Orpheus Phono Preamplifier with LCR RIAA equalizer
In a sentence: Two tube gain stages with zero feedback passive LCR phono equalizer. The phono equalizer is designed to amplify the delicate signal of your phono cartridge without loosing any of it and to apply the RIAA equalization. To achieve this our design accommodates for both MC and MM type cartridges trough the use of a built in Lundahl cobalt amorphous step-up transformer for the MC types. It is switchable for low and high impedance providing for appropriate load and gain to the wide variety of cartridges.
The input stage is a super low noise D3A German tube used as a triode. It provides the necessary gain and output impedance to feed the custom passive LCR RIAA correction. The equalization is done with high precision custom made Sowter coils wound with Cardas wire and Jupiter BeeWax impregnated paper capacitors. As the equalization is passive the next stage is another super low noise tube (a Russian triode similar to WE417) loaded by a Hashimoto transformer to bring the signal up to the necessary level and provide complete isolation of this sensitive circuit from the following components.
The equalization is realized through the use of constant impedance filters. This means that the input stage feeding it sees a constant load. This making its behaviour predictable at any signal level. The filter is out of any gain loops and sits between the two gain stages protected from any unpredictable influences.. To make it properly, very precise high quality inductors are needed. The signal is in series with them so high wire quality and magnetic material with lowest hysteresis are used. This solution was made popular years ago by a Japanese transformer manufacturer called Tango. Due to the difficulty in manufacturing consistently and the high cost associated, penetration of the idea was very limited to just a few manufacturers. Yet this was enough to start a worldwide movement worshiping it. We decided to go a step further, making it from discrete components allowing us to tune it in context of the whole system.
The power supply The power supply of Orpheus uses a very well known but extremely rarely used arrangement. We use a C core transformer with a gap in the core to prevent any saturation of the core from distorted mains or a DC component in the mains. As the power transformer is in the same case we chose a design that has two identical sets of windings (essentially two transformers) on the same core. This way all the nasty magnetic and electrostatic leaks are cancelled and it does not influence any of the other components in the chassis. To reduce power supply noise further instead of the normal arrangement of bridge rectifier and electrolytic capacitor bank we use a choke input full wave tube rectifier arrangement. This practically eliminates switching noise and anything that comes before the choke stays there. The other beauty of this topology is that the choke keeps the voltage on the capacitor bank quite constant. In our case, this allows for a considerably small capacitor bank opening the possibility to use vibration resistant military paper in oil capacitors for the job that would last a lifetime.
The ergonomics. It does not matter how advanced a technology is, if it is not useful. We have tried to anticipate all the possible needs of our clients. Orpheus has 2 inputs on RCA and one on XLR connectors. All inputs are balanced differential unless the grounding option is engaged on the RCA inputs. As we switch the ground plane when switching inputs there are no hum loops and induced ground noises, allowing for large setups with multiple sources powered from different mains outlets in the house. It applies for the outputs as well. We have provisioned for balanced and unbalanced outputs. Switches on the back, which program the unit, set the gain and type of each input. We have included an absolute phase switch with memory for each input.