Graham's Blog Archive
A New Phono Stage Invention
Patent application number: 1407626.9
Although the shape of the frequency response of an RIAA record played using a magnetic cartridge is known (by only a few it would seem) to be a combination of two separately occurring curves, playback EQ has always been configured to deal with the homogenous result.
A number of alternative methods have been put forward as to how this homogenous curve is best equalised for playback – some splitting the homogenised curve into defined sections – it has never been dealt with as the two separately occurring curves which form what is commonly accepted to be the RIAA playback curve. Because very few, if any, manufacturers of phono equalisers (phono stages) understand the underlying principles, their explanations of the reasons for what is thought to be the RIAA playback curve are quite often wide of the mark.
RIAA equalisation is in fact a prescribed frequency correction curve applied to the record to compensate for the most common type of playback pick-up devices which were in use at the time (circa 1953). These pick up devices had an ‘almost-flat’ response and gave a relatively constant amplitude output. They were of the type known as crystal or ceramic phono pick ups.
The need for compensation was twofold: the bass output fell away below 50Hz on average; and due to resonance, the outputs of these pick-ups began to rise above 500Hz and then started to fall away at some higher frequency.
The RIAA recording curve was constructed to compensate for this, and inflection points were set to give boost or cut at particular frequencies described by their time constants. The curve was claimed to give a better match to the phono reproducers of the time and the resulting accuracy was within +/-2dB.
Playback using a magnetic phono cartridge does not produce the same output response. The magnetic cartridge has what is called a ‘rising response’ due to its constant velocity nature. Its output voltage increases at the rate of 6dB per octave (20dB per decade).
The resulting signal is a 45 degree tipped-up version of the RIAA playback response.
This invention corrects for the rising response of the magnetic phono cartridge separately to the record playback response.
By applying its magnetic phono cartridge correction in a negative feedback loop of an amplifier, it has the advantage that input and output impedances are kept constant because the curve of the closed loop gain closely matches that of a ‘real-world’ amplifier’s open loop gain, keeping loop gain constant.
The above puts to rest the age-old ‘argument’ of varying cartridge load impedance due to using an active EQ stage, even though the argument was a relic from the valve amplification days, where valves suffered poor open-loop gain.
Because there is no step change in loop gain in the mid frequencies, which would otherwise occur in a conventional feedback EQ configuration, there is no mid frequency harshness which can blot out a considerable amount of spatial musical detail – a real problem solved!
By arranging the inflection point for the magnetic cartridge response correction to coincide with the record’s low frequency time constant, that time constant does not have to be ‘post-equalised’, making for a much simpler record equalisation filter network. It also allows for a good dynamic range so that circuit noise is minimised. Furthermore, our hearing is much less sensitive to a filter operating at this frequency than it would be at a mid frequency.
Such an approach to RIAA equalisation (or any other phono EQ) yields better audible results than the existing state of the art. Now revealed, this technique could be seen as being so very obvious, except that in keeping with the saying… it is so obvious that it has always been overlooked.
The above describes the reasons for and the basic principles of the invention. As always further development and ‘tweaking’ is required before its launch as a tangible product. However, a basic implementation has been aired to visitors of our recent ‘Road Show’ and the resulting ‘sound’ noted by comments such as… “The sound … it was just so real”
Edit: For more on this topic see "A Different Approach To Record EQ".