OHMS?

June 5, 2006 12:51 PM

Amp/ Cabinet compatibility filter: Help with the OHM!

I have a Kustom 250 head that says "3-4 ohms output". This connects to two seperate cabinets, one 4 ohms and one 8 ohms (2.67 ohms total.)
Will this fry my head?

posted by cometwendy to technology (10 comments total)

Connected in parallel that would be 12 ohms, and pushing it. Connect in series and it should be 6 ohms. Probably not too far out of range.

(Parallel is splitting each wire to run to both, series is looping one wire through both and back again.)
posted by StickyCarpet at 12:58 PM on June 5, 2006

Oops, what you said. But .33 OHMs should be within tolerance. When I measure nominally 8 OHM speakers they are often closer to 6 OHM.
posted by StickyCarpet at 1:01 PM on June 5, 2006

StickCarpet... I'm thinking you're wrong on this. Sorry. :-S

Parallel resistances reduce the combined resistance, series resistances increase it. And the resulting parallel resistance will always be lower than the lowest resistance joined, just as the resulting series resistance will always be higher than the highest resistance in the chain.

If you're not pushing the amp it probably won't blow up, but if you are going below 3 Ohms (as it states) you will be risking damage if you drive the amp too hard, as it will dump more current out than it can handle properly. Of course, a well designed amp will have internal shutoffs (fuses, sometimes resettable, etc.) to handle this. Hopefully yours does!
posted by shepd at 2:14 PM on June 5, 2006

shepd: yeah, I wasn't thinking clearly at that moment. Funny, never happened before.
posted by StickyCarpet at 2:34 PM on June 5, 2006

You probably are OK. Does the amp have shutdown circuitry? I would hate to rely on that, but it should save you if it is there and this amp truly can not handle this low of load. Does the amp get hotter than normal hooked up to both speakers?

I still probably wouldn't do it. Would you really want to have it shut down during a concert? "Ummm sorry folks, we are going to have to stop right here in the middle of this song and take a twenty minute break until our amps cool down." Of course, it could be worse.

You could use a one ohm, 100 watt resistor in series with the speakers but that would probably muffle and kill the sonics.

If you do decide to use this combo you might consider putting a cooling fan into the head if it doesn't already have one. It's all about heat when going this low in load.
posted by caddis at 5:32 PM on June 5, 2006

When you talk about speakers and amplifier ratings, "8 ohms" is impedance, not resistance. You can't measure impedance with an ohm-meter.

Actually, you can't express impedance with a number, either; the proper way to express impedance is with a curve. But what they usually do is to choose an average value on the curve and write that number down.

That said, the problem with using an ohm-meter is that it's using DC (i.e. zero hertz), and the impedance at zero hertz isn't very interesting. (In fact, by definition it isn't impedance at all, which is why the measurement device is called a "resistance meter", not an "impedance meter".)

Generally speaking, the value measured at zero hertz will be lower than the average impedance, because after the first few milliseconds the speaker coil is charged and doesn't fight you any longer. The speaker inductance is the primary component of speaker impedance (and the part which varies by frequency), but what the ohm-meter measures is the resistance of the wire in the voice coil, without the effects of the speaker coil inductance.

If the speaker impedance is substantially lower than the amplifier impedance rating, then it means the speaker will try to draw a lot more current than the amplifier really expects it to draw at a given output voltage. The speakers will be louder at a lower "volume" setting, and you'll start to clip a lot sooner.

If the speaker impedance is higher than the amplifier rating, then the speaker will draw less current than you'd like per output voltage. The sound will be much more quiet even with the volume turned up quite high.

Depending on the speaker and amplifier designs, there can also be effects relating to frequency response, but the main effect is volume.

Having the speaker impedance be too high won't damage anything. Having the speaker impedance be a lot lower than it should be (i.e. 20% of the amplifier impedance rating) can potentially smoke the amplifier final stage if the amplifier is badly designed and if you really massively overdrive everything for long periods of time.

If the rated impedance of the speaker and amplifier are within 50%, then there should be no particular problem. An exact match isn't required.
posted by Steven C. Den Beste at 6:32 PM on June 5, 2006

A little off-topic, but note that the ohm measurement on speakers is an impedance measurement. That is, it's a measurement not of just resistance, but also of capacitance (low for speakers) and inductance (high for speakers). So when you put a multimeter across an 8 ohm speaker and measure only 6 ohms resistance, that's to be expected. It's a bit confusing that impedance and resistance have the same units. It's even more confusing when you learn that impendance changes with frequency, and the value given is a typical or average value.
posted by todbot at 6:37 PM on June 5, 2006

This question is flawed. Consider a standard speaker cable. It has two ends. Each of them is a quarter-inch plug. If you plug one end into a Kustom 250 speaker output labeled "3-4 Ohms," that leaves exactly one end free to plug into a cabinet. If you plug that into a cabinet labeled 4 Ohms (the right thing to do, incidentally), you are then left with exactly zero free speaker cable ends to plug into a cabinet labeled 8 Ohms.

So what are you doing, exactly? Where are the plugs going? How many of them are there? What kind of cracked-up jury rig have you got going where you think you might have connected your cabs in parallel? What kind of speaker cones, how many, how are they rated, and how are they connected inside their individual cabs; and how high do you tend to push your master volume? Are you playing bass or guitar through this rig?
posted by ikkyu2 at 6:51 PM on June 5, 2006

A few answers.
Kustom 250 head is plugged into an svt 410 hlf bass cabinet. This has two 1/4 inch jacks on the back. I was told that when plugging another speaker cable into this and connecting it to another (bag end 2 10inch speaker cab for bass at 8 ohm) that this is actually being connected in parallell and not in series as one might assume, due to the way it's wired.
Also, I am getting more volume at a lower setting, and the head does seem to be getting hotter than usual. The real question is if anyone knows whether a Kustom head (solid state from the early 70's) can handle it? The way I figure it if there is a margin of error it might work. If it goes down to 3 ohms and I'm plugging it into 2.67 ohms of cab's.
posted by cometwendy at 7:11 PM on June 5, 2006

You're right about your amp dual outputs being wired parallel; this is how most amps are wired.

I don't have any experience with Kustom bass heads, but my gut feeling is that you're risking melting your output transformer with this setup.

If I were you, I would internally rewire one of my cabs, changing a parallel connection into a series connection (in a balanced fashion, so no cone gets an excess of current), in order to increase the total ohmage of the setup.
posted by ikkyu2 at 10:58 PM on June 13, 2006