I posted this before but it appears to have dissapeared from the forum.
Like the title says, I have a 4 volt pre-out Alpine head unit and a refurbished 150 watt mono Rockford Fosgate subwoofer amp. I have to turn the gain on the amp at or very near the max to get a balanced output level with the coaxials running off the internal head unit amp. This is with the Subwoofer volume setting at around 7-11 out of 15 max on the head unit. I have a simple multimeter but I was not able to detect *any* voltage out of the RCA outputs on the amp. I was able to read the voltage of the car battery so the meter seems to be fine. The analog meter range goes down to 15 max volts AC or DC at its most sensitive, but I can't get the needle to budge on the RCA outs. I *know* that some power is being made there.
I assume that having to turn the gain up to max is not normal for a 4 volt pre-out. How can I properly diagnose my problem here?
you'll need a "true RMS" DMM to test pre-out voltage, because it's an AC sinewave. using a DC voltmeter will show 0 volts, because that's the average for an AC signal. same reason you can't measure a wall outlet for voltage without a test light or a true RMS meter.
as for why there's no output.. what sub is connected to this amp? 150 watts isn't a lot of power but you should have the gain that high, nor the bass output on the ehad unit that high. it should all be set flat. bass boost, EQs, bass level etc.
what kind of sub box? how is the alpine's internal XO set for this sub? 80Hz LPF, 18dB slope, 0 boost is a good start.
this is something best sorted out by going through it one step at a time
What is a "true RMS" DIMM? My meter does have an AC setting which I tried on the RCA outputs. It does measure wall outlet AC voltage! I just tested it.
I have an Alpine CDA-9811 and a 10" Infinity 1030w single voice coil 4 ohm sub in a sealed box with the dimensions reccomended in the manual. The XO on the head unit is usually set to 80 Hz LP and the slope is non-adjustable. Also the minimum level on the HU for the subwoofer is 0 (no negative numbers) and I like it more in the middle of the range so I can adjust the sub's level up or down depending on my mood or the music. I can also adjust the bass control obviously but being able to adjust both settings up or down gives more flexibility. Turning down the setting to 0 produces very feeble bass, even with the gain set to max. Using a test CD and my Radioshack SPL meter (both originally used to calibrate home theater speaker levels) I get about 4 or 5 on the Subwoofer control and maximum gain on the Sub amp for matching levels. I usually like listening with slightly bosted bass from that reference.
Try disconecting your right rca and listen for a second, then hook your right back and unhook the left. I am guessing you are running the amp briged and for some reason only one side is getting any imput. If I am corect then it will give you some better ideas as for what to troubleshoot.
if that amp is a mono 150 watt @ 2 ohm amp, and you have a 4 ohm sub, you're feeding it 75 watts. that's partly why it sounds like crap.. not nearly enough power to make the sub work well. particularly if the sub is isolated in the trunk of a car, or somewhere else lacking good coupling of air between the sub and cabin.
if you have the gain set all the way up, it's clipping and damaging the sub.
one idea would be to look at adding a line driver or pre-amp from audiocontrol. that'd boost your line voltage. some alpines like the 9811 have a documented issue with low line voltage on the sub outs. this may be part of your problem, as sub pre-outs are closer to 1.7 volts and not actually 4v as advertised.
Wouldn't clipping only occur if the preamp voltage was too high for the gain setting? If the voltage is low and the gain is set high so that the SPL matches using test tones made for matching subs to high's and an SPL meter. why wouldn't that be okay? That is my current situation. How could I tell if it's clipping anyhow? It's easy to tell on high frequency sounds but I am not sure what to listen for on a subwoofer. Should I just forget about it since it seems to be okay or should I look into getting this fixed because it could be causing damage.
I am concerned that it might actually be the amplifier that is not sensitive enough because it was a refurbished unit direct from Rockford Fosgate and they didn't even bother including several neccesary things in the box such as the screws to hold down the wires. I called RF and they thought it was the HU.
Right now, I am wondering if I should just forget about it and leave it as is or get a local shop to have a look at it. Leaning towards the latter...
you'd look for clipping using an oscilloscope and a tone generator. you'll see the sine wave clip, and become a square wave, or flatten out at the peak and trough.
With the volume and subwoofer level *both* set to the absolute maximum, I was able to measure 1 volt AC using a 40 Hz test tone. Anywhere in the listenable volume ranges the needle won't budge.
This has me VERY annoyed because it is the second Alpine HU where I have had that has this problem. I had a new CDA-9825 for a few weeks which I returned because that was even worse! I thought Alpine was supposed to be a good brand. This is crap!
maybe something is damaging the pre-outs.. like your amplifier which may not have been properly tested before it was sent back out after refurbishment. that's an acute possibility.
see if the head unit is turned up all the way you'll definitely get clipping, because the head units line driver only stays clean up to about two-thirds to three-quarters of full volume. that's something else to be careful with. this sounds like you have something damaging the pre-outs though the more we get into it.
If the amplifier is causing damage, it would have done so instantly to both head units. The gain had to be turned up to max the very first time I ran it. You couldn't hear the sub at all at the lower gain levels. Now I definatly will be having this looked at. I'm still within the 90 day warranty on the amplifier.
One last question: How could an amp damage the pre-outs of the HU and how could I test for that? Like I said, Rockford Fosgate thinks this is my HU's fault.
Hey Brian seems like you're having too much problem with the amp. I'm not a fan of refurbished products. Anything that had to be sent back for repair is not worth buying. Used is a different story. I've had similar problem with one of my amp. It didn't respond much unless I had it set the gain to max. It worked for a while then burnt out. It was a faulty amp. I'd return the amp and buy either a used or new one. Not worth the aggravation.
Using a digital multimeter I was able to measure the voltage properly. Using a 40 HZ test tone I turned the volume up to about the maximum that I usually listen at (Volume 22 Sub 10). Guess what? I got 100 mV out of the preamp and the amp is rated for 100 mV at its maximun sensitivity. Turning it up to 23 broght it up to like 140 mV and 25 was around 200 mV, but with music the main 6.5" speakers are sounding rather strained at that point. Using the front speaker pre-outs yielded similar voltages in the milivolt range.
I called Alpine and they were compleatly unhelpful and the local Alpine dealer was not really helpful either. He said he could return it to Alpine for service but it would cost me *at least* $120 which is way too high. So I guess I will just live with it.
how old is that alpine? this is very weird. the amp could damage the pre-outs by causing a short, which would burn out the driver IC in the head unit. I've seen this happen when a cable gets pinned. (RCA cable)
The Head Unit is 1-2 years old. I am not sure since I bought it used. I am using some cheap Radioshack RCA cable. I wonder if that could have caused it... Damn, I would be pissed off to no end if a stupid cable fried both of my HU's. I have actually tried a different cable and noticed no difference though.
ah ok its not in warranty then. well, if the RCA is pinched, the moment its connected it'd short out the preamp driver IC... so unless you tried the new cable with the virgin head unit, that wouldn't help, ya know? just a thought.
That's a good point. I went and took the RCA cable out, which incidentally is the thinnest RCA cable I have ever seen in my life. Seriously! I basically got it for free. Anyhow, it was pinched pretty badly although the wires were not showing so it was not cut. I just went out and bought some nice thick name brand cable along with an extra protective outer plastic sheath and installed it. It's not noticably louder but it does sound better. The lowest frequencies especially benefited.
I don't know if the HU sounds as good as it should but it sounds pretty good anyway so screw it. It's very good for the $134 I paid for it.
maybe next time go Eclipse, if Alpine still leaves a bad taste for ya. hey take that old cable and test it with your meter.. see if you get any continuity breaks or shorts.. see what the resistance is for each pos and neg pin at either end. they should be about equal unless one got 'pinned'
that was an old trick to mess with CB base station people and HAM operators.. stick a nail or pin through a coax cable causing a short. it'd blow out the finals in their radios the instant they key up the mic to transmit, and the finals don't see any load from an antenna. dirty trick, but a good lesson in how important a good cable is.
