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Q. How do I prevent my preamp overloading my audio interface?

By Hugh Robjohns

When hooking a preamp such as this Grace Design m201 AD, which has both analogue and digital outputs, to an audio interface, it’s important to make sure the analogue sensitivity settings of both units are suitably configured — otherwise you can end up with wildly different levels from the digital and analogue preamp outputs.

I purchased two Grace Design m201 mic preamps because Hugh Robjohns really praised the design and its performance, so I seek his help again to resolve an issue and add to my understanding. I’m having trouble understanding how the m201 A‑D converter works. I use a Neumann mic on channel 1 and take the analogue XLR output to a MOTU interface and into my DAW. The DAW signal level is pretty good and very usable. The same Neumann mic signal goes through the Grace’s internal A‑D, and I take the S/PDIF digital output to the same interface and into the DAW, but in this case the S/PDIF signal level is a great deal less — and not very usable in comparison.

I can increase the Grace channel 1 gain to get the S/PDIF out to be nice and usable, but then the Grace’s channel 1 analogue out clips the interface. I think the AES3 and S/PDIF connections give similar output levels and both are much lower than the analogue channels' XLR outs for the same mic signal. I contacted Grace Design, who said that there could be a sensitivity mismatch between A‑Ds, and that I may have to re-calibrate my Grace m201 units; MOTU said their AES and S/PDIF circuit is designed to AES pro standards. What am I doing wrong? Can you explain what’s happening in technical terms, so I can understand how to resolve this issue?


SOS Technical Editor Hugh Robjohns replies: Grace were right on the money: the only ‘issues’ here are different analogue gain structures in the two signal paths, and/or different analogue/digital alignments employed in the Grace and MOTU A‑Ds. The Grace preamp is designed to full professional specifications and has the ability to output very high analogue signal levels — its analogue outputs don’t clip until +28dBu, and the factory calibration of the internal A‑D provide maximum digital level (0dBFS) with an analogue signal of either +18 or +22dBu, depending on which calibration mode is selected (although the internal alignment can be adjusted between +20 and +24dBu, if required). So, to get your mic signal peaking strongly on the Grace’s internal A‑D you’ll end up with quite a high signal level at the analogue output, as you’ve found.

That this appears to be overloading the MOTU’s inputs suggests that the analogue/digital alignment in the MOTU is (currently) set much lower than in the Grace — in other words, the MOTU’s A‑D converter reaches the maximum digital level (0dBFS) with a much lower analogue level of, say, +12dBu. By setting a strong level in the Grace, then, you overload the MOTU, while setting a more conservative level in the Grace gives a weak digital output.

I don’t know which MOTU interface you’re using, but the MOTU 828 MkIII, for example, has adjustable input trims for the rear-panel analogue inputs, and the manual implies that when turned fully down it’s capable of accepting up to +22dBu — which happens to be the same as the Grace’s default alignment. If your interface adopts the same approach, you should be able to turn down the input sensitivity of the MOTU input channels that are connected to the m201. The MOTU would then require a much stronger analogue input level to reach the maximum digital level (0dBFS) — you’d have brought the Grace and Motu A‑D converters into alignment. It might also help if you switch the Grace to use the +18dBu alignment option, as this will also reduce its analogue output level a little.

To comment briefly on your other point: the Grace’s AES3 and S/PDIF outputs will be identical in level, as the differences between these two formats relate only to the physical connectivity (plug types, electrical voltages, balanced/unbalanced, etc) and the ancillary data.

I have to say, though, that it seems rather pointless to run an analogue signal into the MOTU from the Grace m201 when you have the latter’s lovely A‑D converter on board. Personally, I’d just send the Grace’s digital output to the MOTU, not forgetting to set the MOTU’s clocking to sync from the digital input.

Published April 2018