Many of the best‑loved classic equalisers used a passive inductor‑capacitor (L‑C) circuit, in most instances followed by a stage of amplification to compensate for any level loss: the legendary Pultec EQP‑1A is a classic example of this approach to EQ design. According to designer Roger Mayer, a passive L‑C EQ works particularly well on the non‑symmetrical waveforms typically produced by most musical instruments and voices, which are not always treated optimally by a circuit that relies on negative feedback (as found in most general active EQ circuitry).
His Dottore XFM is a two‑channel, true passive L‑C equaliser based on high‑quality audio transformers; one of the transformer windings in each channel provides the necessary inductance. In bypass or boost mode, the Dottore XFM is also able to connect a balanced source into an unbalanced input without any loss in the signal level (typically you’d lose 6dB when feeding balanced sources to unbalanced inputs such as insert returns or consumer devices).
The Dottore is specifically designed to manipulate low frequencies, the ‘boost’ frequency being determined by a six‑position switch with options for 60, 73, 87, 108, 130 or 160 Hz. The two channels are extremely closely matched for stereo applications but have completely separate controls, to allow for dual‑mono operation. A Q control adjusts the character of the boost, while a two‑position rotary switch takes the EQ components out of the circuit but keeps the transformer in the signal path. As the circuitry is passive, the output noise is very low (better than 140dB signal‑to‑noise ratio) but, equally importantly, the wide frequency response of the transformer allows for a very flat phase response (‑3 degrees at 20Hz) and a THD at 1kHz of 0.0016% for a +10dBu output level (0.0018% for a +20dBu output level). Because there’s no powered circuitry, there’s no power supply here to interfere with the audio.
Internally, the unit is engineered to the very high standards I’ve come to expect from Roger Mayer. It uses a six‑layer PCB and has been designed to provide optimum shielding from electromagnetic interference. The case is made from unpainted stainless steel and the four rear‑panel audio connectors are all gold‑plated Neutrik XLRs.
Dottore XFM makes the source sound seem more ‘analogue’ — it makes it easy to get the right sound quickly.
The Dottore can be used between a mic/line preamp and the input to a DAW’s audio interface or, alternatively, it can be used in insert points. Though it offers only low‑frequency EQ, there’s definitely something special about the way this circuit enhances drums, adding weight without making them sound processed, while sources such as acoustic guitar and voice gain added depth and smoothness, without any sense of them sounding muddy. I could best sum up this device by saying that the Dottore XFM makes the source sound seem more ‘analogue’ — it makes it easy to get the right sound quickly. There are many units on the market, both hardware and software, that claim to work as ‘sound better’ boxes, but in the case of the Dottore XFM, the claim is definitely justified.