Help with ohm cabs compatibility

Orange Amps Technical Q&A's

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Pinthar
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Help with ohm cabs compatibility

Post by Pinthar » Sun Feb 11, 2018 10:41 am

I’m not woodshop pro busting out in class with a bunch of technical terms to gain favor from the teacher and my peers. I’m not that dude. I’m not the hotshot. I’m the dude who doesn’t know too much about why amplifiers have different ohm rating outputs and different things like parallel and series. I thought you could just plug a speaker in a cab as long as the watts are low enough and it should work. Now I’m reading it’s not true and I can damage my hea if the ohms are not matching..ugh so, if you want to help read on, if not thanks for making it this far. Have fun in woodshop.

I own a Orange tiny terror with three outs on the back. A 16 ohm, and two 8 ohms. I also own a cheap Marshall MG15MSII micro stack. On the back of the Marshall amp it has two outs that says 16 ohms...for both speakers, and also a warning stating don’t use with loads under 8 ohms...according to the manufacturer, the Marshall cabs are both rated at 16 ohms each so I can either use one, or both....So I am thinking that each output on the back of the Marshall amp are neither parallel nor series...whatever that means...anyhow. They are just single 16 ohm outputs...yes or no?

I want to use the Orange tiny terror with both Marshall speakers...this is where the confusion comes in. Can someone draw me a diagram o something? I don’t want to blow up my orange head, and I sometimes need the smaller Marshall speakers when traveling. Normally I plug the Orange amp into my Bogner 2x12 using the 8 ohm output. Matching perfect. But...

I know it’s best to match ohms, but orange 8 ohms are wired parallel so plugging in two 16 ohm cabs will be 8 ohms...yes or no!

Why is this so complicated...

Rlw59
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Re: Help with ohm cabs compatibility

Post by Rlw59 » Sun Feb 11, 2018 12:02 pm

Quick simple answer -- if you want to use one Marshall 16 ohm cab with the Orange, plug it into the Orange's 16 ohm jack.

If you want to use both Marshall 16 ohm cabs with the Orange, plug each cab into the Orange's 8 ohm jacks.
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One thing that makes it confusing is different manufacturers label jacks differently.

Another thing that's confusing is the difference between solid-state and tubes.

On that solid-state Marshall, the two jacks are in parallel. You can connect one 16 ohm cab, two 16 ohm cabs, or one 8 ohm cab.

On the Orange, the two 8 ohm jacks are in parallel. You can plug one 16 ohm cab into the 16 ohm jack, two 16 ohm cabs into the two 8 ohm jacks, or one 8 ohm cab into one 8 ohm jack.

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Re: Help with ohm cabs compatibility

Post by Jondog » Sun Feb 11, 2018 12:31 pm

I liked woodshop...but we never did guitar cabs :cry:
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Rlw59
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Re: Help with ohm cabs compatibility

Post by Rlw59 » Sun Feb 11, 2018 12:48 pm

Now off to the woodshed:

A tube amp works most efficiently when the output impedance matches the speaker impedance. Both high mismatches and low mismatches lower output power and put more stress on the amp.

That's because tubes have extremely high output impedance -- thousands of ohms. They use a transformer in between the tubes and the speakers to match the impedances.

The simplest transformers are two separate coils of wire wrapped around an iron core. One coil is connected to the tubes, the other coil is connected to the speakers. When the electrons from the tubes move through the "primary" coil, they electromagnetically induce movement of the electrons in the "secondary" coil.

With a tube amp, none of the electrons in the tubes ever reach the speakers. The transformer is like a set of gears -- the movement of the electrons on the primary side make the electrons that are already in the secondary side move.

Other transformers have "tapped" secondaries. You can connect the speakers to the full length of wire in the secondary coil, or to a shorter length. Think of it like a 1-speed bicycle versus a 5-speed bicycle -- you can change the gear ratio.

Like a gear transmission, the two sides are interactive. When you change the speaker load (plug an 8 ohm speaker into the 16 ohm tap) you change the load on the tubes.

Think of the ttrasmission in your car. If you shift into first gear when you're going 100 miles an hour, your engine is forced to spin too fast and blows up. And if you're going 5 miles an hour and shift into fifth gear, your engine is forced to spin too slow and either stalls or lugs (possibly causing damage to the crankshaft bearings). So a big mismatch with a tube amp is very bad. Worst case is running a tube amp with no speakers connected. Some amps have "shorting" output jacks -- zero ohms with no speakers connected. That's like 5th gear at 5mph. Other amps have "open" output jacks -- infinite ohms with no speakers connected. That's like 1st gear at 100mph.

But if you're going 40 mph, you can use third, fourth, or fifth gear without damaging anything. So a slight impedance mismatch with a tube amp generally won't hurt.
Last edited by Rlw59 on Sun Feb 11, 2018 4:09 pm, edited 1 time in total.

Rlw59
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Re: Help with ohm cabs compatibility

Post by Rlw59 » Sun Feb 11, 2018 1:18 pm

Solid-state is completely different. Transistors have low output impedance, so they're connected directly to the speakers. The electrons that flow out of the transistors flow through the speakers.

The lower the impedance of the speakers, the higher the flow of electrons. The amp puts out more power into low impedances and less power into high impedances.

If you short circuit (zero ohms) the output of transistor amp, the transistors attempt to put out infinite watts. They'll be limited by the power supply's available power so they can't achieve infinite watts -- but fuses will blow or things will melt.

But an open circuit (infinite ohms) simply shuts down the transistors. No electrons flow, zero output power. So generally you can safely run a solid-state amp with no speakers connected.

Solid-state amps have minimum impedance requirements. They put out maximum power into that minimum impedance. If you go lower, they exceed their design capability and overheat. If you go higher, they put out less power.

So, your Marshall puts out 15 watts into an 8 ohm load. If you connect a 4 ohm load, it tries to put out 30 watts and overheats. But if you only connect one 16 ohm cab, the amp puts out 7.5 watts and runs cooler.

If you wired your two 16 ohm cabs in series for a total load of 32 ohms, your amp would put out 3.75 watts. You can go as high as you want with impedance on a solid-state amp -- the only consequence is less output power.

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Re: Help with ohm cabs compatibility

Post by Rlw59 » Sun Feb 11, 2018 1:26 pm

Summing up:

Tube amp -- most power and least stress when ohms match. Infinite ohms (open circuit) is catastrophic. Zero ohms (short circuit) is usually survivable but can sometimes cause damage.

Solid-state amp -- the lower the speaker impedance the more power the amp puts out. Higher speaker impedance lowers the output power and reduces stress. Short circuit (zero ohms) is catastrophic. Open circuit (infinite ohms) is safe.

Pinthar
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Re: Help with ohm cabs compatibility

Post by Pinthar » Mon Feb 12, 2018 4:29 am

Thanks mate, I am now ready for woodshop and can use words like Mitersaw, dovetail joint, radial armsaw, and the coveted mortise-and-tenon. :D

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Re: Help with ohm cabs compatibility

Post by a.hun » Mon Feb 19, 2018 10:23 pm

Brilliant analogies from Rlw59 there. I've posted many times here on impedances with valve / solid state amps. About impedance matching always being best with valve amps, but if mismatching low direction mismatches being safer for the amp, etc etc.

Forget all that though, Billy should just sticky this thread!!! :D


Andy.
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Re: Help with ohm cabs compatibility

Post by bclaire » Tue Feb 20, 2018 11:40 am

a.hun wrote:
Mon Feb 19, 2018 10:23 pm
Brilliant analogies from Rlw59 there. I've posted many times here on impedances with valve / solid state amps. About impedance matching always being best with valve amps, but if mismatching low direction mismatches being safer for the amp, etc etc.

Forget all that though, Billy should just sticky this thread!!! :D


Andy.
The point missing though in solid-state is that you should never go lower than recommended. Trying to run a 2 ohm load for example from an amp that says 4 ohm minimum will send the amp into overload and shut down - you can have all kinds of problems if you do....

I could add Rlw59's insight to the current sticky that talks ohms... you do know that there is already a sticky that talks ohms, right?

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