Nagra HD PREAMP preamplifier Sidebar

Tubes and Transformers

While the HD PREAMP was conceived to partner the sublime HD AMP mono blocks [HFN Jul ’18], its design follows a very different path. I discussed the topology of the solid-state (MOSFET) HD AMP in our earlier review but while both amplifiers have a lot of attention lavished on their PSU regulation, reservoir capacitance and RF/EMI filtering, the HD PREAMP, by contrast, combines a tube line buffer (employing NOS E88CC triodes) with a transformer-coupled output. This transformer employs multiple secondaries that are switched in combination to provide very fine adjustment in volume. This is a more sophisticated approach than you’ll experience with an archetypal ‘passive’ transformer preamp although the HD PREAMP does offer the maximum 6dB gain – a doubling of signal level – that is common with such devices. There’s also some change in output impedance and a very slight adjustment to its ultrasonic response as a result of volume (transformer tap) position but, again, this is minor compared with the far higher source impedance of some passive preamps [see Lab Report]. PM

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