Hot Tub Posted March 6, 2008 Share Posted March 6, 2008 OK, I understand Ohm's Law from a mathematical perspective. What I don't know is what it means from the "viewpoint" of the output stage of a power amp. Here's the problem. I have a 100w amp which is designed to "talk" to a 4ohm load, and the 115 cab I'm using is 4ohms. Perfect. But I have the chance of getting a 410 cab on the cheap, and it's 8ohms. If I connect them in series, I'll get 12ohms which (I think) will be pretty quiet, but if I put them in parallel I'll have 2.7ohms. What damage - if any - will this do to my amp? Quote Link to comment Share on other sites More sharing options...
Gazm Posted March 6, 2008 Share Posted March 6, 2008 Assuming it's solid state, the output transistors will overheat and eventually fail unless there's some form of protection circuit in which case it would just cut out. If it's a valve amp, the output transformer will most likely cook. Pete. Quote Link to comment Share on other sites More sharing options...
BOD2 Posted March 6, 2008 Share Posted March 6, 2008 What it means from the viewpoint of your amp is that you should always ensure that the speaker load is within the range specified the manufacturer. That's all you need to remember from a practical sense. So if the manufacturer says "minimum load 4 ohms" then make sure you don't connect anything less than 4 ohms. If it's a valve amp, make sure the speaker selector is set to the same load (ohms) as the cabinets that are connected. If you don't follow these manufacturer's instructions then your amp might well be damaged. Quote Link to comment Share on other sites More sharing options...
bass_ferret Posted March 6, 2008 Share Posted March 6, 2008 Ohm invented Ohms Law. There is a really good explanation of all this on the wiki. Quote Link to comment Share on other sites More sharing options...
nig Posted March 7, 2008 Share Posted March 7, 2008 Ohm's law states that in an electrical circuit, the current passing through a conductor between two points is directly proportional to the potential difference (i.e. voltage drop or voltage) across the two points, and inversely proportional to the resistance between them. The mathematical equation that describes this relationship is: I = V/R where I is the current in amperes, V is the potential difference between two points of interest in volts, and R is a circuit parameter, measured in ohms (which is equivalent to volts per ampere), and is called the resistance. The potential difference is also known as the voltage drop, and is sometimes denoted by U, E or emf (electromotive force) instead of V.[1] FROM WIKIPEDIA. Quote Link to comment Share on other sites More sharing options...
Steve_K Posted March 7, 2008 Share Posted March 7, 2008 You could achieve 6ohms with a little rewiring (Assuming the 410 is comprised of 4*8ohm speakers) Rewire them all in parallel and then run the cabs in series. 1/(1/8ohms*4)+4ohms = 6 It could also be done to bring it to around 3.5, though that is still a bit risky for the amp. Quote Link to comment Share on other sites More sharing options...
BOD2 Posted March 7, 2008 Share Posted March 7, 2008 [quote name='Steve_K' post='153273' date='Mar 7 2008, 05:55 PM']You could achieve 6ohms with a little rewiring (Assuming the 410 is comprised of 4*8ohm speakers) Rewire them all in parallel and then run the cabs in series. 1/(1/8ohms*4)+4ohms = 6 It could also be done to bring it to around 3.5, though that is still a bit risky for the amp.[/quote] Technically that is correct, but it would mean you could only ever pair the 410 IN SERIES with another cabinet that was also unusually wired to allow a series connection. And if you forget and hook up the 410 2ohm cabinet on its own.... It's better (safer) to keep things standardised. Quote Link to comment Share on other sites More sharing options...
Paul_C Posted March 8, 2008 Share Posted March 8, 2008 you could always replace the 15" speaker with an 8ohm one, then the two combined would give you 4ohms Quote Link to comment Share on other sites More sharing options...
Hot Tub Posted March 10, 2008 Author Share Posted March 10, 2008 Many thanks for good advice, muchly appreciated! Cheers folks! Quote Link to comment Share on other sites More sharing options...
alexclaber Posted March 10, 2008 Share Posted March 10, 2008 [quote name='BOD2' post='153398' date='Mar 7 2008, 09:20 PM']Technically that is correct, but it would mean you could only ever pair the 410 IN SERIES with another cabinet that was also unusually wired to allow a series connection. And if you forget and hook up the 410 2ohm cabinet on its own...[/quote] Also running non-identical cabs in series causes all sorts of weird frequency response problems due to their varying impedance across the frequency spectrum acting as a shifting voltage divider. Bad. Alex Quote Link to comment Share on other sites More sharing options...
Steve_K Posted March 13, 2008 Share Posted March 13, 2008 [quote name='BOD2' post='153398' date='Mar 7 2008, 10:20 PM']Technically that is correct, but it would mean you could only ever pair the 410 IN SERIES with another cabinet that was also unusually wired to allow a series connection. And if you forget and hook up the 410 2ohm cabinet on its own.... It's better (safer) to keep things standardised.[/quote] Well that's my hand firmly slapped Quote Link to comment Share on other sites More sharing options...
BOD2 Posted March 14, 2008 Share Posted March 14, 2008 [quote name='Steve_K' post='157092' date='Mar 13 2008, 11:41 PM']Well that's my hand firmly slapped [/quote] Sorry ! Didn't mean it to sound like that. I always feel that if we ever suggest something here that could lead to damage if applied wrongly then we need to mention that fact so that anyone reading the thread later is aware of this. Certainly don't want to discourage input and discussion from anyone, though. Quote Link to comment Share on other sites More sharing options...
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