Contour Enthusiasts Group Archives Forums Modifications Audio Amp gain questions (ATTN: KnuKonceptz??) LONG

Recently purchased a JL Audio 300/4 (75x4) amp. It has a switch on it for input voltage selection "switchable from 200mV-2V RMS
to 800mV-8V RMS". Also got a new head unit (Sony ES CDX-M770) which has selectable output voltage; either 4v or 5.5v). So, I know(think?) that the higher output would be best. But when I asked the guy at CarToys which position I should use on the amp (I've never had an amp with this) he said, and I quote:"just switch it back and forth to which ever one sounds better."! WTF, sounds like a typical salesman answer to me! SOOOO, with out even thinking about it, I've had the output set to 5.5v (default on the deck) and the input set to low(default on the amp). The gain was set to about 10 o'clock (6 o'clock being both min and max) and it sounded good (but what the hell do I know). I couldn't turn the volume up (at the deck) past half w/o shattering eardrums. Sounded VERY clean (JL Audio XR650-CS components up front). Switched the input to High (or whatever) and had to turn the gain up. Now the system sounds weak. OK, now for my questions ('bout time, eh?):

First) Does it hurt any equipment to be running a 5.5v output into an amp that is set to a max input of 2v (gain turned way down)?

Second) What the hell am I talking about here? I'm getting to know way too many terms wothout actually grasping the concepts behind them.

Third) If no one feels like responding, could they at least give me an email address for KnuKonceptz? He seems to know what he is talking about.

Thanks and sorry for the length, I'm not from the MTV generation (well, I am but never watch it) so my attention span is NOT shot to hell!

-Joe

Hey Joe,

Check this link, he does an excellent job of explaining how gains work and explaining the misconceptions regarding setting the gains.
http://www.eatel.net/~amptech/elecdisc/gaincon2.htm

You want to use the highest ranges in both cases. I can't say for sure if your Sony has
a non-clipping pre-amp output. Some decks do. [some high-end Alpines for example] Which means you don't have to worry about distortion being created by the head unit itself by turning the volume up too much. Someone very familliar with Sony ES should know. [Or contact Sony]
If it's a non-clipping design, turn down your gains, turn your head unit all the way up. Slowly turn up a gain control until you here the speakers begin to sound like they are stressing a LITTLE bit. Back the gain down slightly. This should be done with a cd btw., not radio etc. If it's not a non-clipping design, turn the volume control up
about 3/4 of "max". Repeat above procedure.

Don't fret that you're not at "ear splitting" levels at half volume after this. You aren't supposed to be with it properly
set up. You should have much smaller increases in output per volume "step" when you finish, and still be "ear splitting" at the top. The primary reason for voltages getting higher and higher [4, 5.5, 8 etc.]is that the signal passed along is more and more resistant to picking up noise from the car's other electrical circuits. These levels still need to be "matched" from component to component though. If your amps
max rms output is a "real world" 200 watts
with a input of 500mV, If you change the gain to 2V and show it a signal of 2V, it's still 200watts. If you show it 5 volts now, it most likely will produce more power, but the onset of distortion comes rapidly as 2V is surpassed, and it will begin to sound like crap quickly as internal components reach their max operating ranges with damage resulting eventually to the amp, and or speakers etc. The "switch it back and forth" advise is pretty scary. Actually a top installer will match gain settings with a test track on a cd of "all bits high" or the like, and an occiloscope [sp?] or a small self amped' speaker listening for clipping on the outputs of line level components etc.

In any event, you should be able to get close by yourself. Try not to take a long time though. The longer you listen, the more "accustomed" your ears become, and you may find you are turning things up from where you first told yourself to stop. It happens more than you think. Good luck!

This is a good example but it is backwards, the lower the number on the gain (500mV) the higher the output will be but you will be well on your way to achieving a clipped signal.

If you don't understand that little tutorial, I'll summarize what you need to do.

Switch to the 5.5 volt output and try to adjust your amp to 5.5 volts input as well.

Setting it to the minimum (8V) will only allow your amp to produce about 50% of it's rated output while setting the amp at 5.5v will allow it is full unclipped output.

You can use the method Scooby described by setting the volume to 3/4 and then moving the gain until the onset of distortion then back it off slightly ('til the distortion stops).

No, I meant that in the example of going from 2V-2V to 2V gain and 5V signal from head unit. I see how I got everyone confused though. If I had a brain I'd probably take it out and play with it!!!

Ahhh....that makes sense. I'm not sure why i didn't read it that way.

Again, good explanation.

This is an interesting discussion to me as my system has experienced quite a bit of engine related noise on AM radio every since it was first installed.
The installing dealer steadily turned the amplifier gain down in an attempt to correct the problem, but the noise still remains.

Thus my question (& pardon my apparent ignorance).
Should I have the amplifier gain increased to it's optimum setting & either try to eliminate the AM noise by other methods or simply live with it?

this is something i have always encountered in amplified system i've owned. I never had any noise with FM/CD/Cassette, but with not so strong AM signals i get a tremendous amount of noise. Let me check into this because it annoys me to know end.

LOL, that is too funny, sorry I missed the post, looks like everyone helped out and led you on the right path.

I have never had an AM related whine that was really noticeable, nothing like going under hi-voltage lines. Hard to say, my last large system consisted of Alpine's optical setup, it had no noise period.