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:
Computer with a "Line In" input (not "Microphone In")
3 4.7KΩ resistors
Alligator Clips (at least four)
RCA to 3.5mm cable
Digital Multimeter (optional)Required Software:Room EQ WizardZelscope
(Optional, but highly recommended)DIY Audio Test DVDProcedure.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.