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DIY Speakers and Subs

AustinJerry's Avatar AustinJerry
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Quote:
Originally Posted by 3ll3d00d View Post
@AustinJerry one thing just occurred to me, you are outputting from REW via the LFE channel aren't you? ISTR you having trouble with ASIO and hence can only output in stereo via HDMI. If this is the case then you're 0dBFS signal is 10dB low & hence you will clip way sooner than you think (and would be better off switching the jumpers on the minidsp to the 2Vrms sensitivity).
Whew, you had me concerned there for a moment. No, I am outputting with the AVR inn Stereo mode, with the signal going to the sub channel by virtue of bass management. It should not be 10dB low.
3ll3d00d's Avatar 3ll3d00d
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Quote:
Originally Posted by AustinJerry View Post
Whew, you had me concerned there for a moment. No, I am outputting with the AVR inn Stereo mode, with the signal going to the sub channel by virtue of bass management. It should not be 10dB low.
0dBFS on the lfe channel is 10dB higher then that on a bass managed main channel. You will be in danger of clipping the input if you go past -10 in the MV.

If you want to prove this then grab a test disc that has such a signal (e.g. DIY Audio Test DVD) and use that in place of the rew signal generator.
AustinJerry's Avatar AustinJerry
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Quote:
Originally Posted by 3ll3d00d View Post
0dBFS on the lfe channel is 10dB higher then that on a bass managed main channel. You will be in danger of clipping the input if you go past -10 in the MV.

If you want to prove this then grab a test disc that has such a signal (e.g. DIY Audio Test DVD) and use that in place of the rew signal generator.
D00d, not only do I not understand what you are talking about, I can't understand why you are bringing this up at the 11th hour. I am not using the LFE channel. It's like, Jerry, you are almost done with reconfiguring exactly like I told you to do, but I forgot something important that renders everything you have done so far null and void.

Edit: Besides, this is a system for listening to music. How many 2-channel music tracks have an LFE channel?
3ll3d00d's Avatar 3ll3d00d
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Quote:
Originally Posted by AustinJerry View Post
D00d, not only do I not understand what you are talking about, I can't understand why you are bringing this up at the 11th hour. It's like, Jerry, you are almost done with reconfiguring exactly like I told you to do, but I forgot something important that renders everything you have done so far null and void.
yeah sorry. However it doesn't render it null and void as a) now you know how to do this, so b) you won't be faced with a horrible clipping of the minidsp, and c) all is not lost all it means is you are 10dB off.

You had 1.8V at MV=0/SW=0 so +10dB gives you ~5.7V, you now need to switch the sensitivity on the minidsp to 2V and then reduce the trim by bring this down to 2V (5.7V down to 2V is about -9 so a final trim of ~ -9.5)
AustinJerry's Avatar AustinJerry
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I still do not know why this affect me--I am not using the LFE channel in any of the measurements. In case you can't tell, I am seriously upset by this development.
Audionut11's Avatar Audionut11
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I could understand you being upset if we were pretending to be experts on the subject, and charging you for a service. But we are just trying to help as best we can, free of charge.

Knowledge is a powerful thing Jerry, no need to get your knickers in a knot.
3ll3d00d's Avatar 3ll3d00d
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Quote:
Originally Posted by AustinJerry View Post
I still do not know why this affect me--I am not using the LFE channel in any of the measurements. In case you can't tell, I am seriously upset by this development.
I can imagine the multimeter is currently this close to being buried into the wall atm so lets see if we can save your multimeter from that fate, honestly you're really v close to having this sorted out so no need to be vexed.

Take a step back then, what you're doing is trying to put the minidsp into your signal chain between the preamp and the sub(s). The minidsp has a fixed input sensitivity (of either 0.9Vrms or 2.0Vrms) and provides *no* mechanism to attenuate the input. Therefore you *have* to make sure the signal that reaches the minidsp cannot possibly be more than 0.9Vrms (or 2.0Vrms depending on the jumper position). This is your goal for this exercise.

Now take one step back up the signal chain, to the SW preout from your processor, and look at what is on that channel. In a bass managed system, this is a mono signal that is the sum of 2 components;

1) the output of a low pass of each main channel
2) the LFE channel itself

***The key point is that the peak signal strength on the LFE channel is 10dB higher than that on a main channel. ***

Now step back one more step to your signal generator. You are using REW to generate that "peak signal" by using a -3dBFS (rms) sine wave. You are connected as the LR channels so, as far as the processor is concerned, your signal is a main channel. If you had the capability to connect as the LFE channel (either into multichannel analogue inputs or, more commonly, via HDMI and channel 4 using ASIO4ALL) then it would see you as the LFE and hence 0dBFS would be 10dB higher.

In the final reckoning, it just means that the actual max voltage your processor can put down that wire is much higher than you have measured so far. Therefore, to avoid clipping, you can either; restrain the MV to a max of -10 OR change the gain structure accordingly.

AIUI the calculations I gave in my previous post should accurately (subject to rounding errors given that a DMM only goes to 0.1V resolution) describe what you really need to set trims to (though obviously best to verify this with measurements).

Does that make sense?
AustinJerry's Avatar AustinJerry
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Sorry, d00d at al, I don't mean to disrespect anyone by venting my frustration. I will review your latest post and decide what needs to be done going forward. I appreciate everyone's input. As for today, I am done.
TheSeanO's Avatar TheSeanO
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dood,

Made sense to me. I think I have been doing it wrong. I am going to go back and see if I can measure the volts coming out of my LFE before it hits my DSP. I wonder if miniDSP would be able to add a software update that allowed the program to measure the voltage coming in.

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