It is sometimes said that an SUT should be matched to the cartridge (or the other way around).
The reason typically given is the need to have an appropriate amount of SUT gain to bring the cartridge's output up to a level that is comfortable for the chosen phono stage.
However, there is another less-known but equally important reason, which is that the low frequency bandwidth and phase response of the SUT depend on both the inductance of the SUT's primary windings, and the net impedance that the primary windings will see, according to the following formula:
f = Z/(2*Pi*L)
where f is the frequency where the amplitude is 3dB down compared to higher frequencies,
L (in henries) is the inductance of the SUT's primary windings
Z (in ohms) is the net impedance
The inductance of the SUT's primary windings should be measured with the secondary windings left open. (Note that a completed SUT may contain internal load resistors across the secondary windings, therefore it is risky to assume that simply because the SUT is not connected to a phono stage, the secondary windings are truly open.)
"Net impedance" in this particular situation means the phono cartridge's source (self) impedance, in parallel with the impedance that is reflected back from the SUT's secondary windings (when connected to a suitable phono stage). Usually the net impedance will be dominated by the cartridge's source impedance.
The mathematical formula tells us that in order to improve the low-frequency capabilities our options are to lower the source impedance (choose a cartridge with less self-impedance), increase the primary inductance of the SUT (increase the number of transformer windings, increase the dimensions of the transformer core, or use a higher-permeability material for the transformer core).}
In a nutshell, the larger the self-impedance of a cartridge, the more primary inductance will be required from the transformer in order to keep the same low-frequency extension or phase response.
Conversely, if the cartridge has small self-impedance, less primary inductance will be needed for sufficient low-frequency extension or phase response.
Although the target frequency for "f" in the formula above could conceivably be set to the lowest frequency expected to be reproduced, if good phase linearity is likewise a goal, this will require a considerably lower target frequency.
Caveat: although good low-frequency performance from a transformer requires sufficient primary inductance, since high inductance (in the form of more turns, or more wire length per turn, or higher-permeability core materials) tends to pose problems for high-frequency performance, single-mindedly selecting a transformer with as much primary inductance as possible, isn't recommended either.
Summary: When choosing an SUT, be sure that the gain is appropriate for the output level your phono cartridge.
Also be sure that the SUT's primary inductance is suitable for the coil impedance of your cartridge.
When connecting a SUT and phono stage together, try to choose as low-capacitance cable as possible, using as short lengths as possible.
Generally SUTs work better with low-impedance cartridges.
The reason typically given is the need to have an appropriate amount of SUT gain to bring the cartridge's output up to a level that is comfortable for the chosen phono stage.
However, there is another less-known but equally important reason, which is that the low frequency bandwidth and phase response of the SUT depend on both the inductance of the SUT's primary windings, and the net impedance that the primary windings will see, according to the following formula:
f = Z/(2*Pi*L)
where f is the frequency where the amplitude is 3dB down compared to higher frequencies,
L (in henries) is the inductance of the SUT's primary windings
Z (in ohms) is the net impedance
The inductance of the SUT's primary windings should be measured with the secondary windings left open. (Note that a completed SUT may contain internal load resistors across the secondary windings, therefore it is risky to assume that simply because the SUT is not connected to a phono stage, the secondary windings are truly open.)
"Net impedance" in this particular situation means the phono cartridge's source (self) impedance, in parallel with the impedance that is reflected back from the SUT's secondary windings (when connected to a suitable phono stage). Usually the net impedance will be dominated by the cartridge's source impedance.
The mathematical formula tells us that in order to improve the low-frequency capabilities our options are to lower the source impedance (choose a cartridge with less self-impedance), increase the primary inductance of the SUT (increase the number of transformer windings, increase the dimensions of the transformer core, or use a higher-permeability material for the transformer core).}
In a nutshell, the larger the self-impedance of a cartridge, the more primary inductance will be required from the transformer in order to keep the same low-frequency extension or phase response.
Conversely, if the cartridge has small self-impedance, less primary inductance will be needed for sufficient low-frequency extension or phase response.
Although the target frequency for "f" in the formula above could conceivably be set to the lowest frequency expected to be reproduced, if good phase linearity is likewise a goal, this will require a considerably lower target frequency.
Caveat: although good low-frequency performance from a transformer requires sufficient primary inductance, since high inductance (in the form of more turns, or more wire length per turn, or higher-permeability core materials) tends to pose problems for high-frequency performance, single-mindedly selecting a transformer with as much primary inductance as possible, isn't recommended either.
Summary: When choosing an SUT, be sure that the gain is appropriate for the output level your phono cartridge.
Also be sure that the SUT's primary inductance is suitable for the coil impedance of your cartridge.
When connecting a SUT and phono stage together, try to choose as low-capacitance cable as possible, using as short lengths as possible.
Generally SUTs work better with low-impedance cartridges.