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Small yet Mighty. The tale of the 12's


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Thanks @maxmercy, I will need to read up on that tonight and see whats up.  If it helps me identify if I'm clipping I'm all for it.  Like I said, I know I need to ensure my signal chain is setup appropriately so I'm getting all I can out of these drivers, while also ensuring I dont blow them.

One update is that I made some small tweaks to things shown below

  • Audio/Volume: Scale - 0-98* | Limit: 70 (previously 80)
    • *My options for the scale are -79.5db -18db OR 0-98 (which is what I have mine at)
  • Speakers/Crossovers: Front: 60hz (previously 80)
  • Speakers/Bass: Subwoofer Mode: LFE | LFP for LFE: 80hz
  • Speakers/Level: Subwoofer: +4.0 (previously -2.0)
  • Audio/Subwoofer Level Adjust: OFF (previously +6.5db)

Doubt this is going to change much, but haven't had time to measure anymore and wont for the forseeable future.

PS all you folks have got me looking at alternative solutions as well such as employing some 21" drivers in my nearfield boxes and swapping out the 12" drivers slated for my DO Sonotubes (still a WIP) with some local FI Audio 15's lol.....

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The process would be to use the HDMI out from REW for your signal.  You can use EqualizerAPO to 'copy' the signal to other channels in the 7.1 channel bed.  Measure at the subwoofer out, at low AVR volume, with a low signal drive level.  Add AVR volume until you are at your 'max' listening AVR volume setting, keeping track of both the soundcard input signal level (so as not to clip it and overload/fry the card) as well as the waveform on REW.  One or the other will clip as you increase drive level.  You want to protect the soundcard while you find the limit for the subwoofer out.  You may need a voltage divider circuit to do this, some AVRs can generate significant voltage from the SW out.  

MOST AVRs will clip the SW out with a WCS (Worst Case Scenario) signal, especially if the AVR output levels for each channel (esp subwoofer) are at "-0" or at "+anything". 

To make a WCS signal, use channel 4 on the REW HDMI out, and use EqualizerAPO to copy that signal to all other channels.  Be careful, that is a lot of signal to be bass-managed, you have to start low on both drive level (-30dB or below), and AVR volume.  I would disconnect all speakers powered by the AVR to do this.

Once you find what bass-managed signal strength clips the AVR output, you have to make sure it doesn't clip the MiniDSP input.  You can either measure a 60Hz signal with a voltmeter at the SW out to make sure it isn't above 4V, or hook up the MiniDSP and look at it's input levels as you play the 60Hz WCS tone with REW and EqAPO as you increase volume to the point where you clipped above.

Once you have done that, You have to make sure your EQ is not going to clip the MiniDSP output, and that your MiniDSP output is not clipping your Amp input.  Easiest way to do that is look at the MiniDSP out with your craziest EQ scheme and sweep, making sure to protect the soundcard. 

To see if you clip the subwoofer amp input,  you can apply the craziest boost you can to a 60Hz tone again, and use a voltmeter to make sure the V out of the MiniDSP is not clipping, and below your amp's voltage sensitivity rating.  You can also measure directly out of the amp with the amp turned down and a voltage divider to protect the soundcard.

Lots of things can clip in an audio chain, and if anything in the chain does, it will increase distortion VERY quickly.


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What John is describing is very similar to how I do it.

One difference is I run the sine wave looped back through the RTA and monitor the THD. It should be well below 1%. Once it starts to clip you'll see the harmonic content jump abruptly and THD shoot up over 1%. It usually happens very abruptly. If the device has a clipping indicator you may find this happens well before or well after the indicators. You do have to make sure that this is the output voltage clipping and not the input though. To reiterate a voltage divider will probably be required. I cheat and use one of my DCX2496's in the chain usually. They will take an input of >10 volts before clipping and can easily reduce the signal to something that your SC input can handle when looped back. Go through the components one piece at a time starting with the AVR, then adding the EQ unit and finally the amplifier. You want to eventually get the highest voltage cleanest signal that you can to your amplifier and then set the amplifier so that it is the first thing that clips. This also helps keep your electronic noise floor as low as possible.

Definitely disconnect all speakers. They aren't needed for any of this testing. 

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FWIW, for the Denon and Marantz AVRs and processors I know of, if you have bass management enabled, they cannot pass a WCS signal at master volume "0" (or 80 for you) unless the subwoofer trim on the unit is set to the lowest setting of "-12".  This assumes Audyssey is turned off.  If Audyssey is turned on, it's still possible to clip.  If you don't play higher than "70", then you should be OK with the trim as high "-2" without Audyssey.    With Audyssey, you might still want to go with "-12" because Audyssey can boost (IIRC) by up to 9 dB.

These days I use my Denon without bass management and without running Audyssey.  I do it in my own processor downstream.  Hence, I can get away with putting my sub-trim at "-2" in theory.  (LFE channel alone needs -10 dB compared to LFE+LRCS.)  In practice I have mine set to "-7" and mains set to "+3" as this let's me go as high as +5 dB vs. reference, in case I need that.

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4 hours ago, Ricci said:

I cheat and use one of my DCX2496's in the chain usually. They will take an input of >10 volts before clipping and can easily reduce the signal to something that your SC input can handle when looped back.

That is an awesome tip.  I'll  have to use that in the future.


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