Pre/Power Amplifiers

Halcro’s logic- (relay-) governed R-2R volume control ‘number’ reflects the absolute gain in single-ended (RCA-to-RCA) mode but there’s a +5.6dB boost in the fully balanced (XLR-to-XLR) setting. So, ‘0.0dB’ volume represents –0.4dB and +5.6dB gain and ‘+20.0dB’ gives +19.6dB and +25.6dB gain, in RCA and XLR modes, respectively. In practice, the 40x ±0.5dB steps in between are achieved with a mere ±0.02dB error. The next 40 steps down to ‘–20.0dB’ are also achieved with an accuracy of ±0.02dB, increasing to ±0.07dB down to ‘–30.0dB’ and ±0.15dB down to a volume setting of ‘–40.0dB’. Down to ‘–50.0dB’ the error increases to ±0.25dB ending up at an absolute –79.38dB at the final ‘–60.0dB’ volume position. Over the Equinox’s full 160-step, 80dB volume range the channel balance is typically good to within 0.02dB – all these figures being impossible to achieve with a conventional volume pot.

The Equinox’s maximum balanced output is a substantial 16.0V from a flat but moderate 164ohm source impedance [red trace, Graph 1] while the response [black trace] remains independent of volume position, reaching out to –0.2dB/100kHz while rolling away at very low frequencies to –0.1dB/20Hz and –3dB/3Hz. Noise is low, leading to a 109dB A-wtd S/N ratio (re. 0dBV) – possibly the widest I have ever measured from a preamplifier. Halcro also makes claims for low distortion but the rated 0.0002% under-estimates the minimum 0.00003% achieved at 1kHz over a full 100mV-6V output range [vector-averaged and see Graph 2, RMS averaged]. Distortion does increase with frequency but this is still a very low 0.0002%/20kHz and, interestingly, also falls with increasing input level up to the maximum 4.6V tolerated by the Equinox. PM