88_TAOD

I was wondering what the difference between ohmage is. I am going to buy a pair of mtx 8000 subs that are 8 ohm. DO they pound as hard. What is the difference
Will ineed a special amp.
Jason

racer J

Ok here's the deal. The ohm rating only has to deal with what load the amp sees. 8 ohm subs are the same as 4 ohm subs except for the ohm rating. They'll sound the same. The catch is when you wire them up. If you wire the positive wire from the amp to the positive wire of one sub then wire that positive terminal of that sub to the other sub's positive terminal.....then do the same thing with the negative terminal you'll have what the call a parallel load. If you bridge a 2 channel amp down to one channel and hook up two 4 ohm subs in parallel the amp will see a 2 ohmn load. This will make the amp put out more power. If you hook up two 8 ohm subs the same way you'll only have a 4 ohm load. Basically youneed to look at how much power the subs can handle and how much power the amp puts out. It's usually better to go with 4 ohm speakers at a 2 ohm load as described above. JL audio's subs are a better buy though. Two 12's will pound your ears out with the right amp.

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1988 Camaro
305 bored .030 over
Stock TBI
Aluminum radiator
No smog pump
gutted factor cleaner
cam ground to crane specs
timing advanced 9 degrees
no thermostat
other than that its bone stock and ran a 16.4 @ 82 mph with earth shattering wheel hop and a 2.88 60 ft time. (this was with the rebuild and cam only)

IROCET

You are not kidding about 2 12's pounding your ears out. I just got two 2ohm 12" Boston Pro's and hooked them to my Memphis 1000D, running at 1100watts and it is way loud. That is before a box and everything, just sitting on a plate right now.

I think alot depends on compatability. How well will your speakers running through this amp vs. that amp. It is also a lot of personal preference. I would go with the 4 ohm model in you case. You will probably be happier in the long run. Good luck.

Matt

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irocet@hotmail.com

drive.to/Stang_Kilr

88 IROC-Z -- 350 L-98, 700R4, Flowmaster, K&N, MAF Sceens Gone, Airfoil, March Pullies, Comp Cams Magnum 1.6 Roller Rockers, Accel U-Groove Plugs, Accel 8.8 mm Wires, Hypertech Cap & Coil, Hypertech Chip, Hotchkis Strut Tower Brace, One Loud Stereo!

racer J

I was wondering if Boston's subs were any good or not. I guess they are if they are pounding in a plate set-up lol. Out of curiosity what model number are they and what are the tech specs?

------------------
1988 Camaro
305 bored .030 over
Stock TBI
Aluminum radiator
No smog pump
gutted factor cleaner
cam ground to crane specs
timing advanced 9 degrees
no thermostat
other than that its bone stock and ran a 16.4 @ 82 mph with earth shattering wheel hop and a 2.88 60 ft time. (this was with the rebuild and cam only)

[This message has been edited by racer J (edited February 22, 2001).]

The ODB

I prefer 8 ohm speakers for car installations.

You do not get more power or more sound from running low impedence speakers. That is just a farce conjured up to market a new line of products that came out in the early 90's.

The actual impedence of a speaker will change dynamically with frequency.

Ohms law is true if you keep voltage and frequency constant. This does NOT happen with a musical signal.

ODB

Jim85IROC

iTrader: (2)

Sorry dude, but you're wrong. Although the impedance certainly does change with frequency like you said, a driver rated at 4 ohms will have half the impedance of an 8 ohm driver at virtually all frequencies, with it being a tad different only around the resonant frequency. Ohms law ALWAYS applies, even when one or more variables are changing. The resistance of the driver at any given frequency will not change based on input voltage/current. The amplifier will deliver a relatively constant voltage, and the current will change based on impedance.

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stingerssx

Yeah, jim is right. The load that your speaker represents, will change the output from the amp, any and everytime. And although the resistance changes at different frequencies, it won't change enough to make a difference. That is to say, the same difference from an 8 Ohm load to a 4 Ohm load.

No matter what, the lower the impeading load given to the amplifier, the higher output, at the same voltage, always.

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'82 Firebird, dead stock, 9 bolt disc rear, over 200,000 miles and still going strong, more to come...
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The ODB

I will never agree with that. The output of the amp is controlled by gains.
If the output had no controls and was basically a short straight through the impedence of the voicecoil, then yeah ohms law would apply.

If you max your amp out at say 200 watts, and that is all the power supply can support without blowing the fuse, then what happens when you turn up the gain, or lower the speaker load?
My point is that an amp has only one MAX power that it's weakest internal component is designed for. You will not make it past this point by decreasing impendence at the speaker.

Furthermore, once a speaker has reached the limit of travel that it's suspension can support, any extra current turns into heat in the voicecoil.

IMO running higher impedences gives more stable and controlled output to the speakers which sounds better and is more reliable. Reaching maximum power is simply done by matching the gain to the input signal.

If anyone has any physical proof that an ordinary amplifier doubles its output into half the impendence (after it has reached maximum power with its rated ohm load), then I'd love to see the test data. This would be some info that I'd really like to learn, so please either post here or email to me.
thanks,
ODB

stingerssx

What are you talking about man? If you max your amp, then it's maxed. No doubt about that. But when you run a lower impeadance, the power that the amp is able to produce increases, up to it's breaking point, which is the max output. An amp will produce as much power as possible before it blows. The lower the impeadence, the more likely that it will blow, and the more distortion you will have. The more impeadence, the more restriction, the less power. Straight up.

Jim85IROC

iTrader: (2)

Never taken a circuits class have you? The gain on an amplifier only controls the INPUT level to the FETs. The multiplication factor is based ONLY on how the FET is biased, which is based on the output resistance and it's relationship to the source/drain and gate resistances.
Take this headphone amp circuit as an example:

It's been a long time since I've biased one of these circuits, so I'm not going to do the math tonight, but your output characteristics of that FET depend on the relationship of the output impedance, which in this case is in parallel with R5, and their relationship with R2 and R3. RP in that circuit is your Gain control, with R1 just adding further attenuation.

If, with an 8 ohm nominal load your amp puts out 200 watts as you said, then in *theory* dropping to a 4 ohm load would give you 400 watts. (P=I^2*R) In reality, your output can usually increase by 50% to 70% depending on the design of the amplifier. I looked for the birth certificates for my Kicker amps, but they must be burried somewhere. If I can I'll dig them out later and post the actual bench-tested wattage at multiple impedance loads.

------------------
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View the restoration of an 85 IROC</A>
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The ODB

Jim,
thank you for this:
"The gain on an amplifier only controls the INPUT level to the FETs. The multiplication factor is based ONLY on how the FET is biased, which is based on the output resistance and it's relationship to the source/drain and gate resistances.
"

next question,
would you say than in a practical application you could not reach the amplifiers maximum current output into 8 ohms by simple gain adjustments?

I ask this because every test I've done on a "normal" amp shows the same max current output at 2, 4, or 8 ohms. I've even ran a test amp into a 16 ohm load and it still came close to its maximum power output (about 20 - 30% under). Does this not sound right to you? I did the tests around 1991.

I do remember even before that a couple of companies were purposely underrating their amp's max output for use in competition. It was very poplular back then for people to hook up 10 or more large woofers to one Punch45 amplifier @ loads around 1-ohm. It sure did output more than 45 watts and played the speakers, but they sounded like total A$$. Not only that but the amps overheated in a very short time and would often blow their internal fuses or fry any number of internal components.

I still take this whole impedence ballgame as a huge marketing ploy that originally started out as a way to cheat powerclasses at crank-it competitions.

What is your take on this?

ODB

87kevroc

I'll get in on this if for no other reason to find out if I know what I'm talking about.

ODB,

I'll try to summarize what I believe your point to be, correct me if I am wrong:
The maximum power output of an amplifier is not a function of the load on the amplifier.

I do not totally disagree with this point. However -

Concerning Ohm's Law: Ohm's Law is Ohm's Law. It applies in every case and at all times I know of, whether frequency or voltage or any other parameter is dynamic or static. When is Ohm's Law not Ohm's Law? If there is no agreement that Ohm's Law is immutable there is no foundation for the rest of the discussion.

I'll try to illustrate by example. Assume impedance in a hypothetical system to be nominal, that is, impedance does not change appreciably with respect to sonic frequency or other dynamics. 8 ohms. At first, let’s say, gain will be static.

We measure and record the current to the speaker at our static gain. Then we remove the 8 ohm and put in a 4 ohm speaker. We measure current and find that it is higher than with the 8 ohm speaker. If current increases, power increases, provided (this is key) the voltage across the output leads of the amp is the same in both cases. This is Mr. Ohm talking, not me. I believe the amp would maintain the voltage across the output leads (this is where I need some help, I'm not sure about this).

Now to your point. Single 8 ohm speaker. Crank up the gain, all the way. The amp responds by increasing current. The constraint you mentioned above is certainly valid, the amp can only draw as much current as the weakest component can support without burning up or (hopefully) blowing a fuse.
But the fuse does not blow. Now, if we substitute our 4 ohm speaker for the 8 ohm, what happens? As in the case above, when current increases, (thus power draw) increase. Volume increases. The fuse holds out. More power is being drawn by less resistance.

What’s the trade off? Distortion, probably. I suspect the amp will be able to provide less distortion at 8 ohms than it will at 4 ohms, but not as much volume.

I believe racer, stinger and Jim’s posts support what I’m saying here (or I support theirs, whichever). What say all to this?

The ODB

Yes I agree to the hypothetical situation you described.

I do disagree as far as real-world goes. In a properly adjusted system you should reach max power with the load that you have if it's 2, 4, or 8 ohms.

The marketing ploy is to trick people into thinking the amp can do twice as much as what it can actually do. It treats speakers and music like they are simply dead-shorts and simple electric motors. They are motors, but they are very dynamic as is the musical signal. I think Ohm's law has been pimped out.

Jim85IROC

iTrader: (2)

Do you know WHY it overheated? Because it was creating substantially more power, therefore drawing substantially more current. As I hope you know, approximately 40-60% of the input current is dissipated through heat. So, the overheating was a DIRECT result of more current consumption and more power output.

When you turn the gain up on an amplifier, you are increasing the VOLTAGE input to the amplifier. In the specific case of the circuit I displayed above, it is biased for a unity gain in voltage. That means that the voltage output is exactly (or close to it) the same as the voltage input. The current output depends on how the transistor is biased.

In the case of a car amp, the voltage of the input signal is probably stepped up in a pre-amp before being fed to the output transistors. The voltage feeding the output transistors is stepped up as well. When you increase the gain, the voltage increases at the input of the output transistors. Unfortunately, physics still prohibits a transistor from producing an output voltage that is higher than the that of the power feed to the device. Anyway, this is called the 'rail-to-rail' voltage. This is not to be confused with the input signal. When you continue to increase the gain on the input, eventually your output signal is going to exceed the rail-to-rail voltage, and the result is a clipped output waveform. This is obviously bad. By running into a lower impedance load, you are capable of producing more power without exceeding the rail-to-rail voltage. Your biggest limitations here are the ability to keep the FETS cool and to be able to supply enough current.

------------------
The IROC Homepage
<A HREF="http://www.rit.edu/~jli4307/camaro" TARGET=_blank>
View the restoration of an 85 IROC</A>
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The ODB

I did realize the first part. Yes more current but I questioned its usability.

kind of like advertising those alternators that you can weld with... you don't want to weld your battery, just charge it.

I was also assuming the user would have sense enough to avoid clipping the signal.

but that's cool, you have taught me something. It didn't really change my feelings on the matter, just my understanding of it a bit.

I think you're saying it basically comes down to the amplifier design..
if it is to reach is maximum power with a normal input signal, or do they give you leeway to run more load.

What is a bit ironic is from my own experience. Most of the car amps I've worked with enjoyed an 8-ohm loading, but the home amps seemed to perform better at 4 and even a little less.
this had nothing to do with the speakers, as I used the same "car" speakers for all my testing.

It wasn't what I was expecting at the time, but sure enough is how it worked out. I think maybe the inputs were more sensitive on the car amps. think so?

thanks again for your explainations.
ODB

Jim85IROC

iTrader: (2)

Home amps will almost always play just fine with a 4 ohm load, although they don't advertise as such. Most home receivers have connections for 2 pairs of front speakers, with A & B switching on the front panel. Those two pairs are wired in parallel, which would of course create a much lower resistance than just one pair or the other. The home amps are designed to play all day long with 2 pairs of speakers connected.

Home equipment also has the luxury of being fed a fairly standardized input level. Most cd players and other line-level have a fairly consistant output voltage, so you can design your amp around that. With car audio, your head unit may put out anywhere from .5 volts to 8 volts, and there are line drivers that have output voltages up to God-knows-where. Most car amps are still designed with a specific voltage in mind, and it's ultimately up to the user to set that gain properly to achieve that, but the car audio designers know that 95% of their amps will not be set up properly, so they must design it to operate well over a broad input range. Add to that the fact that the 12 volt DC car environment is substantially less ideal than 120VAC, and it becomes apparent that there will be necessary comprimises with car amps that are not present with home equipment.

As for differences in sound quality between running an amp at 4 or 8 ohms, I doubt that there would be any. The only difference in terms of output is that the damping factor is much lower when driving a lower impedance, but any amp of even marginal quality is going to have a damping factor many times higher than what could be considered audible. In general, any damping factor above 4 to 5 is inaudable. A decent amp will have a damping factor over 100 even when driving 2 ohms. Far, far more than what is any bit audible.

------------------
The IROC Homepage
<A HREF="http://www.rit.edu/~jli4307/camaro" TARGET=_blank>
View the restoration of an 85 IROC</A>
"I didn't know a bored out Ford could go so slow" -Shenandoah

chrisfrerick

most high quality amps will double there power as there load is decreased from 4 to 2 ohms. rockford,ppi,mtx,kicker come to mind. when amp manufactors design these amp they usually design them for 2 ohms stereo per channels and they build there power supples to handle the currant at this ohm load. but most amp manufactors advertise there amps at 4 ohms stereo.

the problem in the 12 amp world is that there is no uniform standard on how to rate an amps power like there is with home audio. some tricks amp companys do to make you think the amp can put out more power i follows.

lets say we have a pp1 a600.2 ppi rates thier specs as follows

150wrms x 2 at 4 omhs 11-15v 10-100000 hz
300wrms x 2 at 2 ohms 11-15v 10-100000 hz
600wrms x 1 at 4 ohms mono 11-15v 10 100000 hz
this amp is really designed to produce an over all power of 600w total rms you will not get anymore out of this amp. but ppi markets this amp as 150wrpms x 2 4 ohms in general.

now if ppi wanted to they could advertise the specs as follows
300wmax x2 at 4 ohms
600w max x 2 at 4 ohms
1200w max x 1 at 4 ohms mono

they can rate the amp at
1200w x 2 at 1k
this amp could put out a thousand watss at 1k
for a second and they could rate it as a thousand watts by two at 4 ohms if they wanted two.

there are many tricks they can use to make the amp look more powerful like rating it at a single frequncy or a higher voltage level like 15.5v for a non regulated amp and so on and so.

as far as using the gain to max out the amp it depends on the signal leveal feed into the amp. the higher the signal going into the amp. the less you have to turn up the amp be cause the ratio is changing.

they only point that i agree on with odb is that if an amp is playing an 8 ohms sub instead of a 4 ohm sub its damping factor is better and you could control the sub better in theory.

finally, all amps will try to double there power as the ohm load is halved. eack time you halve the ohm load you are asking the amp to produce four times more current then the previous ohm load. if the amps power supply can produce that curreant it will blow up or shut down from its protection circuits.

Jim85IROC

iTrader: (2)

The same tricks apply to home audio ratings. My 60 watt Denon receiver will tear apart any 100 watt Sony that you can find. In addition to some of the things Chris said, there are other, more subtle ways to fudge power ratings. One thing is to rate the amp's output with only 1 channel driven, which will result in more power than each channel is capable of producing when all channels are driven. You also have to watch to see what THD the power is rated at. If you look at a THD vs. power graph, you'll find a very flat region, then all of a sudden the THD will skyrocket at some certain power level. A quality amp will rate their continuous rms output at a point where the THD is very low, and in many cases still far from the point where it skyrockets. Lesser quality amps will not use this same method and may be rating the power at God Knows What levels of THD.



------------------
The IROC Homepage
<A HREF="http://www.rit.edu/~jli4307/camaro" TARGET=_blank>
View the restoration of an 85 IROC</A>
"I didn't know a bored out Ford could go so slow" -Shenandoah

83CAMAROMAN

i have two speakers that go boom

sorry i had to do that

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KEYSTONE CLASSIC WHEELS.
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