This has ended up being my favourite cartridge step-up transformer (SUT) whenever I am using a Moving Coil cartridge.
The Fidelity Research FRT-4 SUT is versatile, with 4 x Gain/Impedance ratios and it sounds very good.
It is better sounding than my Fidelity Research FRT-3 (dynamic but slightly ‘coarse’ in comparison) or the very good Stevens & Billington TX103 (less extended treble tone).
Fidelity Research operated from Tokyo from 1964 – 1984.
The FRT-4 SUT was designed with a separately encased toroidal transformer for each channel, housed inside a steel box with an outer aluminium casing.
The 4 x switchable Gain / Impedance ratios provided are – 31.1dB (35.84x) – 26.3dB (20.68x) – 25.2dB (18.27x) – 20.0dB (10.55x). Fidelity Research Specifications below:
There are 3 x switchable inputs and a ‘bypass’.
Below is a Fidelity Research table showing recommended optimal cartridge impedances and suitable MC cartridge models for each Gain/Impedance ratio (assuming 47kΩ preamp input impedance):
Japanese SUTs typically specify optimal cartridge coil (source) impedances in this way.
The FRT-4 is assembled with a 18kΩ resistor load across the secondary winding outputs, together with a 1MΩ resistor across part of the secondary winding. (This may be simply to adjust impedances, or to provide some ‘damping’.) As per (my) diagram below:
Testing replacement resistors did result in changes in sound, but the best sonic result was with the 18kΩ resistor removed from the circuit and 1MΩ resistors changed to Caddock TF020 type. (Or other type of your choice.)
The Primary ‘reflected’ impedances will also be changed as a result. My table below shows the resulting Primary ‘reflected’ load impedances, with a ‘standard’ 47kΩ phono-preamp load connected to the SUT Secondary output.
The FRT-4 remains an excellent-sounding and versatile SUT, suitable for MC cartridges with coil impedances ranging from say 3Ω – 40Ω.
There is good tone (extended sweet treble especially), good bandwidth, ‘agile’ dynamics and good ‘transparency.’
Initially’ there was some brightness compared with the S&B TX-103 but, overall tonal balance is excellent & extended.
If your phono-preamp. has ability to adjust input impedance, then any MC cartridge could be optimally matched with a suitable SUT primary load impedance (of 10x to 20x coil impedance).