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Old 04-20-2004, 05:22 PM   #1
MackN666
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help a noob with some amp ?'s

I have 2 mtx 6000 subs that total an advertised 600 watt rms, 1000 watt peak. . .
I am interested in getting a 600 watt amp so that my subs are getting all they can handle with no risk, but in order to power 2 subs, do I need a 2 channel amp, or does a mono subwoofer amp do the same job? Also, what is the deal with OHM's. . .i have no idea what that means, except for the fact that amps advertise different outputs at different OHM's. If my subs are "4 OHM" does that mean i need an amp that advertises 300watt x 2 at 4 OHM?

Ahh, amps confuse me. . .all i know is I want an amp that will power these two subs well. Any help with my questions is appreciated.
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Old 04-20-2004, 06:32 PM   #2
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Ohm is a measure of resistance. More or less is not necessarily better. But, you probably need a 600watts rms X 1 into 2 ohm amp. You can get a mono block amp that is 2 ohm stable. Most amp manufacturers make them, just choose one.
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Old 04-21-2004, 12:21 AM   #3
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The Skiny on Ohms, Current and Power

Hi,

Sorry I cannot answer all your questions directly. I am not a speaker expert... and have not studied speakers before. I am an electrical engineer and I design electronics for cell phones. I am an expert in amplifier design and I know something about electric / magnetic field theory and amplifier design. I can tell you the following that may help you in making an informed decision. This is a bit long... but here it goes.

A speaker contains an iron magnet and a coil of wire. The coil of wire is attached to the speaker membrane which is also surrounded by the iron magnet. When current (from your amp) flows through the speaker coil it creates a magnetic field that is either in the same direction as the iron magnet's magnetic field or in the opposite direction to the iron magnet's magnetic field. When the fields are in the same direction the membrane gets attracted/pulled toward the magnet. When the field is opposite the membrane is pushed away by the magnet. The direction of the magnetic field (push or pull) created by the coil depends on the direction of current flow. The direction of current flow changes rapidly at a rate equal to the frequency of the signal. Audio signals are in the range of a few hundred Hertz (few hundred changes per second) to 20,000 Herts (20,000 changes per second). What does this have to do with current and Ohms?

The amplifier pushes the quickly changing current into the speaker. The current through the coil creates the magnetic field in the coil. The magnetic field created in the speaker coil is proportional to the number of turns in coil and the current pushed into the coil. Thus, if the amplifier could double its output current you would get twice the magnetic field in the coil and perhaps twice the speaker membrane movement.... making it louder.

The speaker coil does have some "resistance" because the copper wire coil is not a perfect conductor. It may have 1 ohm or so resistance to direct (non-changing) current. If you pushed like 100 units of direct current (called an Amp ... not "amplifier") into the 1 ohm resistance coil it would light up like a toaster wire... then melt. However, the Ohms you refer to for power actually has to do with the speaker's action/dissipation of energy while creating the magnetic field. Pushing current into the coil creates the magnetic field. Like every thing in nature, everything at rest want's to stay at rest. To create the magnetic field in the coil (that is currently at rest) with current from the amp, the coil pushes back on the amplifier to resist the creation of the magnetic field. The faster you try to change the magnetic field in the coil the more opposition the coil gives to the amplifier. This phenomena is refered to Inductance... the same as objects in motion want to stay in motion and objects at rest want to stay at rest. The coil's magnetic field want's to keep the current flow constant to keep the field constant and the amp wants to change this current and change the magnetic field. The coil's opposition to change in amplifier current change is Reactance (not Resistance), which also has the unit Ohms. This is not dissipated power (like resistance) but power which is drawn from the amplifier to change the magnetic field in the coil. This electrical power / energy is then dissipated as heat in the speaker (from the above toaster effect) and also lost in the membrane in causing it to move. Although I have not researched the characteristics of speakers, my guess is that a larger speaker with larger coil (more turns of wire) will have larger inductance and thus a larger Ohm rating like 8 Ohms instead of 2 Ohms.

The amplifier pushes current into the speaker. The (fast changing) current in the speaker will create a voltage across its coil. The amplifier is hooked up to a 12V battery and thus the voltage across the speaker can't exceed 12V (actually it could theoretically go to 24V but that's beyond the scope of this discussion). The amplifier will be powered up and dissipate power of its own. The amplifier will be optimized to minimize the power it dissipates while being able to deliver the maximum current to the speaker and not exceed 12V. Reactive power delivered to the speaker (to get the magnetic field to change) is Current*Current*Ohms.... in this case 8 Ohms or whatever the speaker is rated at. The bigger the Ohms the more power it dissipates for a given current. Because an amplifier is optimized for a given Impedance (Ohms) I would suggest picking the correct amplifier for the correct speaker size or Ohms. Otherwise you may not get the power out of the amp that you expect, even if it is rated higher than you need.

As for Watts peak ... vs Watts RMS? That is some lingo that some amplifier makers use. RMS is Root Means Squared which is a value that pertains to the equivalent average voltage or current that would provide the same amount of average power in a purely resistive (heating element) type of load. RMS is used to describe the voltage or current value that delivers power, not power value. When they speak of RMS power I guess they are pertaining to the average power delivered by the amplifier most of the time. As for peak power??? I suppose they mean the maximum power that could be delivered by the amplifier for an instant of time... which could exceed the average power.

As for number of channels? This could be the number of amplifier outputs? I don't know. I personally refer to channels as Left speaker being 1 channel and Right speaker being another channel. Mono is usually only 1 channel and thus only one speaker (ie: AM radio). The number of channels is determined by the type of radio and the type of demodulation it performs on the signal. If the signal has multiple channels encoded in it (ie FM Radio) then you can demodulate (extract) two independent channels (left speaker/ right speaker) and drive left speaker and right speaker with 2 different independent signals.

You have sub-wofer, mid-range and tweater (parden the spelling). The low frequency (slowly changing) currents (ie: 100Hz to 1000Hz) get filtered (picked off) and sent to the sub-wofer for the Bang-Bang. The mid-frequency signal currents (ie: 1000Hz to 10,000Hz) get filtered and sent to the mid-range for the voice signals, etc. The high frequency (fast changing)
signal currents get filtered out and sent to the tweater. The tweater coil is much smaller than the sub-wofer and so its magnetic field is much smaller. However, the frequency is much higher and it has to change the magnetic field in the speaker at a faster rate... thus it sees a more opposition to magnetic field change and thus for the given smaller size the Ohms is still high like 8 Ohms, comparable the Ohms of the sub-wofer.

I may be wrong on some of the above concepts. Again, I am not a stereo speaker expert and not a car stereo expert. Hope this helps.

Cheers,
John
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Old 04-21-2004, 08:02 AM   #4
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You should teach car stereo 101.
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Old 04-21-2004, 08:57 AM   #5
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Quote:
Originally Posted by MackN666
I have 2 mtx 6000 subs that total an advertised 600 watt rms, 1000 watt peak. . .
I am interested in getting a 600 watt amp so that my subs are getting all they can handle with no risk, but in order to power 2 subs, do I need a 2 channel amp, or does a mono subwoofer amp do the same job? Also, what is the deal with OHM's. . .i have no idea what that means, except for the fact that amps advertise different outputs at different OHM's. If my subs are "4 OHM" does that mean i need an amp that advertises 300watt x 2 at 4 OHM?

Ahh, amps confuse me. . .all i know is I want an amp that will power these two subs well. Any help with my questions is appreciated.
Go to my website http://home.comcast.net/~customcarstereo/ for detailed explanation about all car stereo related issues, including amplifiers, subwoofers, etc.
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Old 04-21-2004, 09:39 AM   #6
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Amps / Ohms - The missing part above

Hi,

I just want to clarify what I said above regarding Ohms. After reading it I noticed a few holes in what I said.

The unit of currents is Amp or [A]
The unit of voltage is Volts or [V]
The unit of Impedance or (Resistance plus Reactance) is [Ohms].

Why a speaker has a RESISTOR value in Ohms.The speaker will have an EQUIVALENT resistor value depending on the power or intensity it delivers to the room. Above I said that the speaker has a reactance in Ohms. This reactance (in Ohms) is the spekaers opposition to the amplifier changing the magnetic field in the coil. If we disconnect the speaker membrane from the coil the speaker will not produce any sound. The energy the amplifier puts into the coil's magnetic field will stay in the coil. As a result, when the amp pushes current into the coil the coil will oppose it. However when the amp draws current out of the coil the coil will push current back toward the amplifier. The net overall effect is that there is no power dissipated by the amplifier. The coil just stores the amplifier's power (and feeds it back to the amplifier) just like a capacitor stores electrons to keep a constant voltage acorss its pins. When we re-connect the membrane back up to the speaker... now driving current into the speaker will cause the membrane to move. The air pressure around the speaker vibrates and you hear sound. Because the coil's magnetic field caused the membrane to move some work had to be done to move the membrane. The constant work to move the membrane over time is the power the speaker absorbs... and it absorbes it from the amplifier. This can be MODELED as a resistance (representating the amount of power dissipated).... but there is no physical resistance that you can measure with a multi-meter. The resistance (not Reactance) number represents the work over time (power) the speaker draws from the amplifier to move the speaker membrane. This is similar to a cell-phone antenna. The transmitter will eventually run the battery dead by drawing power from the battery and pushing it out the antenna. An antenna is just a piece of wire with no significant Resistor value - measurable with a multi-meter. However, pushing an alternating (high-frequency) current into the antenna creates an Electro-Magnetic field around it. This EM field radiates out into space... and can be detected by other antennas. Because the EM field radiates it is not stored by the antenna ... but delivered to the envoronment. This EM field energy loss to the environment (during the call) required power. This power transmitted by the antenna can be MODELED as a resistance, while there being no physical resistor that is measurable with a multi-meter.

Amplifier Power - Current and Ohms
Above we said that if the amplifier drives current into the speaker the speaker reactance (inductive impedance in Ohms) will generate a voltage across its terminals. This voltage can't exceed 12V. The maximum current out of the amplifier depends on the pre-amplifier or the receiver... or whatever drives the power amplifier. Whatever drives the amplifier is limited to be some maximum value... lets say 5V output. The power-amplifier will have some gain in it but it isn't designed to have much gain. It is designed to deliver maximum power to the speaker. Maximum power is delivered to the speaker when the current out of the amplifier is at its maximum possible value AND the voltage generated across the speaker is also at its maximum possible value (12V). Power = RMS_Current * RMS_VOLTAGE in Watts. Also, RMS_Voltage = RMS_Current * Speaker_Ohms. If the speaker_Ohms is too high the voltage across the speaker will reach 12V Maximum BEFORE the amplifier output current reaches its maximum possible value. Therefore Current*Voltage will not be maxiumum. If the speaker_ohms is too low then when you have MAXIMUM current out of the amplifier the voltage across the speaker will be LESS than 12V. Therefore Current*Voltage again will not be maximum. The conclusion is that the amplifier is designed to deliever current to a specific speaker_ohms value for maximum power. If the speaker_ohms value is off (not what the amplifier is designed for) you won't get the rated power out of the amplifier.

Sorry for the long explanation.

I also just checked out cal1242's web page. He/she does a good explanation of how to hook up the stereo..... with a few pictures and equations.

John

Last edited by bearing01; 04-21-2004 at 09:49 AM.
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Old 04-21-2004, 03:04 PM   #7
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Quote:
Originally Posted by bigjae1976
Ohm is a measure of resistance. More or less is not necessarily better. But, you probably need a 600watts rms X 1 into 2 ohm amp. You can get a mono block amp that is 2 ohm stable. Most amp manufacturers make them, just choose one.
Can you recommend certain amps that have that power output? thanks in advance...
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Old 04-21-2004, 03:23 PM   #8
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There are few amps that have exactly 600 watts per channel @ 4 ohms or 1200 watts mono @ 2 ohms. Manybe you might want to consider 1000 watts or more, depending on brand.

It really is a matter of budget. How much are you willing to spend? Are you on a tight budget that will steer you towards lower end brands, or can you afford high-end equipment? Most people will suggest whatever their "prefered" brand is, and most shops will suggest whatever they sell.

I would suggest going to a couple local car stereo places and see what they got. As far as brands, pretty much everyone offers a high power subwoofer amplifier. It seems that 99% of the people here go for JL amplifiers. As a generalization, for amps, even though there are numerous exceptions, american/european companies are best.
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Old 04-21-2004, 06:55 PM   #9
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Quote:
Originally Posted by MackN666
Can you recommend certain amps that have that power output? thanks in advance...
IMO...Bang for the buck but high quality? I'm sold on US Amps 2 channel amps, not the multi channel. Why? They cost less than $.75 per watt, won't shutdown, VERY reliable, cool looking, plenty of dynamic headroom, and just all around sound great.

But there is so much variety out there, you'll get a lot of personal preferences. US Amps is mine...they are worth a listen though.
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