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All Phono Stage Preamps >> RIAA Equalization

The Phono Preamp

As most of today's hi-fi amps don't have an input socket for RIAA equalization of a turntable - a phono preamp is what you need to play a turntable into your hi-fi amp.

The exception to the above rule is that some turntables now have a built-in phono preamp and are line-out - ready to plug straight into your amp, but that limits your ability to upgrade your turntable's performance.

Reflex M Phono Preamp

Often these turntables are fitted with a switch so you can switch between line out and the output of the cartridge - and in that case you can use an outboard phono preamp. However, on some turntables the switch leaves the in-built phono preamp load-resistors in circuit! That's a real shame because the cartridge will be double loaded by it and the outboard phono preamp, and it won't sound at all right.

Also there are USB output turntables. They not only have a built-in phono preamp, they also have an analogue to digital converter (ADC) to produce the USB bit stream. It gets its clock signals from a 12MHz crystal oscillator, and quite often interference is fed back into the sensitive pick-up cartridge. It's obvious that this type of turntable won't give you the best performance.

For the best vinyl record reproduction you don't want either of the above - you need a regular hi-fi turntable fitted with a good pick-up cartridge and a good separate phono preamp.

What The Phono Preamp Does

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. Just to make it even more confusing for the beginner there are many descriptions for what is essentially the same thing: phono stage, RIAA preamp, phono equaliser, turntable preamp — they're just different ways of saying phono preamp.

A phono preamp is therefore a separate box you 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 this article guides you into making the right choice.

The problem is that the output of the pick-up cartridge on a turntable isn't sufficiently large enough to drive a modern line only input amplifier. 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. The phono preamp corrects this.

So what phono preamp should you buy? 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 really 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 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.

RIAA Equalization - from cutter head to replay

To reiterate, the phono preamp's job is to correct for the tinny treble and quiet bass as well as the tiny signal the pick-up cartridge outputs. It wasn't always like this in the days of crystal pick-ups (a long time ago) but then the magnetic cartridge came along. The following explanation is quite complicated but explains all - the reasons why things are, and why they're done the way they are.

Basic Cutter Head Response

Basic Cutter Head Response

The cutter head cuts the groove of the record. Put more accurately, it cuts a lacquer which is used to make a stamper which presses the record with a copy of that groove.

Left alone, the cutter head exhibits this output curve because of it's inductive behaviour. This is of no use.

Linearised Cutter Head Response

Linearised Cutter Head Response

The cutter head has to be linearised by the application of feedback through feedback coils, and produces this straight line graph with slope y = -x.

Electrical energy is being converted to mechanical energy. This slope has constant velocity. The rate is -6dB/octave. It slopes downwards - it's still of no use.

Ideal Response

Ideal Response

We have seen above that the cutter head output is falling at -6dB/octave with increasing frequency, but what we want is a flat output — constant amplitude as shown here.

To acomplish that we'd need infinite boost at a rate of +6dB/octave to cancel-out the falling cutter head response.

The record companies knew that was impossible! Infinite boost from a virtually zero frequency, say 1Hz up to 100kHz, for sufficient margin to ensure the entire audio band (20Hz - 20kHz) is reproduced perfectly flat, would require a dynamic range of 100dB just for that purpose.

That doesn't take into consideration some 60dB (at least) signal to noise, or the 14dB of headroom a recording requires — in total 174dB of dynamic range which is beyond what physics allows even to this day.

RIAA Response

RIAA Response

As we have found, the cutter head amplifier is not capable of infinite boost but it can sustain a 40dB boost "budget". The RIAA recording curve distributes this "budget" to provide the flattest output on playback when using a cross section of consumer (crystal) p.u. cartridges.

The turnover frequencies and time constants used provide boost from 50Hz/3180uS at +6dB/octave to 500Hz/318uS; and then boost again from 2122Hz/75uS at +6dB/octave, to an undefined upper frequency, which will depend on the cutter head amplifier's dynamic range. Modern disc mastering services can reach ≥25kHz.

The boost takes the downward slope of the cutter head and lifts it to provide two flat portions which roughly fit the response of a crystal cartridge which most people had in 1953 when the RIAA standard was laid down.

The choice of 50Hz offsets the lower bass droop exhibited by most consumer crystal p.u.'s and the 500Hz to 2122Hz "shelving down" offsets the gradual increase in output from crystal p.u.'s on their approach to their resonant frequency at around 15kHz. With this type of p.u. the RIAA curve gave a reproduction accuracy of +/-2dB.

 

Magnetic Cartridge Response

Magnetic Cartridge Response

Unlike the almost flat response of a crystal cartridge, all magnetic pick up's produce a rising response — the slope has constant velocity, but this time mechanical energy is being converted to electrical energy producing a slope y = x. The rate is +6dB/octave. It slopes upwards.

RIAA Response Via A Magnetic Cartridge

RIAA Response Via A Magnetic Cartridge

When played using a magnetic p.u. the RIAA response, due to the +6dB/octave magnetic p.u. response, is therefore tilted up at +6dB/octave. If played as such the listener would hear mostly treble with attenuated mid frequencies and vastly attenuated bass.

This curve is often falsely taken to be the output of the record itself. As we have shown, it is not! Such explanations that say so are misinformed, and go on to explain characteristics as to why this is, which bear no resemblance to the truth.

Phono Stage Response

Phono Stage Response

To produce a flat output from the RIAA response via a magnetic p.u. the phono stage must provide the exact inverse of the frequency response chart shown above. The curve shown here does that.

Some refer to the portion of the curve between 0 and 50Hz as the "rumble shelf". It is no such thing. It is simply the inverse of the previous curve as this article has shown.

The Resulting Output Now Flat

The Resulting Output Now Flat

The phono stage may also add further filtering to attenuate unwanted signals at either end of the frequency range, such as subsonics produced by record warps or a worn platter bearing (rumble) but this is not specific to the RIAA standard. Subsonic filtering can introduce undesirable phase shifts and it is better to deal with these problems at their source, for example: the pick up cartridge and arm compliance should be chosen such that their resonant frequency is mid-point between warp frequencies and the beginning of the audio frequencies at 20Hz. The turntable platter bearing can be made quiet by refurbishment.

Passive EQ

Phono equalisation (EQ) can be done using one of two techniques and the phono preamp you buy will use one or the other. The techniques are active and passive.

Active means that the EQ is done inside an amplifier or gain stage negative feedback loop. Passive means that the EQ is done between two or three gain stages outside a negative feedback loop. The EQ itself, in both cases, comprises one or more resistor-capacitor networks.

Originally, the equalisation was done the active way, and at that time phono preamps used valves (tubes). One exception was a passive design by RCA which placed a resistor-capacitor network between two valve gain stages.

Around the mid sixties onward it became fashionable for designers to be different claiming they were offering something new. In reality there was nothing new, just designs that had been long forgotten and brought back to life - it's a marketing trick which has worked well for many.

By the late seventies passive solid-state phono preamp designs became the latest "fad". Even so, RCA had done it many years before as I stated above, but "passive" became the buzz-word for all things good in the hi-fi world, and to this very day the customer has generally been brainwashed into believing passive is good and active is bad.

Unfortunately for customers, they don't have the knowledge of a highly experienced audio electronics engineer, and that's very fortunate for the passive phono preamp manufacturer, who only has to say the word "passive" to sell on a large scale.

In truth, both ideas, active and passive, can have their failings. However, you will be surprised to learn that it's hard to find many good points about passives when the hype surrounding them is removed.

Because the gain stages of a passive phono preamp are flat, and the input from a magnetic cartridge is rising in voltage with rising frequency, it will, at some point, result in clipping of the input gain stage. If the input gain stage has its gain reduced to compensate for this, it then contributes more noise (hiss).

Getting it just right is a fine balance, and that limits it to a narrow range of phono cartridge outputs. True, you will see in specifications massive overload claims suggesting the opposite, but this is referenced to the 1kHz mid frequency. At 20kHz there's just one tenth of that overload margin due to the rising output of the magnetic cartridge, and at "click and pop" rising edge frequencies the overload margin will be a negative figure meaning record noise will be clipped, highly distorted and exagerated.

On your most perfect records, especially simple recordings, the passive phono preamp can have the edge over active, but on "busy" recordings with lots of high energy high frequency program, the passive phono preamp shows it's distress, and your well-worn favourite records can often become unlistenable.

There is also a quirk related to passive EQ networks. Have you ever witnessed the "sliding sound stage" with a passive phono preamp? It's quite weird but female vocals in particular, can drift and often flick dramatically side to side in the sound stage. It's because large capacitor values are needed in passives, often in the order of 0.047 to 0.1 micro-farad for the bass EQ. At this size it's difficult to obtain the really high voltage rating needed to have a relatively constant capacitance-frequency curve, so the EQ actually varies with pitch. And being passive there's no negative feedback to correct it.

Active EQ

In an active phono preamp, even if it used high value capacitors, the negative feedback goes a long way to correct the sliding sound-stage problem. Because of recording technique there will always be some drift, but a good active phono preamp doesn't confuse the ear quite as much. The active phono preamp can be made to use low value capacitors which can be obtained in higher voltage ratings leading to greater capacitor versus frequency linearity, so that EQ hardly varies with pitch.

(Amongst other things, tone controls also earned their poor reputation because of the varying pitch problem)

Active phono preamps aren't driven into clipping all that easily compared to the passive stage. The negative feedback gives a constant overload margin at all frequencies which the passive phono preamp cannot boast. The same overload margin extends way beyond the audio spectrum including where the leading edges of "clicks and pops" exist, so, if designed properly, the active phono preamp can actually sound like it has noise reduction – even though it hasn't.

Active phono preamps have been given a poor reputation because active EQ is the type used in cheap phono preamps. This is because the component count is smaller than that of a passive phono preamp - they can be built smaller and hence more cheaply.

Now hold onto that thought! By the same virtue, a really good - dare I say high-end - active phono preamp would still have a smaller component count and for the same or better performance, can be built smaller and hence more cheaply — if you know how.

Realising the many virtues of an active phono preamp, it made sense for me to explore just how far the technology could be pushed. It was a Rotel designer called Michi who rekindled my interest in active phono preamps, not in person I must add – I read an article in a hi-fi magazine.

There are countless Audio Engineering Society white papers, and more from other sources on active phono preamp design hidden in the annals of time due to the preocuptation with passive design. Put every little bit of information gleaned into an active phono preamp design and it changes the outcome considerably.

Normally, a cheap active phono preamp has problems driving its negative feedback loop because it is capacitive at high frequencies. Usually that uses up open-loop gain and that lowers input impedance, making a mess of the highs. It can also send the gain stage into instability... there are many traps an active phono preamp designer can fall into. But wait a minute! We've already learned that an active phono preamp can overcome the shortcomings of the passive phono preamp. So if all those "traps" were turned into positives, an active phono preamp would make music like no other!

And that's exactly where we are today - making active phono preamps that realise the full potential of your vinyl record collection.

Moving Magnet versus Moving Coil

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 can suffer 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 lifelike. 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 which compensates the moving magnet's problems.

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. This kind of balances the equation between moving magnet and moving coil.

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. Usually two dedicated phono stages will cost less than a "one box does all" for the same musical quality.

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 (the quantity of electron charge per unit time) 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.

With this knowledge in mind a number of phono preamp burn-in devices have burst on the scene. Are they really necessary? And do they speed-up burn-in? In reality no. They only inject signal. They cannot speed-up the forming of electrolyte on aluminium capacitor foil edges. Only direct current from the power supply can do that. The best thing is to leave it on — in fact, leave it on always so it will always be ready to play at its best.

Phono Preamp output ready reckoner

Outputs for different cartridge sensitivities using our phono preamps (1kHz reference frequency). The values given below are a guide only and are approximate within 6% and rounded to the nearest 5mV.

MM Phono Preamp Outputs

(for the Gram Amp 2 Communicator which has a lower gain, multiply the given output by 0.7)

Cartridge Sensitivity gives output:

  • 1.5mV : 180mV
  • 2.0mV : 240mV
  • 2.5mV : 300mV
  • 3.0mV : 355mV
  • 3.5mV : 415mV
  • 4.0mV : 475mV
  • 4.5mV : 535mV
  • 5.0mV : 595mV
  • 5.5mV : 650mV
  • 6.0mV : 710mV
  • 6.5mV : 770mV
  • 7.0mV : 830mV
  • 7.5mV : 890mV
  • 8.0mV : 950mV
  • 8.5mV : 1.01V
  • 9.0mV : 1.07V
  • 9.5mV : 1.13V

MC Phono Preamp Outputs

(for the Gram Amp 3 Fanfare which has lower gain, multiply the given output by 0.63)

Cartridge Sensitivity gives output:

  • 150uV : 180mV
  • 200uV : 240mV
  • 250uV : 300mV
  • 300uV : 355mV
  • 350uV : 415mV
  • 400uV : 475mV
  • 450uV : 535mV
  • 500uV : 595mV
  • 550uV : 650mV
  • 600uV : 710mV
  • 650uV : 770mV
  • 700uV : 830mV
  • 750uV : 890mV
  • 800uV : 950mV
  • 850uV : 1.01V
  • 900uV : 1.07V
  • 950uV : 1.13V

References:

"Audio/Radio Handbook" National Semiconductor, 1980

"The Art of Electronics" Horowitz and Hill, 1980

Creative Commons Licence
The Phono Preamp by Graham Slee is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.