I dont know a hell of a lot about amps,,never needed to own one being all I do is play at open mics where the amps are supplied. Anyway,,I understand that cabinets can be of of different ohms.. so do amp heads have ohms ratings as well..do you need to match the head to the cabinet with the same amount of ohms>? And what happens if you run two cabinets,,? And what happens if you run two different ohm ratings? Confused about this stuff
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Need Help Understanding
- Thread starter Pbasslovernyc
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Yes, most amps can take 8ohm or 4ohm, very few can also go down to 2ohm.
Yes. With tube amps you need to select the correct impedence, it's automatic with solid state amps.
Two 8 ohm cabs show a total 4 ohm impedence to the amp,
Two 4 ohm cabs show a total 2 ohm impedence.
If you mix a 4 ohm and a 8 ohm cab, the amp will see 2,666 ohm, like if you mix three 8 ohm cabs.
The 4 ohm cabs will receive 2/3 of the power, and the 8 ohm cab will receive 1/3.
- Feb 12, 2006
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- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
If the amp has a solid state output section, you need to know the minimum load it is rated for. As has been said, 2 ohms and 4 ohms minimum is common. Check the amp's specs.
If the amp has a tube output section, show the amp the expected load. Some tube amps expect one specific impedance. If the amp has a 4 ohm output, then hook up one 4 ohm cab or two 8 ohm cabs in parallel.
Other tube amps have two or possibly three options. For example some tube bass amps have 2, 4, and 8 ohm outputs.
The way the jacks are labeled can sometimes be misleading/confusing. Make sure you understand the correct way to hook up cabs, and ask an expert if you get confused.
Here's an example (Orange AD200B MkIII:
The labelling is a bit confusing as it appears there are two 4 ohm outputs and one 8 ohm output. The amp actually has a 4 ohm output and an 8 ohm output. However the 4 ohm output has two jacks in parallel. The load should be 4 ohms total for both of these jack.
- Run one 4 ohm cab on either 4 ohm jack.
- Run two 8 ohm cabs, one speaker connected to each 4 ohm jack.
- Combined load of two 8 ohm cabs in parallel is 4 ohms.
- Daisy chain two 8 ohm cabs to one of the 4 ohm jacks.
- Combined load of two 8 ohm cabs in parallel is 4 ohms.
- Run one 8 ohm cab on the 8 ohm jack.
- Do not run two 4 ohm cabs as the total load would be 2 ohms.
It depend on if the connection is series or parallel. In most instances the connection will be parallel.
If the connection is series, simply add the impedance of each cab. 8 + 8 = 16 ohms. 8 + 4 = 12 ohms, 4 + 4 = 8 ohms, etc.
If the connection is parallel and both cabs are the same impedance, simply divide the impedance of one cab by the number of cabs. For example if you have a pair of 8 ohms the impedance is 8/4 = 2 ohms.
If you connect two cabs with different impedance in parallel, use this formula where Z is impedance:
For example if you have an 8 ohm cabinet and a 4 ohm cabinet:
When you use multiple cabs you also need to consider how power is shared.
If both cabs are the same impedance then they share the power equally.
It's preferred to use matching cabs. Adding a matching cab in parallel gives the following results:
1. Combined Impedance = 1/2
2. Combined Power Handling = 2X
3. Combined Sensitivity = +3dB
If the impedance is not the same, the way the share power depends on if the connection is series or parallel.
If you have 8 ohm and 4 ohm cabs in parallel, the 4 ohm cab gets 2/3 of available power and the 8 ohm cab gets 1/3. Make sure to consider how much power is available and the power rating of each cab.
For example if the amp can make 600W and the 8 ohm cab is rated 400W and the 4 ohm cab is rated 200W you can't use all of the available power. At full power the 8 ohm cab will get 200W, which is safe. However the 4 ohm cab will get 400W, which double it's power rating. In this example, you can only use 300W. 200W goes to the 4 ohm cab and 100W goes to the 8 ohm cab.
It's generally not a good idea to use dissimilar cabs in series. The math says the 8 ohm cab will draw 2/3 of the power and the 4 ohm cab will get 1/3. But series connections can produce confounding results because the actual impedance at a given frequency varies by speaker design.
Also note that the vast majority of cabinets are wired such that their input/output jacks are wired in parallel, so simply daisy chaining from amp to one cabinet, and then connecting a second cabinet to the first cabinet DOES NOT produce a series connection - the cabinets are still in parallel.
Usually you need to use a special speaker cable to connect cabinets in series.
Usually you need to use a special speaker cable to connect cabinets in series.
Just for the sake of argument...is there a particular amp you used and / or enjoyed at an open-mic? Trying to set-up a real world scenario with known hardware.
Riis
Always impressed with the detail of your replies, @Wasnex. Your patience and your knowledge is impressive.
honestly I never know the exact models of what I am playing through,,its been everything from Mark Bass to Ampeg to Crate..to Fender
Not really,,it sounds very different when you are standing right next to it,as opposed to in the crowd
