Pre/Power Amplifiers

Look beyond the glitz and glamour and there's a very clear-minded circuit philosophy behind darTZeel's amplifiers – minimum active devices (transistors), minimum feedback, all local in situ signal routing and no Zobel network on the output of its power amps. While we discuss the evolution of the NHB-468 and NHB-18NS in our main review, there's a hidden gem in the form of designer Hervé Delétraz's 8th-generation MC phono stage.

'We use eight paralleled pairs of low noise discrete transistors [no step-up transformer] at the input', says Hervé. 'This reduces noise by about another 9dB, and what remains is pure hiss – there's no hum because of the battery PSU. This allowed us to increase the onboard gain up to +72dB – actually, as the phono stage is fully discrete, with a pure passive RIAA EQ in between two identical amplification sections, the internal gain at 20Hz is +92dB!'

Offering the multitude of gain and loading options while still adhering to darTZeel's NS (No Switch, or no contact in the signal path) ideology was challenging. 'The solution was to use shorting relays as close as possible to critical gain and loading components while still keeping switches out of the signal route. The relays are all governed via a micro.'

A CPU on a phono board? 'Yes', Hervé confirms, 'we designed a new 6-layer PCB, with massive ground planes. The analogue circuits are on one side, the CPU and other logic on the other side. And, for total isolation, when the settings are done, the CPU goes into sleep mode after ten seconds'. PM