Question about Qu-5 input impedance: 1.5kΩ vs the old 5kΩ?

Qu Qu General Discussions
1

I’m looking at picking up a new Qu-5D. I was digging through the spec sheets and noticed something that’s raising some alarm bells for me.

The original Qu-16/24/32 series and the SQ series list a mic input impedance of >5K-ohm. The new Qu boards are listed at >1.5K-ohm.

I’m definitely not an electrical engineer, but from what little I know about mic loading, this looks like a step backward. I get that 1.5Kohm is totally fine for active DIs, wireless receivers, or condenser mics. But I still rely on wired SM57s and other dynamic mics.

My understanding is that a 57’s impedance isn’t perfectly flat and spikes pretty high at its resonant frequencies. If the preamp is only 1.5K, wouldn’t that fail to bridge properly and overdamp the capsule? I’m worried it’ll act like an electromagnetic brake and dull the transients and choke the top end right at the jack.

Even the Zed consoles seem to have a 3.3K input impedance. 1.5K is quite unusual at this price tier.

Was this a necessary engineering compromise to make the new padless preamp design work with lower thermal noise? Or was it just a space/cost-saving measure for the new chassis?

I’d love to hear from the A&H engineers on the rationale behind this change.

Also, has anyone actually A/B tested a dynamic mic on a snare between the older 5K-ohm preamps and these new ones? Will I actually hear a blunted transient, or am I totally overthinking the math here?

Genuinely happy to be proven wrong, I just want to understand the tradeoffs before pulling the trigger. Thanks!

2

Interesting question. Also curious to hear what it means. I’ve been using a QU5 for 3/4 of a year now, with all kinds of mics (mostly Beyer Dynamic though) but it really sounds great.

3

Note that you’ll only see this if you use the local IO directly or through an analog snake. A DX168 masks the problem. Furthermore, the difference will only manifest at the high-end of the frequency spectrum. It might be audible on a snare’s sizzle, but probably not on vocals.

It’ll still probably sound great – in fact, rolling off the top-end may give it a slightly warmer vintage character. But that’s usually not Allen & Heath’s “thing”. My perception is that A&H’s engineering objective is to usually go for a hyper-accurate capture and leave it to their users to dial in their tastes.

To me, the bigger issue is consistency – the fact that the console may sound slightly different depending upon whether I’m using local IO or a DX168 is an irritation that I’ll have to constantly remember.

Once again, I am not an electrical engineer and am probably riding the crest of the Dunning-Krueger curve with all this theorization. I would love for a real engineer to correct me.