This review page is supported in part by the sponsors whose ad banners are displayed below
The sound of these monos is exactly like or better than the highest-quality SET. Except these do 250 watts of pure class A with 24 amperes of current capability and very high bandwidth. That's unique. That was Stavros Danos of Cypriot valve electronics house Aries Cerat who also designed my reference Gladius speakers.
The project brief PDF of his amps read so interesting that I felt inspired to relate it here by way of an informal—unpaid unsolicited—'infomercial'.
Vaguely reminiscent of Audiopax's parallel-connected twins in the Model 88, the Aries Cerat Contremo consists of two discrete single-ended OTL parafeed amplifiers in one chassis. Those are balanced-connected through a toroidal output transformer. Each of the two SET OTLs runs at 3 amperes idle to produce 6A idle current per channel.
Each mono amp runs two banks of 16 Svetlana 6C19p for 32 high-current low-resistance triodes per side driven from a hybrid direct/reactor-coupled input stage with one Siemens E280F per bank. For power each of the independent SET amps runs double-choke regulation, choke-regulated filaments and massive high-speed capacitor banks. The same applies to their driver stages. This clocks the combined iron at 16 chokes and 5 toroidal transformers per monoblock or twice that for a pair. Full-power bandwidth is given as 20Hz to 90kHz with an impressively low noise floor of -110dB unweighted.
Stavros explained. The very low ratio of the output transformer contributes to the amp's sonic virtues. Push-pull designs usually use high step-down ratio transformers. This suffers very high parasitic capacitance and inductance to affect the high-frequency spectrum on power levels and distortion. Any slight current imbalance between the two push-pull banks also causes severe problems in the low end, usually with high distortion in the bass and very high intermodulation artifacts over the entire audio range. Our design has none of these serious drawbacks.
The two OTLs are AC-coupled to the output transformer which instead of the usual 25:1 ratio only runs a 4:1 ratio. Our parasitic capacitance and inductance values thus are about 36 times lower than the best of the conventional push/pull transformers. Output resistance is a very low 0.25 ohms without feedback anywhere in the signal path and this circuit can deliver 24 amperes continuous without the slightest sag on its power supply. The Contremo monos are our statement on how high-power tube amplifiers must be built.
How deep did his take on the bleeding edge cut into the wallet? Retail is €38.000/pr. At about half of Audiopax's current high-power statements, that's still very serious money but given the raw power specs of pure class A tube operation probably as close to value as any such extreme efforts can get.