Still don't get the ohm/impedance deal

[sr2000]

Not new to music, but new to guitar - so I am not completely understanding the purpose of 4ohm vs 16ohm. I know that when I plug my RM100 from 16 ohm to the 16 ohm input on my Randall cab, I'm getting a solid sound. When I plug the 4ohm output into the 4ohm input - no sound. Maybe the 4ohm option is completely irrelevant - can anybody clarify this?

 

[JKD]

All you need to know about impedance is that you should match whatever you set the output on the head to whatever the input on the speaker is...that said:

It sounds as if the jack plate on your cabinet is screwed up.....

All the Randall 4x12 cabs I have owned are wired with 16Ohm speakers so each side has pairs of speakers wired in parallel to make a total of 8Ohms per side for Stereo operation.

For the amp to run on mono, thees pairs are then wired in parallel or series for a total of 4 or 16 Ohms (Mono) depending on which side you plug into.

Stereo - 2 speakers on each side working independently
Mono - all 4 speakers working together.

If the 4 Ohm input isn't working then that's really weird...are you getting sound from all four speakers in 16Ohm mode?

Can you put the cab into 'Stereo' mode and check each side running at 8Ohms...make sure there's sound from both sides?

 

[sr2000]

Good to know that the main thing is to match the impedance from the head to the speaker - I had assumed that, but couldn't tell for sure. Thx

I've only used it in mono so far - hadn't checked to see if all 4 speakers are putting out sound - but I'll check that next

Even if it were to work, what would be the advantage of using the 4 ohm input/output instead of the 16?

 

[JKMV12]

the matching of impedances comes from a concept in electronics called maximum power transfer. Basically if you have any circuits (like this amp circuit for example) you can simplify it down to just a voltage source in series with a theoretical device with the total impedance of the circuit. This is called a Thevenin Equivalent Circuit. You can also simplify it to a current source in parallel with a device with the total impedance of the circuit. That is called a Norton Equivalent Circuit. There are various methods to find the values for these elements depending on the nature of the circuit. So to get maximum power transfer from the thevenin equivalent circuit to the load, the impedance of the load has to be equal to the complex conjugate of the total thevenin impedance. I know....what the hell is a complex conjugate?? It is a term in complex math...here is an example.

16+5.5i (which in circuits refers to a 16 ohm resister in series with an inductive element with 5.5 ohms of reactance)
16-5.5i (which is a 16 ohm resister in series with a capacitive element with 5.5 ohms of reactance)

I only know the basics so i could be applying some of these concepts the wrong way...but the concept of maximum power transfer is the reason why you match impedances.

EDIT: if you are confused about what impedance and ohms are...impedance is a term that describes how much resistance a circuit element gives a flowing current. ohms are the units used to measure impedance. A simple example is water being poured into a funnel. This funnel is restricting the flow of the water much like a resister is restricting current flow. electrical current is a term that describes how much electric charge flows through a single point over a certain amount of time. more specifically it is the derivative or rate of change of electric charge with respect to time.

 

[kc2eeb]

JKMV12, Excellent!
It's also OK, with tube amps, to run a amp into a higher impedance than
the output transformer is wound for, but, NOT THE OPPOSITE.
You can run the amp impedance selector at 8 ohms into a 16 ohm speaker
but not 16 ohms into a 8 ohm speaker.

 

[JKMV12]

JKMV12, Excellent!
It's also OK, with tube amps, to run a amp into a higher impedance than
the output transformer is wound for, but, NOT THE OPPOSITE.
You can run the amp impedance selector at 8 ohms into a 16 ohm speaker
but not 16 ohms into a 8 ohm speaker.

Thanks! I guess that basic Electric Circuits class last semester did me some good. Too bad i won't be learning more b/c i am going into mechanical/aerospace and not electrical. The good stuff i will have to learn on my own.

an impedance mismatch where the load (speaker) impedance is lower than the thevenin impedance will overload the speaker, correct? depending on how much lower it is, the speaker will either instantly fail due to fracture or eventually fail due to accelerated cyclic fatigue. a higher load impedance is alright b/c all it is doing is resisting the source more. doesn't this result in a lower output?

 

[kc2eeb]

In 1950, yeah, maybe. Today, and in the recent past, a 100 watt rms
rated 8 ohm speaker run with a O.T. set at 16 draws more current but
with todays coil form material and cone construction it could probably easily
handle it. The transformer would draw more current and get hotter.
A 16 ohm speaker just has more turns around the coil.
Also the ohm rating is nominal and frequency dependent.
An 8 ohm speaker will typically be up to 16 ohms in a narrow frequency range and as low as 4 in another.
What kills most speakers is heat. And......distortion. A distorted signal, yes
like a high gain channel, is less like a sine wave and starts to approach
DC. That makes heat. That's why guitar speakers had to be "invented."
And yes, a higher impedance speaker will produce lower output and the frequency response can also be different, and both are not necessarily a bad thing.

 

[GuitarGod87]

I haven't read the other replies, but this should help a little. I think, lol. http://www.youtube.com/watch?v=sdjLPkJcxXM

 

[Daryl]

I haven't read the other replies, but this should help a little. I think, lol. http://www.youtube.com/watch?v=sdjLPkJcxXM

I haven't seen that video in years. That is a good primer for someone wondering about impedance loads.