Hello everybody, recently i bought car amplifier for repair and decided that im gonna replace my Sony XM-4020 that i have been running as my subwoofer amplifier for a year now. I took this opportunity to write instructable on how to properly hook car amplifier for use inside of your house (connected to mains voltage) and what you should be careful about.
There are many reasons why you would want to use car amplifier, but if you can choose between proper 120v-240v amplifier and car amplifier i would suggest you to pick house amp, its power supply is well designed and you will get proper power output out of it. Car amplifiers are mostly rated at 14,4V and lower impedances (1 or 2 ohm for example) while some home amplifiers can only go as low as 4 ohms. Since you will be running it off 12V power supply (more about that in further steps) you might not get as stable power as you would in your car.
If you however still choose car amplifier then continue reading as i will try to provide as many information about it as i can.
I've been running car amplifiers in my room for 6 years or so, it used to be hard to get proper amplifier and if u were like me you probably had spare ATX power suplies so you could save a few bucks on that. These days it's much cheaper and easier to just get 24V chip amp from ebay and have decent setup. Even designing and building your own, internet is full of schematics that are just waiting to be built.
Step 1: Choose Your Warrior
If you already have amplifier you can just skip this step.
If you still haven't got one and you are thinking about buying it there are few things to consider.
Mainly when choosing amplifier you are gonna choose between Class AB and Class D designs. Most people are using class AB on mids and highs (midrange speakers and tweeters) while going for class D on subwoofers. Reasons for this are that class AB has better sound characteristics. There is alot of discussion about this so im not gonna go there, but main difference between them is power consumption. While class D has excellent efficiency of +80%, class AB might be only about 50-60% which means they will also heat up alot more. This is thing to consider when choosing PSU and desired output power. If you have smaller amplifier (less than 250W) it may not be problem since even with 50% efficiency it would only draw 500W, however if you have bigger amplifier it would draw insane amounts of power so you will need bigger and more expensive power supply.
Step 2: Choose Your PSU
Since you have your amplifier, you need 12V power supply to power it up.
PC ATX and Server power supplies offer a lot of power and they can be cheaper than other solutions. Only downside is they provide only 12V and not 14,4V. This means you are gonna get a bit less power than your amplifier is rated at (some are rated at 12V and at 14,4V, some only at 14.4V). Other alternative is to get 12V power supply for LEDs or other things. These have small potentiometer so you can adjust voltage a bit higher, usually from 11-14V.
PC ATX usually requires green wire to be connected to negative terminal to power on. You can use simple switch to short them together. You will also need to cut off connectors and bundle together all 12V wires and Ground.
Modifying ATX power supply for 14V output can be done if you have some electronic knowledge, but if you dont then you shouldn't be opening one. They contain capacitors that are charged up to mains voltage and can be deadly. For this reason i'm not gonna go further, if you are good with it you wont have any problems with it since its as easy as changing few resistors.
How much power?
Now we come to a part where difference between AB and D class comes in play.
Add together power of all channels, for example if your amplifier is 2x250W you get 500W, if you have monoblock 1x500W or just bridge 2 channels use power that its rated at. Good rule of thumb is to multiply that by 1.5 for class AB and 1.2 for class D. So your 500W amplifier actually draws 750W if its class AB or 600W if its class D. Add about 50W of headroom to that. Divide that by 12V and you roughly need current of 67A for class AB or 55A for class D. Check if your rating is a bit higher than your fuses on your amplifier. If you go under it, it may not be enough to blow fuses which could lead to damage of your wires, power supply or even burn them down. Now you see why its better to get class D for higher amounts of power, that's 200W difference for 500W amplifier.
Step 3: Wiring
Use max current rating of your power supply to choose your wire gauge. Its always better to get as big wire as you can connect to your power input that is as short as possible and then use longer wires to your speaker. With higher power amplifiers its good to also include fuse that is recommended with your power wire.
Use online charts to roughly calculate your wire gauge, but its always better to get bigger gauge as they will have less drop in voltage with higher current draws.
I included diagrams for connecting psu to amplifier but if you have any questions just ask me in comments since your setup might be a bit different.
You can use switch between REM and Positive input to turn on inputs, however best would be to use DPDT switch and use one side to short REM to Positive and other one to short Negative to Green (PS_ON) wire on your ATX PSU. This way your PSU would turn on at the same time as your amplifier. Alternatively you can use simple SPDT switch to short Negative to Green (PS_ON) and just put jumper on REM and Positive input.
Ground is not same as Earth on your PSU so don't connect Ground to PSU case! Case of your amplifier is negative or ground terminal but PSU is earth, shorting them out will cause ELCB or other breakers to break.
For audio input you need RCAs. Simple 3.5mm to RCA will do the trick if you want to connect it to your pc or laptop. If you already have receiver or other device that has RCA output then you need male to male RCA cable. You can get them in any bigger shop or even online.
If you have bass knob it simply goes inline with your RCAs.
Step 4: Batteries?
If you have high power amplifier and simple PSU wont get you enough power you could consider getting 12V batteries. Don't get open types since they can produce a lot of hydrogen while charging which is not good considering you are going to have them in your room.
SLA or Gel types are good for this since they are mostly closed and have better discharge capabilities (deep discharge). This would require a bit different approach since you will need to get 14V-15V (0.5V drop from diode) from your power supply and add diode in series. Diode should be high power, rated at least few amps more than your max charge current, high power bridge rectifier could also work since higher current ones can be screwed to heatsink. You will need to put switch on mains input of your PSU if it doesn't have one since overcharging lead acid batteries is never a good idea.
As safety precaution you will need to add fuses since they can provide much higher currents (up to 200A for single 10Ah battery!) for shorter periods which means it can definitely burn even higher gauge wires.
Unless you are doing something crazy just skip this step, in my opinion it isn't worth it for daily listening at normal levels.
Step 5: Setting Your Gain, Matching Power
If you wired everything correctly, you can turn on your amplifier now.
See, that was easy. Now comes the tricky part, since all setups are not equal, you might get less or more power than advertised. Setting gains isn't that hard if you have SMD DD-1 or oscilloscope. Turn every filter, bass boost and gain down and turn bass knob or volume knob all the way up and leave your audio source at about 50%. Get 40Hz test tone for subwoofer amplifier, or 1kHz for full-range amplifier. Its safest to tune it at 0dB since that way you can't clip output signal even if you listen to rebassed or bass boosted music. If you dont listen to those types you can get -3dB or -6dB but you might clip it with some music. Hook your audio source to amplifier, play test tone and turn the volume up. Your audio source might clip so firstly checking outputs with DD-1 or oscilloscope should reveal any distortion or clipping, if everything is ok you can raise your gain until waveform starts looking distorted (as on pic 2) or clipping LED lights up on DD-1. This should be done while your speakers are hooked up and using few test tones. Since your speaker has impedance which varies by frequency you are playing it at, its always best to play different tones to see if amplifier distorts.
I don't have SMD DD-1 or oscilloscope, what should i do?
Well you can do this by ear but it wont be as accurate. Simply do everything as described before, but when you raise volume up listen to change in sound. It's hard to describe it but you will notice that your speaker will change sound when it starts to distort. Again make sure you do this at multiple frequencies to ensure output is completely clean.
EXO made a good tutorial about that, so if i'm not clear enough watch this video or ask me in comments.
Step 6: Problems?
My amplifier makes much more power than my speakers can handle, will that be problem?
Some speakers can handle more power than rated without getting damaged, some may not. To ensure you are not getting more power to them, use nominal impedance rating and ohms law to determine how high should voltage on output be. For example if you have 4 ohm speaker that can handle only 100W, play test tone and raise gain until you get 20V on outputs and make sure waveform isn't distorted since clipping hurts speakers more than getting too much power to them.
My power supply shuts off when raising gain, what could be the problem?
Getting power supply from china or suspicious sellers might not mean you will get exactly what you wanted, your power supply might not output as much current as it states on it. Try measuring voltage on outputs of your PSU, if it drops too much it may trigger undervoltage protection to prevent any damage.
My amplifier gets into protection when raising gain, what could be the problem?
First make sure your amplifier is stable at impedance of your speakers. Dont run 1 or 2 ohm speakers on amplifier that doesn't say its stable at those impedances. Check voltage on power inputs of your amplifier, if it drops under 10-11V it may trigger undervoltage protection on your amplifier. Some amplifiers have multiple colors of LEDs to indicate so check your manual.
I'm gonna expand this list so comment if you have problems.