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Discussion Starter #1
I'm a newbie here, so if I've posted to the wrong forum, please direct me appropriately.

My listening setup for 2.1 audio is as follows:

FLAC source over wired Ethernet, into
Raspberry Pi B+ with HiFiBerry DIGI+, running XBMC, Toslink, into
NAD C390DD amp, into
Pair of Axiom M80 towers and 12" SVS sub

Astute readers will observe that the entire signal path remains in the digital domain until the last possible moment, just before delivering analog to the speakers. When I use my laptop as source, I play FLAC files via JRiver Media Center through USB to the NAD amp using a NAD supplied driver.

The NAD amp can be characterized as a DAC with USB/Toslink/Coax inputs and a (class D?) power amp inside one box.

I have more or less established best speaker and listening positions wrt room dimensions.

Now I wish to adjust amp and sub parameters to optimize room performance. The sub has variable crossover, gain and phase adjustments. The amp can boost/cut at 6 different frequencies from 40 to 240 hz and in narrow or wide mode for each. A fair bit of adjustment to do by ear alone. With everything set to zero, I sense that the result is less than optimal.

I have installed REW on my laptop and have played with sweeps using laptop microphone and USB out to the C390DD.

Now I am confused. The REW instructions suggest I should first join the audio out to the audio in and do a loopback pass to calibrate the sound card. I can do this with the analog jacks on my laptop, but REW cautions that these are noisy. I could invest in a standalone "soundcard", but it seems to me that I would then go analog in the sound card. My amp has no analog inputs, so I would need an ADC to get back to the digital domain. I don't know how to do a USB loopback, and at any rate nothing in my laptop participates in USB audio except the NAD driver. By using the Raspberry Pi, I avoid the possible deficiencies of USB audio anyway. I have no need for a soundcard in normal playback (do I?).

All I really want to do is have REW do its sweeps through the NAD USB driver to the amp, the amp through the speakers, and the speakers output picked up by a microphone, preferably a USB mic to avoid the anomalies of the analog laptop input. Once I have the amp and sub adjusted optimally, the laptop won't even play a role in playback.

Am I missing something?

Thanks for any help anyone might provide.


Super Moderator
9,256 Posts
The loopback calibration is really only needed if you have a so-so sound card. Everything I’m hearing these days says that most late-model laptops have soundcards with linear response. So you can skip that step and use the laptop’s headphone output “as is.”

Your best bet is to just connect the laptop’s headphone output directly to the sub’s input and run REW like that. You’ll need an outboard parametric EQ to equalize it.

If that’s not to your liking you’ll have to get some additional hardware. Shouldn’t be a big deal since your NAD has cards available with HDMI and analog inputs. But even with that, the built-in equalizer isn’t all that impressive since it only has two bandwidth settings. IOW, I’d recommend an outboard EQ for the sub even with the NAD’s equalizer.

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