Impedance?

[Drei]

First of all, I apologize in advance if this question has been answered before, but I tried some searches and couldn't get the answer I needed.

I'm a fairly new bassist, and have been looking to upgrade from my little practice amp. I have a pretty good understanding of wattage, but I also noticed that there are impedance ratings given to amps. I was just wondering what the difference between 8 ohms and 4 ohms is, what that does to the sound/power, and why one might be prefereable to the other.

 

[BenderR]

Impedance is a characteristic of an AC circuit that accounts the DC resistance of the circuit as well as the capacitance and inductance of the circuit which only come into play with AC and are affected by the frequency of the AC.

All you need to know is that the impedance of you speakers must match you amp's output needs. With solid-state amps this becomes critical. Most tube amps will tolerate some level of impedance mismatch but SS can't handle it. Some speaker cabinets are switchable so that they can be matched to a variety of amps. I wouldn't suggest one impedance over another, just make sure that you can match the impedance of your speakers and your amp.

There are differences in output power if the same amp is driving a different load. An amp that produces 100 watts into 8 ohms will produce 200 watts into 4 ohms. This is actually very little difference in volume because the relationship of volume to power is logarithmic and not linear. The effect of doubling the wattage is barely discernable.

A greater factor in volume is speaker sensitivity which is measured in Decibels per Watt measured at one meter. The higher the sensitivity the more volume. A 100 watt amp playing through a set of speakers that are rated at 98 db/watt would be percieved as ten times louder by just going to 99 db/watt speakers. This would be the same effect as going to 10,000 watts and keeping the same speakers.

 

[MuzikMan]

That was WAY more information than asked and is riddled with inaccurate statements.

The impedance ratings on amps are basically indications of what load is acceptable and what wattage the load will produce.

The load is provided by the speakers.

Lets say an amp is rated at 350watts @4ohms and 200watts @8ohms. If you hook up a speaker cab (or cabs) that provide a combined load of 8 ohms, the amp will provide 200watts to the speakers.

In this example a 4 ohm load is the minimal safe load for the amp. If you tried to go lower the amp will provide more watts but will be in danger of overheating and damage.

 

[BenderR]

The ratio of load to output is just that, a ratio. I'm not talking about ratings here, I'm talking about Ohm's law.

MusikMan: If you've got something personal send me a private message. Otherwise, why not keep it polite.

 

[Joris]

.. with inaccurate statements.

Hardly. I'd say it was a pretty accurate description of impedance and related subjects.

I agree on the more info than the original posted asked for.

One doesn't have to understand impedance to be able to work with it. All you need to know is whether everything matches. Keep the (combined) load equal to or higher than the minimum rating of the amp. Lower impedances usually give discernably higher amp output power. However, adding speakers is a better way to increase sound pressure.

 

[Muhwi]

More accurate info about calculating Ohm loads can be found with the search option, but the basics you'll probably need the most:


The load produced by two 8 Ohm speakers wired in paraller:

8 Ohm + 8 Ohm = 4 Ohm

The load produced by two 8 Ohm speakers wired in series:

8 Ohm + 8 Ohm = 16 Ohm

 

[BenderR]

If I went to deep with my answer I apologize. I was simply trying to cover the end of the original post regarding "what the difference between 8 ohms and 4 ohms is, what that does to the sound/power, and why one might be prefereable to the other". Pardon my passion for the subject but designing circuits is what I do and I love it.

 

[Bob Lee (QSC)]

All you need to know is that the impedance of you speakers must match you amp's output needs. With solid-state amps this becomes critical. Most tube amps will tolerate some level of impedance mismatch but SS can't handle it. Some speaker cabinets are switchable so that they can be matched to a variety of amps. I wouldn't suggest one impedance over another, just make sure that you can match the impedance of your speakers and your amp.

Most of your message is good, but this paragraph IMHO is a little off. Tube amps must have load impedances approximately equal to what they are set up for. Many will have multi-tapped output transformers to allow you to select the output that is optimum for the impedance of your loudspeaker(s). Trying to drive too high a load impedance may damage some amps.

Solid state amps can handle any load impedance equal to or higher than their minimum. Into higher impedances, however, a SS amp will put out less power with any given gain settings and input signal level. That is because they put out a signal voltage proportional to the input signal voltage, and the current they put out is proportional to the output voltage and inversely proportional to the load impedance; power is a function of the output voltage and current. But there is no danger in driving high impedances. Instead, driving load impedances that are too low may trigger current-limiting protection in better amps or cause damage in more poorly designed amps.

 

[notanaggie]

Hardly. I'd say it was a pretty accurate description of impedance and related subjects.

And I agree with Bob that SS is quite tolerant of "mismatch". In fact, it hardly exists as a meaningful term with SS.

"Mismatch" CAN occur with a tube amplifier. The result may be anything from no effect to amp or tube damage. Depends on the amplifier.
Some tube amps sound better with a 4 on the 8 tap, or an 8 on the 4 tap. Others might be damaged.

So that statement was not only inaccurate, it was precisely *opposite* to the facts.

As far as sound, aside from the effects of mismatching, the impedance has no direct influence on sound. It will affect power in an SS amp, but not on a tube amp with suitable taps.

It may be difficult to physically make a 4 and an 8 ohm speaker exactly equal in sound qualities, due to some technical reasons. But that is different from a direct effect of the impedance.

The power with an SS amp will depend on speaker impedance, lower normally giving more power, if the amp can handle it.

A tube amp with taps for two impedances will give essentially equal power at either.

So pick your speaker impedance to give a suitable amount of power with your SS amp.

For a tube amp, I would pick the highest impedance that a tap is available for, typically 8 ohms. That way you can add another 8 ohm speaker later, and have a 4 ohm combination, for instance.
(It won't add power, but it may improve sound.)