if i am running a 1000-1200 watts system, how many farads should i go with? i was thinking 1.5 farad or so. is having a bigger cap better?
Replies (18)
danielm87 on 02/8/2005 22:11:55
Get ready for a debate, but if you've already made the decision to buy a cap, then theres nothing wrong with going forward with it.
The issue is debated, but the fact is that a capacitor does a small job in supplying power for the amplifier, and the difference is noticeable and can often help. The permanent and obviously more expensive solution is to buy a new alternator and high rated battery.
Having a bigger cap is always going to be better, regardless of wattage. The surge from a cap is a very small fraction of that as opposed to a good batterys surge, but as a rule of thumb from Crutchfield,
.5 farad for systems up to 500 watts
1.0 farad for systems up to 1000 watts
2.0 or more for larger.
The chart isnt necessarily perfect, but your assumption of a 1.5 farad cap is correct. You can use anything from a 1 to 2.5 farad cap or larger with a powerful system like yours.
Expect comments from others, thanks.
Daniel
uochronos on 02/8/2005 23:55:48
cap is a good choice if you haev
A. large system and you have already upgraded barttery,wires, and alternator
B. small system with very little dimming
Now with that said with the size system you have if you havent upgraded your Alternator and Battery then that is a cheaper and better first upgrade,,, not saying a capacitor wont help but it well put a larger strain on your Alternator and you could risk bruning it out and getting stranded somewhere.
i can expalin further if you would like me to but alot of people get angry when we tell them a power cap isnt the awnser but hoenstly a larger battery or even better a larger Alternator well help your system alot... i run around 1800watts RMS in my system and i dont run a cap at all just a huge alternator and battery.
Chronos
gnehc1 on 02/9/2005 11:05:33
Chronos, i don't know what i'm doing wrong then. i upgraded my alternator to a high output 150AMPs, i was running a 2.5 farad cap, i got a 1000 CCA rated battery, and i upgraded the big 3 wiring (battery to alt, negative to ground, and valve cover to chasis). at idle or low speeds, the amp goes on and off. check out my previous post
http://www.clubknowledge.com/Car_Audio_FAQ/?g2958_1541
hope you can help me touble shoot this. the inly thing i can think of is the output of the alternator, for the power drops off substantially under low speeds and at idle.
under Technical Info at motorcityreman.com it states:
An alternator's output is dependent on speed, but this can be deceiving because this output is not linear. Instead, it follows a curve. Each alternator has a unique curve, and at idle small changes in the alternator's speed can make a big difference in its output capacity.....The output of high amp alternators can drop off substantially under 2400 rotor RPMs.....
gearhead on 02/9/2005 11:18:21
If you drive the car "normally" (don't redline it on a regular basis, drag race, etc.) you can alleviate the problem somewhat (depending on what size the stock pulley is) by putting a smaller pulley on the alternator, or a larger pulley on the crankshaft. A smaller pulley on the alternator will only increase the dpeed of the alternator. A larger pulley on the crankshaft will spin everything that is driven by that pulley faster, unless you also install larger pulleys on the components you don't want to speed up (water pump, power steering, etc.). This "may" shorten the life of the alternator bearings depending on your driving habits, but it won't be by much.
MrBrownstone on 02/9/2005 16:56:46
In order to actually USE a capacitor, it's ESR/ESL must be low enough for it to discharge to actually have an impact on your AUDIO system. I believe Mr Meyagi said something to the effect of 0.015 ohms to have an impact in a 12V system.
with those digital monitor ones, it's basically audio jewelry.
On a side note, most capacitors will help you keep your lights from dimming.
To answer your question directly, probably 5F @ 0.014 ohms...but one doesn't really exist.
The main thing to remember is that the capacitor isn't a power supply, it's a power storage product. about 50W of power per farad is all it's capable of. With the duty cycle of the amplifer being 10-25%, that's 200W per farad/second....not a lot of power.
If you USE all 1kW of power, 5 Farads is necessary to have any impact on your system. The grand question is, "..If you add a capacitor and can't hear the difference, was there any?.." That question is costing most Cap users $100/Farad (retail) for no good reason.
The source of power in a car audio system is the alternator. Larger/upgraded wiring assures that you're not having a resistance problem getting the juice there.
uochronos on 02/9/2005 20:34:27
1200watts on a 150amp alternator is pushing it... my 1200watt amp is a class T which is more efficient then even a class D and it can pull over 100amps..... if your amp is pull 100-120amps when you have it loud then thats only 30amps left for your engine, lights, etc.... thats not much...
if your amp happens to be a A/B or a less efficient class D then you could be pulling upwards of 120-150amps on big bass spikes...
I have a 200amp alt that puts out 100+amps at idle... and full power at 3000rpm....
you know between 600 and 1200watts there is only about a 3db gain in volume at high volume you cant tell 3db differnce really. i suggest turning your gain back just a little for instance if its set at 2volts now then turn it to about 2.4volts maybe a bit higher this well make your amp stay more in the 800watt range... and drop its power consumption down a bit it may fix your problem without loseing much volume at all... you might not be able to tell the differnce even.
Chronos
P0werLifter on 02/10/2005 03:12:11
What class is this amplifier? If its a class AB then the current demands are much higher than a Class D amplifier.
1200watts divided by .9 (efficency of class D [90%]) = 1333.33
1333.33/14 (charging voltage while running) = 95.23amper draw for that 1200watts.
150amp alt - 95.23 = 54.77 amps left for the auto to run in.
(NOTE; A typical car takes 50-60ampers to power the stock
electricals)
Class AB now:
1200 divided by .5 (50% efficient) = 2400
2400/14= 171amper draw for that 1200watt system (more than your alternators full output)
Keep in mind class AB amps run from about 50-60% efficient while class D run 90% ++ efficient
Notice that you have no leeway here in either option. If you've upgraded your Big 3 (alt to batt, bat to gnd, engine to gnd), upgraded to a 800+ CCA batt, and your alternator Id Venture to suggest upgrading to a higher alternator (I run a 200amp, soon to be 300amp).
Also what guage is your wiring?
uochronos on 02/10/2005 16:54:32
also its possible for a class D to be less efficient then PL said and pull more then 100amps... most 1200watt amps i see are fused at like 120amps or more... an amp at full power can usualy pull withen 5-10 amps of what its fused for..
Chronos
MrBrownstone on 02/10/2005 20:51:56
Class D & Class T are basically very little difference in efficiency. Basically, Class T plays full range, D, up to about 250Hz effectively.
A 100 Amp fuse can withstand 200 amps for 1/2 second without going out, so don't besurprised if 400 amps go through for 1/16th second on that same fuse. Amplifiers can put out up to 250% of it's RMS power at full 100% distortion, so imagine the current.
As as far as 100 amps on a standard alternator. Try to remember that the amplifier is only 'ON' 10-25% of the time. The other 75-90% of the time, the amplifer is just there listening, recharging, and not drawing much current above idle current.
The truth of the matter is that 60 amps of net current from an alternator can suffice 240 or more amps of draw considering how long the amplifier is pulling full out power. That amplifier isn't going to be pulling all 100 or more amps 100% of the time, so the standard alternator is probably fine. This is where we get into the capacitor debate.
Efficiency of amplifiers at less than 50% output is less than 50% in most cases, and closer to 70-75% at full RMS power. Efficiency at 100% distortion is like 90%!!!!! Efficiency ratings are not a factor. Efficiency of AB amps is ~10-20% at nominal and 44-48% at full RMS....75% efficient @ 100% distortion. You're not getting 90% out of any amp clean power, unless it's from Mars.
Class D is a class D. T is basically a D. T = Tripath chip allowing full range. Nothing more admirable than that.
gnehc1 on 02/10/2005 23:29:52
i'm running a class D amp. i think i'm gonna get a smaller alternator pulley so that it can spin a bit faster, and i'm thinking about adjusting the idle so that it will spin faster also. hopefully after i do that, it will fix my problem.
uochronos on 02/11/2005 02:52:38
ya smaller alternator pull might very well solve this problem cheaply for you thats awlays nice..
Mr.B i'm interestind in undertsanding this whole thing where the amp isnt pulling current all the time? i know its not pulling huge amounts of current all the time... with a 1200watt amp running at half power around 600watts. and another amp running 250watts rms... and the alternator could not keep up it would flat out kill the car at an idle at a stop light...
Seems to me it had to be pulling close to 60amps almost constantly to do that? so it seems to me a 1200watt amp would pull close to 80amps constantly and well over 100amps on spikes.. and especialy with rap, dance and hip hop the bass just keeps pounding seems that it would really pull the current..
i'm not argueing with your logic infact i think your explanation was one of the best i have seen.
just seems to me with the hard hitting bass music thats popular and my own experience shows that it pulls alot of power...
Chronos
MrBrownstone on 02/11/2005 13:06:14
You'd be amazed at how little work an amplifier actually does. It works 10% of the time. Kinda like an olympic sprinter...particularly the Class D. That's why they don't need cooling fans
MrBrownstone on 02/11/2005 13:08:46
One more example. a 250kW weather radar averages 250W of power....less than the average microwave oven. Both have microwave transmitters, just that one averages 800W and puts out 800W. The other one, puts out 250,000 watts for 0.001 second, then waits for 0.999 seconds for responses. the net is 250 Watts.
The main issue is whether or not a CAPACITOR will do this guy any good, and the truth of the matter is that it won't affect his audio system. It's just helping the lights from dimming. The amplifier has already made it's discharge before the cap does it's work.
uochronos on 02/11/2005 15:10:39
ok ya that makes sense... i get what your saying... took me a bit but i understand now... the bursts from your amp only last a second or less usualy most the time the amp isnt drawing... that makes sense...
woohoo learned something new:)
i was looking at it a differnt way but now that i'm seeing it from a differnt perspective it makes alot more sense... sometimes you have to flip things around to see them correctly:) thanks for the help.
Chronos
MrBrownstone on 02/14/2005 14:30:45
Your amp is operating for 0.1 second, and at full 100% distortion, about 0.25 second. That's why it's mostly a waste to own a cap unless it's extremely low esl/esr AND storage is quite high (like 5F).
It's hard to believe, but your amp is quite a lazy machine. It gets off the couch less than you do!!!
Pinch on 02/14/2005 16:38:04
Quick sanity check, POwer. If I am reading your post correctly, a 1200 watt amp at 90% efficiency would produce 1200 divided by .9 watts. As you mentioned, 1200 divided by .9 is 1333.3333 Watts. Then you go on to show that a class A/B amp is only like 50% efficient, which turns out to be 1200 divided by .5 which is 2400 Watts. Here is my problem. This method shows that a less efficient amp squirts out MORE power than a more efficient one. I guess my question is this. Shouldn't you multiply power output by efficiency to get actual output? As in, the Class D would produce 1200 * .9 or 1000 Watts, while the A/B would produce 1200* .5 or 600 Watts. This makes more sense to me because we know that the output capabilites of the amplifiers are 1200 watts in both cases, but the Class A/B and D amps dissipate 600 and 200 watts in heat respectively due to internal losses. Just curious on everyone's thoughts regarding this subject.
Pinch
gearhead on 02/14/2005 16:47:17
I'm pretty sure they're talking about how much amperage the amp will draw, not how much it will put out. Class D won't draw as much amperage to supply 1200w as a class AB amp.
swez on 02/14/2005 17:30:43
This is a tough subject for some folks to grasp initially... but Mr. B. is correct in his assertions that a typical audio amp is often cruising along at moderate power (even when we kick it up) due to the "duty cycle" it operates within.
Most newer Class D amps use high speed pulsed switching power supplies to develop their power. They also have large CAPs inside the power supply to store power between sonic bursts. The efficiency ratings are typically in the upper 80's - 90%.
We can also get a pretty good estimate of actual watts out to a speaker
by knowing the ohmic value of the speaker and input voltage.
Example: 4 ohm sub load, voltage measured at sub terminals is 40 volts. How many watts are we getting into the sub?
The math: Ohm's law states Volts^2/Resistance = Watts
40 x 40 = 1600/ 4 = 400 watts
FYI: When measuring voltage off a sub or speaker, the speaker resistance changes notably with frequency. If we substitute a " non-inductive dummy load", the readings are more accurate on how many volts the load will show. However, the early mentioned method is more real world as nobody I know, listens to music off a dummy load. That would be just plain dumb! GRIN
Finally, average music of most kinds mentioned, do have sustained bass tones of a given amplitude. If we really want to press the amp into a good workout, test tones of 5 seconds or longer will do the trick. That's a steady sine wave with lots of energy behind it.
Swez
Replies (18)
danielm87 on 02/8/2005 22:11:55
Get ready for a debate, but if you've already made the decision to buy a cap, then theres nothing wrong with going forward with it.
The issue is debated, but the fact is that a capacitor does a small job in supplying power for the amplifier, and the difference is noticeable and can often help. The permanent and obviously more expensive solution is to buy a new alternator and high rated battery.
Having a bigger cap is always going to be better, regardless of wattage. The surge from a cap is a very small fraction of that as opposed to a good batterys surge, but as a rule of thumb from Crutchfield,
.5 farad for systems up to 500 watts
1.0 farad for systems up to 1000 watts
2.0 or more for larger.
The chart isnt necessarily perfect, but your assumption of a 1.5 farad cap is correct. You can use anything from a 1 to 2.5 farad cap or larger with a powerful system like yours.
Expect comments from others, thanks.
Daniel
uochronos on 02/8/2005 23:55:48
cap is a good choice if you haev
A. large system and you have already upgraded barttery,wires, and alternator
B. small system with very little dimming
Now with that said with the size system you have if you havent upgraded your Alternator and Battery then that is a cheaper and better first upgrade,,, not saying a capacitor wont help but it well put a larger strain on your Alternator and you could risk bruning it out and getting stranded somewhere.
i can expalin further if you would like me to but alot of people get angry when we tell them a power cap isnt the awnser but hoenstly a larger battery or even better a larger Alternator well help your system alot... i run around 1800watts RMS in my system and i dont run a cap at all just a huge alternator and battery.
Chronos
gnehc1 on 02/9/2005 11:05:33
Chronos, i don't know what i'm doing wrong then. i upgraded my alternator to a high output 150AMPs, i was running a 2.5 farad cap, i got a 1000 CCA rated battery, and i upgraded the big 3 wiring (battery to alt, negative to ground, and valve cover to chasis). at idle or low speeds, the amp goes on and off. check out my previous post
http://www.clubknowledge.com/Car_Audio_FAQ/?g2958_1541
hope you can help me touble shoot this. the inly thing i can think of is the output of the alternator, for the power drops off substantially under low speeds and at idle.
under Technical Info at motorcityreman.com it states:
An alternator's output is dependent on speed, but this can be deceiving because this output is not linear. Instead, it follows a curve. Each alternator has a unique curve, and at idle small changes in the alternator's speed can make a big difference in its output capacity.....The output of high amp alternators can drop off substantially under 2400 rotor RPMs.....
gearhead on 02/9/2005 11:18:21
If you drive the car "normally" (don't redline it on a regular basis, drag race, etc.) you can alleviate the problem somewhat (depending on what size the stock pulley is) by putting a smaller pulley on the alternator, or a larger pulley on the crankshaft. A smaller pulley on the alternator will only increase the dpeed of the alternator. A larger pulley on the crankshaft will spin everything that is driven by that pulley faster, unless you also install larger pulleys on the components you don't want to speed up (water pump, power steering, etc.). This "may" shorten the life of the alternator bearings depending on your driving habits, but it won't be by much.
MrBrownstone on 02/9/2005 16:56:46
In order to actually USE a capacitor, it's ESR/ESL must be low enough for it to discharge to actually have an impact on your AUDIO system. I believe Mr Meyagi said something to the effect of 0.015 ohms to have an impact in a 12V system.
with those digital monitor ones, it's basically audio jewelry.
On a side note, most capacitors will help you keep your lights from dimming.
To answer your question directly, probably 5F @ 0.014 ohms...but one doesn't really exist.
The main thing to remember is that the capacitor isn't a power supply, it's a power storage product. about 50W of power per farad is all it's capable of. With the duty cycle of the amplifer being 10-25%, that's 200W per farad/second....not a lot of power.
If you USE all 1kW of power, 5 Farads is necessary to have any impact on your system. The grand question is, "..If you add a capacitor and can't hear the difference, was there any?.." That question is costing most Cap users $100/Farad (retail) for no good reason.
The source of power in a car audio system is the alternator. Larger/upgraded wiring assures that you're not having a resistance problem getting the juice there.
uochronos on 02/9/2005 20:34:27
1200watts on a 150amp alternator is pushing it... my 1200watt amp is a class T which is more efficient then even a class D and it can pull over 100amps..... if your amp is pull 100-120amps when you have it loud then thats only 30amps left for your engine, lights, etc.... thats not much...
if your amp happens to be a A/B or a less efficient class D then you could be pulling upwards of 120-150amps on big bass spikes...
I have a 200amp alt that puts out 100+amps at idle... and full power at 3000rpm....
you know between 600 and 1200watts there is only about a 3db gain in volume at high volume you cant tell 3db differnce really. i suggest turning your gain back just a little for instance if its set at 2volts now then turn it to about 2.4volts maybe a bit higher this well make your amp stay more in the 800watt range... and drop its power consumption down a bit it may fix your problem without loseing much volume at all... you might not be able to tell the differnce even.
Chronos
P0werLifter on 02/10/2005 03:12:11
What class is this amplifier? If its a class AB then the current demands are much higher than a Class D amplifier.
1200watts divided by .9 (efficency of class D [90%]) = 1333.33
1333.33/14 (charging voltage while running) = 95.23amper draw for that 1200watts.
150amp alt - 95.23 = 54.77 amps left for the auto to run in.
(NOTE; A typical car takes 50-60ampers to power the stock
electricals)
Class AB now:
1200 divided by .5 (50% efficient) = 2400
2400/14= 171amper draw for that 1200watt system (more than your alternators full output)
Keep in mind class AB amps run from about 50-60% efficient while class D run 90% ++ efficient
Notice that you have no leeway here in either option. If you've upgraded your Big 3 (alt to batt, bat to gnd, engine to gnd), upgraded to a 800+ CCA batt, and your alternator Id Venture to suggest upgrading to a higher alternator (I run a 200amp, soon to be 300amp).
Also what guage is your wiring?
uochronos on 02/10/2005 16:54:32
also its possible for a class D to be less efficient then PL said and pull more then 100amps... most 1200watt amps i see are fused at like 120amps or more... an amp at full power can usualy pull withen 5-10 amps of what its fused for..
Chronos
MrBrownstone on 02/10/2005 20:51:56
Class D & Class T are basically very little difference in efficiency. Basically, Class T plays full range, D, up to about 250Hz effectively.
A 100 Amp fuse can withstand 200 amps for 1/2 second without going out, so don't besurprised if 400 amps go through for 1/16th second on that same fuse. Amplifiers can put out up to 250% of it's RMS power at full 100% distortion, so imagine the current.
As as far as 100 amps on a standard alternator. Try to remember that the amplifier is only 'ON' 10-25% of the time. The other 75-90% of the time, the amplifer is just there listening, recharging, and not drawing much current above idle current.
The truth of the matter is that 60 amps of net current from an alternator can suffice 240 or more amps of draw considering how long the amplifier is pulling full out power. That amplifier isn't going to be pulling all 100 or more amps 100% of the time, so the standard alternator is probably fine. This is where we get into the capacitor debate.
Efficiency of amplifiers at less than 50% output is less than 50% in most cases, and closer to 70-75% at full RMS power. Efficiency at 100% distortion is like 90%!!!!! Efficiency ratings are not a factor. Efficiency of AB amps is ~10-20% at nominal and 44-48% at full RMS....75% efficient @ 100% distortion. You're not getting 90% out of any amp clean power, unless it's from Mars.
Class D is a class D. T is basically a D. T = Tripath chip allowing full range. Nothing more admirable than that.
gnehc1 on 02/10/2005 23:29:52
i'm running a class D amp. i think i'm gonna get a smaller alternator pulley so that it can spin a bit faster, and i'm thinking about adjusting the idle so that it will spin faster also. hopefully after i do that, it will fix my problem.
uochronos on 02/11/2005 02:52:38
ya smaller alternator pull might very well solve this problem cheaply for you thats awlays nice..
Mr.B i'm interestind in undertsanding this whole thing where the amp isnt pulling current all the time? i know its not pulling huge amounts of current all the time... with a 1200watt amp running at half power around 600watts. and another amp running 250watts rms... and the alternator could not keep up it would flat out kill the car at an idle at a stop light...
Seems to me it had to be pulling close to 60amps almost constantly to do that? so it seems to me a 1200watt amp would pull close to 80amps constantly and well over 100amps on spikes.. and especialy with rap, dance and hip hop the bass just keeps pounding seems that it would really pull the current..
i'm not argueing with your logic infact i think your explanation was one of the best i have seen.
just seems to me with the hard hitting bass music thats popular and my own experience shows that it pulls alot of power...
Chronos
MrBrownstone on 02/11/2005 13:06:14
You'd be amazed at how little work an amplifier actually does. It works 10% of the time. Kinda like an olympic sprinter...particularly the Class D. That's why they don't need cooling fans
MrBrownstone on 02/11/2005 13:08:46
One more example. a 250kW weather radar averages 250W of power....less than the average microwave oven. Both have microwave transmitters, just that one averages 800W and puts out 800W. The other one, puts out 250,000 watts for 0.001 second, then waits for 0.999 seconds for responses. the net is 250 Watts.
The main issue is whether or not a CAPACITOR will do this guy any good, and the truth of the matter is that it won't affect his audio system. It's just helping the lights from dimming. The amplifier has already made it's discharge before the cap does it's work.
uochronos on 02/11/2005 15:10:39
ok ya that makes sense... i get what your saying... took me a bit but i understand now... the bursts from your amp only last a second or less usualy most the time the amp isnt drawing... that makes sense...
woohoo learned something new:)
i was looking at it a differnt way but now that i'm seeing it from a differnt perspective it makes alot more sense... sometimes you have to flip things around to see them correctly:) thanks for the help.
Chronos
MrBrownstone on 02/14/2005 14:30:45
Your amp is operating for 0.1 second, and at full 100% distortion, about 0.25 second. That's why it's mostly a waste to own a cap unless it's extremely low esl/esr AND storage is quite high (like 5F).
It's hard to believe, but your amp is quite a lazy machine. It gets off the couch less than you do!!!
Pinch on 02/14/2005 16:38:04
Quick sanity check, POwer. If I am reading your post correctly, a 1200 watt amp at 90% efficiency would produce 1200 divided by .9 watts. As you mentioned, 1200 divided by .9 is 1333.3333 Watts. Then you go on to show that a class A/B amp is only like 50% efficient, which turns out to be 1200 divided by .5 which is 2400 Watts. Here is my problem. This method shows that a less efficient amp squirts out MORE power than a more efficient one. I guess my question is this. Shouldn't you multiply power output by efficiency to get actual output? As in, the Class D would produce 1200 * .9 or 1000 Watts, while the A/B would produce 1200* .5 or 600 Watts. This makes more sense to me because we know that the output capabilites of the amplifiers are 1200 watts in both cases, but the Class A/B and D amps dissipate 600 and 200 watts in heat respectively due to internal losses. Just curious on everyone's thoughts regarding this subject.
Pinch
gearhead on 02/14/2005 16:47:17
I'm pretty sure they're talking about how much amperage the amp will draw, not how much it will put out. Class D won't draw as much amperage to supply 1200w as a class AB amp.
swez on 02/14/2005 17:30:43
This is a tough subject for some folks to grasp initially... but Mr. B. is correct in his assertions that a typical audio amp is often cruising along at moderate power (even when we kick it up) due to the "duty cycle" it operates within.
Most newer Class D amps use high speed pulsed switching power supplies to develop their power. They also have large CAPs inside the power supply to store power between sonic bursts. The efficiency ratings are typically in the upper 80's - 90%.
We can also get a pretty good estimate of actual watts out to a speaker
by knowing the ohmic value of the speaker and input voltage.
Example: 4 ohm sub load, voltage measured at sub terminals is 40 volts. How many watts are we getting into the sub?
The math: Ohm's law states Volts^2/Resistance = Watts
40 x 40 = 1600/ 4 = 400 watts
FYI: When measuring voltage off a sub or speaker, the speaker resistance changes notably with frequency. If we substitute a " non-inductive dummy load", the readings are more accurate on how many volts the load will show. However, the early mentioned method is more real world as nobody I know, listens to music off a dummy load. That would be just plain dumb! GRIN
Finally, average music of most kinds mentioned, do have sustained bass tones of a given amplitude. If we really want to press the amp into a good workout, test tones of 5 seconds or longer will do the trick. That's a steady sine wave with lots of energy behind it.
Swez
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