Input Voltage Question

Hello to all,

I have a question regarding the input voltage setting on my amplifiers when used in conjunction with my head unit. I have an Eclipse 8454, with 8 volt preamp output; the amplifiers are the Jl 300/4 and the 1000/1.

I was setting the gains today using a multimeter, test tones and the amplifier manuals. In the manual under the instructions for setting gains, it directs you to switch the "Input Voltage" to "Low." You then proceed to adjust the input sensitivity to reach the target voltage for the respective channels. The manual then states that, "If excessive voltage is read on either set of channels with the control on minimum, switch the appropriate "Input Voltage" switch to "High" and re-adjust."       

Earlier in the manual, it appears to provide contradictory information. It says that the "Low" position on the "Input Voltage" switch accepts an input sensitivity between 200mV and 2V; the "High" position selects an input sensitivity range between 800mV and 8V.

With the Eclipse head unit, shouldn't the "Input Voltage" switch be set to high? I set my gains as per the manual's instruction but I'm concerned the amps are more susceptible to clipping with the switch in the "Low" position. Doesn't it increase the output of the amplifiers? Thanks in advance for any help!      

The statements are not contradictory.  In the statement "If excessive voltage is read...", the voltage they are referring to is the AC voltage you are reading at the amp's terminals, not the input voltage.

With the majority of head units, starting with the Low position will keep you from having to turn the gain excessively to match the head unit.  With your Eclipse, however, the input voltage will be higher than 2 volts so you would forego that first step and just proceed with setting the gain with the input position on High.

There is no increase or decrease in amplifier output with either of the two positions, but one position will work better than the other.  This is just a way to give the gain control two different "feels".  Ideally, you want to be able to turn the gain to some degree, for the sake of accurately meeting your target point, but not very far.

Look at it this way:  look at the gain control on the amp.  Turn it fully to the left, or to the minimum position.  With the control set to Low, that position represents 2 volts.  With the control set to High, that position represents 8 volts.  That should give you a better picture of what that control is doing.

But...as I re-read your post's last paragraph....you successfully set the gain in Low position?  Meaning you were able to turn the gain to some degree to reach your target voltage?  That indicates, if you were conducting the test properly (head unit at full clean output and pro-recorded test signal) that the input voltage was under 2 volts.  Double check your test and if it comes up good you're done.

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

stevdart,

The gains were set as was suggested in the manuals, using a 50 Hz and 1kHz test tone signal for each amplifier. It was set while in the "Low" mode, with target voltage reached at just past the halfway mark for each of the input sensitivity knobs. The only exception was that I turned the head unit volume only to 3/4 (60 on a scale of 80). I can go past that point of reference and still have clean output.

I'm just wondering why they don't differentiate between the two type of signal sources (preamp output) for when you're adjusting the gain. Both manuals have the same set of insructions, with no variation: set the gain with the input voltage switch in the "Low" position. If I redo the test and get the same results, should I leave it as is or set the gains while in the "High" mode? Thanks.       

I would say you could leave it as it is, or you can take another day and spend some time finding what you truly think is the maximum clean output of the head unit.  That should be your reference point for setting gains.  Voltage has a way of increasing dramatically when you are at the uppermost limits.

If you were able to turn the gain all the way to the halfway position while in Low, then you shouldn't go to High at all.  You would be turning it even further.  Remember that you are setting the voltage that is the extreme of what you will allow to be played...even if you never listen to it at that level.  You're setting the extreme voltage, so use the extreme deck setting (and thus maximum output voltage), just not to the point of clipping.  And finding the maximum clean output of a deck is easier said than done.  You may have to listen several times to finally arrive at the magic number.

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

I think I'll leave it as is and focus on fine tuning the rest of the settings. As it is, I don't think I could listen to it at 3/4 volume. I've become a bit paranoid about clipping; apparently if my subwoofer begins to distort, it's most likely damaged (Brahma). Thanks for the advice.

Dave
"lakers08"

P.S.- The Jl manual says that you shouldn't mount the amplifier on the roof of your car. Now I have to change that too!