Phono Preamplifiers

We’re already very familiar with the cutting-edge technical design of Grimm Audio’s digital products [HFN Dec ’20 and Apr ’25] which push at the boundaries of both subjective and measured performance. However, those of us expecting the PW1 to be cut from the same ‘vanishingly low distortion’ cloth should think again, for designer Peter van Willenswaard’s ‘Phono Wizard’ is borne of, shall we say, more Zen-like principles. You can read Peter’s thoughts on his blog (https://www.grimmaudio.com/blogs/designing-the-pw1-phono-preamplifier) which is a powerful advocate for minimalism.

In the PW1, MCs are treated to either +20dB or +30dB gain before the signal joins the MM path where two FET gain stages sandwich a passive RIAA network. There follows an op-amp output buffer that’s switchable between unity (0dB) and +10dB extra gain. I discuss the MM/MC gain options in my [see PM's Lab Report] and while these are accurately specified by Grimm, so too is its candid admission of a relatively high 0.56% THD (re. 5mV/MM input). This represents a 358mV balanced output from the PW1 where, in practice, THD is a little lower at 0.3% – a figure that increases to 0.65% with a 10mV input, 1.35% with a 20mV input and a full 3.5% with a 50mV input [Graph: MM, red; MC 20dB, pink; MC 30dB, black].

The latter represents a realistic output from a 5mV-rated MM when tracing a groove modulated at +20dB (re. 1kHz/5cm/sec). The ‘24dB overload margin’ cited by Grimm would realise ~5.4% THD at 1kHz and a whopping 20%+ at 20kHz. While these are massive figures when judged by current standards [see HFN Aug ’25], it should be appreciated that the PW1 is a solid-state emulation of Peter’s personal, low-feedback tube phono preamp. In both designs, passive RIAA eq soaks up a lot of gain while overall circuit linearity is viewed through the lens of typical MM/MC pick-up distortion which, as we know from HFN’s own exclusive tests, rapidly increases at treble frequencies. So the PW1 is, arguably, the first phono preamplifier designed for ‘hi-fi realists’! PM