JBL PB12 subwoofer shuts off at normal volume

[jimrockford]

I recently moved from Washington to Florida and ran into an issue with my subwoofer (JBL PB12) when I set up my home theater. The auto turn-on feature works properly when I turn up the receiver volume really high, but shuts off at a normal listening volume. This occurs regardless of the audio source (DVD, TV, radio, etc). The sub has two LEDs on the front that switch from red to green when it turns on so I'm positive it is actually shutting down, not just too low to hear.

It looks like the sub isn't sensing the signal from the receiver until I crank it up. I'm not sure if something is going out in the sub or if there might be a sensitivity adjustment inside the sub that I could fiddle with. Is anyone familiar with this particular sub or with this type of sensing circuit?

I've tried a few things to narrow down the problem:

1. Swapped single Monster Cable RCA with a dual (L/R) RCA. Receiver has two subwoofer RCA outputs; sub has two RCA inputs. I tried every combination of hooking those up with the LFE switch in both positions - No change.
2. Verified all subwoofer settings and tried about every combination (Level, LFE, Phase, and Crossover Frequency) - No change.
3. Verified proper receiver configuration. Subwoofer output is turned on and receiver indicates signal is being sent to sub - No change.
4. Connected iPod to sub RCA inputs via miniplug->RCA cable - sub instantly turned on. My guess is that the headphone jack on the iPod puts out a stronger line level signal than the sub output on the receiver. On the other hand, is the sub output on the receiver going out?

When the sub does turn on, it sounds great which leads me to suspect the turn on circuit in the sub, rather than the subwoofer output in the receiver. But, I'm no expert. That's why I'm here. Any expert opinions for me?

 

[MDS]

2. Verified all subwoofer settings and tried about every combination (Level, LFE, Phase, and Crossover Frequency) - No change.

You've tried everything I would have suggested, but for number 2 there was that the level of the sub or the receiver's subwoofer pre-out level? It's usually best to have the receiver's level set to zero or above to make the auto-standby feature work reliably, but using a Y adapter will double the voltage and should have improved things.

If the sub works fine directly connected to the iPod, then it does sound like there is an issue with the receiver's sub pre-out.

 

[jimrockford]

I adjusted the output level of the sub but also went through the menus on the receiver. On the receiver, the subwoofer is enabled and the level control is turned all the way up (for testing - all the way up is not very listenable). However, it's not really a pre-out adjustment I don't think. It's the adjustment used to set the volume levels of all of the speakers relative to the listening position. Does that have the same effect as a pre-out adjustment?

 

[Nuglets]

The subwoofer level adjustment in the reciever will adjust the subwoofer pre-out level. Set this to around 0dB or a little above, and then adjust the volume of the sub using the Subwoofer Level control on the subwoofer's amplifier.

The subwoofer will shut down when the voltage doesn't get above a certain level for a period of time. The pre-out level on the reciever is how you will control the voltage of this signal. The subwoofer level control on the subwoofer's amplifier controls how much that signal will be amplified.

If your level on your subwoofer's amplifier is set to high, you will require less volts across the RCA cable to make the sub sound loud, but it will also allow the sub to shut down because the voltage is below a certain level for a period of time.

 

[j_garcia]

I'd personally get rid of this sub. The amps in these subs are very well known for going up in smoke (literally) and are generally poor quality.

 

[jimrockford]

Nuglets -

Thanks for the clear explanation. That makes sense. However, it sounds like since I have the receiver's subwoofer level set as high as it will go, the sub should stay on, right?

j_garcia -

I hear what you're saying, but my whole system is of "generally poor quality". My receiver is a Sony, my front speakers are Infinity, and center/surrounds are Acoustic Research. Unfortunately, replacing everything at this time isn't in my short term financial plan...

 

[Tod]

I've also got a low-end JBL sub (although I'm not really using it any more). It's had a couple of problems. First, some time ago the capacitors or diodes or cathode ray tubes or tasers or something (I'm still working on my correspondence PhD in electrical engineering, so be nice) went out. It just sat there humming loudly until I unplugged it. Apparently that was a common problem, based on what the repair guy told me. I think he put in better parts.

The other thing was that the level control knob on the front started going bad. This would result in the subwoofer going off completely unless I had the knob just right so that it still made contact. I ordered a new one from JBL, but then found out I couldn't take the old one apart so had to wire it in externally. Now my level control knob is hanging off by a couple of wires and looks stupid. But at least it works.

Don't know if either of those relates at all to your problem, but yeah I wouldn't trust my life to the long-term reliability of JBL subs at the moment.

Edit: Ignore me if you want - I was thinking this was a different ancient thread I came across recently that was actually talking about a malfunction.

Also, on my JBL it plays even when the red diode is on, which should be standby. The green light comes on when it has to play loud, but it's still going even before that.

 

[Nuglets]

However, it sounds like since I have the receiver's subwoofer level set as high as it will go, the sub should stay on, right?

Yes that is correct. If it still shuts off then this obviously isn't the problem you are having. Does your sub have more than one RCA input? If so, try to use a Y-adapter on the pre-out and run a twin RCA cable to the sub. This will increase the line voltage also so you will have to adjust the level again. If the sub only has 1 input and the reciever is outputing it's max signal, then the subwoofers input is very insensitive or the reciever's pre-out signal is weak.

EDIT: I just read in your first post about the 2 subwoofer pre-outs on your reciever so I guess you are using 2 RCA's. Are one of the pre-outs specified as 'mono'? If so, try only using that one with the Y-adapter. If not, I guess try one at a time and see if that works.

 

[j_garcia]

You don't have to scrap the whole system, just the sub $114 + shipping will get you a 10" powered sub from Parts Express that will at least equal if not outperform that sub. They also have a 12" version for ~$140.

If it turns on immediately with the I-pod, I would then suspect your receiver. Do you have any friends that might let you borrow a sub to try out and see if you get the same thing? Or maybe go buy one a CC or BB and try it out and return it?

 

[Nick250]

If it turns on immediately with the I-pod, I would then suspect your receiver. Do you have any friends that might let you borrow a sub to try out and see if you get the same thing? Or maybe go buy one a CC or BB and try it out and return it?

I missed the i pod part on my first read. My thought was similar to garcia's, which is to buy a receiver for testing purposing and then return it, or keep it for that matter if turns out to be a receiver problem. The i pod part really makes me think it's a receiver issue.

Nick

 

[markw]

Have you given thought to running it from the speaker level inputs?

It might take the urgency from having to run out tomorrow and buy, in haste , something that is less than you really want or need.

 

[jimrockford]

All good suggestions...

Tod -

That's strange that your sub works when the red lights are on. Mine definitely doesn't do anything until the green lights come on.

Nuglets -

I originally had a Monster Cable sub RCA running from one of the receiver outputs to one of the sub inputs. The sub was set to LFE which I believe is the sub's equivalent of the mono setting. During troubleshooting, I replaced the cable with a dual RCA and tried the LFE switch in both positions.

j_garcia/Nick250 -

you may be correct about the receiver outputs being bad. However, I suspect the output from the iPod's headphone jack might just be a higher voltage than the receiver's pre-out.

Regardless, I have an old receiver I use in the garage that I could hook up to the sub to narrow down the problem. I hadn't thought of that. I'll give it a try when I get off work today.

markw-

I did consider running it off of the line level inputs but honestly have never hooked one up that way. I'd have to break out the instruction manual to see how to do it.

 

[Nuglets]

Wait, so you're saying you have the cables plugged into the subwoofer input on the sub, but there is a setting for LFE? Isn't there a separate LFE plug on the sub? My understanding is that you would use the LFE input to bypass the subs internal crossover so it relies on the reciever's bass management to filter the frequencies.

 

[j_garcia]

The iPod headphone output is likely less than 1v, where your receiver should ideally be something like 3-5v, so I don't think the iPod has higher output.

 

[MDS]

Wait, so you're saying you have the cables plugged into the subwoofer input on the sub, but there is a setting for LFE? Isn't there a separate LFE plug on the sub? My understanding is that you would use the LFE input to bypass the subs internal crossover so it relies on the reciever's bass management to filter the frequencies.

Recent JBL subs have only one input and a switch and it works exactly as Nuglets described. When the switch is set to LFE, it bypasses the sub's internal xover and when set to Normal the sub's internal xover is active. If you use the receiver's bass management you want the switch in the LFE position.

 

[Nuglets]

I looked at the user manual for your sub. If you have it set to LFE, use the Left OR right input NOT both of them. You could try using both of the inputs and setting it to Normal insead of LFE. If you do this you will want to set the crossover to it's highest setting so it doesn't interfere with the reciever's bass management.

Another thing. When I was talking about a 'mono' pre-out, this is on the reciever pre-out, not the sub input. Try using only one pre-out on the reciever and one RCA cable with the sub set to LFE. If that doesn't work, try the other subwoofer pre-out. Still nothing, then try one pre-out using a Y-adapter and twin RCA's going to the sub's input with it set on Normal. Then the other if there is still no improvement. If none of that works I am inclined to believe that your reciever's pre-out's give a very weak signal.

 

[jimrockford]

It looks like the receiver is to blame. I used a multimeter to measure voltage at the receiver's sub outputs and only managed readings of .4 to .6 volts. The manual states the subwoofer output should be 2 volts.

Also, I connected the sub to my garage receiver and it worked perfectly. So, it looks like I may be in the market for a new receiver.

Unfortunately, I'm also shopping for bedroom furniture and a dining room table. The old stuff won't do in our new house (so sayeth the wife). So, If I do go for a new receiver, I'll be on a budget.

Does anyone have a recommendation for a decent receiver in the $500 range? I'd prefer to buy online and would love to find a killer deal on a closeout or year old model. I see that Yamaha and Denon are highly regarded here. If I go with one of those brands, is there anything I should keep an eye out for?

My home theater consists of low end equipment:

Panasonic TAU 36" standard definition TV
Sony STR-DE845 Receiver
Infinity RS425 front speakers
Acoustic Research center channel and surrounds
Pioneer DVD
TiVo
Old VCR
Brighthouse Cable TV box (Scientific Atlanta)

All video signals run through the receiver using S-Video cables (except the VCR). Audio is via digital coax or optical (again except for the VCR). I don't expect to upgrade to HDTV for a while since my TV is still fairly new and my entertainment center maxes out at a 36" TV.

Thank you for all your help.

 

[jimrockford]

Update...

Well, I hooked the sub up through the speaker level inputs thinking that would get me through until I could buy a new receiver, but I'm still having the same problem. At this point it's got to be the sub.

The front speakers still work, but the sub won't stay on until I turn up the volume. So, I guess I'm looking for a new sub now.

 

[bandit]

One thing you may consider - to buy yourself time is to just turn the sub to the on position instead of auto on if you have that option on that amplifier. One mistunderstood thing about most subwoofer amplifiers is how the "auto on" signal works. If you really get into them - it turns out that the "auto on" circuitry on most amps actually is just an input signal switch allowing the sub to operate when signal is present but disabling the input circuitry when no signal is present. In most cases the output sections including the power supplies and final stages of amplifcation remain powered at all times unless the sub is unplugged or if the switch on the amplifier has an "off" position.

Bandit.

 

[Nuglets]

Update...

Well, I hooked the sub up through the speaker level inputs thinking that would get me through until I could buy a new receiver, but I'm still having the same problem. At this point it's got to be the sub.

The front speakers still work, but the sub won't stay on until I turn up the volume. So, I guess I'm looking for a new sub now.

Bummer. If your budget allows it would be a great time to upgrade to something a little higher up in the food chain. Or at least not another JBL like that one. There is really no excuse for a malfunction like that and I would be very dissapointed if that ever happened to a sub of mine. Is it still under warranty? If so, maybe you can get it fixed or get a replacement, then sell it and use the money to get an even better sub.

When you checked the line voltage of the RCA out, are you sure you were measuring it correctly? I don't have much experience with a multimeter so I really don't know how to go about checking the voltages of pre-outs. Anyways, if the sub preout is out of commision you will be forced to use the speaker level inputs for any sub you try unless you get a new reciever. Not that using speaker level inputs is a bad thing, it just requires a few more wires and a different setup in the reciever. In other words, you won't be downgrading by any means using speaker level inputs so there is no need to feel pressured into a new reciever right away. Wait until your budget allows and upgrade slowly and do your research and you will be much better off than if you spontaneously buy things.