Solid State Power Amp Reviews

Sidebar 1: Ultimate Class-D

There's a clear evolution in technology and performance from Hypex's original and innovative "self-oscillating" UcD through to NCore and now to the Purifi Eigentakt class-D module used in the Buckeye amplifier. The original UcD circuit married an input comparator, a power stage, and LC filter with feedback looped back between output and input "undoing" enough of the LC filter's phase shift to ensure stable operation. Loop gain was a respectable 30dB, but designer Bruno Putzeys has said this was as far as he could go and still guarantee the amp's stability.

The later NCore modules combined a UcD amp with a filter that simulated its response, the difference between the two outputs being the distortion of the power stage alone. This error signal drove the corrective feedback with a limiter now added to the correction to prevent over-reaction when the NCore amplifier clipped. So NCore broke the link between stability and loop gain, offering over 50dB of feedback and with the same load-invariant frequency response that distinguished UcD from run-of-the-mill class-D amps.

The Eigentakt circuit superficially resembles NCore's but now includes a low-pass filter added at the input, all encompassed within the Eigentakt's "global" feedback regime. Loop gain is extended yet again, but the control circuit is now so complex that component values had to be derived by modeling software and not tweaked by hand. The result? A full 75dB of feedback, unconditional stability, a vanishingly low 0.006 ohm output impedance, and flatter, more extended response.—Paul Miller