More ado about EQ

All About The Phono Preamp

As most of today's amplifiers or pre-amplifiers don't have an input socket marked turntable, a phono preamp is what you need to connect a turntable to your audio or hi-fi system.

(picture shows a graphical representation of the RIAA replay curve taken from a National Semiconductor Handbook - more on this below)

Phono Preamp and Hi-Fi System Basics

The phono preamp is an electronic circuit or stage that deals with the correct amplification of the phono cartridge and sends it to the input of an amplifier, a pre-amplifier, or an audio system. It is commonly called a phono preamp, phono stage, phono preamplifier, RIAA preamp or turntable preamp - just to confuse matters further for the non-technical.

A phono preamp therefore is a separate box you would have to purchase and connect between your turntable and your amplifier, but what does it do? And with so many phono preamps or phono stages at several different prices to choose from, will any do?

By answering "what does it do", I hope it should guide you to making the right choice.

Firstly for those who don't know, I'll explain in the most basic terms I can, what a preamp in general is. A preamp is short for pre-amplifier - a little amplifier that deals with tiny or small signals, boosting them up so they're big enough to drive a power amplifier which then drives your loudspeakers. A general purpose hi-fi amplifier is a one box unit that comprises a pre-amplifier and power amplifier. Its proper name is an "integrated" amplifier because it integrates the functions of pre-amplifier and power-amplifier in one box.

In yesteryear before the digital revolution, people had tape decks or cassette decks, FM radio tuners, turntables (or record players), and maybe an old eight-track or the output of a video player which they'd connect to "the stereo" and hear through the speakers. The amplifier was therefore presented by a number of different signal voltages from all these musical sources which it had to pre-amplify by different amounts to make them all the same so the power-amp stage could get on with the job of driving the speakers. And therefore the inputs would be marked accordingly: "tape", "tuner (or radio)", "turntable (or disc, or records)", and "aux" to cover anything that was auxiliary such as the eight-track or video player.

Then CD (compact disc) came along, and amplifiers started to feature an input marked "CD". Some amplifiers featured a "CD-direct" button that bypassed the preamp stage completely, delivering the signal from the CD player directly to the volume control just before the power-amp stage.

As the use of vinyl records declined, many integrated amplifiers stopped featuring an input for the record player or turntable. And today, with virtually all signal sources being of digital origin, with similar signal voltages in the region of half to one volt, the preamp-stage itself is no longer required, being replaced by a switch or bank of switches that simply select one of a number of identical inputs, feeding them to a volume control and from there directly to the power-amp stage.

Henceforth was born the "passive preamp" for those who like the preamp and power-amp stages in separate boxes. But there is absolutely no preamp in a "passive preamp", just switches, relays, or solid state devices that route the chosen signal to the power-amp.

The Phono Preamp

The problem is that the output of the pick-up cartridge on a turntable isn't sufficiently large enough to drive the power-amp stage. And there is the added problem that the output from a magnetic pick-up cartridge isn't "flat" which, in a nut-shell, means the treble is louder than the mid-range, and the mid-range is louder than the bass. So all you'll hear by hooking-up the turntable straight to an amplifier's "line" or "aux" input is tinny but quiet treble.

Unless, that is, you buy one of those new fangled "line-output" turntables that have a phono preamp built in! The problem there is you are stuck with the sound quality or performance the turntable manufacturer has decided upon in their choice of phono-stage electronics. Unless it is quite an expensive line-output turntable that includes a good state-of-the-art phono preamp, you would be better buying a turntable that simply uses the output from the pick-up cartridge itself; a conventional turntable.

That leads us to the accessory called the phono preamp or phono stage. All you have to do is obtain one of them, hook-up the turntable to it, and hook-up the output from the phono preamp to your amplifier - done!

The performance you'll get will depend on a few different factors. Firstly, it is doubtful that you'll get any satisfaction at all from a "dirt-cheap" phono preamp. You could go the other way and just throw money at it, but that isn't a guarantee of satisfaction either - there are a surprising number of "dirt-cheap" phono preamp circuits dressed in expensive looking boxes just waiting to relieve you of your hard-earned cash and leave you very disappointed. Somewhere in-between there are some (a minority I guess) excellent performers.

Now to get technical...

Phono Preamps and RIAA Equalisation

The difference between an ordinary preamp and a phono preamp is its frequency response: The phono preamp has to "equalize" the different frequencies which are reproduced by magnetic cartridges at different levels, so they come out "flat".

So why don't they make phono cartridges whose output is "flat" to start with?

Well they did, but magnetic cartridges rule-the-roost and records are cut for magnetic cartridges and that's a fact of life. So even though the output of a ceramic or crystal cartridge which used to feature on mass produced British record players, is flat, the record, being cut for the magnetic cartridge isn't, causing a dip in the response right in the mid frequencies where our hearing is most sensitive (which you couldn't really tell on a cheap record player). They too could be equalized but that costs more money, so the better-bet was on the "more common in USA hi-fi circles" magnetic cartridge - and it won.

Fig 1. RIAA record output from a "flat" device such as a "perfect" ceramic cartridge. Actually it's the graph of the output from a typical magnetic cartridge tilted 45 degrees to the right - the output of a magnetic cartridge rises with frequency because it is a constant velocity device rather than constant amplitude like a ceramic or crystal cartridge.

Fig 2. RIAA record output from a velocity device - a magnetic cartridge (MM or MC). The hump in the middle of the curve is because of the cutting head inductance, peaking around 700Hz - a property put to good use to "build" the RIAA standard curve to fit the dynamic range of older magnetic phono stage preamps. [Graph taken from "Audio/Radio Handbook", National - Semiconductors, 1980]

Fig 3. The RIAA replay frequency response plot of the Reflex M (green curve) can be compared to Fig 2, where it can be seen the two curves cancel to give a flat output between 20Hz and 100kHz.

Examination of the RIAA standard reveals a tolerance (or leeway) of ±2dB which is audible. So no matter how accurate the frequency plot of a phono stage preamp is, it's the inaccuracies caused elsewhere, in the mastering suite, that decide if a record's tone is acceptable or not.

Moving Magnet and Moving Coil Phono Preamps

There are cartridges where the magnet moves inside a fixed coil (Moving magnet) and where the coil moves around a fixed magnet (Moving coil).

They have different outputs: moving magnet in the region of 2 - 10 mV and moving coil in the region of 0.2 - 1.0 mV (there is a type of moving coil made to produce the same output as a moving magnet, but it's a bit of a compromise because it loses some of the usual moving coil qualities in the process - this type is called a "high output moving coil" or HOMC).

Moving magnet cartridges suffer from restricted high frequency extension (because their higher output requires many more turns on their fixed coil windings making them highly inductive), which although is beyond the hearing range, can curtail the important harmonic structure of the music making it less life-like. However, because of their higher output the phono preamp needs less gain and doesn't have to be as low-noise as a moving coil phono preamp. This gives the designer the possibility of using electronics with better high frequency performance and linearity.

Moving coil cartridges have superb high frequency extension, but to obtain the signal to noise ratio required means using electronics that don't have such good high frequency extension. Low noise electronics don't have the same linearity either.

It can be seen from the above that a phono preamp that's switchable between both cartridges will be a compromise. Either that or it will favour one type of cartridge over the other. Therefore it's best to select a dedicated phono preamp for whatever type of cartridge you're using.

Phono Preamp Burners/Burn-in

The last word really belongs to a thing called "burn-in". Are there any moving parts in solid-state electronics? Yes, electrons! As current flows in new components they undergo changes that can last weeks. It would be no good making a phono preamp that sounded good at first use only to fade with time. Much better to make a phono preamp that when fully burned-in sounds great. Therefore, from new and from switch on things may not sound the way you expected, but they will do as time marches on.

Take a look at our phono preamps

Also, take a look at these References (especially "Great Sounding Records" by Kevin Gray - it will open your eyes!)

Great Sounding Records, Kevin Gray, 1997 (online)

Audio/Radio Handbook, National Semiconductor, 1980 (page 2-23 to page 2-25)