HANGAR18 Posted December 9, 2012 Posted December 9, 2012 Let's say that you have an amp with an output selector switch which will allow you to choose 4 ohms, 8 ohms or 16 ohms, AND you also have a speaker cabinet with an input selector switch which will allow you to choose 4 ohms, 8 ohms or 16 ohms. Obviously you will want both switches to be set to the same impedance for the most balanced output. But given that you can choose anything you want in this scenario, 4 ohm, 8 ohm or 16 ohm... What is your preference? The reason I ask is because I was talking to a guy who I believe has had many years of gigging experience and he said that he always chooses the 16 ohm setting because the amp doesn't have to work as hard and the tubes don't get as hot and the amp lasts longer and so on and so forth. So, in my quest to continue learning about amplifiers, I would like to learn what the majority of everyone's preference might happen to be and why. Thank you in advance.
DetroitBlues Posted December 9, 2012 Posted December 9, 2012 Never thought about it... Is there really a difference?
LK155 Posted December 9, 2012 Posted December 9, 2012 Um, well, that sounds backwards to me. Ohms are a measure of electrical resistance. 8 ohms is twice the resistance of 4, etc. I believe that's why amplifiers (at least in the audiophile world) usually are rated as being able to deliver more power (watts RMS) into 4 ohms rather than 8. And into 8 ohms rather than 16. Less electrical resistance, more work being done by the amplifier. Having said all that, I'm not going to be surprised if one of the electical engineers in the membership tells me I'm dead wrong. It wouldn't be the first time.
skydog52 Posted December 9, 2012 Posted December 9, 2012 More resistance = Less current flowing, given the same voltage. They are all directly related in this formula. P=Watts E=Electromotive Force or Voltage I=Current or Amps R=Resistance or Ohms
Blunote Posted December 9, 2012 Posted December 9, 2012 I'm not a that savvy about electricity but am pretty sure that less power is required to produce the same level of sound when the speakers are at a higher resistance. An amp rated at 25w/channel 4 ohm, would be rated 50w at 8ohm and 100w at 16ohm.
DetroitBlues Posted December 9, 2012 Posted December 9, 2012 So 100w 4ohm is going to be a really loud amp?
Kuz Posted December 9, 2012 Posted December 9, 2012 So 100w 4ohm is going to be a really loud amp? Yep, but probably not enough for you!!! ha, ha,....
mars_hall Posted December 9, 2012 Posted December 9, 2012 It doesn't matter. They sound the same if the amp impedance matches the speaker impedance. The switches are there to offer more choices in the use of extension speaker. In all cases, you must match the amp impedance to the connected speaker impedance. So you balance the amp impedance with the cabinet impedance. Only then is energy delivered by the amp balanced between what is internal and what is external.. If the balance is not maintained, more energy is dissipated within the amp and efficiency is lost. Problems arise when the amp can not dissipate the increase in wattage dropped internal to the amp. Unless the amp has thermal shutdown mechanisms in its output stage, the amp has the possibility of destroying or prematurely wearing its components.
Kuz Posted December 9, 2012 Posted December 9, 2012 It doesn't matter. They sound the same if the amp impedance matches the speaker impedance. The switches are there to offer more choices in the use of extension speaker. In all cases, you must match the amp impedance to the connected speaker impedance. So you balance the amp impedance with the cabinet impedance. Only then is energy delivered by the amp balanced between what is internal and what is external.. If the balance is not maintained, more energy is dissipated within the amp and efficiency is lost. Problems arise when the amp can not dissipate the increase in wattage dropped internal to the amp. Unless the amp has thermal shutdown mechanisms in its output stage, the amp has the possibility of destroying or prematurely wearing its components. Are you sure about that? On the Jazz guitar forums you will read about an SS amp capable of; 80 watts at 16ohms, 110 watts at 8ohms, and 135 watts at 4ohms. Now I am making these figures up here, but the same amp has a higher output rating with a lower ohm speaker/cab. You are saying the dBs aren't higher?
kbp810 Posted December 9, 2012 Posted December 9, 2012 Are you sure about that? On the Jazz guitar forums you will read about an SS amp capable of; 80 watts at 16ohms, 110 watts at 8ohms, and 135 watts at 4ohms. Now I am making these figures up here, but the same amp has a higher output rating with a lower ohm speaker/cab. You are saying the dBs aren't higher? Soild State power amp works different from power tubes - in an oversimplified nutshell of an explanation, the output transformer in a tube amp has a ratio that balances the draw/load on the tubes to the draw/load of the speaker. An Output transformer that has multiple OHM outputs has a seperate winding tap for each of those ohm ratings, so each winding has a different respective ratio. So connecting a 4ohm load on the 4ohm tap reflects the same on the power tubes as would connecting a 16ohm load on the 16ohm tap; as long as the ratio is balanced you have happy tubes that are operating within their intended specification. This is also why it's a bad idea to connect to the wrong speaker load to the wrong tap in a tube amp. That being said, there are many that believe the 16ohm tap (or more so, the largest "tap" available) is desirable to use because it is using more of the transformer (longest winding).
GuitArtMan Posted December 9, 2012 Posted December 9, 2012 Um, well, that sounds backwards to me. Ohms are a measure of electrical resistance. 8 ohms is twice the resistance of 4, etc. I believe that's why amplifiers (at least in the audiophile world) usually are rated as being able to deliver more power (watts RMS) into 4 ohms rather than 8. And into 8 ohms rather than 16. Less electrical resistance, more work being done by the amplifier. Having said all that, I'm not going to be surprised if one of the electical engineers in the membership tells me I'm dead wrong. It wouldn't be the first time. You'd think that way as it seems logical, but that's not how it works. With a lower resistance, more current will flow and more heat will be generated. Ever short out a battery? Things can get real hot real fast - it's the same principal. With tube amps however you have a transformer between the speakers and the tubes and there's something about reflected voltages that I can't remember - that was 27 years ago and I've been a software weenie ever since. Maybe KBP810 will jump in here.
GuitArtMan Posted December 9, 2012 Posted December 9, 2012 Well Gerald Weber (Kendrick amps) suggests using the largest tap available on your transformer (i.e. if your amp has 4, 8 and 16 ohm outputs use the 16 ohm output) as you are using the full transformer and not just a tap. Of course you have to have a cabinet to match. He also has a preference over series/parallel vs. parallel/series but I can't remember what the difference was. Something to do about the branch inductance if I remember right. I've also heard some people swear they can hear a difference between and 8 ohm version of a speaker and a 16 ohm version. For instance the original Celestion Alnico Blue only came in 8 ohms - I know of one manufacturer who wont use the 16 ohm version.
Kuz Posted December 9, 2012 Posted December 9, 2012 KBP & GAMan, thanks for the education. I definitely learned something new today.
Steiner Posted December 10, 2012 Posted December 10, 2012 That being said, there are many that believe the 16ohm tap (or more so, the largest "tap" available) is desirable to use because it is using more of the transformer (longest winding). Well Gerald Weber (Kendrick amps) suggests using the largest tap available on your transformer (i.e. if your amp has 4, 8 and 16 ohm outputs use the 16 ohm output) as you are using the full transformer and not just a tap. Exactly what I have read and utilized throughout the years.
DetroitBlues Posted December 10, 2012 Posted December 10, 2012 You know a lot about moving some air at your ranch Steiner!
JohnCovach Posted December 10, 2012 Posted December 10, 2012 Exactly what I have read and utilized throughout the years. Me too. For tune amps at least, always go for the highest impedance that matches the cab. My AC-15 has an 8/16 ohm switch so I put the 16 ohm blue in it when I swapped out speakers.
JeffB Posted December 10, 2012 Posted December 10, 2012 I quite often run a 16ohm cab from 8ohm speaker out. Its slightly darker sounding and has a slightly different feel.
Blunote Posted December 10, 2012 Posted December 10, 2012 I quite often run a 16ohm cab from 8ohm speaker out. Its slightly darker sounding and has a slightly different feel. I'm told that you can do that but that it's not good for the amp -particularly the tubes.
JeffB Posted December 10, 2012 Posted December 10, 2012 I'm told that you can do that but that it's not good for the amp -particularly the tubes. Ive been doing it for 26yrs to the same amp. Not many tube swaps in that time. I believe the amp runs cooler doing this. I could be wrong again though.
kbp810 Posted December 10, 2012 Posted December 10, 2012 Running a bigger ohm speaker then what the amp is set for will run the tubes cold, typically a safe move, but in many cases the amp might not sound as good as it perhaps should; so as in the example above, running a 16 ohm cab on an 8 ohm amp setting is fairly safe. The opposite is where you are going to run the tubes (and often the OT) hot, and more likely to cause damage and/or shortened component life; for example running an 8ohm cab on an 16 ohm amp setting. Though...depending on the amp and speakers in many amps you can often go one step in either direction and still be safe (perhaps worth noting: 4, 8, or 16 ohm is just the general classification, there is variation between speakers - for example one speaker might be 6.1 ohms, and another might be 7.2 ohms and they'd both be labled as an 8 ohm speaker). Going two steps in either direction (i.e. setting to 4ohm and running a 16 ohm cab, or vice versa) is never a good idea.
mars_hall Posted December 10, 2012 Posted December 10, 2012 Though...depending on the amp and speakers in many amps you can often go one step in either direction and still be safe (perhaps worth noting: 4, 8, or 16 ohm is just the general classification, there is variation between speakers - for example one speaker might be 6.1 ohms, and another might be 7.2 ohms and they'd both be labled as an 8 ohm speaker). Going two steps in either direction (i.e. setting to 4ohm and running a 16 ohm cab, or vice versa) is never a good idea. What often is ignored among non-technical folks is the fact that impedance in speaker labeling is actually the sum of the DC and AC components. The speaker is labeled 8 ohms and you measure it with a meter and find a DC resistance of 6.1 ohms. What's up with that, you might ask? The rest of the rated impedance of the speaker is made up of its reactive/inductive components (read AC) which varies with frequency. The frequency used in this measurement for the purpose of labeling is typically 1 Khz. The speaker coil itself is a large inductor which initially blocks current from moving and, once moving, tends to keep it moving for a period of time. Think of this behavior as a sort of electron inertia. This AC component is added to the DC resistive component to yield the nominal impedance.
HANGAR18 Posted December 11, 2012 Author Posted December 11, 2012 I really appreciate all the feedback! Always a learning experience for me here in this forum.
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