Phono MM MC
MU-2 was starting point in our adventure with tube amplification. Experiments with transistor phono preamps was not as satisfactory as we expected. We wanted to verify if such old fashioned electronic component as tube meet high requirements of Moving Coil (MC) cartridges preamplifier. MC cart generates very low 0,2-0,5mV signal that has to be amplified 1000 times (60dB) and in addition corrected according to RIAA curve what increases the required open-loop amplification to 10000 (80dB)!. Of course we’d like the sound is free from background noise and hum. That’s nothing to transistor but for noisy, low efficient, high voltage (200-500V) supplied tube is a challenge and looks impossible to do.
Indeed there is no tube only MC preamplifier on the market. We decided to use J-FET transistor in parallel with 6922 tube in input stage. Together with 6922 tube second stage we achieved 51dB amplification factor. In practice it fits 0,4mV and higher voltage MC carts and of course all Moving Magnet (MM) carts.
Thanks to advanced 250V stabilized power supply the sound of MU-2 is surprisingly good: low hum and noise much lower than generated by vinyl record itself.
Side to side comparison to transistor preamps was positive for MU-2. It’s subjective opinion but MU-2 is more dynamic and neutral. When additionally pure “analogue” design will be taken into account then vinyl record listening satisfaction is guaranteed.
MU-2 has the following additional features:
- two completely separated path for left and right channel, with one triode in each. It allowed to decrease inter channel crosstalk level
- input gain switch for MM and MC cartridge
- input impedance switch for MC cartridge
- resistors and capacitors in audio signal path with 1% and 5% tolerance respectively for RIAA correction curve accuracy.
- microprocessor controlled operation
Front panel was made of polished glass. Incandesced tube’s bulbs can be observed via two oval viewing insets. Preamplifier was equipped with superb quality 6922EH triodes made by Electro-Harmonix. It guarantees ideal symmetry between channels. MU-2 chassis was made of polished acid-resistant stainless steel. Power supply module together with toroidal transformer were separated from the rest of electronics by additional metal shield. Like in all of our products case elements and transformer was produced by polish manufacturers and manually assembled. We are very proud of it.
MM/MC switch is located in the rear side. I’s in our opinion optimal approach, much more comfortable than set of micro switches used by other audio equipment manufacturers. Those micro switches are often located inside or underside the case and thus very hardly accessible. Exposing such switch on the front panel is also questionable from aesthetic point of view especially that it is used occasionally. In MC mode there is possibility of input impedance regulation in range 50Ω – 1kΩ, depends on cartridge manufacturer recommendation and listener sound preferences. Such range will be sufficient for almost every phono cartridge available on the market. MU-2 gain and input impedance level can be changed during listening of the records thanks to microprocessor control. It taking care of output relays switch off just before change and a switch on again when preamplifier is back to stable state. Thanks to that loud pops unpleasant for ears and dangerous for power amplifier and speakers could be avoided. This operation is completely inaudible. Similar task is performed by microprocessor when power off the preamplifier. Output is switched off immediately. Microprocessor is also responsible for switching output on after 40 seconds tubes warm-up time when power on the amplifier.