Q I have a problem with my sub power. I am using a JBL A601GTi (600 RMS @ 2 ohms) connected to a 1.5-farad capacitor. Recently, I switched from a JBL Power P1220 subwoofer to two JBL W12GTi's in a vented box. Running the P1220 at 2 ohms my amplifier put out 550 watts max (I can see this on the meter that comes with the A601GTi sub amplifier). However, when I changed to the GTi's I expected the amplifier to put out 900 watts max, but instead is only making 670 watts. I don't understand where the 230 watts went. The GTi subs are 3 ohms each (dual 6 ohms) so if the amp can give 300 watts RMS at 4 ohms and 600 at 2 ohms, it should give me 450 watts RMS at a 3-ohm load. The power cables, battery and the capacitor I think are quite capable of giving enough power, but I have 10mm wire for the sub and this measurement is for 400 watts. Should I update and put 20mm speaker wire for the GTi sub? Or do the subs need to be played and when they get softer, accept 900 watts. Also, I played with the input gain but nothing. Please give me some advice because I really want this sub to kick as hard as I expected-that was the reason that I bought it. To have 900 watts and not 670 watts because I don't get any improvement over the P1220 in power.Thanks,AlexandarRussia
A I had been to Moscow many years ago to conduct installer training and wrote about my experience in Car Audio and Electronics magazine. It was an amazing trip that I will remember for years, and one day I would like to visit again to see how car audio has changed since my last trip. At the time I was shocked at the lack of professional installers and the lack of retailers that sold car audio product. I was also sorry to discover that there were no good car audio magazines in the country. It seems at least that has changed!
I would first suspect that the little meter that plugs into your amplifier is influencing your ideas. Without knowing its accuracy, you need to make a few tests to confirm whether it is giving you valid information or just a pretty light show. Status monitors on any good product can be relied on for a certain level of information, but they're not precision measurement devices.
With a rating of 600 watts at 2 ohms and 300 watts at 4 ohms, you could make a balanced guess that at 3 ohms, you should see the difference between 2 and 4 ohms, or 450 watts. One P1220 sub is rated at 2 ohms, which would encourage the amplifier to put out its maximum power, and your meter seems to be close to the spec. I don't see how you can expect to get 900 watts out of a 600-watt amplifier. If you were to use a pair of 3-ohm speakers wired in parallel, the resulting 1 1/2-ohm load would lead you to believe that 900 watts was achievable. That's pushing it.
An amplifier that can truly double its output when moving from 4 ohms to 2 ohms is great, but don't think that the same rule applies when you go from 2 ohms to 1 ohm. There is a point at which the amplifier will simply max out, delivering what you paid for, and a bit more. For instance, you are seeing readings of 670 watts, which to me is well above what you should expect from any 600-watt amplifier.
You can check the actual output with a digital multimeter (DMM). Get your hands on a signal generator or a CD with test tones on it. Without knowing the impedance of the speaker, you will need to make a couple of assumptions. If you play a bass tone, say around 70 to 90Hz, without knowing the impedance, you can still measure the power. Connect a decent-quality DMM to the amplifier input using a Y adapter and play your test tone into the amplifier while the amplifier is off. Watch the voltage reading on the DMM and set the volume to about 2 volts on the AC voltage setting with the input gain control set to minimum on the amplifier.
Once you have that done, record the volume setting on your CD player and turn off the unit. Now connect the DMM to the speaker terminals, fire up the amplifier and play the tone into the amplifier while turning up the input gain. You will see the voltage reading on the DMM rise until it stops rising. If it did not stop rising before you hit the end of the input gain control's rotation, turn up the CD player a bit. This should only be done briefly, and you will probably need hearing protection. Note the maximum voltage reading you see and also note the power shown on the amplifier's meter.
If you do a quick calculation by squaring the voltage and dividing by the impedance of the speaker, you will arrive at a wattage reading that may be reasonably accurate. In other words, if the voltage reading was 30 volts and the speaker was 2 ohms, the wattage would be 450 watts. This assumes that the speaker is really 2 ohms, and without the impedance graph or a test, we can't be sure. Usually a subwoofer would be near its impedance rating at 70 to 90Hz, but not always. An error of 1 ohm could throw your results off by 150 watts.