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This is a discussion on How a car amp works (pic) in the Car Audio / Security / Electronic Accessories forum
There seems to be a little confusion on how amplifers work. I have found out the hard way there is as much bs in the audio industry as there is with preformance parts for cars. Like when you buy a cold air intake off of ...
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|12-19-2004, 02:58 PM||#1|
There seems to be a little confusion on how amplifers work. I have found out the hard way there is as much bs in the audio industry as there is with preformance parts for cars. Like when you buy a cold air intake off of ebay that is guarenteed to give you 25hp from your honda. In contrast have have seen amplifiers rated at 2000watts but have are fused at 20amps, how companys come up with this number and how they can legally sell a product like that is beyond me, but anyway. To help to remedy this problem I have been doing a small writeup on the basic parts, theory and troubleshooting of a car amplifier. I choose to do a car amplifer rather than a house stereo or pa amp because this is a site based on automotive industry.
**WARNING: whenever you work on an amplifier, ALWAYS remove power whenever possible. Most amplifiers are not high voltage but are HIGH CURRENT, which is JUST AS DANGEROUS. Some of the troubleshooting requires some electrical knowledge, if you do not understand some of this, or are not comfortable with electricicty, do not attemot to troubleshoot! All this assumes you know how to at least proporly hook up an amplifier at the very least!
There are three basic sections to a car amplifer
1. Power supply
2. Audio input
3. Amplifer section
Here is an image to help you figure out where everything is, it is a Pioneer amp I have been using for parts: :
1. Power supply
The power supply does just what it says. It takes in the power from the car and distributes it to the audio input and the amplifier.The amplifier runs off of a higher voltage (ie 40V) then what the car runs off of (13.8V). The power supply must take the voltage off the line and boost it up.
It takes the DC (direct current) Voltage and converts it to AC (alternating current) through some high power mosfet transistors. A separate circuit is used to drive the mosfet transistors at a high frequency causing the DC signal to oscillate. There is a coil inside the power supply section, this coil is a transformer. In a car amplifier they usually look like a large circle with thick wire wrapped around it. A transformer can effectivly raise AC voltage while lowering AC current, as a ratio. The high frequency voltage runs through the transformer and is boosted to a higher voltage. The current will be lowered, but since there is an abundent source of power from the alternator of the car there is not usually much of a problem.
The power for the amplifier circuit needs DC voltage again so it is converted through a simple bridge rectifier. A bridge rectifier consists of 4 diodes. In most amplifiers they usually look like two transitor components. They look like transistors but are actually two basic diodes in each component. Some companys are nice enough to label them with a diode symbol telling you which is the anode and which is the cathode. After the AC passes through the rectifier, it is becomes a pulsating DC, the signal is further smoothed out using large capacitors. If anyone wanted to know, this is also how a power inverter works, a device you can plug a standard 110V device into your car with. The only diffrerence between the one in the amplifier and the one for a power inverter is that the signal is not converted back to DC again.
2. Audio input
This section is responsible for taking in the audio signal from the head unit and sending it to the audio amplifier. The audio input contains the gain, and sometimes a crossover. The gain controls the signal level going into the amplifier section. If the signal is two low, the amplifer may not be able to effectivly drive the speakers. If the signal is to high, it will cause the amplifer section to “clip” the AC wave. For the duration the AC wave is clipped the amplifier remains on all the time, sending a pulsating DC signal to the speakers, which is VERY bad.
A speaker coil is basically an inductor. An inductor has diffrerent proporties to AC and DC signals. The higher the frequency, the more the inductor will become an insulator (non conductor), the lower the frequency the more the inductor becomes a short. The equation x=2*pi*f*l illustrates this. Pi= 3.14, l is your inductor, say it is 3mH which is .003 H. If you fill in those variables and pop in diffrerent values for f you will see that x will become lower for lower frequencies, and higher for higher frequencies. The unit for x is ohms,the unit for resistance. So if you are sending DC through which is 0 Hz, the whole equation becomes x=0 ohms, a dead short. There is obviously some DC resistance in the wire, ie 4 ohms, just 0 impedance, but you can get the idea, the short will make the coil very hot and will eventually damage it or the speaker piston.
Typically you want to keep the gains set someplace in the middle, and not more than ¾ of the maximum. The reason being is that if you have the gain up all the way, and all of a suddon a very loud sound comes from the head unit, it can send a clipped signal to the speaker and possible blow it out. You may have been listning to your music at a moderate level, but a loud sound can come from something simple as a scratched cd. Allways keep it safe, keep these under ¾.
The crossover is a simple filter circuit, it in itself is a gain, but can control bass, treible and/or midrange. It allows you to boost or lower certain frequency ranges to your desired level.
This section is the basic function of the amplifier, it takes the available power from the power supply, and boosts up the audio signal using it. Most car amplifiers are a “Class B” type and require two power transistors to make it work. Transistors allow you to use a very small signal and control a very large one with it. One transistor compliments the other. The first transistor takes care on the top half of the signal and the other takes care of the bottom half of the signal. The AC signal from the head unit controls the transistors.
Biasing a transistor means that it is set up such that an incoming signal will not cause it to turn on or off. Turning the gains up to high, and clipping the signal is an easy way to turn the transistor on fully. Most transistors can act as an amplifier or a switch. Almost all the current flows between the collector and the emitter of the transistor. When the transistor is completely on, it will allow all the current to flow between the collector and emitter, an effective short, ie switch. There is some resistance in the transistor call beta. When you clip the signal, you turn the transistor on all the way and send a DC signal directly to the speaker.. Common power transistors in amplifiers usually start with an A or C, each one compliments the other.
Amplifier safeguards and troubleshooting
Amplifiers, like cars do have some of their own built in safeguards to help protect itself, you and the equipment it is connected to. Amplifiers have a power on test, thermal protection, fuses and sometimes more. When you first turn the amplifier on, you might notce a delay. This delay alows the amplifier to turn on, let everything come up to par and prevent and unwanted signals from going to the speakers. The amplifier can check to make sure the output transistors are ok or not as well. If they are bad the amplifier will not turn on or some will display “protection” on them You can check these transistors yourself without even opening the case with a simple multimeter.
If you don’t have a multimeter you can pick a cheap one up at RadioShack or Sears. Make sure it has at least 4 digits of resolution before the decimal point, or I wouldn’t buy it. It doesn’t make a huge difference, but it indicates a more accurate meter. Nowadays I would not even bother with an analog dial type, they are more difficult to read, less functional and less accurate.
To check the transistors, put the meter on diode check (diode symbol). The meter uses this check to see at what voltage the circuit will conduct electricity. A “perfect” silicon diode will conduct electricity at .7V. The range can vary probably between .4V and .9V in some cases, there might be other parts in the circuit throwing it off. TURN OFF AND REMOVE POWER FROM THE AMPLIFIER Put the leads on the positive and negative of one of the speaker terminals, and look at the display of the meter. Then flip them around. Check each output doing the same. The meter should never indicate 0V or something close to that. If it does, that is a very good indication that the output transistors are bad. When I was explaining how the amplifier worked, I explained how the transistors are hooked directly to DC power. When they indicate 0V on the meter, it means that it takes 0V or close to 0V to make the transistor conduct, the same as a piece of wire, which is a short. So therefore it will take that DC power and send it to the speaker. The only reason you usually do not see your speakers smoke is because the amplifier checks itself when it first kicks on, if it didn’t it would be the same as putting the positive and negative of the speaker right to your car battery.
If your meter indicates a short, you must open up the amplifier. When you open the amplifier, visually look around in it, look to see for black marks from smoke, or a cracks on the transistors, all these can indicate bad parts. Use the same diode check to ckeck the leads of the output transistors. Sometimes you must check the 3 leads of the transistors in diffrerent combinations to find the short. If you are not sure whether or not a short is there, run over to the other side of the amplifier for the other set of transitors. If you check the same leads the exact same way, and get something totally diffrerent, it is a good sign it might be bad.
If the output transitors are ok, and you still have no power or output it is most likely the power supply. You can check all of the mosfet transistors and the bridge rectifiers the same way. None of these should be shorted as well.
First you must find replacement parts. If your amplifier is Pioneer, RadioShack, Jensen, some JBL, crystal audio….and a whole bunch others, your in luck. Most of these share similar parts and can be interchanged a lot of the time. If you have a Kenwood, Fosgate, Planet Audio, high end Orions, etc, GOOD LUCK. You will pay an arm and a leg for parts. Unless you paid a significant amount of money for these, its usually not worth it. I once tried to fix a Kenwood for somebody, I had to get the parts from Kenwood upwards of $60, only to find out there were more broken parts, the amp was worth about $200, not worth it. If you have a broken one you can use for parts, open it up and see if the transisors are the same. If you cannot find the right parts then the real seach begins. The easiest way, and most expensive way is to go right to the company for parts. Ex. 1-800-PIONEER You can try transistor cross references, NTE products, etc..
First off, get a rag. There is usually heat conductive grease all over the amplifier and is very messy. Start by removing the cover, the knobs, screws for connects and such. Then remove the parts holding down the transistors. KEEP IN MIND that there are small “strips” of non electrical conductive behind most of the transistors. DO NOT loose these, they must go back behind them. They keep the transistor insulated from the case, if you forget these and put the transistors right against the case, they will short out.
If you don’t already have these, go to RadioShack or local hobby store and pick them up.
1. Soldering iron
2. Solder sucker, or desoldering iron
4. Heat sink grease, if you need it
Flip the amplifier board over so you can see the solder and traces. Heat up the desoldering iron (or solding iron if you are using a sucker). Heat up the part and suck off the solder from each lead of the transistor take a small set of pliers, and CAREFULLY wiggle the lead to break it loose from the board. You can now pull the transistor out. LABEL where it went, it is very important to replace it with the correct part.
When you replace a part, push it back into the holes. Heat up the soldering iron. Heat up the lead and trace of the transistor for a few seconds and apply the solder to the lead. Most people try to apply solder to the iron, which is incorrect and will not flow to the lead. Solder will only flow where there is heat, if the lead is hot, it will flow to the lead. Put the amp back together, put the case back on, and try it out. Hook up just the power first and turn it on, check for smoke. Hook up the speaker and audio last. Be careful to watch it for smoke, strange noises etc. Immedially kill the power if you see anything wrong.
Anything beyond this requires special equipement, ie, ocilliscopes, power supplys etc. There is a lot more to troubleshooting but it is beyond the home mechanic. If you have any questions pm me and I may be able to help, but keep in mind special equipment is sometimes needed.
How to tell whether or not the rating on your amp is BS or not
The fuse size is one of the BEST ways to see if the amp is overrated or not. Wattage is total power, the power you take in, must equal the power you take out, it is always completely converted. Whether it is mechanically used by the speaker, gone away as heat, lost in the amplifier somewhere, it is always converted. The power you take out therefore CANNOT be greater then the power in. The equation for wattage is P (watts) = V (volts) * I (current). Take 13.8 (maximum voltage from car) and multiply it times the fust size (ex 40A) The rms power of the amplifier CANNOT exceed this number. If it did, the fuse would blow. But wait, your amp says 2000watts and has a 20Amp fuse, where did they come up with this number? Well they have different ways of converting these numbers to confuse people, and sadley most people will believe these numbers. Sometimes I have no idea where they even come from they are so out there. To get peak power, take the volts, and fuse size and divide them by each by .707. Then take those two numbers and multiply it together, that is your peak power. If you want peak to peak power, first divide each of those numbers again, then multiply each by two. Then take those products and multiply them together. They are not lying, its just not propor to give wattage in these terms. Sometimes I wonder if they dipped the amp into liquid nitrogen and cranked the voltage from 13.8 to 20 with a 1 ohm load to get these numbers. When looking at wattage, always compare rms power! Look in the manual or look at the labels for it.Everybody seems to have diffrerent ways of displaying their power, or what it means to them. Ex. Speakers might say peak power is maximum amount of power in a single instant, that’s not truly what it means but, the company will say it that way. So always always always rms power.
There is also something called total harmonic distortion or THD. It is measured as a percentage. They say under 1% you cannot hear it, I say I can’t tell a flute from a piccalo, so I won’t notice much of a diffrerence. Some amps give a rating of power @ z% distortion. It might be a higher power amp but sometimes at higher power it won’t sound as great, some people care, some don’t.
What makes one amp better? Different manufacturers use diffrerent technologys, diffrerent ways of building their amps. A good sound quality amp will have a very regulated power supply, will use good transistors and will be checked to make sure crossover points and on the amplifier itself is good, etc.. A amp build for sound level will simply try to get the most power out, with disregard for anything based on sound quality. Just like a car a 10 second car might not be great to drive in city traffic, a audio system that puts 160dB at the dashboard might not be all that appealing to the ears some peoples ears.
I hope this helps a few people, it took me a little bit to write this, wouldn’t mind if it got pinned but, if it doesn’t that’s alright. If I have any grammer errors please tell me, I’m not a spelling wiz. If you have any questions or comments please feel free to ask.
"The only man who makes no mistakes is the man who never does anything. Do not be afraid to make mistakes providing you do not make the same one twice."
Last edited by cheese9988; 09-24-2006 at 03:04 PM. Reason: Grammer mistakes, wow I'm bad....
|12-19-2004, 04:15 PM||#4|
this should be stickied......intresting......
Ive only taken one apart, to replace a resistor which was easy to spot(burnt) and I just replaced it, I never knew what it was for, now I do(part of the preamp)
|12-19-2004, 08:08 PM||#5|
yeah working on PCBs (printed circuit boards) is fun stuff. just make sure you are properly grounded before working on them. believe it or not, just a small of static electricty that you cannot feel, can burn up a part and/or the PCB you are working on. you have to be careful. i have 5 years in electronics manufacturing in doing everything from assembly to testing to quality control and i have seen some wild shit through.
that one pictured above is a VERY simple layout. ive worked on FAR more complicated stuff that that.
|12-21-2004, 03:42 PM||#9|
Ya, radioshack used to be good at one point for parts, they got out of alot of that stuff though, they used to be affiliated with ecg, but ecg was bought out by nte.