Current Amplifiers – How they work

Our Moving coil products – How they work.

A phono cartridge is essentially an electric motor. It turns the mechanical power that the record puts int moving a stylus into electrical power. It does this by placing a magnet nest to a coil of wire and moving one with relative to the other. This varies the magnetic field in the coil and produces electrical power.

The two types of phono cartridge
In a Moving Magnet cartridge the coil (two for stereo) is fixed rigidly into the cartridge body and the two magnets are connected to the stylus cantilever and move in response to the record groove thus producing electric power.

In a Moving Coil cartridge, as the name implies the magnet is fixed into the cartridge body and its the coils that are attached to the stylus body. MC cartridges are more difficult to manufacture and are therefor more expensive that MM types but can have significant advantages.

You may be surprised to know that the earliest magnetic phono cartridges for 78 RPM records were in fact MC and not MM!
This is because early magnets were very weak, very large and very heavy so any hope of attaching them to a stylus was out the window as the coils could be made much smaller and lighter.

As LP vinyl records grew in popularity in the 50’s, smaller and much more powerful magnets were available which could be attached to a stylus and the lower cost of manufacture made the MM cartridge the natural choice for a quality phono cartridge.

In the 1970’s the MC cartridge re appeared as the coils once again became smaller and lighter then the magnets and the lower mass of the MC stylus assembly responded more quickly to the sound in the record groove and gave a more faithful representation of the music and became the “High end” choice for music lovers everywhere.

Since then MC and MM cartridges continue to evolve and leap frog each other in terms of quality so that today we have exceptional MM cartridges and very exceptional (and expensive) MC cartridges.

High and Low output – A misunderstanding!
At the beginning of this article we said that a phono cartridge was an electric motor that converted mechanical power into electrical power so for the same mechanical input both MM and MC cartridges should produce the same amount of electrical power (and in fact they do) so why are MM cartridges described as “high output” and most MC cartridges “low output”.

The terms high and low refer to the output voltage from the cartridge not the output power.
Output voltage is determined by the number of turns of wire in the coil which equals extra weight so most MC cartridges have only a small number of turns of wire on the coil so produce a proportionally lower voltage (about a 10th) than a MM cartridge.
However electrical power is voltage x current so for the same mechanical input a MC cartridge can deliver 10 times the current of an MM cartridge.

So a MM cartridge is a high voltage, low current “motor” whereas a MC cartridge is a low voltage, high current “motor” and both produce the same electrical power.

The MM phono stage
Most electronic amplifiers are voltage amplifiers in that the multiply the input voltage by the gain of the amplifier to produce a certain output voltage (A typical MM phono stage with line level input has a gain of about 450x (40dB).

As MM cartridges are high voltage, low current motors the current can be ignored (from the amplifier standpoint) and a simple voltage amplifier can be used to generate the required signal with very little loss of performance.
This was fine until MC cartridges came along and their much lower output voltage required more gain in the voltage amplifier and disproportionately more noise! However most “switchable” Phono stages continue to rely on voltage amplifiers to amplify the output from an MC cartridge!
There’s an even worse problem. The only way to do this with a voltage amplifier is to feed the MC cartridges output to a load resistor and to amplify the voltage produced across the resistor. This resistor has to be a low value to match the output impedance of the cartridge and so typically half the output power of the cartridge is lost in the resistor. This has to be compensated for in the voltage amplifier resulting in a higher noise floor than would be ideal.

Solution 1 – The Step UP Transformer (SOT)
An electrical transformer is a power converter and can convert high voltage, low current into low voltage, high current and vice versa with a very small loss of power in the transformer.
A step up transformer does just this. It takes the low voltage, high current output from an MC cartridge and “transforms” (hence the name) it into a high voltage, low current output that can be sent to a traditional MM voltage amplifier.
As there is no electrical amplification in the transformer there is no extra electrical noise (assuming the transformer is properly screened)