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Guitar Amp Speaker Ohms


What Ohm Speaker Cabinet Can I Use With My Guitar Amp?

1x12-cb-750.jpgThis is the most common question I get asked!  The short answer is that you can plug any cab of any impedance into a Mack amp.  There is no combination of impedances that will cause damage to your Mack amp.  I realize that may sound heretical based on what I imagine you have read on the Net so please allow me to explain.


Why Are Mack Amps OK With Any Ohm Speaker Cabinet?

There has been a lot written about matching the impedance of the speaker to the amp's output and the potentially disastrous results if you don't.  However, if an amp's power is relatively low (say, under 50 watts), its design is sound and the output transformer and tubes are of good quality, there should never be an issue with mismatching impedances.

The speaker jacks in Mack amps are wired to the 8 ohm tap on the output transformer.  However because of the relatively low power output of our amps (under 50 watts) and the high quality of the output transformers (Hammond) and the tubes (JJ), they are perfectly happy to have any impedance speaker cabinet plugged in.  At the voltages and power levels that we run, the output tubes and the output transformer never come close to being damaged.

Mack Amps Speaker Ohm Testing

Because we get this question so often, we actually tested all or our amps by applying an input signal to the front of the amp (at a level higher than any guitar pickup could produce), turned the volume to '10' and left the amp running for over an hour... with nothing plugged into the speaker jack and then with a dead short across the speaker jack!  Theses tests simulated the absolute worst case scenarios.  

We took measurements throughout the test periods and confirmed that at no time did the output tubes or output transformers see voltages and currents anywhere near their maximum limits - they were just fine.  So, if you ordered any of our amps, you would get two jacks that can each be hooked up to any cab or speaker impedance.  

Mack Amps Are OK With Impedance Mis-matches, But Check With Other Manufacturers

I can't speak for other amp brands, but given that they meet the above criteria of sound design, etc., any amp should behave the same way.  However, before you mis-match impedances with another amp check with the manufacturer to make sure!!

Guitar Amp and Speaker Ohms and Impedance

Warning: skip this section if you really don't care about the details!!  No harm, no foul!

To make sense of this topic we need to know what 'ohm', 'impedance' and 'impedance matching' really are.

Ohm is a unit of electrical resistance.  Apply a voltage across a device called a resistor and there will be a certain amount of current that will flow.  Change the resistance and the current will change - more resistance, less current flow and vice versa.  The resistor 'resists' the flow of current.  How much resistance a resistor posseses is measured in ohms.

Impedance is sort of like resistance, but not quite.  This term is used to describe the electrical resistance of a device where its resistance varies according to the frequency of the signal that passes through it.  For example, an electrical device might have more resistance at low frequencies and less resistance at high frequencies.  The voice coil of a guitar speaker is such a device - the resistance of the speaker varies as the signal from the amp varies.

However, speakers are rated in ohms, which implies that they're impedance is not frequency dependent.  The ohm rating of a speaker is an industry standard convention that provides a nominal value with which to compare speakers.

Impedance matching can be defined as follows: "In electronics, impedance matching is the practice of designing the input impedance of an electrical load or the output impedance of its corresponding signal source in order to maximize the power transfer and minimize reflections from the load." -Wikipedia: http://en.wikipedia.org/wiki/Impedance_matching.

Generally, electronics designers worry about matching the output impedance of a device (in our case a guitar amp) with the input impedance (guitar speaker cab) so that maximum power is transferred from the upstream device to the downstream device.

Sounds like a good idea and it is.  But, what happens if you mis-match guitar amp and speaker impedances?

Three things occur: the power delivered to the load (speaker) will change, the tone will change and the electrical operating characteristics of the guitar amp's output transformer and tubes will change.

The change in power transfer is really not that noticeable so forget about that.  The tone change is noticeable if you listen hard and compare back and forth - this can actually be a useful experiment if you have different impedance cabs and/or your amp has an output impedance selector switch.

The third change, operating characteristics, is where the controversy comes in.  Some amps will not like mis-matches and may be damaged as a result.  However, read the above information again to find out how Mack amps don't, in fact, care about impedance mis-matches.