Amp overheating with subwoofer (this problem defies all logic)

[goingdef 10+ year member]

I sure hope I didn't damage the amp... The way I set the gain was the following:
1. Turn the gain on the amp to the minimum value.

2. Turn the volume up on the head unit playing a 40 hz tone until I hear clipping from the sub.. The way I determined if it was clipping was if the pitch seemed to jump from one volume notch to the next.

3. At that volume where clipping happened, I go one back. Now turn the gain on the amp up until I hear the clipping from the sub again.

4. Go back a tad till it is eliminated.

5. Done.

So did I do it right? I've been reading about impedances online.. Isn't it possible that the sub is rated for 4 ohms but often falls under that during normal operation? I read that impedance varies with frequency.

oh you did it by ear, yea thats not very accurate get a dmm and set it with a -4db tone and you will see a big difference in the way it gets hot! Your ears can't detect clipping before there's enough to do damage or at least get things hot.

 

[xenofactor07 10+ year member]

oh you did it by ear, yea thats not very accurate get a dmm and set it with a -4db tone and you will see a big difference in the way it gets hot! Your ears can't detect clipping before there's enough to do damage or at least get things hot.

What is a dmm?

 

[goingdef 10+ year member]

What is a dmm?

digital multi meter, there VERY useful in CA such as checking voltage setting gains checking grounds like I said very useful! advance has a good one for 50.00 and it's dead nuts accurate.

 

[xenofactor07 10+ year member]

digital multi meter, there VERY useful in CA such as checking voltage setting gains checking grounds like I said very useful! advance has a good one for 50.00 and it's dead nuts accurate.

Walmart probably sells them for cheaper than that. Can I buy just any dmm? Or should I look for one with a certain setting/function?

 

[THATpurpleKUSH 5,000+ posts]

i would use a 0 db test tone, it leaves you a little conservative in the end, i have seen music with get up to -2db

 

---------- Post added at 01:48 AM ---------- Previous post was at 01:47 AM ----------

 

Walmart probably sells them for cheaper than that. Can I buy just any dmm? Or should I look for one with a certain setting/function?

usually cheaper equipment means not as accurate of a reading

 

[xenofactor07 10+ year member]

0 dB test tone? Can you explain what that is? I thought test tones only are constant frequencies.

 

[THATpurpleKUSH 5,000+ posts]

it has to do with recording quality, but yes test tones are a single note

 

[goingdef 10+ year member]

it has to do with recording quality, but yes test tones are a single note

I found a 0db won't do anything but make me crank HU settings up to get the sub to become responsive but I also run my eq at flat and won't go above that if anything I attenuate the settings to get what I want.

 

[hispls 5,000+ posts]

Class A/B amps run very hot. Even more so at lowest impedance bridged doing sub duty. Getting too hot to touch after 20 minutes of sub duty is about par for the course, and the primary reason why the industry has moved into class D.

 

[xenofactor07 10+ year member]

Class A/B amps run very hot. Even more so at lowest impedance bridged doing sub duty. Getting too hot to touch after 20 minutes of sub duty is about par for the course, and the primary reason why the industry has moved into class D.

Is there a difference in sound quality from AB amps compared to D amps? Particularly when it comes to subwoofers.

 

[goingdef 10+ year member]

Is there a difference in sound quality from AB amps compared to D amps? Particularly when it comes to subwoofers.

yep little more distortion from a/b to d but it's so little you can't tell by ear, not even a little except the class d will seem to have more authority and get louder without pulling as much power or heating up!

 

[sam_b03 10+ year member]

the 0 or -4 test tone is the recording volume that is plays. Alot of recorded music is produced a -3 so. It is basicly the signal strength. A 0 is going to be louder at any given volume setting on the HU than the same tone recorded at -4. Another tip when setting gains is to set your HU at 3/4 volume because the HU will start producing a "dirty" signal when it is pushed hard just like your amp. Set your amp gains properly with the 3/4 signal from your HU and don't use the volume above that again.

 

[TheUnderFighter 10+ year member]

Walmart probably sells them for cheaper than that. Can I buy just any dmm? Or should I look for one with a certain setting/function?

WalMart or Radioshack will have them for around $15, and it will do what you need it to do. Not the most accurate, but it will get the gains set proper, and you can use it for other things too. It's a very useful tool to have around in general.

I still think something is wrong if the amp is getting so hot it will burn you. You're only running 200w on it, at 4 ohms. I ran my 500w JL bridged for years. It certainly got warm, but not so that I'd fear for my health.

 

[zako 10+ year member]

When you bridge an amplifier and connect 4ohm load, then the channels are running at 2ohm internally. That alone puts more stress on the amplifier. The GRA-802 a cheap _compact_ class A/B amplifier that's primarily meant for powering full range 4ohm loads. That you can power a 2ohm load with such an amplifier is merely a theoretical possibility. I always had thermal stability issues with an A/B/ channel running at 2ohms. There is nothing wrong with using bridged channels for subwoofer duty if the amplifier is fit for it, but I'd recommend a Class D amplifier in that role.. with some exceptions (For example, Boston Acoustics 2-channel GT-Class amplifiers, which are Class A/B, are supposed to be fit for subwoofer duty bridged).

 

[quackhead 10+ year member]

When you bridge an amplifier and connect 4ohm load, then the channels are running at 2ohm internally. That alone puts more stress on the amplifier. The GRA-802 a cheap _compact_ class A/B amplifier that's primarily meant for powering full range 4ohm loads. That you can power a 2ohm load with such an amplifier is merely a theoretical possibility. I always had thermal stability issues with an A/B/ channel running at 2ohms. There is nothing wrong with using bridged channels for subwoofer duty if the amplifier is fit for it, but I'd recommend a Class D amplifier in that role.. with some exceptions (For example, Boston Acoustics 2-channel GT-Class amplifiers, which are Class A/B, are supposed to be fit for subwoofer duty bridged).

This is likely the case. It is a cheap class A/B. Don't get me wrong, it sounds good on a decent set of components...you need a sub amp. Look for a Class D that does the rms power you need at 4 ohms. A DMM is not going to verify where you are clipping your amp. Only an Oscilloscope will do that. I commend you for correctly setting your gains by ear. It is the only sure way I will do it. A DMM will detect a problem such as low output at max volume. The way you described setting your gain, I am certain you are not clipping(outside of media clipping)....and you know when you do. You are far ahead of the game compared to many here in that aspect. I would suggest finding someone who can "scope" your system, if you doubt your gain. Using a DMM with that little amount of power could likely hinder more than help, if you do not have access to a RTA to see where you are peaking, or use multiple test tones and a dmm to try and find what frequency presents the lowest ohm rise.....I think you are fine on your gain though.