Amplifier Cuts Out at High Loud Notes, but Subwoofer Doesn't Miss a Beat!

OK, so I have two JBL Stadium GTO620 speakers in the front, which are 2 ohm.
I have Alpine SPS-610 speakers in the back, which are 4 ohm

I have a Pioneer DEH-x9600bhs head unit.

Using the four preamp RCA jacks on the head unit, I connected into an Alpine KTP-445u amplifier. It’s the little Class D one.
Also using the two preamp RCA jacks on the head unit, I connected a Kenwood KSC-SW11 powered enclosed subwoofer.

It sounds absolutely wonderful, most of the time.

The problem I’m having is when the radio is loud, the sound on the four speakers will cut out at very high, loud notes. The odd thing is that the subwoofer is fine. It doesn’t miss a beat. The four speakers will resume musically after the high note has passed, so no resetting, or repowering of anything is necessary.

  1. The yellow power line that I use to power the Alpine KTP-445u amplifier comes from the back of the head unit.
  2. I tapped into that wire to power the Kenwood KSC-SW11 subwoofer as well.
  3. Neither the head unit, nor the amplifier, nor the subwoofer share a ground point. They all have their own.
  4. The head unit is in charge of crossovers. I have a low crossover set at 63Hz. I think the high crossover is the same, 63Hz.
  5. Head unit spec sheet never indicates that the radio can handle 2 ohm speakers, just 4 ohm.

Can anyone point me in the right direction?

​​​​​​​Thanks.

Since you are using the preamp outputs and not the speaker outputs, the head unit doesn’t know that you are using 2 ohm speakers on the amp.

I’d try hooking another head unit temporarily to see if it’s a headunit preamp issue, or an amp issue. Id also try changing the crossover settings, using full range on the headunit, and varying them on the amp. I may be that conflicting crossovers are fighting each other.

I don’t have another head unit to hook up…

The amp has no crossover enabled. I did try to vary the High Frequency Cutoffs, with minimal changes. I even disabled the cutoff on the head unit entirely. No valuable change.

That amp will also accept speaker level inputs. Try them. Have you double checked that the dipswitch settings on the amp are correct?

Do I read this correctly that you are pulling power for both of your amps from the head unit?
Or do you mean from the same wire that supplies the head unit?
Either answer makes me nervous…
What is the path from each amp to the battery?

Yes. I did check to make sure amp dipswitches are all set to HPF Cutoff “OFF”. Other options are 60Hz, 80Hz, and 120Hz.

Only other dipswitch settings are set to 4 Channel (vs 2 Channel) and RCA (or speaker level). I’ll give speaker level a go when I get back in there.

Thanks for the suggestions.

You are.

They are both Class D amps. The harness that comes from the back of my head unit has a yellow wire that I tapped for both the Alpine 4 channel amp and also the subwoofer.

I know where you’re going with this. The path back to the battery goes through the head unit…

I am wondering if the high-end amp is seeing low voltage or some sort of overload which is causing the cut out. Couple of ideas:

Try temporarily disconnecting each pair of speakers from the high end amp and see if that changes anything. That amp should handle those speakers fine, but who knows. If running the high end on only one pair of speakers eliminates the problem then it’s some sort of amp overload issue. If the problem persists, then I’d suspect head unit.

Also test by disconnecting power the the subwoofer amp and just running the high end. That can tell you if your sub amp is causing a voltage drop which affects the high end amp. Yes, I know the sub amp does not cut out, but it may be less sensitive.

Ok. Respectfully I would not do that. I’d run direct to battery with a good fuse close to the battery. I get that they are class D and all that, just sayin that in this game clean tight DC power is a must. You could temporarily run a line and see if it helps. even if this is not the problem I would re-wire your power source. My .02.

Do you have an ipod or basically anything with RCA outputs that you could jack into the amps with some Y adapters? Then crank it up. That could completely eliminate the head unit as a possible source.

Sorry for all the posts, just brain dumping…

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That’s a great idea!

It’s rule no.1…

I too believe that the Amp is starving of juice and cuts out.

Thank you. I will try that first, as it is indeed the easier solution to attempt!

It does kind of kill the upside of a Class D amp. Since I have to run a new power line, any old amp would do, I guess.

Well, the compact design is the second plus. It currently resides in the knee bolster where most people would expect a glove compartment. (It does render the thermometer sensor for the heating and cooling useless, so my HVAC panel is always set on manual)

There are still advantages to class D in that they don’t consume as much current as an AB. My amps are powered by four gauge cable into heavy duty fuses run directly from the battery. Overkill for a class D.

Regardless, not to beat a dead horse, it seems like you’re pulling all the current for entire system through the head unit power wire. I assume that is where the yellow wire gets its power.

And if the head unit is hooked up to a factory power wire somewhere, versus directly to the battery or bus, there are even more choke points between the amps and the battery.

If you really want to nerd out, you can take a few voltage measurements. What is the voltage across the battery terminals compared to the voltage at the amp power wires? Even with no load, I bet you’re seeing a drop.

I am leaning more and more towards the amp seeing low-voltage and cutting out, I’ve never seen a head unit cut out when only the preamp outputs are being used. Not saying it can’t happen, I’ve just never personally seen it. The one thing that gives me pause there, is, I suppose it’s conceivable that the amps are pulling enough current via the head unit to cause the preamp section of the head unit to cut out. But that still feels like a long shot.

Keep us posted, I am super interested to learn what you discover!

So I connected the amp direct from the battery using an 8AWG wire. No difference.

The odd thing here is that the wire harness that came with the amp uses a 16AWG wire for power…I wonder what would happen if I attempted to upsize that.

I think I might need to upsize the amp?

The spec on the amp says it’s good for two Ohms, four channels. So you’re not even working it as hard as it could be worked, with 2 ohms on the front and 4 ohms on the back.

Can you switch the amp to two channel mode, and just run the four ohm speakers? See if that helps at all. That would reduce the load on the amp quite a bit, and maybe indicate if we are in fact seeing an overload condition.

The other thing you can do is try setting the high pass cut off on the amp to 60. Yes, the deck is in charge of crossover’s, and you could try it with both crossovers enabled, on the deck, and the amp, or just the amp. It will probably sound funky with both enabled. Point being, enabling the crossover setting on the amp might reduce the work the amp is doing.

It might very well be that your Amp has a problem.
I think the only easy way to find ou would be to try an other Amp, or an other Head unit to see if the problem persists.

Switched to two channel. No difference whatsoever.

I am feeding only the amp on the 8AWG wire. I’ve left the subwoofer attached to the power line from the radio. Subwoofer never misses a beat, and I’ve been unable to make any differences to the amp.

Speakers are 75W rms, 225W peak…so I think the speakers are fine? They’re on the stock/OEM 22 gauge wire.

Gain on the amp is set at 3 out of 7. N (7 being maximum)

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Working on the theory that the high-end amp is being starved of voltage for some reason:

You mentioned upgrading the 16ga power cable. I would prefer to only have that 16ga for the last 12 inches or so, and a 10ga or larger from there to the battery. But you’ve already eliminated the positive power supply as a problem with your 8ga test, so I would not worry about the 16ga cable.

Is there any sort of indicator on the high end amp - power light, clipping light, etc., that does anything while the problem is occurring?

Does this happen both with the engine off and the engine running?

Are you 100% sure that the ground for the high end amp is clean, tight and has a good connection?

EDIT: Per your post above running just the high end amp on 8ga eliminates this possibility. Does the problem persist when you run ONLY the high-end amp? (So the subwoofer isn’t hogging power).

The other possibility is that the amp is being over-driven and is going into some sort of self-protect mode.

EDIT just saw your post above - you switched to two channel running which speakers? All four, or jsut two? You could test for this by running just the 4 ohm speakers in 2 channel mode, then just the 2-ohm speakers in 2 channel mode. It should handle either of those loads easily.

Speaker power capacity is likely not a factor, the amp doesn’t care. Feeding 75w speakers say 125w over time will overheat the voice coils and fry (“blow”) the speaker. They will sound like hell but typically won’t hurt the amp.

Amp gain sounds good, near the middle of the range like you have it is ideal.