Solid State Power Amp Reviews

Manufacturer's Comment

Thanks to Jason and all at Stereophile for such a great review of our amplifier.

All Infigo Audio models are designed to create the ultimate musical experience. Of course, measurements are important, and we thank John Atkinson for uncovering some things that will lead to demonstrable improvements.

Scrutinizing the bias circuit to find and fix what caused those issues revealed a small undershoot of idle bias current right after the zero crossing of the signal. This undershoot deepens with increasing frequency and with total signal-current drawn. This leads to a new phenomenon, which I dub "cross-after" distortion because it happens a defined time after the signal's zero crossing.

The minute amount of negative feedback used caused this anomaly to show up in measurements at higher frequencies and power. Adding large amounts of negative feedback would suppress this to the low levels commonly present in amplifier designs. However, doing so would kill the amplifier's musicality.

Typically, this anomaly plays an inaudible role, which is why it wasn't noted in previous listening sessions, especially at audio shows.

The deepening of this cross-after current drop with increasing signal current limits the output power at higher currents, which partly explains the discrepancies in measured power between 8 ohm and 4 ohm loads. Thanks to John's input, we are incorporating a redesign of the topology of the bias circuit that is free of this anomaly and will fix the measurement discrepancies. Starting immediately, every Method 3 will feature the new topology.—Hans Looman, CEO and Chief Designer, Infigo Audio