A Matter Of Equalisation!

A matter of equalisation!

Assuming one uses some kind of 'magnetic' phono cartridge one needs a phono preamplifier with the 'standard' RIAA playback equalisation in order to properly 'process' the equalised signal cut into the groove of a record and bring it to so called 'line' level. The basic standards for RIAA playback equalisation were set back during the mid 1950's and probably just as well too as before hand, there were a multiplicity of different standards depending on the recording company! The RIAA curve has remained unchanged except for the occasional 'revision' such the IEC recommendation for additional bass roll off in 1976. Thankfully, this recommendation has been mostly ignored as in my opinion, this recommendation represented a real loss of bass. Engineers do come up with odd things at times, but hey that's just my personal opinion!

Actually those who came up with the original RIAA standard got it right as evidenced by long time conformity and agreement, the idea of bass cut and treble boost on record 'cutting' and the reverse on playback. For those interested in the whys and wherefores, the internet has many useful resources on this subject. In the world of vinyl, phono preamps occupy an interesting market and from a circuit point of view there are quite a few different ways of implementing the RIAA playback curve to a high standard, with low noise and distortion along with tight adherence to the RIAA playback standard.

Having built a few phono preamplifiers myself I admit that this is an area of considerable interest to me and yes I'm of the opinion that some circuit approaches are better than others. Designing a sonically good phono stage is not an easy task. As I once read in a book on hi fi circuit design, 'it is a demanding application'. Yes it is!
The arrival of the op amp was an absolute godsend to designers of phono stages as the use of frequency shaping equalising networks in the 'feedback' loop of the op amp is a relatively straightforward design procedure, and in fact many of the cheaper and even not so cheap phono stages use this approach and admittedly this does work well given a high quality op amp device and good design and construction.

However this is not my favoured circuit approach for several reasons and mainly to do with the accuracy of the treble equalisation roll off. The original RIAA specification calls for 'ongoing' 6 db/ octave rolloff in the treble range. Although most of the phono stages that utilise the simple op amp non inverting circuit appear to provide accurate treble roll off in accordance with the RIAA standard, in actuality they do not as the gain of that circuit configuration eventually drops to 'unity' (1) and no lower. There are ways of providing the required ongoing treble roll off through additional treble networks, but inherently this circuit configuration with simple feedback equalisation has its limitations. There are other op amp circuit configurations occasionally used such as the so called 'inverting' gain stage which does provide ongoing treble roll off but this circuit has the disadvantage of being appreciably 'noisier', however it is favoured in some audio quarters.

However one needn't use op amps at all. One can use transistors and/ or field effect transistors and even valves. Valve phono preamps are popular but it is rather difficult to build a really 'quiet' phono stage based on valves, but it can be done. Circuit noise is a real limitation with valve circuitry along with the problems presented by valve 'aging'. Regarding the particulars of 'equalisation' I personally favour the use of 'passive' treble equalisation which I regard as sonically superior. 'Full' passive equalisation is quite problemetical to produce for reasons of signal loss, potential distortion, impedance matching and optimisation and noise, but I have built one full passive equalisation phono stage design using FET's combined with bipolar transistors and it sounded nice! However my preferred phono stage, the Elliott Sound Products P06 design uses a combination of active (bass) equalisation and passive treble equalisation, which I consider to be the best of both worlds. The P06 is my 'reference' phono stage.

All of the discussion so far has centred around phono stages for use with 'moving magnet' cartridges of reasonably high 'voltage' output. The popular 'moving coil' cartridges (unless so called 'high output' versions), have a very low 'voltage' output requiring additional voltage 'gain' preamplification before the equalisation networks. This complicates the noise and potential distortion issues considerably! High quality 'step up' transformers are frequently used in this service or very low noise 'active' electronic preamplification. Low noise preamplification for this kind of application is truly 'state of the art' and some esoteric circuit designs have been used. It is a challenging area of electronics design. Personally, I think I'll stick with my beloved moving magnet cartridges! More to come! Felix.