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oldbluzer
11-18-2012, 10:07 PM
I know just enough about elektrics to be dangerous, so I have a question that may be very evident to everyone else?
I see amps that say a certain wattage at 4 ohms and a somewhat lower wattage at 8 ohms and then it is suggested that to get more volume you can add a second speaker. If you have two speakers at 8 ohms don't they have to be wired parallel? This is never mentiond anywhere that I am aware of. Or is it just me?

Ditto
11-19-2012, 01:06 AM
I have a MESA BOGIE combo with 2X12 inch speakers. I wanted to add 2X10 cab to get the response of a 10 for that MID range punch. I called MESA and they let me talk to the TECH guys. It was way cool as the guy talked to me for 30 minutes about the things I "CAN DO" with the amp that I have. I have 600 WATTS at 4 ohms and 300 WATTS at 8ohms. So If I buy a 2X10 CAB at 4ohms with my 2X12 at 4ohms it would drop my output to 300 WATTS. Still more than I will ever need :) The TECH guy said I could wire my combo in parallel and add the 2X10 cab in 4 ohms. Problem is they make that cab in 8 ohms and I would have to special order it on 4......

I do not know what kind of AMP you have, but maybe call them and ask the TECH guys what you can do with YOUR amp.

Basslad
11-19-2012, 09:58 AM
You will find good info on the subject by following this link: http://www.tunemybass.com/bass_amps/how_to_hook_up_heads_and_cabinets.html

TobiasMan
11-19-2012, 12:44 PM
Most amps have different power ratings at 8 or 4 (or sometimes 2) ohm loads. Because the load or resistance face by the amp is higher for the 8 ohm cab, it will produce less power at that load than into a 4 ohm cab. This means that your amp will provide the rated wattage to a single, 8 or 4 ohm cab. If you are using a single cab, try to find one that is also rated for a similar power handling capacity.

Most amps and cabs are wired for parallel connections, allowing you to connect more than one cab to the amp. If you are considering using more than one cab on your amp, you must determine that the resulting load will not be lower than your amp's rating. The summed load from two cabs with the same ohm rating is 1/2 of the ohm load of the cabs you have connected. Two 8 ohm cabs, connected in parallel, would be a 4 ohm load and two 4 ohm cabs, connected in parallel, would result in a 2 ohm load. There are other combinations that can be used. Two, 8 ohm cabs and one 4 ohm cab combine to produce a 2.6 ohm load and should be safe with an amp able to drive a 2 ohm load.

The actual calculations for ohm loads is a bit more complicated when using multiple cabs and especially when using cabs with different ohm ratings. This link (http://www.tunemybass.com/bass_amps/how_to_hook_up_heads_and_cabinets.html) provides a good overview of cab/amp connections and provides a formula for calculating the actual ohm load created by different cab combinations.

You should never connect cabs to your amp that will result in a lower ohm rating than it is rated for - this is rapid way to release the "magic smoke" and find yourself going directly to your amp tech for major work!!!!!

What does adding more cabs to your amp achieve?
If you connect two 8 ohm cabs to an amp, the amp will be seeing a 4 ohm load, and will provide the higher, 4 ohm rated power to the two cabs, which will share that power. Even though the power is shared between two cabs, so each cab would receive less power than a single, 4 ohm cab, the result will be significantly louder because you have doubled surface area that is producing the sound.

If your amp is rated down to 2 ohms, then you could connect two 4 ohm cabs (resulting in a 2 ohm load) and feed the 2 ohm power level to the cabs. You could also connect four 8 ohm cabs to that amp (also resulting in a 2 ohm load) and deliver the 2 ohm power output to those four cabs. Again, although the power to each individual cab will be less than in the two cab setup, the result will be much louder volume (much more surface area).

A general rule of thumb is that it takes double the amp power to increase volume output by 3 dB and adding an additional cab will achieve a similar volume increase from the original amp power level.

If you amp is a single channel amp, but with two power outs, they will normally be connected in parallel inside the amp. You could add one 8 ohm cab to each output and the amp would see a 4 ohm load. If your amp is a single channel amp, but with only one output connector, you could add two, or more cabs to that single channel connection (as most cabs are wired for parallel connections - amp -> cab 1 -> cab 2, etc) and your amp would see the combined load of all the cabs you have attached. Never attach cabs to your amp that will result in a load that is less than that for which your amp is rated.

If you have a two channel (stereo) amp you can add a single cab to each output and each channel will provide the power for which it is rated for the loads attached to it. If your amp channels are rated accordingly, you could connect two cabs (in parallel) to each side, and each power channel would provide the power for the new ohm load to the cabs on each channel. Eg. If your stereo/dual channel amp is rated at 4 ohms minimum, you could connect two, 8 ohm cabs to each channel, (for a total of four, 8 ohm cabs) resulting in the rated 4 ohm load on each channel and be pushing a LOT of air and volume.

If your stereo/dual channel amp is able to run in Bridge Mode, in that mode it will combine the power of both channels into a single output with the maximum power provided at the minimum cab load (typically 1/2 of the minimum load for the two channels operating separately). In this way, two 8 ohm channels would be combined in the amp and provide their combined power to a 4 ohm load. An amp rated at 4 ohms per channel, in Bridged Mode, would typically provide its maximum power into a 2 ohm load.

A very few cabs/amps are wired for series connection. Series connections would have the hot wire go from cab to cab and be terminated by the wire returning from the last cab to the output of the amp (very unusual). Some cabs use series wiring internally to achieve the design cab load from the drivers installed.

Always confirm the minimum load rating of your amp before connecting a cab to the amp, and always determine the actual ohm load of the cabs you are connecting before connecting them to the amp. A little math can provide additional safe options for the addition of more cabs and more volume,while avoiding the horror of watching you amp go up in flames (so to speak).

Bassix
11-19-2012, 01:03 PM
You should never connect cabs to your amp that will result in a lower ohm rating than it is rated for - this is rapid way to release the "magic smoke" and find yourself going directly to your amp tech for major work!!!!!

Don't you hate it when that happens? ;) Nice, detailed write-up!

DaveB
11-19-2012, 01:53 PM
I replied to a similar posting a while back and my posting was converted into an article. This article can be found here:

http://www.thunderrow.com/content.php?1176-A-Visit-from-the-Wizard-of-Ohms

Bass amplifiers (heads) with two or more speaker connections and combo amps with an external speaker connection will generally connect the speaker cabinets in parallel (check your owner's manual to be sure). The total impedance of parallel connected cabinets can be calculated by dividing the product of the cabinet impedances (cab1 X cab2) by the sum of the cabinet impedances (cab1 + cab2). For example, I have a Carvin MB210 combo amp in which the speakers that are are internally connected to the amp that provide an 8 ohm load. The amp also has a connection for an external speaker cabinet. If I connected a cabinet to the amp via this connection that was rated at 8 ohms I would have a total impedance to the amplifier of 4 ohms. In this case the product of the parallel impedances is 8X8=64. The sum of the impedances is 8+8=16. So, the total impedance is 64/16=4. Some cabinets have a second connector to allow you to "daisy chain" more cabinets in parallel. If I added another 8 ohm cabinet in parallel to the previously calculated 4 ohms the total impedance would drop to 2.66 ohms. In my case, this would be a "BAD THING" because the MB210 is not rated to handle a load less than 4 ohms. NEVER go below the minimum impedance rating of your amp unless you enjoy the smell of fried electronics.

oldbluzer
11-19-2012, 06:48 PM
Hey guys, thanks for all the information in your replys. I thought I had tried to find some of the answers before I posted, but evidently didn't look hard enough. Your answers confirmed all my questions and suspicions.
Thanks again, and please look over my inexperience in(or ignorance of) using the forum.

DaveB
11-19-2012, 11:02 PM
Thanks again, and please look over my inexperience in(or ignorance of) using the forum.

This is exactly what the forum is for. There is a very diverse membership on the Row. I don't believe that I've ever seen a question on the forum go unanswered. There is always someone on the Row who has the necessary background to provide an answer to just about any question anyone could come up with regarding all things bass (and probably just about any other subject as well).

brettpobastad
11-20-2012, 01:22 PM
Good info... thank you. The whole ohm/watts/amps/digibytes thing just kills me.

dgracia
01-14-2013, 08:41 PM
Good info... thank you. The whole ohm/watts/amps/digibytes thing just kills me.

Actually, if you just remember that hooking up cabs in parallel reduces the ohms load and hooking them up in serial increases the ohms load, you'll be fine.

Ohms is a measure of resistance to current flow. Higher ohms is like hooking a 1/2 inch hose onto a water faucet that is built to use a 3/4" hose. If you constrict the hose, less water (or power for an amp) gets through. If you use lower ohms, that means less resistance to current...like hooking a bigger hose to the water faucet so much more water gets through. Lower ohms means lest resistance to current and more power comes through the connection.

As the T-man said, NEVER let your ohms load drop below the minimum ohms load your amp is rated for or you may burn it up or melt things from overheating.

A fairly common mistake is to hook both an 8 ohm cab and a 4 ohm cab to an amp that is rated for a minimum load of 4 ohms. Hooking an 8 ohm and 4 ohm cab either directly into the amp or by daisy chaining (one cab into the amp and the second cab into the first) is normally a parallel connection and that drops the ohms load to just 2.67 ohms. That will seriously damage or destroy an amp rated for a minimum of 4 ohms.

Very few cabs have a serial out connections. The standard is a parallel and for safety's sake you should presume that hooking 2 cabs up to an amp (or to one side of a stereo amp) is a parallel connection. Running a stereo amp in dual mono give you a lot more flexibility to run different cab connections because whatever is hooked on one side has no influence on what's hooked on the other channel.

Dan