the truth about the gain knob
Printed From: the12volt.com
Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=90526
Printed Date: June 10, 2024 at 11:15 AM
Topic: the truth about the gain knob
Posted By: rabreedlove
Subject: the truth about the gain knob
Date Posted: February 15, 2007 at 5:36 PM
I am a member of another forum that's specific to the vehicle I drive. We have an audio section, and people very often post messages that sound something like this: "I have a XXX amp with my YYY sub." Investigation then reveals that XXX amp is rated for 600W, while YYY sub is rated for 250. The proposed solution is to turn down the gain knob on the amp and everything will be fine. Now, I know that the "gain knob" is intended to be set to match the input signal voltage supplied by the head unit. I know that the term "gain knob" is a misnomer, because the amplifier's gain is fixed based on its internal circuitry. I also know that this knob is in no way intended to be set to a point that protects the speaker. However, I really would like for one of the electronics experts here to give me a good write-up about the theory behind this issue so I can more adequately address why this is a bad idea. Thank you in advance for your help.
Replies:
Posted By: haemphyst
Date Posted: February 15, 2007 at 7:56 PM
It's NOT a bad idea, correctly implemented. IF the "gain control" is set at a point sufficiently low enough to produce ONLY 250 watts, based on the average input voltage fed to the amplifier by the headunit, then this will be a perfectly acceptable situation. It will simply allow for adequate headroom, and put a clipping condition that much further away from the picture. It is a perfectly safe idea, but it just has to be WORDED correctly. If the "gains" are set using an o-scope, the safe level can be set fairly easily, and as long as that level is never passed, it'll work forever, with never a complaint.
I do it all the time in my systems. For example, in my current system my tweeters (JL silk domes) are rated for 75W, driven by (a potential) 125W, my mids (Vifa 4 inch - specially modified by Harmon, nothing you can get) are rated 20W driven by (a potential) 125W, and my midwoofers (Adire Extremis 6.8) are 200W driven by (a potential) 370W. The 10" TCSounds woofers are driven by (either) 600W RMS @ 8 ohms, or I can flip a switch in the dash which activates relays and changes the woofers (they're DVC) to 2 ohm, and the amp then has (about) 1800 to 2000 watts on tap - per woofer. All amps are Eclipse. I don't USUALLY recommend everybody attempt to drastically "overpower" a system to these extremes, I have been doing it for a LOOOONG time, and I can hear drivers in distress, and I have NEVER blown a driver. (I set all my gains by ear, BTW.) I simply state this to show that it CAN be done safely.
You are absolutely correct in the assertion of the "gain control" not being a gain control at all. It is a level matching stage.
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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
Posted By: forbidden
Date Posted: February 15, 2007 at 9:53 PM
Another thing to consider here is the limitations on the input voltage gain and the cd player output voltage. If amp XXX has a gain setting up to 4 volts and you have yourself a Eclipse 8volt unit, you will hit maximum input voltage in relatively short order even with the gain all the way down. This is as example of why I tell people that it is not a volume control and should not be used as such. ------------- Top Secret, I can tell you but then my wife will kill me.
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