This design has been arrived at during several years by Mika Kalm aka MrCurwen. It’s quite well thought out, the individual parts complement each other. It was originally designed to avoid using boutique parts and instead use less known, still mostly undiscovered high quality but low price parts. Furthermore the design was made in such a way that diminishes reliance on parts.
In Oblivion the input stage is single ended while Citadel’s input stage is balanced. After the input stage, in both amps, are balanced CCS loaded source followers that drive the output tube grids.
The grid drivers serve two functions:
- Relieve the previous stage of any drive responsibilities. This includes the coupling capacitor; the capacitor is basically completely transparent when placed in front of a FET gate.
- Properly drive all the parasitics of the tube grid. All tubes draw some amount of grid current much before grid going positive. For best results, this must be accounted for, and the way to accommodate it is not thru a capacitor which is the traditional way.
The grid drivers improve transient response, they make the sound more ‘fast’ and dynamic. The technical reasons for this are 70% relating to the grid and Miller capacitance and 30% related to the coupling capacitor. The follower deals with both of them in a brute force manner.
All tubes have gyrator plate loads that provide a practically flat loadline for the tubes. This eliminates individual tube distortions. Also the balanced stages have a very powerful tail CCS, which eliminates practically all imbalances caused by tube mismatch. Matched tubes are not necessary with this design.
Everybody knows that a tube amp lives or dies by it’s output transformer. A not-so-good output transformer will traditionally always lead to a not-so-good amp. All OT’s are a bit problematic technically, so you should always buy the most expensive one.
These amps utilizes a technical solution which gets around this. OT price is no longer an issue! This is achieved by two design choices: no DC is passed through the OT primary, and the signal to the OT primary is driven by a low impedance source. These choices were not readily available in the past, but now they enable us to get rid of OT related imperfections.
Here you have two imperfect solutions (‘weak’ follower and an OT) that in effect cancel out each other’s weaknesses. The OT primary is such an easy load that the weak follower is overkill for the job (even a tube can manage it!) and produces no distortion, and the imperfect OT is so ‘overdriven’ that it’s parasitics disappear. It doesn’t produce any distortion. The idea of OT’s as distortion producers is a relic from ages past where you only had very compromised ways of driving the primary. Since the output is parafeed there’s no DC flowing in the OT primary.
The OT doesn’t need to be of boutique quality because it is driven to transparency as it is.
Both the B+ and B- power supplies are each regulated with a two FET regulator which keep the PSU output voltages rock solid, they don’t fluctuate with the signal on heavy transients. That combined with the constant current draw from the audio circuit make the power supply practically “perfect”. Meaning it doesn’t affect the signal in any way.
Both Oblivion and Citadel are designed with the same ideology in mind. The major difference between the two models is lower THD and less harmonics in the all balanced Citadel vs Oblivion. It’s a matter of taste; some prefer a touch of “tube sound” while others prefer full transparency.
Citadel without any optional add-ons
Oblivion with optional XLR inputs and updated full aluminum chassis
Oblivion with speaker outputs
NOS Russian film capacitors
Citadel tubes, 6P31S (EL36) & 6H9C (6SL7)
Oblivion tubes, EL81 & 6AT6