How To Determine your Receiver's Maximum LFE Output for Gain Structuring

In preparation for my upcoming DIY project, I figured out my receiver's maximum LFE output level the other night. I did it in a way which I haven't seen described on the forums before, so I figured I'd share my process.

The reason for wanting to determine one's maximum LFE output level from their receiver is so that they can set their LFE trim such that the signal can be fed into a pro audio amplifier at the highest level possible, without clipping. Basically, this is a good first step in setting up your "gain structore". However, I also discovered that this can be handy even if you are simply connecting to a commercial subwoofer.

The short version is that I discovered that when fed a -.5dbFS signal, my Onkyo TX-NR809 AV receiver can output a maximum of 4.5V RMS at a LFE trim level of -2db. And as an aside, the -3db point is 3Hz.

Required parts:

AV Receiver
Computer with a "Line In" input (not "Microphone In")
RCA Cable
3 4.7KΩ resistors
Alligator Clips (at least four)
RCA to 3.5mm cable
Digital Multimeter (optional)

Required Software:

Room EQ Wizard
Zelscope (Optional, but highly recommended)
DIY Audio Test DVD

Procedure.

WARNING: YOU WILL BE USING REFERENCE LEVEL TEST TONES AS PART OF THIS PROCEDURE. TURN OFF YOUR AVR AND DISCONNECT ALL SPEAKERS BEFORE PROCEEDING!
Ok, you don't have to disconnect all your speakers. I disconnected mine because 1) the test tone DVD doesn't stop after each tone, and 2) just in case I clicked on a mains test tone rather than an lfe test tone.

Note: This procedure can be performed without Zelscope. But I like to be able to see a waveform, and not just an SPL spike on an RTA.

Connections Setup:

After disconnecting all your speakers, connect an RCA cable to the LFE Output (may be labeled as "Sub Out") on your receiver.

Connect an RCA to 3.5mm cable to the "Line Input" of your computer. Adjust the volume of the line input level to a low level. One click above 0 will work. If the level is too high, the waveform will clip within the sound card, and this could be misinterpreted as the receiver clipping. (Windows 7: Start-->Control Panel-->Sound-->Recording tab-->Select "Line In"-->Click Properties-->Levels tab-->Adjust slider all the way left, and then go one notch right. Other Operating Systems: Good Luck)

Set your Line Input as your default recording device. (Windows 7: Start-->Control Panel-->Sound-->Recording tab-->Select "Line In"-->Set Default)

Connect three 4.7KΩ resistors in series.

Connect an alligator clip from the center conductor of the RCA cable going to the receiver to one end of your resistors.

Connect another alligator clip from the shield side of the RCA cable going to the receiver to the other end of your resistors.

Connect an alligator clip from the shield side of the RCA cable going to your computer to the end of the resistors connected to the shield side of the cable going to your receiver.

Connect an alligator clip from the center conductor of the RCA cable going to your computer to the other side of the resister connected to the shield wires.

What you end up with should look like this:



Note: The reason we are doing this with resistors is because if you connect your AVR straight into your line input on your sound card, you may clip the input of your sound card. This could be falsely interpreted as your receiver clipping. What we are doing here is setting up a simple voltage divider so that we don't clip the signal coming into our sound card.

Software Setup:

Room EQ Wizard:

Once Room EQ Wizard has started, go to Preferences-->Preferences.

In the drop down menu under "Input Device and Input" select "Line In"

In the drop down menu under "Input Channel", select right or left, depending on which RCA plug you have connected to the resistor network. (Red = Right, White = Left)

Close the preferences window.

Click the "RTA" button.

Click the red circle near the top right corner of the window that pops up.

Now you should see a real-time spectrum from your Line In.

Zelscope:

Click the Play button located just underneath "File" in the top left corner of the window.

Click the Ch1 and Ch2 buttons to turn on both channels. You can turn off the unused channel once you determine which channel corresponds to the input you are using.

V/Div you can adjust later. However, note that any voltage reading you get from the scope is NOT VALID unless you first calibrate the software with a sine wave of a known voltage level. For our purposes here, we don't need to know what the voltage level is.

Under the "Horizontal" section, set "MS/DIV" to 2.

After having disconnected your speakers, turn your AV Receiver on.

Now, start the Audio test DVD, navigate to "Extras", and select one of the sweeps in the LFE section. If everything is set up properly, you should see a sine wave appear in Zelscope. Adjust V/Div to a comfortable viewing level, and turn off the un-used channel.



The Actual Test:

Now, turn your receiver up to 0. (assuming your volume display is set to db. If it is, as you turn up the volume, the numbers should go down) As you do this, the waveform in Zelscope should get bigger.

If the waveform you are getting at this point is not clipped, go into your AV Receiver's setup, and turn the LFE trim setting up and re-check until you see clipping at the output.

If the waveform you are getting at this point is clipped, adjust the "Line In" volume setting in your computer. If the clipped waveform displayed on the scope gets bigger, it is your receiver clipping. If the clipped waveform gets even more clipped, your sound card is clipping. Turn down the level of your "Line In".



Now, using the -.5db sweeps in the Audio Test DVD, you can continue adjusting your receiver's LFE output trim until it is just below clipping. On my receiver, that level is -2db.

Optionally, at this point you can hook a DMM across all three resistors. This will give you your receiver's maximum output level before clipping in volts RMS. This reading will help you determine whether or not your receiver can drive a pro audio amp. My amp outputs a clean 0db signal at 4.5V RMS. It clips at 4.8V RMS.

At this point, you can turn to the Room EQ Wizard RTA window. To verify you are not clipping, click the "THD" button. Your THD should be under 1% with no clipping. With clipping, it will be more than 1%.

If you know your way around REW, you can calibrate it's SPL meter to the 95db pink noise on the audio test DVD. Once you do this, the RTA should read just under 115db (ideally 114.5db) when you run the -.5db test tones.

Having accomplished this, you can now play the individual 1Hz to 20Hz tones and plot out your receiver's low frequency response.

NOTE: Ideally, this would give you an accurate reading. However, your sound card will also have a low frequency rolloff. In reality, your receiver's actual low frequency rolloff will likely be a little less steep than what you measure using this method. In order to get an accurate reading of your receiver's low frequency rolloff, you would need to measure using an actual oscilloscope.

Here is the plot I got for my receiver:



Now that you have this information, what do you do with it? For one, decide how much headroom you want. Ideally, your LFE output will never output anything greater than a 0dbFS signal. However, it is possible to see a greater than 0dbFS signal if a similar signal appears on the other channels and gets re-routed to the LFE channel by way of the receiver's bass management. And so if you want an extra margin for safety, you could set your receiver's LFE trim to 3db below the level at which you get clipping with a 0dbFS signal. However, I have heard numbers as high as +9db FS (124db SPL) thrown around as theoretically possible. (0dbFS signal at the same frequency, in phase, to all 7 channels at the same time in a 7.1 soundtrack). And so how much headroom you want to leave is up to you.

Now you have determined what level to set your AVR to when setting up your gain structure.

I suppose that would have been quicker, in that I would simply subtract the master volume value at the onset of clipping, and trim the channel down that much. Whereas I did the reverse and found the trim level by trial and error. So that method would be quicker, but both get you to the same destination.

Zelscope is a free (for a 14-day trial, $10 to buy) program which uses the inputs of your sound card as the inputs to a software oscilloscope. Basically, it's a software oscilloscope.

I had raw resistors in my electronics toolbox, so I simply twisted the leads together. It wasn't until after I did this that I found my breadboard.

Just one RCA to 3.5mm is required. I didn't butcher the ends of any cables. I just used some alligator clip leads to connect the plugs to the resistor network.

Yes, in an ideal world we would just set the trim to just below clipping. That brings up an interesting question. How hard can a sub be clipped without damaging the woofer? Of course, the answer is it depends... on how much power the amp has, on whether or not the amp has protection against a clipped input, on how much power the speaker can handle. I'll have to research these when I determine how to set up my gain structure. If my amp will protect against clipped inputs, then I'd just prefer to set my receiver trim to just below clipping a 0db FS signal, and if a poorly mixed movie comes through my system, I'll just accept that I won't hear >115db of bass.

Anybody have an answer to this? I know clipping can kill tweeters, but how about woofers?

Thanks for spelling out the theoretical maximum! I didn't know the exact formula. I just added up the channels. +3db for the first channel, +3db for the next two, and +3db for the last four. Either way you slice it, 10db of headroom is a lot of headroom. Building a subwoofer system capable of 125db would be really expensive. My two LLTs I plan to build with SI HT18 drivers should easily achieve 115db. But I'd have to build four or more to achieve 125db!

Wow, that's scary! The worst case scenario (shy of 7.1) available for download. I may have to try that out. Looks like I get to tweak around some more.

Most of what I learned about gain structuring is from this article:

Gain Structure for Home Theater

I used Audyssey to set the distances and levels of the individual speakers. If you want to use the level Audyssey sets up for you, that can easily be achieved by running Audyssey, and then using a test tone to determine the level it set your subwoofer at. Next, see where Audyssey set your trim at. If it set it at a level below your maximum clip level, you are probably good and can leave it as is. If it set it at a level above your maximum clip level, adjust the trim down to the level you determined, run the test tone again, and turn up your sub amplifier to achieve the same level which Audyssey determined. (In this case, you're using the subwoofer amp instead of the receiver LFE trim to achieve the desired level).

Also, it is important to know the clip level in case you are in a situation like mine. As it turns out, Audyssey set my trim to +2.5db. But my clip level (using a -.5db FS signal) is -2db! So if I left my receiver at the level Audyssey determined, I would actually never achieve 115db peaks, but would rather achieve 110.5db peaks, at which point I would be sending a clipped signal to my subwoofer! This makes me want to believe there was something wrong with my methodology, and I may indeed try this again to see if I get the same results.

On a sidenote, I set my receiver to "pure audio" mode in order to get a flat non-EQed response. I'll have to try this again with DEQ enabled to see if there are points where my signal is being driven to clipping by the DEQ.

As far as gain structuring, as I understand it, the theory is that by sending a signal just below clipping (at 0db FS), you are maximizing the dynamic range available to you. The article linked above explains this with pictures. Basically, if you send a signal to a pro audio amp which is lower than what your receiver can put out, you then have to turn the amp up more in order to achieve the desired SPL. This raises the noise floor of the amp, which in turn decreases dynamic range. However, as I understand, noise is generally above subwoofer frequencies, so noise floor is not a huge issue when gain structuring for a subwoofer.

Agreed, it normally isn't a problem to drive multiple amps off one output since the input impedance is nominally 10Kohms. Even if you were to drive 10 amps off one output, the impedance seen is still 1K, which shouldn't load down the circuit that much.