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  #1  
Old 2007-04-17, 03:25
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Sycophant Sycophant is offline
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A guide to matching impedances/setting up a Solid State rig

Okay, so I wrote sort of a little guide regarding all this SS head/ohms/cabs/wiring stuff. I certainly didn't know any of this stuff before I decided I wanted a VH140c, and alot of you dudes helped me out in understanding what the deal is, so for future reference I hope this helps anyone interested so more people won't be discouraged from getting a killer early 90's Crate or Ampeg, Randall Cyclone or any similar solid-state head. Hopes this helps you as well Peter.
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A guide to matching impedances/setting up a soild-state amp rig:

1) I'm looking at the back of my amp head, what the hell does all this stuff mean?

The back of the amp will tell you what the lowest resistance that the amp wants to see hooked up to it. So using a Crate GX-130 head as an example, it says:

65W RMS @ 8 Ohms
4 ohm Min. Load

The RMS and ohm information applies to each poweramp of the head. Each poweramp in the head dishes out 65 watts for a combined wattage of 130. Each poweramp has 2 outputs, and each one is rated at 4 ohms (hence the "@ 8 ohms" label.) The minimum load is the load where the amp puts out the most power. So what this is saying is that the head wants no lower than a 4 ohm load. You don't want to go below the minimum that the amp tells you.

2) Understanding the outputs:

These soild-state heads we're talking about have stereo outputs. Usually the outs look like:

LEFT RIGHT
LEFT RIGHT

The Ampeg VH140c's outs look a bit different.

Basically we've got two left outs and two right outs regardless of the amp.
As you can see from the pic of the VH140c's back panel, the RMS and ohm information applies to each stereo side of the head. So using the Crate GX-130 as an example, each two pairs of outs is rated at 65 watts RMS with a minimum 4 ohm load (the VH140c's is the same actually, just add 5 watts.) Stereo heads like these need stereo cabs. Stereo cabs have two mono inputs. So, you'll need two cables for your setup, one going from one of the head's LEFT channels into one of the stereo cab's inputs and one from the RIGHT going into the other input on the stereo cab.

3) Okay, now onto the cab wiring:

Here's where things get odd, and I'll try to make it as easy as possible.
Remember when I said that the minimum load is the load where the amp puts out the most power? This is why you need to make sure you're getting the right cab for your soild state head. The GX-130 and VH140c's back tell you that the minimum ohms for each side are 4. Now let's look at the back of a MESA Rectifier 4 X12. We see there are two mono inputs rated at 4 ohms each, it also tells us that these are split for stereo. If the back of our amp heads tell us each sides minimum ohm rating is 4, and we have this cab wired stereo for 4 ohms, we have found the best cab for use with our head, because it matches the minimum ohm rating for the head.

Now you can kind of see what is going on.

- 4 OHM LEFT OUTPUT ------------------ AMP 4 OHM INPUT
SS HEAD
-4 OHM RIGHT OUTPUT ----------------- OTHER 4 OHM AMP INPUT

We're basically playing a matching game. All is happy and brutal if each side of our stereo solid state head's minimum ohm requirement is equal to the inputs of our cab.

4) What if I want to use two cabs?

Say you want to use your SS head with TWO cabs instead of one.
These heads usually have two poweramps, when combined, will give you the total wattage the head can push out. This is why you need either one cab with stereo inputs OR two regular mono cabs.

Why two regular mono cabs? Lets just say your head was rated at a minimum of 8 ohms a side. In that case, you would need two 16 ohm cabs, because two 16 ohm cabs makes an 8 ohm load to the amp, which satisfies the requirement of most power.

Most SS amp heads like the GX-130 or the VH140c are rated minimum 4 ohms a side. If we'd want to use two cabs with them, we'd need to get two regular 8 ohm cabs because two 8 ohm cabs together makes a 4 ohm load to the amp. We would not need them to be stereo because we've got cables going from each side (left + right) into each of the two cabs' single input.

As you can see from the good 'ol MESA pic, this cab has got an 8 ohm input. If we'd want to run our solid state head with two cabs we'd need to get two cabs with that same kind of input (most regular guitar cabs come with only one input rated @ 8 ohms anyway. Perfect example being the Marshall 1960 4 X 12.)

5) That doesn't make sense! If each output (4 outputs in total) on my Crate GX-130C is 4 ohms, wouldn't I want one left output of 4 ohms going to a 4 ohm cab and the right output of 4 ohms going to the 4 ohm second cab? Why 8 ohm cabs when each output (1 left and 1 right) is only 4 ohms?

Chalk this answer up to the laws of physics & electronics. Two 8 ohm loads are the same as one 4 ohm load. Just like:

one 8 ohm cab = two 16 ohm cabs
one 16 ohm cab = two 8 ohm cabs

If you added two 16 ohm cabs together you do NOT get a 32 ohm load, you get an 8 ohm load. It sounds ridiculous, but it's true, it's just how impedance works. The "matching game" that we saw work in one-cab situations does not apply to two cabs.

6) What if I only want to get one cab now but I want to add another later?

Most solid state amps will allow you to go above the lowest ohm load but usually not below the lowest ohm load. Check with the amps owners manual or contact the amp company for advice for their particular amp. If your amp has a minimum 4 ohm load, and you want to get one cab now and leave the door open to expand and add another cab later, then get one 8 ohm cab now and another 8 ohm cab later. When you run a single 8 ohm cab on an amp that can go down to 4 ohm, you lose a little power that only translates to about a 3 db drop in volume which is just barely perceivable, but you can expand and add another 8 ohm cab later which will not only give you more speakers to move more air, but your amp will then be at optimum power at the 4 ohm total load. (from www.avatarspeakers.com)

7) How does all this ohmage and stuff apply to tube amps?:

With tube amps it's really best to try to match the amps exact load requirement, because a tube amp with the wrong speaker ohm load connected to it could sound fine but could be stressing the tubes and transformer causing them to prematurely wear out.

8) What are some well-known albums that feature solid-state amp distortion?

I'm making this list because alot of people ask, and it was a big persuasion for me into exploring the world of solid-state amps.

Crate GX-130 : Jack Owen from Cannibal Corpse was known to use these.
Ampeg VH140c: Suffocation - Effigy Of The Forgotten & Pierced From Within. Gorguts - Considered Dead, Erosion Of Sanity and Obscura. Immolation - Dawn Of Possesion. Cynic - Focus. Dying Fetus, Assück, Misery Index, and Internal Bleeding used this particular amp on mostly all their recordings.

Thanks to Soeru, the_bleeding, Bloodsoaked666, Josh, Adam Quick (Vader Cabinets), Avatar Speakers and anyone else that helped or contributed to this guide.
___________________________________________

Last edited by Sycophant : 2007-04-17 at 21:35.
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  #2  
Old 2007-04-17, 04:47
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Good stuff, I was considering making a guide similar to this but geared towards a list old discontinued tube and solid state Peavey, Crate, and Ampeg amps that can be had cheap these days instead of the whole ohmage thing. Might wanna try changing the title to "Guide to matching impedances" rather than SS rig because many tube amps too particularly poweramps need to run in stereo. But otherwise it's a might damned fine thread.
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  #3  
Old 2007-04-17, 08:16
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Quote:
Originally Posted by Sycophant
4) What if I want to use two cabs?

Most SS amp heads like the GX-130 or the VH140c are rated minimum 4 ohms a side. If we'd want to use two cabs with them, we'd need to get two regular 8 ohm cabs because two 8 ohm cabs together makes a 4 ohm load to the amp. We would not need them to be stereo because we've got cables going from each side (left + right) into each of the two cabs' single input.


This does not make sence to me.

If each output (4 outputs in total) on my Crate GX-130C is 4 ohms would'nt I want one left output of 4 ohms going to a 4 ohm cab and the right output of 4 ohms going to the 4 ohm second cab?

Why 8 ohm cabs when each output (1 left and 1 right) is only 4 ohms?

If I was using 2 left outputs that would be 8 ohms and if I was usings 2 right outputs that would be 8 ohms but if I was only using 1 of each (1 left and 1 right output) that wold only be 4 ohms each.....this has got to be right.


Peter
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Old 2007-04-17, 08:23
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I don't know the exact reason, it's some law of physics/electronics. 2 8 ohm loads are the same as 1 4 ohm load. Just like:

1*8ohm cab = 2*16ohm cabs
1*16ohm cab = 2*8Ohm cabs

It sounds ridiculous I know but it's true, it's just how impedance works. You add 2 16 ohm cabs together and you do NOT get a 32 ohm load, you get an 8 ohm load.
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  #5  
Old 2007-04-17, 09:59
ThornsOfHeaven200 ThornsOfHeaven200 is offline
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This is very helpful. Thanks Sycophant.
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  #6  
Old 2007-04-17, 19:05
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I'm glad you guys like my little guide. I've edited the title of it (I can't edit the title of the thread) and threw in a few more things. When someone in the future starts asking all these impedance questions hopefully all they will have to do is read this to get a clear picture as to wtf is going on.

I know Peter, it's fucking wierd that the "matching game" doesn't apply when you use two cabs! It's some scientific thing. I've also added that question to the guide.
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Old 2007-04-17, 19:08
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Quote:
Originally Posted by Sycophant
I'm glad you guys like my little guide. I've edited the title of it (I can't edit the title of the thread) and threw in a few more things. When someone in the future starts asking all these impedance questions hopefully all they will have to do is read this to get a clear picture as to wtf is going on.

I know Peter, it's fucking wierd that the "matching game" doesn't apply when you use two cabs! It's some scientific thing. I've also added that question to the guide.


Very helpfull dude. I think I got a better understanding of it now!!!

Once I get my cab I will let you knwo how it sounds. You have the same Crate right?
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Old 2007-04-17, 19:39
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When I wrote this thread I was using a Line 6 PODxt Live into a Carvin DCM 600 poweramp into a old Marshall cab. I'm really glad you said that guide helped you out.

Last edited by Sycophant : 2013-05-17 at 18:18.
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Old 2011-04-23, 15:39
neptoess neptoess is offline
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OK. Necro thread, but the law of physics you're referring to is equivalent resistance with parallel resistors. It states the 1/Req = 1/R1 + 1/R2 + .... + 1/Rn. This means if you have 2 8 ohm loads in parallel (which is how two 8 ohm cabs would be) you have 1/8 + 1/8 = 1/?. So it's a 4 ohm load. HOWEVER, the amp has a minimum load of 4 ohms to each side. This means you need a 4 ohm load on the left AND on the right for max power output. So, if you have a 412 with 16 ohm speakers, wire them all in parallel, and you've got 1 4 ohm cab. Otherwise, if you have 8 ohm speakers, you can do two jacks. Each jack has 2 speakers connected in parellel, giving a 4 ohm load. Best option for 2 cabs is 2 4 ohm cabs, or 2 cabs with double jacks, each jack at 8 ohms. Tube amps and solid state amps also have no problems going above the minimum ohm requirement. You just get less output. For a GX-130C, for instance, you get 95 W per side @ 4 ohm, but 65 W per side @ 8 ohm. This is why tube amps and their hardcore output transformers are so awesome, because you can set them for 4, 8 , or 16 ohm output and it's always the same wattage.
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Old 2013-05-06, 19:34
necrotico necrotico is offline
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can u help m out???

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Old 2014-03-17, 05:16
laoniu laoniu is offline
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I know Peter, it's fucking wierd that the "matching game" doesn't apply when you use two cabs! It's some scientific thing. I've also added that question to the guide.

Last edited by Chris Rezendes : 2014-03-20 at 08:30.
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