With its ability to deliver anything from super-subtle saturation to downright despicable distortion, could this be the only valve preamp you'll ever need?
The amazing technical progress of the last 25 years or so has delivered audio equipment that's reached an incredible level of accuracy and transparency, which is great for all of us. But hand in hand with that goes the loss of what are often musically attractive distortions, and so all manner of plug-ins and hardware devices have appeared to allow saturation and distortion to be applied in a more controllable way. But sometimes it's just more inspiring to add this coloration at source, and that's very much the raison d'etre of Canadian company Sonic Farm.
Sonic Farm have consistently impressed me with their musically involving range of mic preamps, and their Xcalibur pentode-valve 'saturation preamp', launched at Summer NAMM 2016, was designed from the ground up specifically to provide a broad range of controllable distortion while tracking. This big two-channel preamp, which weighs a hefty 6.6kg and extends 33cm behind the 1U rackmount ears, is still available but Sonic Farm recently released a slightly more elaborate version, the Xcalibur JC. This upgraded model, which is reviewed here, has a couple of extra features and a slightly modified signal path, all of which are the result of suggestions from celebrated engineer/producer Joe Chiccarelli.
The Xcalibur JC's primary purpose is to add character but it is fundamentally a clean-sounding, two-channel, transformer-coupled mic preamp. The primary gain stage is based around an EF86 pentode valve, in a topology that omits negative feedback — so broadly similar to the company's Creamer (reviewed in SOS May 2013: https://sosm.ag/sonic-farm-creamer) but with a higher anode/plate voltage (320V, compared with the Creamer's 285V).
According to the specifications, around 68dB of gain is available for the mic input, and both channels also feature rear-panel balanced line and front-panel unbalanced instrument inputs, with up to 48dB of gain on tap. The mic and line inputs are all transformer coupled, but the instrument input is connected directly into the pentode's control grid; the circuit topology and 2.2MΩ input impedance are intentionally very similar to those of a traditional guitar amp. All four input transformers are made by Cinemag, with CMMI-10Cs for the mic inputs and CMLI-15Bs for the line inputs.
The output section shares the facilities of most other Sonic Farm products, with switches to select a solid-state, op-amp-based output driver or a Cinemag output transformer. Also like other Sonic Farm products, when ordering the Xcalibur you can opt for a slightly softer-sounding 100-percent iron-cored transformer instead of the standard CM13104 Ni-Fe alloy model.
What sets the Xcalibur apart from the company's other preamps is the addition of a second amplifier stage that's dedicated to creating saturation/distortion. This is based around another EF86 pentode but it works in concert with a FET which affects the way in which it approaches clipping; the arrangement is optimised to produce rich, smooth and 'creamy' distortion. There's also an adjustable high-pass filter to determine which parts...