Amp Question

[alex_dorsten]

What do the ohms mean on both the amp head, and the cab? What amp head ohms go with what cab ohms?

[Swampy]

Re: Amp Question

The matching ones.

4 to 4
8 to 8
16 to 16

[alex_dorsten]

Re: Amp Question

But does a different ohm head line up with a different ohm cab? Or will it hurt the head?

[Swampy]

Re: Amp Question

If it's a tube head, matching the ohms is important. Example- If you don't have a 4 ohm out on your head, and the cab is 4 ohm, I wouldn't. Some say you can go one way, up or down....can't think of which, and you'll only wear out tubes quicker.

[beaubrummels]

Re: Amp Question

Technically, the cab needs to be equal or lower ohms rating.

If the cab is higher resistance than the amp, you'll wear out your amp fast. If the cab is lower resistance than the amp, there's a possibility you might blow out some speakers or shorten their life. The wattage rating and RMS also matter as to whether damage will occur.

It's wiser to match both amp and cab with the same values all around.

[alex_dorsten]

Re: Amp Question

Does the wattage between an amp and cab matter?

[Brandenburg]

Re: Amp Question

Does the wattage between an amp and cab matter?

yes.. you generally want a cabinet to hold more wattage than the amp is capable of producing... mix matching Impedance will change how much your speakers can reliably handle as well

[dystrust]

Re: Amp Question

Technically, the cab needs to be equal or lower ohms rating.

If the cab is higher resistance than the amp, you'll wear out your amp fast. If the cab is lower resistance than the amp, there's a possibility you might blow out some speakers or shorten their life. The wattage rating and RMS also matter as to whether damage will occur.

It's wiser to match both amp and cab with the same values all around.

I'm pretty sure you've got that backwards. For example Mesa Mark series amps don't have a 16 ohm tap on the output transformer, but their manual states that it's perfectly safe to run a 16 ohm cabinet on one of the 8-ohm outputs. OTOH it also says to never hook it up to a load less than 4 ohms.

[aleclee]

Re: Amp Question

I'm pretty sure you've got that backwards. For example Mesa Mark series amps don't have a 16 ohm tap on the output transformer, but their manual states that it's perfectly safe to run a 16 ohm cabinet on one of the 8-ohm outputs. OTOH it also says to never hook it up to a load less than 4 ohms.

As a general rule, BB had it right. The way to remember is that it's not safe to run a tube amp with no load (infinite impedance). Higher is less safe for tube amps and the opposite for solid state.

[koshikas]

Re: Amp Question

I'm pretty sure you've got that backwards. For example Mesa Mark series amps don't have a 16 ohm tap on the output transformer, but their manual states that it's perfectly safe to run a 16 ohm cabinet on one of the 8-ohm outputs. OTOH it also says to never hook it up to a load less than 4 ohms.

this is correct, from what i know. the output impedance (head) <= input impedance. resistance and impedance are not the same thing though both are measured in ohms, the no load scenario should not be explained from a resistance point-of-view. if the head output impedance is higher than that of the cab, thats when you run the risk of frying the OT of the amp. if the cab has the higher value, then your amp/cab will not be operating at its best efficiency (power loss), thats all.

as for the wattage rating , if the amp in question is a tube amp, then the rule of the thumb is to use a cab with rated wattage that is almost twice as the output wattage of the amp to handle high operational transients.

[aleclee]

Re: Amp Question

I'll stand by my earlier assertion:

http://www.hugeracksinc.com/forum/viewtopic.php?t=87063&p=843991

[PFDarkside]

Re: Amp Question

As a general rule, BB had it right. The way to remember is that it's not safe to run a tube amp with no load (infinite impedance). Higher is less safe for tube amps and the opposite for solid state.

As a student of solid state electronics I never understood this... Dealing with voltage or current amplifiers, ohms law still holds. V=IR. P=V^2/R, as resistance decreases, power (and heat) increases. A dead short kills power transistors.

Then I realized that amplifier designs with output transformers (like tube amps) function in a different manner. The output transformer "reflects" the load presented back on the power tubes. As the speaker impedance decreases, effective load on the power tubes increases. Conversely, as speaker impedance increases (toward the proverbial "open" or infinite ohms) the reflected load on the power tubes goes to 0 (short) which is the damaging case.

That's an incredibly simplified version, power tubes don't function like power transistors, even considering the output transformer. However, it's a good way to understand the rule of thumb not to present a tube amp with no load.

That all said, I still only match amplifier to load impedance.

[DrNewcenstein]

Re: Amp Question

Concerning multi-speaker cabs, if you have a 2x12 with each speaker rated for (hypothetically) 50w, does the method of connecting them (i.e. series or parallel) affect their power-handling capability? Say, 2x50w in series = 100w, while 2x50w in parallel = 50w max?

My Carvin 2x12 states a 300w power rating (though I'm sure that's maximum sustained burst, and not recommended to have 300w running into it for several minutes at a time).

[PFDarkside]

Re: Amp Question

Total power handling always sums. Two 50W RMS 8ohm speakers will always add to 100W RMS, regardless of if you wire them in series (16ohms) or parallel (4ohms). Your 300W figure could be RMS or Peak Wattage, best to check the actual speakers.