Help with amp size

[osprey92]

I am in the process of replacing my Android head unit, and in the process decided to replace current aftermarket speakers and add an amplifier. However, I am having a hard time understanding the correct sized amp I need to install. Speakers are dash: 40w, 20wrms, 4ohm; Doors, front and rear: 200w, 40wrms, 4 ohm. No subwoofer currently planned. I want to be able to balance power from front to back, left to right; the head unit has this capability. An amp with auto-on capability (power up when head unit powers on) would be good. Can someone break this down for me and recommend the correctly sized amp and channels (4 or 6)? Suggestions on brands and best mounting locations appreciated.

 

[Rogan68]

well, here's a quick and dirty...
If it were me, I'd run the dash speakers off head unit power, and the doors on amp-power.
I've never seen speakers with 200W max/40 RMS rating (usually max power rating is 2x RMS). All amps are auto-on (triggered by the amp turn-on wire from the radio, usually blue or blue w/white on aftermarkets). Run the 4 doors to a 4 channel, 40 or 50wx4. Keep your amp ground under 18" long. Amp power (fused) direct to battery, and you can run the remote power turn-on with the RCAs from radio to amp.

I used to do this for a living, years ago, and was an MECP-Certified Master Installer. My body said 'no more crawling under dashes', so I obliged.

I, personally, would go a completely different route, but judging by your questions, the above would be the easiest/safest route.

 

[osprey92]

Of course I am open for suggestions. Nothing is done at this point.

Doors:
JBL Stage3 527, 40W RMS (200W Peak Power) 3-ohm Impedance

Already in dash:
JBL 3020, 20W RMS, 60W peak, 3-ohm Impedance

Generic Android head unit. I have been unable to get specs. Performance has been fine until power was killed to door speakers, and now dash sounds like it is pushing low volumes (speech begins to distort). I would assume something like 47w, 4 channel is the power off head unit.

 

[Rogan68]

what gear do you currently have, radio/speaker/amp-wise?

 

[WY-Dave]

lotsa good advice there, couple of things to add.


Many moons ago, I had a 2005 Magnum that I would compete in the Hi-Q contests locally in the Omaha area. It had 3.5" dash and 4 6x9 door speakers. (added a 12" then upsized to a 15" sub when got tired of the contests). I did the dash powered by a Alpine HU then a 4X amp for the doors and then the same amp mono'd for the sub. On the dash speakers I would put a "bass-blocker" on it. The JBL 3020 frequency response: 90-20,000 Hz, but seeing a 3.5" do 90hz is yeah ok.

Everyone has their own preferences on what cutoff freq should be (for 3.5" Crutchfield @ 600hz, PAK @ 2.8khz). I would always go atleast 2X the bottom freq from the response listing, sometimes even 4X. With impedance change, the capacitor value changes for a 1st order highpass filter. Now bass blockers are generally listed at a freq. of 4ohms and rarely tell you the value. Here is a chart that has the values.

Let's say you wanted a 200hz (green) cutoff, a 3ohm cap would be 268 ufd (micro farad). So a 4ohm with the same value would be advertised as a 150hz bass blocker. If you went with a 400hz cutoff, a 133ufd cap would be used. So you would look for a 300hz 4ohm bass blocker.



Another thing, Peak power is garbage rating, use only RMS. Now again has their own preferences on matching power to speakers. Since I did play in the dbRaces (loundness) a few times, I would use this thought on designing a system. An amp will play clean up to a certain power, changing input level has all kinds of fun with it. An amp with a good input (4v IIRC, I ran 3v for Hi-Q) might play crystal clean up, to let's say, 85% of RMS rated power. Then clipping(distortion) would start, all subject to input. So my logic was that clean power is good until 75% of rating. No scientific measuring, just my logic. So for a 60wrms speaker, I would go with a 80wrms amp. 60 is 75% of 80. Now you have to be careful not to go full blast.

With the 3ohm JBL loads, you will not likely see an RMS rating for most amps. You can take a average RMS value of the 2ohm and 4 ohm ratings. Like @4ohm 60w and 2ohm 40w, so at 3ohm it would be around 50w RMS.

 

[89grand]

I'd personally use either a 6 channel amp, or a 4 channel and two channel. The dash speakers produce the majority of the sound so they benefit from an amplifier for sure. In my setup, my dash speakers play from 350hz-20khz. My subwoofer and midbass combined only play from 20hz-350hz.

Sure, the subwoofer and midbass frequencies require more power than the upper frequencies, but you'd be a lot better off using an amplifier on all 6 speakers. Otherwise, your dash speakers will end up being the limiting factor in overall loudness.

Also by using an amp on all channels, you have access to crossovers and also the gain to dial in the balance between the doors and dash speakers. That's a very important thing to be able to do.

 

[Graygoose]

agree, get a 6 channel amp. Make tuning easy.