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I just bought an older car (1982 BMW 528e) and I was looking to upgrade the car audio system. It currently has an aftermarket 2011 era Sony stereo in it. The car can only run 4x6 speakers and to run larger speakers, I would need to do some metal cutting which I do not want to do. My plan is to replace the stereo with the Sony MEX-XB120BT which has a built in class D amp outputting 45W RMS x4, which is sufficient for my goals of just getting good sound out of four 4x6 speakers. I do not plan to have any sub and would prefer not adding the complexity, wiring, and cost of a standalone amp. The speakers I was considering (all around) were either the Infinity REF-6432cfx 4x6 or the Polk db461p 4x6. The Inifinity is rated at 45w RMS and the Polk at 50w RMS. Is there anything I'm missing, or any critiques or suggestions I should be considering?

Thanks!
I have this same HU installed in a '03 Corvette Z06...one of the main reasons I selected it was for the power, I didn't want to fiddle with an amp either. It sounds great after a little tinkering with the settings.
 

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Some of the posters jumped the gun to discount the OPs post without researching Sony’s head units with built in amp line.
That's me!

I stand corrected. That is pretty cool. I still hate Sony's UI. They have a wonderful BT codec if you had compatible devices.
 

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Discussion Starter #25
Following up on this, I got the HU and the ruse is a 15A fuse. This kind of implies that the power rating is a bit weak here right? For example, if i connect the power wire directly to battery, that means I'm getting ~12v. Since voltage x current = wattage, 12 x 15 is 180, which would be the max power I could get, assuming 100% efficiency before the fuse blows. Since this is rated 180w RMS (45w rms x 4), I think the rating of the HU and the fuse are contradictory. Does that make sense?
 

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Discussion Starter #27 (Edited)
If the HU is directly wired to battery, is it still receiving 14v? I know the alternator produces 14v but battery is around 12v.
Assuming 14v, this would push the max wattage to 210w before fuse blowing. And the amp is a class D, so assuming 75% efficiency you’re likely still blowing fuse at rated rms power of 180w right?
 

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You never get 100% when playing music .
Playing tones you can.
 

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Discussion Starter #29
What's weird, is in the actual booklet it says that "During full-power operation, a current of more than 15 A will run through the system. Therefore, make sure that the wires to be connected to the +12 V and the GND terminals of this unit are at least AWG 14". How does that square with the built in 15A fuse?
 

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Does not square well...
But I doubt that you can get as much power laying music as a test with tones can consume.
 

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The numbers do not square with the manual.

CD, USB, etc... usually sound like music.
You would be challenged to get more than 10W out of music.

To get those numbers you need some tones or square waves, and the former I suspect not interesting... and the later case is not worth listening to either.
 

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For what it's worth, most of the small 45Wx4 class-D amps I've seen also use a 15amp fuse (Alpine KTP-445U, for example). Seems pretty common to me.
 

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For what it's worth, most of the small 45Wx4 class-D amps I've seen also use a 15amp fuse (Alpine KTP-445U, for example). Seems pretty common to me.
Yup, because music is rare to get anything closer than 6db to the peak clipped value, so that’s a quarter of the 45wrms when a single channel is driven, so now your at 12 wrms, and then that’s only ever on peak massive bass heavy stuff... which door speakers won’t like, so you use a hpf and cut off low bass, now you end up with half the power, so approx 6 watts rms, sure if you play a zero db test tone you may get over 30watts rms... but personally driving around listening to test tones sucks ass

as I said in my first post, you will likely be getting <12 watts rms at any given point in actual music, anyone who says any other is deluded 👍🏼 Clipping and impedence rise will naturally limit power

as an example lots of tweeters are rated at 10 watts long term... they are often put on 150 watts rms amplifiers and survive... why do you think that is? It’s certainly not because the amp Puts 150 watts rms into the tweeters... likely that 150w amp only puts 1-5 watts into the tweeters, because something has a rating, it doesn’t mean it will do it all day everyday, it’s not how music reproduction actually works 👍🏼
 

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Yup, because music is rare to get anything closer than 6db to the peak clipped value, so that’s a quarter of the 45wrms when a single channel is driven, so now your at 12 wrms, and then that’s only ever on peak massive bass heavy stuff... which door speakers won’t like, so you use a hpf and cut off low bass, now you end up with half the power, so approx 6 watts rms, sure if you play a zero db test tone you may get over 30watts rms... but personally driving around listening to test tones sucks ass

as I said in my first post, you will likely be getting
as an example lots of tweeters are rated at 10 watts long term... they are often put on 150 watts rms amplifiers and survive... why do you think that is? It’s certainly not because the amp Puts 150 watts rms into the tweeters... likely that 150w amp only puts 1-5 watts into the tweeters, because something has a rating, it doesn’t mean it will do it all day everyday, it’s not how music reproduction actually works
So are you suggesting that there is no need to ever run an amp with more than, say, 12W RMS per channel (for non-subwoofer-channels)??

I have a hard time believing that midbass speakers will never use more than 12W RMS while playing music.

Unless I'm misunderstanding what you are saying...
 

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So are you suggesting that there is no need to ever run an amp with more than, say, 12W RMS per channel (for non-subwoofer-channels)??

I have a hard time believing that midbass speakers will never use more than 12W RMS while playing music.

Unless I'm misunderstanding what you are saying...
Not at all, although you don’t need lots of power for tweeters, if I could get a zapco z50.2ap I would be happy to run it on my tweeters, and still have it knocked back to 5 or so watts at full pelt...

I can tell you for my Emma setup I have the gains on minimum on my z150.6ap so set for 9 volts, my dsp outputs only 6v at +5db and I have the tweeters level set at -6 and -11db... likely I only use a fair bit less than 5watts even at full pelt

You can’t run a 12 watt amp for all especially if running passive as you would need more power for midbass for example, if you’ve ever seen the energy dispersion of pink noise (basically how we hear and needs the energy distributed) you would know where I’m coming from, the higher the frequency the more power you will need for equal loudness

The only speaker I run near flat out is the front left midbass off of 250 rms... and even then it won’t see more than 60wrms due to the dispersion of afore mentioned energy distribution in the audio spectrum and also impedence rise, and I can get loud enough

In reality I run the zapco as it’s a solid bit of kit, I compete and it does offer me advantages over a cheaper amplifier and I need every edge I can get in competition... I’d be deluded if I thought I’d need

Pink noise falls off at 3db per octave, so if 20-40 uses all 45 watts, 40-80 will use 22.5 watts, 80-160 is 11.25 watts

so in my case the midbass play 80-200, but let’s say 80-160 watts

so we have 11.25watts if the music is reaching 0db... from testing when I did db drag bass race I measured some very bass heavy music (lil John etc...) and it was rare that the bass freq got past -6db, so let’s be generous and say the midbass is also at that level, even though we know it’s probably less...

so the 45 watt amplifier now does 11.25 watts -6db so to play lil John at full pelt with no clip the midbass would need 2.75 watts of energy to keep up with the subs at 45 watts... don’t forget that the gain is fixed in the headunit so unlike a proper amp you can’t turn it up more (less energy therefore voltage input vs full range with a midbass amp) if it’s needing to make less energy for sub bass due to potentially clipping the preout into the subwoofer also

I hope that all make sense and goes some way to explain what I was trying to get across... in simple terms even if you have a 150 rms your likely only using less than that by a good chunk 👍🏼
 

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So are you suggesting that there is no need to ever run an amp with more than, say, 12W RMS per channel (for non-subwoofer-channels)??

I have a hard time believing that midbass speakers will never use more than 12W RMS while playing music.

Unless I'm misunderstanding what you are saying...
12W RMS is like 250W peak,
 

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I'll be honest - I'm not following.... Are you saying that a 45W RMS amp can really only produce ~12W RMS when it comes to music?

I also saw a figure earlier in this thread (by @Holmz) saying that a 100W RMS amp can only supply about 4W RMS without clipping.

I'm not following - at all... :)

I completely understand that tweeters and midrange speaekers won't use as much wattage as midbass speakers - I get that - not questioning that. Although, I will say, the gains for all channels on my 75W RMS x 6 amp need to be set the same in order for my wideband mids (or 3' coaxs) to be at the correct level for my midbass speakers - especially after EQ (usually have to bring the windbands/coaxials down quite a bit via EQ to get them "flat"). I may adjust a channel by a dB or two in the DSP, but generally speaking, the gains for all amp channels are the same on the amp.
 
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