Help with amp issues!

kev000 · 11-03-2009, 08:13 PM

A little about my setup:

- 2 x 12" Kenwood subs of unknown model. The only identifying marks on the visible part I can see are "Kenwood" and a big "W" in the center of the cone.
- Kenwood (claims 400W but idk) Amp KAC-748 (owners manual here: http://www.kenwoodusa.com/UserFiles/...als/kac748.pdf), 4 channel amp.
- Xenarc MDT-X7000 in dash lcd with AMP

The carpc outputs to the xenarc. The xenarc powers the 4 stock speakers and has 1 line out for the sub.
Looks like i have the subs connected to the amp via 200W each. IDK if my subs are 4ohms or 2 :-/

The problem:

before, i had the sensitivity output turned all the way up the bass power was good, but distorted at times. I turned the input sense down and now power output sucks. I can barely hear it. What do I need to do to get good power AND good sound?

justchat_1 · 11-03-2009, 08:18 PM

Throw an ohmmeter across the sub terminals...even if you don't have the speaker model all you really need is the impedance.

Technically you have 100 watts per channel if bridged and 2ohm subs. Realistically this is a 35w RMS amp...not exactly a sub pusher.

PhilG · 11-03-2009, 08:43 PM

if your measuring the "resistance" across the voice coil, you need to multiply that number by 1.3 to get an approximate "nominal impedance". for that driver. It really doesn't matter though, that amp is only capable of bridging to 2 channels so you are going to wind up putting one sub on each bridged channel anyways so "it is what it is". The amp is capable of a 2 ohm load so even if the subs were wound for 2 ohms (which I am sure they are not) your amp should survive. Make sure you have set the filters up right to knock down the high frequencies, or that will cause distortion. maybe you should consider only running 1 sub (if they are not mounted in the same box) and run 2 channels to the mid/high speakers. That would likely improve your SQ considerably.

kev000 · 11-03-2009, 09:06 PM

so a preamp won't help me? I'm leaning towards getting a new sub/amp set anyway. maybe i'll have to get it sooner than later.

jessekilner · 11-03-2009, 09:06 PM

after reading the specs..lol, being bridged it only shooting @ best 50RMS but 35 is likley per sub RMS is the biggest factor as it's what 95% of your music listening is heard in. The peak is 100Wx2..PEAK this is considerably right on par as Just_chat said earlier.. as not enough juice for subs.. you will get bump on highest settings but your taxing that amp's ability and therefore the distortion is promininent. Just like a car would overheat under heavy stress...your amplifier is doing the same thing.
I would suggest a different amp for subs and use this one to power your compnents 6x9's and such

jessekilner · 11-03-2009, 09:11 PM

[QUOTE=PhilG;1353872]The amp is capable of a 2 ohm load QUOTE]

in 4ch not 2 channel notice they dont mention that...

PhilG · 11-03-2009, 09:22 PM

[QUOTE=jessekilner;1353876]

Quote: Originally Posted by PhilG

The amp is capable of a 2 ohm load QUOTE]

in 4ch not 2 channel notice they dont mention that...

Yes, your right! I saw the line saying "Rated Power Output ( 2ohm) and thought that it belonged with the line above it saying 2 channel, but it actually belongs with the line below saying 4 channel LOL.
It's amazing though that this little amp is claiming a 28amp draw. It can;t be very efficient or its under-rated.
If a new mono block is in the cards to drive the subs, thats the way to go!

justchat_1 · 11-03-2009, 09:27 PM

Pre-amp may help you but so may a different configuration. If those are 4ohm subs they can be wired in parallel and deliver twice as much power. I would say the best option is to see where your starting from.

PhilG · 11-03-2009, 09:32 PM

Quote: Originally Posted by justchat_1

Pre-amp may help you but so may a different configuration. If those are 4ohm subs they can be wired in parallel and deliver twice as much power. I would say the best option is to see where your starting from.

Like we discussed above though, that amp is not 2 ohm stable unless ran in 4 channel mode, so that doesn't work

jessekilner · 11-03-2009, 09:40 PM

I beleive he has it 4 ohm on each channel, I doubt those kenwwods are 2 ohm.. my Jl's are single 2 ohm but Im doubtful kenwoods are. Its just s simple matter of not enough power. reguardless of input range he sticks in it, it will only produce 100W Peak 35-50RMS per channel bridged and thats under the minimal operating range for the sub Im sure.

justchat_1 · 11-03-2009, 09:41 PM

I'm tired so I could be off but wouldn't a 2ohm load at 70 watts deliver more power then a 4ohm load at 100 watts (P=I^2R)? Granted thats peak...RMS would be equal but I would think that would still make a difference?

kev000 · 11-04-2009, 04:26 PM

Quote: Originally Posted by justchat_1

Pre-amp may help you but so may a different configuration. If those are 4ohm subs they can be wired in parallel and deliver twice as much power. I would say the best option is to see where your starting from.

The were originally wired in parallel. So they may just be 4ohm subs.

soundman98 · 11-04-2009, 04:43 PM

Quote: Originally Posted by kev000

The were originally wired in parallel. So they may just be 4ohm subs.

...which would be a 2ohm load, and would cause it to overheat a lot, and cause it to shut off

my sony sub amp is the same way-- it is not 2 ohm stable, but i have hooked it up for fun in 2 ohm mode for 15-30 min and then it will go into thermal protection...

time to break out the DMM and double check.

justchat_1 · 11-04-2009, 08:56 PM

Quote: Originally Posted by soundman98

time to break out the DMM and double check.

audiophyle_247 · 11-05-2009, 03:19 PM

What is the crossover filter set to on the amp?
Ive seen people bring amps in because it sounded like **** when the problem was because they had the gain & bass up to max & the crossover set all the way down to 20hz.

The amp isnt a powerhouse, but Kenwood subs dont need a lot of power anyway.

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