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An Audio Masterclass visitor has a turntable and a mixing console. But they are not sounding good together...

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It makes a pleasant change to receive a straightforward question that has a straightforward answer, and will really make a clearly audible difference. I am more normally asked whether Preamp A has better 400-800 Hz punch than Preamp B. My answer to that is, "Make sure your singer has the right number of sugars in their tea. That will make more of a difference to your recording."

Back to the turntable... So you have a turntable with three wires hanging from it - two with phono connectors on the ends, one just bare at the end. Where do they connect to a typical mixing console, which has microphone inputs on XLR connectors and line-level inputs on jacks?

It's worth considering what would happen. Let's say you made a pair of phono socket to jack cables so that you could connect your turntable to the line inputs. (The bare-ended wire, as yet, seems to have nowhere to go.) Well you can hear audio from the record as it plays, but it is very low in level and lacking in bass.

OK, let's try the microphone inputs. Make sure you switch the phantom power off, or something's going to blow. This time the level is OK, with a tweak of the gain controls, but the bass is still severely lacking. And there seems to be some noise, and possibly some hum.

Special preamplifier

The answer to this problem is that your turntable, or more accurately the cartridge in the pickup arm, needs a special preamplifier that is exactly matched to the characteristics of its output.

Firstly, the output of the cartridge is weak. To get the best performance, the input impedance of the preamplifier needs to be exactly right - 47,000 ohms. This is the value decided on by cartridge manufacturers sometime in the late 1950s. They make their cartridges so that their performance in terms of noise and frequency response is optimized with a 47 k preamp load.

In contrast, the microphone preamplifiers of the mixing console will have an impedance something more like 1000-2000 ohms. The line inputs will have an input impedance that could be 10 kohms up to around 100 kohms. These values won't stress the cartridge, but it is optimized to perform best at a fairly precise 47 k. The available gain of the line input will however almost certainly be insufficient. Indeed, some line inputs offer no control over gain at all. And on top of all of this, the bass will still be very weak.

Bass boost necessary

The reason why the bass is weak using either microphone or line inputs is that the bass is intentionally low in level actually on the record, and must be boosted on playback.

This is because in the development of vinyl record technology it was found that low-frequency content caused the groove to swing widely from side to side. If the bass was cut at full level, the turns of the groove couldn't be packed tightly enough to get a reasonable playing duration per side.

One possible answer was to put the signal through a simple high-pass filter with a 6 dB/octave slope to cut onto the record. This however was too much - the bass ended up very low in level - around 60 dB down - and was noisy when boosted. So, a more complex filter was designed that cuts the bass by a lesser 40 dB or so. An inverse filter must be used on playback to correct for this, otherwise the bass end will be very light - to an extent that anyone would notice, not just audio enthusiasts.


To handle these various aspects of the output from the cartridge - low level, requiring 47 kohm input impedance, low bass - a special preamplifer is necessary, which is known as an RIAA preamplifier, because it was mandated by the Recording Industries Association of America. Every hifi amplifier that is intended to work with a turntable has one. There are many outboard examples available too. Here's one that I use myself...

ART RIAA preamplifier

Oh yes, that extra wire - I almost forget. It's an earthing wire. RIAA preamps have a screw terminal to attach it to. Cartridges are sensitive to hum, and the bass boost of the RIAA preamplifier has the capability to make it much worse. Connect the wire and you should have no hum problem.

P.S. Everything above applies to moving-magnet cartridges. Moving-coil cartridges have other requirements, but they are more commonly seen in hifi rather than pro audio.

P.P.S. The turntable illustrated is the EMT 948, which is an absolute beast of a turntable. If you aspire to own such a classic, then AudioScope would be a likely place to go to.

By David Mellor Wednesday March 27, 2013
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