Increasing the impedance of subwoofers

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trustyrusty

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Joined: August 04, 2006

Location: United States

Posted: August 05, 2006 at 2:16 AM / IP Logged

I recently acquired a JBL BP1200.1 (the older series, all black with a silver JBL logo in the middle). the manual says it puts out 600rms at 4 ohms, 1200 at 2 ohms, then 1200 at 1 ohm also. It is a mono amp, but my subs still connect to it in a 2 channel set-up. JBL uses a parallel set-up i think. Read the manual on JBL's site if you want more information.

(should save time)

http://www.jbl.com/car/products/product_detail.aspx?prod=BP1200.1&ser=POS&cat=AMP

My subs are Cerwin Vega VMax's (older model) that are DVC 4 ohms. I wanted to run them at 1 ohm from the amplifier, but everytime i did, it would go into protect. The amp runs at 1 ohm it says, but im thinking that it is just being pushed to far and thats why it is going into protect. I've already regrounded (ground cable is about 2 feet long), checked the temperatre of the amp (its still pretty cool), but it still goes into protect. Then like 2 seconds after it shuts off, it will play for a little bit, then turn off again. Do you guys think the 1ohm load might be the problem? i have them running at 4 ohms now and no such problems, so im pretty sure its the impedance. Is there any way to increase the impedance of the subs? Cerwin vega spec sheet says that the nominal impedance is 3.68ohms for each sub. Does that mean anything?

Just trying to see if i can really use all the power from my amp or if i'm trying to push it to far. If you want more information, just ask. Thanks!

Steven Kephart

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Posted: August 05, 2006 at 4:21 AM / IP Logged

What size power and ground wire are you using? You should also check the gain to make sure it is set correctly.

Nominal impedance is the lowest impedance produced.

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stevdart

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Posted: August 05, 2006 at 5:10 AM / IP Logged

Check the amplifier out with another pair of subs, or use any speakers like old coaxials if you have some stashed away. You want to try to put a 1 ohm load on it. So far, since this is an untested ("recently acquired") amp, it is the prime suspect.

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

coppellstereo

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Posted: August 05, 2006 at 9:13 AM / IP Logged

where did you get the amp? Are you in texas?

DYohn

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Posted: August 05, 2006 at 10:13 AM / IP Logged

A few points. The JBL is a mono amp so there is no such thing as a "stereo setup." It has dual speaker connections, but that means nothing other than instead of one set there are two. Next, the JBL BP series is not rated to handle a 1-ohm load. They usually CAN, but they are designed for a minimum 2-ohm load. Not all of the BP amps are stable at 1-ohm. Plus, when you do try a 1-ohm setup you must be VERY careful how you set the input gain, as small changes will make large differences, and make sure the bass-boost circuit is off. Thirdly, the amp has a low-voltage shutdown so if your vehicle's electrical system is being overloaded (or if you have a poor ground connection) it will go into protection and can exhibit the "on and off" behavior you describe. What's the voltage at the amplifier terminals when is shuts down?

Oh, and by the way, there is no way to "increase the impedance" of a voice coil.

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trustyrusty

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Joined: August 04, 2006

Location: United States

Posted: August 05, 2006 at 11:44 AM / IP Logged

I run a phoenix gold amp kit. 4 guage. 2 foot ground, and i think a 17' power cable. I have recently acquired the amplifier and i know the amplifier works at 2 ohms, however, he had 2 2ohmDVC's, so i haven't seen it tested at 1 ohms other then when i have run my subwoofers on it. JBL says the amplifier has a minimum speaker impedance of 1 ohm, did i read that wrong? When i monitored my voltage, it didn't get below 12V, but i can always recheck that, that was awhile ago. The bass boost is turned all the way off and the gain is set at the normal 3/4 tuning set-up. Should i really turn down the gain a lot more then that? My ground so far has been tested and is doing fine. I even regrounded it just to be sure of that. Rescraped the paint away and all of that.

My next step is to borrow some 2ohm DVC subs from a friend and try running the amp at 2 ohms on my car. Then i'm going to take my amplifier and move it into his car and depending on if his worked in mine or not, run it at 2 ohms with his sub or 1 ohm with mine in his car and see what happens. His electrical system is pretty big compared to mine. IF none of those yield results, i will take his 1ohm stable amp and try running that in my car with my subwoofers. Am i going in the right direction? or what else should i be trying?

thanks!

DYohn

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Posted: August 05, 2006 at 12:04 PM / IP Logged

trustyrusty wrote:

the gain is set at the normal 3/4 tuning set-up.

There is no such thing as a "normal" gain position, and 3/4 is most likely way too high especially for a 1-ohm load. Who gave you this idea?

Turn the gain all the way down. Disconnect your subs. Play a test tone at about 70Hz. Set your head unit to about 3/4 volume. Measure the AC voltage output from the amplifier's speaker terminals. Slowly raise the gain adjust until you read about 34.5 volts. This corresponds to approx. 1200 watts into a 1-ohm load (review Ohm's Law if you don't understand this.) Leave the gain there. Don't touch it again. Turn the HU volume down and turn it off. Reconnect the subwoofers. Try it. If it still goes into protection, either the amp cannot handle the 1-ohm load, the system voltage is falling too low, the ground is bad, or the amp (or the speakers) may simply be defective.

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Steven Kephart

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Posted: August 05, 2006 at 8:55 PM / IP Logged

Yup, Dyohn is exactly right. I have seen MANY subs fried from that amplifier because of improperly set gains. The problem comes from those people having high voltage head units. If your head unit puts out a 4 volt pre-out, then the gain should be almost all the way down with that amplifier or it will clip the signal.

trustyrusty

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Joined: August 04, 2006

Location: United States

Posted: August 06, 2006 at 4:37 PM / IP Logged

K, i just got it all figured out I think...it was the problems with the gain and the ground both i think. I adjusted the gain first and it still sort of did it but way less. Then i regrounded once more, found a closer one (one foot, 4 inch ground with new gold plated connectors for it) and so far it looks like it finally is working fine. Thanks for all the help you guys!

Steven Kephart

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Joined: November 06, 2003

Location: Oregon, United States

Posted: August 06, 2006 at 4:56 PM / IP Logged

If you didn't, be sure to grind off all paint and corrosion at your ground point so you are connected to bare metal. This will ensure a good, solid ground.

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Increasing the impedance of subwoofers

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