Resistance! Lets get my Ohms Straight!

[Raskolnikov]

I don't remember my physics class well enough to do the figuring here...

My Sunn 1200s is 1,200 watts 2 ohms minimum total.

My speaker cab is 8 ohms. If I were to add a second speaker, how would/can this work?

PS: there are four speaker outs in parallel.

[zepp_rules]

i just took that test is physics today...

however, i think i failed it, so sorry no help here

[Lordathestrings]

Resistances that are connected in series are simply added.

The more common connection is in parallel. The total resistance Rt is calculated as;

1/Rt = 1/R1 + 1/R2 + 1/R3 + ..... + 1/Rn

For example:

Two 8 ohm cabs in parallel gives you:

1/Rt = 1/8 + 1/8 = 1/4
Rt = 4

An 8 ohm cab in parallel with a 4 ohm cab gives you:

1/Rt = 1/8 + 1/4 = 3/8
Rt = 8/3 = 2 2/3

Series/parallel connection gets a bit tricky, but if you break it up into subsections and then combine them, it makes sense. For example:

A 4-speaker cab usually has 4 identical speakers in it, connected as two separate pairs in series, which are then connected together in parallel. Lets assume that all four of them are 8 ohm speakers.

So, for each pair:
8 + 8 = 16 (remember, series resistances add)
connecting two 16 ohm pairs in parallel gives you:
1/Rt = 1/16 + 1/16 = 1/8
Rt = 8

Note that I use the word resistance here, instead of impedance. The two terms are not interchangeable. However, for estimating the net load a speaker setup will present to an amp, these calculations are close enough.

Matching the load to the output transformer setting is very important for keeping a tube amp healthy. Since the Sunn 1200s has a transistor output stage, matching is much less critical, as long as the load is not less than 2 ohms. The above example of a 4 ohm and an 8 ohm cab in parallel come dangerously close to this value, but you can see that two 8s or an 8 and a 16 is just fine.

[Lordathestrings]

I'd like to add a few words about cables while we're talking speaker impedances. Some things get overlooked.

NEVER use a guitar cord to connect an amp to a speaker cab!!!

Guitar cables are designed to carry the signal produced by a couple of pickups. A set of active pickups with a preamp turned up full might kick out a couple of volts. The impedance of the amplifier input circuit would limit the current flow to much less than a milliamp. Passive pickups produce even less.

Raskolnikov's amp is an extreme example, but the numbers are fun to play with. 1200 Watts into a 2 ohm load implies 49 Volts at 24.5 Amps! To deliver that power to the speakers you need at least #10 AWG wire. Anything smaller will just piss away those Watts as heat. This would not only be frustrating, but also potentially dangerous. A guitar cable wouldn't have a chance! With very low impedance speaker systems, the cables can become a major part of the total load. And you don't need shielded wire!

Don't get led down the garden path by some sales-type with a set of high-priced 'golden' cables. For anything less than 200 Watts, a twisted pair of #14 AWG wires less than 20 feet long is ideal. For lower power, or shorter runs, a #16 AWG 'lamp cord' is just fine.

[Raskolnikov]

Thanks! Some of that hurt, but I think I'll abosorb it before I can afford another cabinet.

EDIT: Oh, and they mentioned that daisy chaining cabinets together would be a very bad thing for me to do with this amp.

[Edited by Raskolnikov on 02-25-2002 at 10:14 PM]

[Raskolnikov]

Originally posted by Lordathestrings
Raskolnikov's amp is an extreme example, but the numbers are fun to play with. 1200 Watts into a 2 ohm load implies 49 Volts at 24.5 Amps! To deliver that power to the speakers you need at least #10 AWG wire. Anything smaller will just piss away those Watts as heat. This would not only be frustrating, but also potentially dangerous. A guitar cable wouldn't have a chance! With very low impedance speaker systems, the cables can become a major part of the total load. And you don't need shielded wire!

The speaker cable they gave me with the amp is as big around as the power cord you plug into the wall...

[Lordathestrings]

By 'daisy chaining' cabs, you end up with a bunch of speakers in parallel. As the examples show, it wouldn't take long to get close to that 2 ohm lower limit. I recommend getting an 8 ohm cab to go with the one you already have. That way, you can freely use one, the other, or both together, without worry.

[Lordathestrings]

Originally posted by Raskolnikov
...The speaker cable they gave me with the amp is as big around as the power cord you plug into the wall... Take a look at the actual wires. They should be almost 1/8 inch in diameter. You'll be needing some serious connectors, too.

(I'm still shuddering at the thought of standing in front of speakers that are being fed that much power... Its been good to know ya! :D

[Incidents Happen]

Yikes! it is true!


You do get wiser with age!

[Lordathestrings]

... a combination of clear thought and sound judgement.

These characteristics are usually gained through a process of collecting experiences.

Experience is often gained by not thinking clearly, and exercising faulty judgement! :D

[Raskolnikov]

Originally posted by Lordathestrings
(I'm still shuddering at the thought of standing in front of speakers that are being fed that much power...

You'd be stunned at the back massage they give if you lean against them while somebody plays a nice fast slap bass line.

[Lordathestrings]

Originally posted by Raskolnikov
You'd be stunned at the back massage they give if you lean against them while somebody plays a nice fast slap bass line. I'm thinking about the vertigo and disorientation resulting from having my inner ears reduced to frothy mush. :eek:

[Akira]

*resurrects thread*

Hey guys, i'm looking for a 2x12 cab for use with my Crate GX212 120W Solid State Amp. I heard knowing something about ohms is important to understand which cab is right for the amp, but i'm confused by all these terms and how to calculate whatever i'm supposed to be calculating. I need help on this. Sorry for such a stupid question. :o

Thanks in advance. :o

[Lordathestrings]

As I said earlier in this thread, transistor amps aren't as fussy about matching the load. The only critical factor is the minimum load impedance. What is the spec on your amp?

[Akira]

As I said earlier in this thread, transistor amps aren't as fussy about matching the load. The only critical factor is the minimum load impedance. What is the spec on your amp?

Found this online:

"GX-212 AmplifierCrate continually develops new products, as well as improves existing ones. For this reason, thespecifications and information in this Crate manual are subject to change without notice.

Output Power Rating: 115 watts RMS @ 5% THD 4 ohms load 120VAC

Speaker Size and Rating:GX-212: 2 Crate Custom 12”, 8 ohms

Input Impedance: 470k ohms

Maximum Input Signal Level Accepted: 7 volts, peak-to-peak

Total System GainOverdrive Channel: 110dB all controls @10, Gain 2; 88dB Gain 1 Clean Channel: 58dB all controls @10

Overdrive Channel: Low Control:11dB Range @ 80Hz, Shape Control: Proprietary Circuit, High Control:10dB Range @ 10kHz

Clean Channel: Low Control:22dB Range @ 80Hz, Mid Control:14dB Range @ 600Hz, High Control:28dB Range @ 10kHz

Input Power Requirements120 VAC, 60Hz, 95VA100/115VAC, 50/60Hz, 95VA230VAC, 50/60Hz, 95VA

Cabinet Size and Weight:GX-120: 17-1/2” H x 19-1/4” W x 9-3/4” D, 44 lbs.GX-212: 21” H x 26-1/2” W x 11” D, 50 lbs."

The thing is i'd like to know what all of this means. Maybe Doc could do an "idiots guide to ohms" or something. I really do not know much to do with this sort of thing, I never was that good at electronics in school; it's an area that i'd really like to improve upon.

[Lordathestrings]

These two lines are what you need;

Output Power Rating: 115 watts RMS @ 5% THD 4 ohms load 120VAC
There's usually a spec for the minimum load impedance, but in this case we can assume that 4 Ohms is it. The 120 VAC is the mains power, not your amp output.

Speaker Size and Rating:GX-212: 2 Crate Custom 12”, 8 ohms
This shows that the standard cab has two 8 Ohm speakers wired in parallel to give a 4 Ohm net impedance.

So you can use any 4 Ohm cab that will handle 115 Watts rms. Because your amp has a transistor output stage, you can also use any 8 or 16 Ohm cab, but the amp will deliver less power to a higher impedance load. If you decide to run two cabs, they will be connected in parallel. You can use the formulas I posted earlier to calculate the net impedance of these cabs. Just be sure that the net impedance is at least 4 Ohms.

[Akira]

Thanks for the info.

I'm confused about some of the terms you used though. :o

[Akira]

A-ha!! Got it sorted. I talked to a friend of my mums about it who plays bass, and who knows alot about this sort of stuff. He helped me gain more insight into my problem and consequently I now know what sort of cab I need for my amp. :)

Thanks for the info too. I think a nice addition to the site would be some sort of tutorial on this sort of stuff. Anyone else agree?

Thanks. :D