Amp questions

[RobertP]

I've just got my head unit back (irritating display switching now turned off) and while in the shop splashed out on a kenwood 5 channel amp (959) and some 17cm infinity components for the front.
I have not had an amp before. It all looks straight forward enough...except the power on trigger lead. Looking at the diagrams for the head unit (kenwood z838w) the lead marked for telling the amp to turn on is also marked as being for the power to the electric aerial. i asked in the shop if it would be ok to pick up off the aerial (save yet another wire from dash to boot) and the guy said no - it must come straight from the head unit.
Am I right that its the same wire for power on and aerial?

I intend to mount the amp on a plate under the middle of the rear parcel shelf between the 6x9's. There won't be much natural convection of air. Does an amp make much heat at comfortable volumes? Do I need to worry about cooling?

I got a 5way amp to make the option of adding a sub easy. I intend to give it a listen without one first. Stupid q maybe - I've never had a car sub before - do you just lay the thing in the boot and let the bass come through the back seat! ? Do I need to trawl through reviews for a good one? Good to me = clean sound not louder:)

Thanks

[lynx]

Firstly, i'm not familiar with the hu so ... It should be fine to attempt your suggestion, however be aware that some units only signal a short pulse to the aerial rather than a continuous signal ( which your amp will need).

[G1OSE]

you do as far as i no need to take a cable from the hu to the amp because as lynx said its not constant as ythe arial is only powered for a few seconds whilst it goes up
the amp is best not being upside down on the parcel shelf but you can get away with it if used at low volumes but it does give of a fair bit of heat
the sub needs to be boxed and in the boot best facing away from the back seats towards the bit that opens so it then uses the whole boot as a box and if the right sub is used you will feel the bass not hear it through the car without it being in the passenger part
hope that helps
BTW if you buy a sub and box mount the amp to the back of the box it is a lot easier than cutting holes everywhere and can be removed easier
ok

[RobertP]

Thanks for the info.
I'll stick a voltmeter on the aerial wire and see if it stays on. I'm guessing it will as the manual for the head unit clearly says the wire is for aerial *and* amp turn on.
I'm making a custom aluminium plate to hinge down and clip up to the parcel shelf underside - so the amp will be the right way up.. but with restricted airflow. I could stick a computer type fan on the end if needed.
I want to be able to easily unplug and remove a boxed sub (if I get one) so mounting the amp to it is not an option.
"Feel the bass not hear it" :grin: I was wondering how a sub worked being shut in the boot!
What sort of budget would get me a reasonable sub?
the amp is 4 x 40W rms + 1x120W rms for the sub.....

[micb3rd]

Most headunits have an assessory lead at 12V to power the remote turn on of an amplifier.


Subwoofers and bass in cars....

A car cabin is like a very small room so there huge gain in the bass region.

Whe the subwoofer is facing away from the seats the Bass waves fire through the open boot space hitting the rear then reflecting back through the boot area passing throught the seats (which make very little resistance) and into the main cabin area.

[lynx]

Originally posted by RobertP
I'm making a custom aluminium plate to hinge down and clip up to the parcel shelf underside - so the amp will be the right way up.. but with restricted airflow. I could stick a computer type fan on the end if needed.

I'd tread cautiously with the amp mounting.I assume the planned sub will be 4 ohm ? If so running it in a mono configuration ( amp bridged ) will see 2 ohms at the output devices which when driving a sub, moderate to hard, will result in heat build up which will require to dissapate.This problem will be compounded if the amp is mounted upside down as the heat will dissapate through the board ..... not good.

[RobertP]

Originally posted by lynx
I'd tread cautiously with the amp mounting.I assume the planned sub will be 4 ohm ? If so running it in a mono configuration ( amp bridged ) will see 2 ohms at the output

The amp has 5 channels and does 120W rms into 4 ohms on the sub channel.
I've noted whats been said about heat and will check the temperatures. The amp has thermal protection...just in case.

[Dave20]

didnt read the other posts.. the kenwoods have a remote in and a remote out to "daisychain" easier

[Dave20]

Originally posted by lynx
.I assume the planned sub will be 4 ohm ? If so running it in a mono configuration ( amp bridged ) will see 2 ohms at the output devices which when driving a sub, moderate to hard,

wrong heh :) if you put a 4 ohm sub on a 4ohm amp it will be 4ohms load and = standard power. when you have a dual VC 4ohm or 2 x 4ohm subs, this is when you can drop down to 2ohm


but mounting upside down would be a "no no" but its your car/amp so do as you will :rolleyes:

[lynx]

Originally posted by Dave20
wrong heh :) if you put a 4 ohm sub on a 4ohm amp it will be 4ohms load and = standard power.
I stand by my statement. You have either misread my post or don't fully understand the principles.
A 4 ohm load presented to an amp with 2 channels bridged will be presenting 2 ohms to the output device.

[lynx]

If you're using a mono-bridged amplifier to power this driver, the amp needs to be stable with 2 ohm stereo loads. The load on the amplifier is 4 ohms, but when bridged, each channel "sees" exactly one-half the net load as seen at the amplifier's terminals.

Source (http://www.jlaudio.com/tutorials/wiring/1svc.html) :)

[Dave20]

Originally posted by lynx
I stand by my statement. You have either misread my post or don't fully understand the principles.
A 4 ohm load presented to an amp with 2 channels bridged will be presenting 2 ohms to the output device.

ok, basically your saying, for eg : a 2 x 100watts amp, bridged for 1 4ohm sub = 2ohm load???(1x400watts)

well in 7 years of professionally installing car audio (probably 1000+ amps) i never knew that.. even after the countless books and manufaturer manuals i own/read saying otherwise.. so i still stand by my statement :)


edit: including a JL 12- w6 manual heh

[Dave20]

but i must admit, i have never installed a mono sub amp. only 2,3,4,5,6,8 chan amps

[lynx]

Dave20, the principle is applicable to two channel amps ( indeed any multi channel amp, forget the JL link for the time being ).Any ohmic load presented to an amp is a nominal value ( there are many variables which mean it's not a constant ), however if a load of 4 ohms is connected across the two bridgable terminals then as far as the amp is concerned it will (electrically) 'see' a load of ½ which the speaker is connected.This is not open to debate or interpretation, it is simply laws of physics. :)
Have a read through your many books again as this subject *will* be covered.

[lynx]

From Phoenix Gold site,
If an amp is rated to handle 4 ohm bridged, then it can handle 2 ohm stereo. It's the same total power. The minimum stereo load is always 1/2 of the bridged capabilities. Each amp channel must deal with 1/2 of the total load (1/2 + 1/2 = 1). So whether it's running 2 ohm stereo or 4 ohm bridged, the amp responds to the load the same. That's why you see power specs like 200 watts x 1 at 4 ohms bridged OR 100 watts x 2 at 2 ohm stereo. It's still 200 watts.


Q. I’ve heard that when you bridge an amplifier at 4 ohms, the amp really sees half that. It seems to me that a 4 ohm woofer can’t magically change into a 2 ohm load! So what’s the deal?

A. Imagine a bridge spanning a river. If the bridge’s only support is each bank of the river, then each river bank must support 50% of the load. So a more correct statement would be that each channel sees (supports) half the load. In this case 2 ohms. This is also why you see amp specs that say 100 watts x 2 into 2 ohms stereo and 200 watts x 1 into 4 ohms bridged. 100 + 100 = 200. This is also why the minimum per channel load an amplifier can play is always half of the bridged spec.

Hope that helps.

[micb3rd]

No offence guys but you seem to make quite a hash up of a very simple situation.

When wiring subwoofers in series it increases the ohms, reduces load and decreaces the power provided by the amplifier.

Wiring in parrellel decreases the ohms, increases load and the power from the amplifier.

Most 2/3/4 channel amplifiers are rated to certain ohm level either in a stereo mode or bridged mono mode.

So a 2 channel amplifier can be presented a load of 2 ohms in stereo can be run safely in bridged mode at 4 ohms mono.

If you ran the said amplifier at either 1 ohm stereo or 2 ohms bridged mono it could cut out and possible burn/catch fire.

Good monobloc (single channel) amplifiers are often very flexible, they can give greater power into lower ohm loads safely (e.g. 1/2 ohm stereo and 1 ohm bridged mono) so for example a big mono amplifier can run 300 RMS to single 4 ohms subwoofer, or 600 RMS to two subwoofers wired in parrellel to 2 ohms or 1000 RMS to three 4 ohm subwoofers wired in parrellel to a 1.33 ohm load.

[lynx]

Originally posted by micb3rd
No offence guys but you seem to make quite a hash up of a very simple situation.
What is it i've made a hash of ?

[RobertP]

I've been a bit busy installing. Demolished the car trim running wires....even had to drill the bulkhead for the power cable. Still cables are run now, amp is mounted (right way up) on a 5mm thick aluminium plate. Might get time to actually get to connect something tomorrow!
Interesting way this thread has gone. I'm no car audio expert but from reading the Kenwood amp leaflet I think I can see where the bridged/stereo and ohms confusion is.
A 4ohm load is a 4ohm load whatever its connected to. When an amp is bridged (at least it looks that way in the leaflet) the amps are in SERIES.... which will add their output voltages. Increase the voltage and more current will flow. Hence the comparison to a 2ohm load on a single amp. each half of the high voltage bridged amp will be supplying current as if it were connected to a 2ohm load.

For the record the kenwood leaflet says the front channels are 40W rms into 4ohm as separate stereo amps or 120W rms into 4ohm with that stereo pair in bridged mode.

Ignoring frequencies P=I²R so stereo mode I=3.2A
V=IR V=12.8V for one amp
Bridged mode 120=I²x4 I=5.5A
V=IR V=22V for 2 amps in series.

I think the site Lynx linked to was misleading.
I typed this after a beer or two so if I've offended anyone I'll apologise now;)

[micb3rd]

Originally posted by lynx
What is it i've made a hash of ?

Sorry mate, I have re-read the posts and I made a mistake in my assumption first time around.


No hard feelings.

[lynx]

Group hug :)

[Pyara]

You should be fine doing what you suggested. The remote lead from the HU stays at 12V whilst the unit is operational (on!). The powered arial has a connection to the battery via a fuse from which it actually powers itself, the 12V remote from the HU just tells it when to go up or indeed go down, it doesn't actually power itself from their.

Logically speaking if you took another wire from the HU, it's the same as bridging the wire which goes into the powered arial from the HU!

On my system, the 12V remote wire goes to the boot and switches on the AMP as normal. In addition, I've also connected it to a relay at the front which powers my fog lights, under car neon and in-dash thermometer/digital compass. Furthermore, it's also connected to a relay in the boot which poweres two green neons and knight rider style red lights.

As I have a remote ignition system I can turn the engine on/off using a remote control this in-turn activates the stereo and hence turning everything else on, I can also do the reverse and turn everything off with the remote!

[RobertP]

It is working fine:)

In fact in the small print the amp leaflet says it will still work without the turn on wire! in that case it would sense to turn on from the input signal. Thought it might be slower responding so I did fit the wire.

I have a few thoughts about my new equipment (now including a sub) which I'll post when I get time....