremely low-noise to prevent hiss being clearly audible, and in my opinion that gave rise to the final patent 4644517 which uses the input transformer.

With my proposal and because of the “slight compromise” by including the 50Hz record EQ turnover into the integrator stage, standard cartridge loading is preserved (and may be altered)!

Not only that, but up to date input protection can be realised where there is chance to “drop the volts” of an incoming spike – as required by EMC regulations. Another requirement being substantial radio frequency immunity which without any voltage drops simply can’t be made to happen. The proper place for I/V conversion that US patents 4470020, 4547819 and 4644517 describe is in close proximity – like in CD players – not at the end of an antenna (the arm cable)!

Other posts on this topic:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5