Weird Results Tuning Amp's Gain with 1 KHz Sine Wave Test Tone?

[JawKnee]

In addition to JL Audio's tutorial at http://mobile.jlaudio.com/support_pages.php?page_id=143 , I also found a message forum discussing a similar method to tuning the gains on an amplifier.

My Phoenix Gold Xenon 100.4 amplifier is rated at 100 watts a channel (4) -- I don't have the birth certificate, but I wouldn't be surprised if actual output was close to 120 wrms per channel at 4 ohms.

Since I'm driving the MMC6500s straight wired (no biamping), the 2 front channels are seeing a 4 ohm load right? As a result, I'm aiming for to set the amp's gain to see 20.0 V -- square root of 4 * 100 (4 ohms x channel output).

Using a 1 KHz sine wave test tone at 0 db, I had my stock unit's volume set to 30 out of 40 and adjusted the gain for the front channels that drive the MMC6500s to read close to 20.0V. I'm actually around 19.7x V. My end result was the gain ended up around the 2 o'clock position.

Now does that sound like it's too much??? Does it make sense?

Or is the fact that I'm using a line output converter (PAC OEM-2: http://www.pac-audio.com/products/oem1.htm) causing some sort of complications? For what it's worth, the gains on the LOC are set at halfway, which is the default.

Oh, I'm using a $90 multimeter that is true RMS digital multimeter.

[MacLeod]

That does sound like too much. Ive never had cause to go beyond the noon position.

I cant really help ya on this one bro cause Ive never used anything other than my ears to set the gains. I wouldnt have the first clue how to use an RTA or such to set the gains.

The old fashioned way has always worked fine for me.

[neomagus00]

the multimeter should be accurate enough... you're not necessarily driving into a 4-ohm load, it's actually probably more than 10 ohms... so you're doing say 110 watts into 10 ohms, for a voltage of V = sqrt(P*R) = 33.1 volts... the LOC will likely give you a hideously low voltage, so i'm not surprised that your gain is so high... a better way to do it is use an oscilliscope to find the maximum volume on the HU, then the maximum gain on the LOC, then the maximum amp gain...

[JawKnee]

Neo,

So why wouldn't I use a 4-ohm load in determining the desired voltage output? I calculated 20.0 V from the square root of 4 * 100.

In any event, does 2 o'clock sound like it's too much for my speakers?

[neomagus00]

i would not say that 2 o'clock is necessarily too much, no... and it's not 4 ohms because the resistance of a speaker is not constant as frequency varies, it's a rather complicated function of frequency... the upshot of it is, that '4-ohm' value is close to average value, but in reality it will vary from 2 to 40 or more (!) ohms, depending on frequency... this frequency dependance is called 'impedance' rather than 'resistance', and is caused by the construction of the speaker - everything from materials used to the number and diameter of the turns of the voice coil... what i did was plug the speaker into WinISDPro, and it spits out a graph of the impedance, which comes out to roughly 11 ohms at 1kHz...

[JawKnee]

Well, I have readjusted the amp's gain so the AC voltage now reads around 19.9 with the HU's volume at 30 (max is 40). For what it's worth, the amp's input sensitivity is 200 millivolts to 8 volts.

All I know is that there's no way I'm going to turn my HU close to 30 with the amp set this way.

I set the gain with the Bass and Treble set to 0 when playing the test tone, and I think I should drop both near their minimum of -6 should I try to play it around volume levels of 10 - 20.

Does the fact that I feel I should drop both Bass and Treble (especially Bass) with the amp's gain set the way it is an indicator of anything?

I had somewhat of a lengthy discussion with a JL Audio tech earlier today regarding this and he said that this method is definitely the proper way of doing it and that everything I did was correct. However, I'm not sure I'm 100% totally at ease with the settings.

I may try to stop by a local car audio shop to see if they're willing to "tune" or listen to my system. We'll see how that goes...