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A power amplifier takes an input signal, usually a preamp level signal, which has both low current and low voltage characteristics, and produces an output which will have higher current and voltage levels. The power supply available to the audio output IC in a head unit is limited to the battery voltage of the vehicle. This means that the head unit can produce an audio signal with a limited (by the battery voltage) voltage swing, and therefore a limited power output to the speaker. Most amplifiers have a special circuit (switching power supply) to boost the available battery/charging system voltage to a higher voltage. The higher voltage developed in the amplifier's internal switching power supply will allow the audio output voltage swing to be greater. This allows the amplifier to produce more power into the speakers connected to the amplifier's output terminals.

Gain Controls:
Most amplifiers will have some sort of level or "gain" control. This control is used to match the output of the head unit to an amplifier. The maximum audio output voltage from different head units will vary. If there were no gain controls, some head units would not be able to drive the amplifier to its maximum power level. Other head units may drive the amplifier to full power at a fraction of its volume control's range. More gain info can be found on this page.

Battery, Ground and Remote Connections:
Virtually all amplifiers have battery, ground and remote connections which must be connected for the amp to operate. The battery connection is the high current +B source that's connected to the battery via a properly fused wire. The size of the power wire is determined by the current the amplifier draws and the length of the wire (from the battery to the amplifier). The ground is another high current connection and is connected to the chassis (body/floor pan) of the vehicle. The ground wire is typically as large as the power wire. The remote connection is a low current control input that tells the power supply of the amplifier to power up.

Remote Input Current:
The remote input current for amplifiers varies with the amplifier and the model. Some draw minimal current. Others draw a little more. The upper limit of a properly functioning amplifier is approximately 50ma (0.05 amps). If you're using/controlling more than 2 amplifiers, it is (in my opinion) much better to use a relay to control the amplifiers. Actually I really prefer having a relay in the remote circuit (no matter how many amplifiers I'm using) because it protects the head unit's remote output circuit in case of a short circuit. The following chart shows the remote input current for various amplifiers I had laying around the shop.

Manufacturer Model Current Draw (mA)
MTX 2300 14
Jensen LXA300 43
Pioneer GMX602 1.5
Autotek 7150 16
Punch 200x2 14-45
Autotek 200x1 17
Coustic Amp162 22
Orion 275SX 28
Crossfire CFA1000D 5
Lanzar Vibe 250 17
Test conditions: 14.3vdc; Fluke model 79 DMM; The meter was inserted in the remote supply line.

Punch amplifiers may draw slightly more current when the power supply fuse blows. This generally causes no problem because the increase in current is still below the current normally drawn by other amplifiers.

There is at least one very popular brand of amplifier that draws as much as 500ma of current when the amplifier fails. This is enough to damage the remote output switching transistor in the head unit if the fuse is missing or is of the wrong value. A relay in the remote circuit will completely eliminate the possibility of damaging the head unit in this situation.

Amplifier Input Circuits:
The input circuit (sometimes called the 'front end') generally employs a noise cancelling circuit which compares the signal on the center conductor (the audio signal) to the signal on the RCA shield (which generally has little or no signal and is only used as a reference) and amplifies the difference between the two.

Input Impedance:
The input impedance is the impedance (that the signal source 'sees') from the center conductor to the shield on an unbalanced input circuit. A typical input impedance would be ~10,000 ohms but some amplifiers may have an input impedance of more than 50,000. If the input circuit uses a mini DIN type connector, the input impedance could be measured from one signal terminal to the other or from the signal terminals to the shield ground. Ideally, the impedance should remain constant throughout the audio band. More than a few amplifiers employ some sort of high frequency noise filter which will cause the input impedance to fall slightly at the upper end of the audio spectrum. These filters are designed to reject high frequency noise from the amplifier's switching power supply. It should also remain constant regardless of the position of the gain control. Some amplifiers (especially budget amplifiers) will have varying input impedance when the position of the gain control is changed. Head units (or equalizers, crossovers...) with low output impedance will handle these variations better than standard head units. Generally, a head unit with high output impedance will have reduced high frequency response if the amplifier's input impedance isn't consistent across the audio spectrum.

Unbalanced Input Circuit:
This type of circuit has a shield ground that's not directly connected to the chassis ground but may have only a few hundred ohms of impedance from the shield to ground. This type of circuit would be designed to accept a single ended signal (signal only on the center conductor). This diagram shows the signal on the unbalanced connections.

Balanced Input Circuit:
Some Amplifiers have balanced inputs. This means that both the center conductor and the shield (if they're using RCA type connectors) can accept an audio signal. If the amplifier uses RCA type connectors and has balanced inputs, it likely uses the chassis ground as a reference (which is a testimony to the noise rejection abilities of a balanced input circuit). If the amp uses a mini DIN or some sort of professional audio connector, the connector will have provisions for two audio signals per channel and a dedicated ground (reference) connection. The following diagram shows that there is signal on both of the RCA connector's terminals. Note that the center conductor's signal is 'normal' and the shield's signal is 'inverted'. Remember that we said that the input circuit amplifies the difference between the center conductor and the shield. If the the signals were the same and it amplified the difference (a-a=0), you'd have no signal. The inverted signal allows you to have a-(-a)=2a. From this, you can also see that the signal has double the magnitude of the single ended signal (if the input signals have equal magnitude). For more info on balanced signal devices, go to this page.

Amplifier Output:
In the diagram below the red sine wave shows how far the audio output voltage can swing in a head unit. It can only swing a total of about 12 volts. The actual voltage is determined by the vehicle's charging system. This means it can swing only 6 volts above or below the reference (indicated by the 'C' arrows). The amplifier's power supply takes the 12 volts from the battery and boosts it. The amount which it's boosted is determined by the design engineer and ultimately determines how much voltage the amplifier will produce at its speaker terminals. The higher the power supply (rail) voltage, the higher the possible power output. The cyan (light blue) sine wave shows how the signal would be clipped if the head unit's (or any power amplifier's) output capabilities were exceeded. The violet sine wave shows how the previously clipped wave is now able to be produced 'cleanly' due to the higher rail voltage (indicated by the 'B' arrows) .

If you remember the formula P=(E*E)/R, you will see why more voltage produces more power. P=the power that would be delivered to the speaker. E=the voltage of the audio waveform. R=the speaker's rated impedance. Impedance is basically the same as resistance except that impedance is used to describe a device's opposition to "AC" voltage. If the resistance/impedance remains constant and the voltage increases, power increases.

And here's a schematic version. You can right click on the image and zoom in on various sections. Click here to make it fill this window. When your cursor is on the schematic, you can click and drag the page to see the entire thing (only when zoomed in). You will have to use the back 'button' (right click won't work) or click on the amplifier link in the directory to return to this page. If there are any terms you don't understand, do a site search for them.

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Click HERE to make this image fill this window.

Getting a Good Ground:
Get a 3/8 inch bolt, nut and lock washer, find a place on the body that can be accessed from the inside of the vehicle and out. You must be able to get to both sides so that you can hold the nut from turning when tightening it up. Drill a 3/8" hole for the bolt, making sure NOT to drill through any fuel lines, brake lines, the gas tank or anything else. Scrape the area under the bolt (inside the vehicle) to remove ALL paint and primer then bolt the ground wire's ring terminal down with the 3/8 inch bolt.

For grounding devices that draw only a few amps (like crossovers, head units and equalizers), you can use virtually any type of screw. Many people warn against using the black oxide coated screws but it won't make a big difference because the electrical connection is between the ring terminal and the metal surface that's been sanded clean and not through the screw. The screw simply holds the ring terminal to the metal.

Amplifier mounting:
DO NOT mount an amplifier on your subwoofer box. I know that there has been a great deal of discussion over mounting an amplifier to an enclosure and many people do it all of the time with no problems but those people probably build good enclosures from 3/4" (or thicker) MDF with extensive bracing. Most people (especially young impatient people) are too lazy to do that and build un-braced enclosures from 5/8 MDF. These enclosures will flex considerably more than a proper enclosure and will likely cause amplifier failure if the amp is mounted to the enclosure.

When the woofer(s) moves in or out, the box flexes and therefore causes the sides of the box to vibrate. This vibration is transferred to the amplifier mounted to the box. All of the electrical components in the amplifier have mass. Inertia (an object in motion tends to stay in motion, an object at rest tends to stay at rest) tells them to stay at rest, the box vibration is trying to make them move. The energy from the box's vibration is transferred to the components through the electrical leads which are soldered into the circuit board. All of this will cause the components to break loose and therefore cause the amplifier to fail prematurely. Basically, the amplifier will commit suicide! :-) I'm not telling you this because someone told me it was bad. I've been repairing amplifiers since ~1985. Virtually every amplifier that's come into my shop with parts rattling around inside them have been mounted on the speaker box. It causes the legs of the semiconductors to break (which causes amplifier failure). It causes the capacitors to break off of the board (which can cause catastrophic amplifier failure). It causes solder joints to break on the semiconductors mounted to the heat sink. It causes transformer windings to grind into one another (which causes lots of smoke to pour out of your amplifier). People who repeatedly tell others to mount their amps on the speaker box because they've never had a problem remind me of people who drink and drive and say there's nothing wrong with it because they've never crashed their vehicle. Eventually, in both cases, problems will arise.

Mounting the amplifier on the enclosure also allows someone to steal BOTH your amplifiers and speakers at the same time with no extra effort. It's bad enough to have one or the other stolen but losing amps and speakers (and anything else mounted to your speaker box) is really going to suck.

For Those who Absolutely have to Mount Their Amplifier on the Enclosure

The top of the enclosure has been removed to show the bracing.

Absolute Worst Situation:
This diagram shows the amplifier mounted in the center of the largest wall of the enclosure. You will notice that there are no braces under the amplifier's mounting points (red arrows). Because this part of the enclosure will flex more than any other, this will cause more physical stress (from panel flex and vibration) to the amplifier than any mounting position. This would be made even worse because the enclosure has no center brace/divider.

Worst Position

Somewhat Better Position:
As you can see, this enclosure has a center brace which will help to reduce panel flex. You should also notice that the amplifier's mounting points are very close to the braces or on top of the wall of the enclosure. These points will have less vibration from panel flex (when compared to points farther from the dividers or walls).

Somewhat Better

As good as it gets (with amp on box):
This method of mounting will relieve most of the stress from the amplifier because the wood braces will connect the top of the enclosure to the bottom of the enclosure and won't allow the panel to flex. The braces should be glued (with a good wood glue) and screwed on both ends. The other mounting points are directly on top of the walls or dividers which will vibrate VERY little when compared to the an un-braced panel.

Very Good Bracing

Amplifier Installation Notes

  • When installing an amplifier:
  • Disconnect the ground wire from the battery. It doesn't really matter which one is removed because removing either connection from the battery (positive or ground) will break the circuit but if you let the wrench touch to ground (any metal surface) when removing the positive wire, you may do significant damage or seriously injure yourself. If you let the wrench ground out when removing the ground wire, you won't have any problems (except maybe scratching the paint).
  • If you don't remove the wire from the battery, at the VERY least remove the fuse (or open the breaker) from the power wire which delivers power to the amplifiers. If you don't have a fuse (or breaker) in the main power wire, PLEASE read the fuses page.
  • When making the power and ground connections on the amplifier, connect the ground wire first. I know it is tempting to connect the RCA cables first because it is instant gratification (having made a connection) but you may damage the head unit or the input section of the amplifier if the amplifier tries to ground through the RCA shield connection.
  • If the amplifier has screw down terminal blocks which are designed to accept either bare wire or spade terminals, use the spade terminals. If you insert bare wire into the blocks, you may have a strand or two of wire touch to the neighboring terminal which is easily enough to convert the amplifier into a paperweight.
  • Mount the amplifier down before moving the vehicle. If the amplifier falls or slides against anything, there is a chance that it will be damaged seriously enough to warrant a trip to a repair shop. I know how cool you are (because I know how cool I was at 15 or 16 years old) and nothing could possibly happen but... mount it down anyway.
  • When making the ground connection for the amplifier, the floor pan of the vehicle is a better choice than some of the braces and other metal structures that you may want to use for ground. Braces and other such structures are sometimes connected to the vehicle's chassis (body) by a few spot welds which will provide a less than optimum ground return path.
  • If the amplifier's ground is properly connected to the body of the vehicle, it will provide a better return path to the charging system's ground than will a ground wire run back to the battery. This is especially true if the ground strap from the engine block to the chassis is upgraded. See this page for more info.

Installing an Amplifier

Making the Connections to the Power Source:
Most amplifiers draw significant amounts of current (too much current to be drawn from the vehicle's fuse box). This means that you will have to make a connection to a more suitable point. Most of the time, the main power wire is connected to the battery's positive terminal. Since the battery is capable of supplying more than 1000 amps of current and your power wire can't likely safely carry that much current, you MUST have an inline fuse (this can not be stressed enough). The fuse must be rated to protect the power wire being used. If the power wire was un-fused and accidentally shorted to ground, the wire and the vehicle would soon be fully engulfed in flames. The fuse holder should be between 6 and 18 inches from the battery. If it's too close to a standard lead acid battery, the fuse and holder may corrode like the positive battery terminal does. If it's too far away, it will not provide the required protection because there is too much unprotected wire (between the battery and the fuse).

Routing the Power Wire to the Rear of the Vehicle:
In many vehicles, there are unused rubber or plastic plugs in the firewall. These plugs can be removed and a small hole (size determined by the outside diameter of the wire) can be cut or drilled in it. The plug can then be replaced and the wire can be passed through the plug. If the angle of the plug will allow water to run around the wire and into the vehicle, use some silicone sealant to seal around the wire. Sometimes, there are extra 'nipples' on rubber plugs where the speedometer or other cables pass through the firewall. The tips can be cut off of the nipples and the wire can be passed through the firewall.

If there is no place to run the wire (not likely), you will have to drill a hole. If you can not CLEARLY see both sides of the firewall where you want to drill the hole, DO NOT drill. There are many hazards to drilling through the firewall. You could puncture fuel lines, brake lines and wires. If you 'think' you know where the drill will pass through the firewall, you can use a center punch and make a small dent on one side then go around to the other side to see if you were right about the location. Many vehicles have 2 layers of sheet metal in the firewall area so this technique won't always work. After you drill the hole, make sure to use the appropriate grommet in the hole to prevent the wire from being cut on the sharp metal. If you don't have a grommet, you could possibly use a piece of heater hose or similar material (siliconed into place to prevent it from falling out of the hole). I know that some people will say that you should just get a grommet, but knowing human nature, I know that some people will try to run it through the firewall without a grommet (or any protection) if they don't have one handy. The piece of rubber hose isn't a perfect solution but it may help protect the power wire for those too lazy to do it right.

Keep it Clear of the Pedals:
When routing the wire through the firewall and under the dash, tie it out of the way of the steering and brake components. Make sure that you run all of the brake, clutch, accelerator and steering components through their full range of motion to determine whether or not the power wire will be in the way of any of these controls. When routing the wire under the carpet, make sure that it's routed so that it will not be crushed or pinched or punctured when the seats (if removed) and rocker molding are replaced. When routing it under the rear seats, make sure that the seat will not put undue pressure on the wire when people sit in the seat. When routing it into the trunk (boot), it will likely have to pass over a sharp piece of metal and make a sharp bend. If this is the case with your vehicle, I recommend covering the wire with a small piece of heater hose where it passes over the sharp edge. If you're only installing one amplifier, the power wire can be connected directly to the amp. If you are connecting the main power wire directly to the amp and the amp does not have an on-board fuse, install the appropriate fuse at the battery (the main fuse will protect the vehicle, wire and amplifier).

Routing Wires from the Head Unit:
To prevent radiated electrical noise from entering your system's signal line, you should avoid running the RCA patch cords along the the main power wire. It is OK for the power wire and the RCA cables to cross each other or even run together for a very short distance but you shouldn't run them together for any significant distance. One common solution is to run the RCA cables down the opposite side of the vehicle. The remote output lead from the head unit may be run down the same side as the main power wire (away from the RCA cables). Any speaker wires that run from front to back can be run down either side. Some people go the extra step and run speaker wires down the center of the vehicle but I don't believe it's necessary. They won't pick up any noise from the power wire and won't induce noise into the RCA signal lines. Again... Make sure that none of the wires will be damaged when the vehicle is reassembled.

I strongly recommend that you insert a 1/2 amp (one-half amp) fuse in the remote lead (as close to the radio as possible). This will help prevent the radio from being damaged if the remote lead is shorted to ground.


  • Additional Information
  • If you have any signal processors such as equalizers or crossovers, I would recommend grounding them at a different location than where the amp(s) are grounded. I know that some people advocate grounding all of the audio components in one place but the amplifiers will introduce a significant amount of noise (some inaudible) at the grounding point. A different ground (a foot or more away from the amp ground) will be much quieter. If the audio components are of good quality, the difference of ground will NOT introduce any noise. The ground connection does not need to be as robust as the one used for the amplifier. Just make sure to sand down to the bare metal at the grounding point.
  • Some people say that an amplifier's case must be isolated from ground (mounted on a piece of wood). This, in my opinion, is a load of @#$%! Most amplifiers connect the case of the amp to ground internally. If there is no connection between the case and ground inside the amp, the case will be completely isolated from ground (and everything else) so it will not matter if you connect it to ground by mounting it down to a grounded piece metal. Do you think they would actually build an amplifier in a metallic case if it could not be connected to ground?

Installing Multiple Amplifiers

Main Power:
When installing multiple amplifiers, you have a few options. If you already have a moderately large wire (8 gauge) for your first amp, you can run a second (appropriately sized) fused wire for the second amp. If you don't plan to add any more amplifiers in the future, this will work fine. This will not require that you buy a distribution block. The second solution would be to install a large wire (suitable to supply both/all amplifiers). A 4 gauge wire is the most common size wire for the main power wire. For a 4g wire, you can use a main power fuse as large as 150 amps. I don't recommend anything larger. The main power wire would run through the vehicle as before but when it gets to the back of the vehicle, it will be connected to a distribution block. The main power wire would go in the larger connector and the smaller connectors would go to the amplifiers.

It is OK to ground all of the amplifiers in the same place (if the ground wire doesn't need to be extended to more than about 18 inches). If the amplifiers are mounted on opposite sides of the trunk, I'd probably use 2 ground connections (with 3/8 bolts to the vehicle's floor pan).

Distribution Block:
A distribution block generally has 1 or 2 large wire connectors (4g is common) and 4 smaller connections. All of the connections are bound down with socket head set screws (the screw head is a six sided socket with no flange). If the block has no fuses, any of the connections can be used as inputs OR outputs (although, the larger sockets are used for the supply wires (from the battery)). This diagram shows an un-fused distribution block.

Fused Distribution Block:
The left side of the fuses are connected to the input terminals. The right side of each fuse is connected to it's own independent output terminal. The gray area is an electrical insulating material. Without the insulator, the block would not allow the terminals and fuses to operate independently (they would be shorted together). This diagram shows glass fuses but they are also made for Maxi-Fuses (which I prefer).

Distribution blocks (with photos) were covered on the Installation Primer and Fuses pages. If you haven't read those pages (you should have if you're reading the site in the proper order), go back and read those pages (and any others that you missed) now.

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You should remember:
1.A car audio amplifier takes a low voltage low current signal and increases both the voltage and current drive abilities of the signal.


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