Musky Posted June 20, 2008 Share Posted June 20, 2008 I know that in theory if you half the impedance, say from 8 ohms to 4 ohms, you should get double the wattage. So an amp that gives 150W at 8 ohms should produce 300W at 4 ohms. Only in practice that rarely (if ever) seems the case. For instance, my 250W (into 4 ohms) HH Bass Machine produces about 180W into an 8 ohm load. And I've seen the specs to other amps that state that using an extra cab (ie a 4 ohm load) only gives an increase in power of about a third. Any idea of why this should be? The only thing I could think of was that maybe the power supply couldn't cope with with the extra demand. Off the top of my head I can't think of any amps that actually give twice the power with half the impedance, with a 50/60% seeming like more the norm. Or am I missing something obvious here? Quote Link to comment Share on other sites More sharing options...
alexclaber Posted June 20, 2008 Share Posted June 20, 2008 [quote name='Musky' post='222991' date='Jun 20 2008, 03:08 PM']The only thing I could think of was that maybe the power supply couldn't cope with with the extra demand.[/quote] Exactly! Alex Quote Link to comment Share on other sites More sharing options...
Bill Fitzmaurice Posted June 20, 2008 Share Posted June 20, 2008 [quote name='Musky' post='222991' date='Jun 20 2008, 10:08 AM']Or am I missing something obvious here?[/quote] Nope. You're pretty much there. Scenario 1: You run your 500 watt/8 ohm SS amp with a 28v output into an 8 ohm speaker. It will deliver 100 watts. Add a second 8 ohm speaker, leaving all the settings the same. The voltage swing will still be 28 volts, the current will double into the halved impedance load, and you will get 200 watts. Scenario 2: Run your 500 watt amp at full power into an 8 ohm cab. The output swing will be 63 volts. Add a second 8 ohm cab. You won't get 63 volts, because the amp's power supply isn't capable of delivering maximum voltage into all impedance loads. It generally drops by a factor of .7 for each halving of the load impedance, so instead of 1,000 watts into 4 ohms you might get 700. So while you will get a doubling of power with a halved impedance load if the amp is well below full power you won't get it at full power. Quote Link to comment Share on other sites More sharing options...
lowhand_mike Posted June 20, 2008 Share Posted June 20, 2008 i've always known my max on my amp @ 4ohms is 307watts (what ever that actually means) and that @ 8ohms its less but not too bad. that was enough for me. but at least that above makes sense off the bat. Quote Link to comment Share on other sites More sharing options...
Musky Posted June 20, 2008 Author Share Posted June 20, 2008 Thanks for that everyone - a simpler answer than I thought! It does beg another question though. With the amount of boutique, high end, expensive amps out there, why aren't at least some manufacturers making amps with power supplies that would be capable of delivering the kind of power needed to make the most of the amp? Expense and/or weight? Or am I answering my own question again? Quote Link to comment Share on other sites More sharing options...
lowhand_mike Posted June 20, 2008 Share Posted June 20, 2008 [quote name='Musky' post='223115' date='Jun 20 2008, 05:40 PM']Thanks for that everyone - a simpler answer than I thought! It does beg another question though. With the amount of boutique, high end, expensive amps out there, why aren't at least some manufacturers making amps with power supplies that would be capable of delivering the kind of power needed to make the most of the amp? Expense and/or weight? Or am I answering my own question again? [/quote] it would probably require more than twice the volts for 8ohms to get twice the wattage in 4ohms, which would mean heavier and more expensive, and probably the transformer would have to be able to step up in power too er Bill? Quote Link to comment Share on other sites More sharing options...
ben_eat_pie Posted June 20, 2008 Share Posted June 20, 2008 does this mean my ashown combo runs at less than 307W at 8 ohms or is that at 4 ohms.... confused, as i'm after buying a new amp, my ashdown with an extension cab gives not quite enough power, so i've been thinking of going a fair bit bigger... any point? cheers [quote name='lowhand_mike' post='223121' date='Jun 20 2008, 05:50 PM']it would probably require more than twice the volts for 8ohms to get twice the wattage in 4ohms, which would mean heavier and more expensive, and probably the transformer would have to be able to step up in power too er Bill?[/quote] Quote Link to comment Share on other sites More sharing options...
Bill Fitzmaurice Posted June 20, 2008 Share Posted June 20, 2008 [quote name='Musky' post='223115' date='Jun 20 2008, 12:40 PM']why aren't at least some manufacturers making amps with power supplies that would be capable of delivering the kind of power needed to make the most of the amp? Expense and/or weight?[/quote]Physics and the reality that there is no such thing as a free lunch. In any event, power is not the limiting factor of how loud an amp will push a speaker. A speaker's output is determined by voltage swing, not power consumed. The entire concept of going to as low an impedance load as one can to 'get all the watts out' of an amp is intrinsically flawed. The main result of so doing is to cause the amp to generate higher heat levels, shortening component life. Quote Link to comment Share on other sites More sharing options...
BB2000 Posted June 20, 2008 Share Posted June 20, 2008 [quote name='Musky' post='223115' date='Jun 20 2008, 05:40 PM']Thanks for that everyone - a simpler answer than I thought! It does beg another question though. With the amount of boutique, high end, expensive amps out there, why aren't at least some manufacturers making amps with power supplies that would be capable of delivering the kind of power needed to make the most of the amp? Expense and/or weight? Or am I answering my own question again? [/quote] Most instrument amps are not designed to drive a load continuously at their rated output power. For instance the Genz Benz shuttle 6.0 (which uses a 500W rated class-D module) will go into thermal shut down if asked to deliver a continuous load of greater than 70W into 4 ohm, or 200W into 8 ohm. Manufacturers do this because of the nature of the signal to be amplified (large dynamic range with low average signal level). Quote Link to comment Share on other sites More sharing options...
Musky Posted June 21, 2008 Author Share Posted June 21, 2008 Cheers everyone. I was asking out of idle curiousity, but it's nice to know all the same. And Ben, I'd imagine your Ashdown will be knocking out about 200W without an extention cab. Quote Link to comment Share on other sites More sharing options...
bnt Posted June 21, 2008 Share Posted June 21, 2008 Another small factor I should mention has to do with increased power losses due to the higher current. If we were to assume that the voltage was constant, and the impedance at any frequency was halved, then the current would be doubled. (I=V/R). However, there is resistance in the connections between the amp and the speakers, including internal resistance in the valves/transistors themselves. The power lost as heat in these components is proportional to the [i]square[/i] of the current (P=I²R), so if you double the current, the losses are [i]quadrupled[/i]. This is also the reason why we have high-voltage power lines: it keeps the current low, meaning less power loss in the lines. Quote Link to comment Share on other sites More sharing options...
Bill Fitzmaurice Posted June 21, 2008 Share Posted June 21, 2008 [quote name='bnt' post='223644' date='Jun 21 2008, 10:51 AM']The power lost as heat in these components is proportional to the [i]square[/i] of the current (P=I²R), so if you double the current, the losses are [i]quadrupled[/i].[/quote] That's part of the overall no free lunch equation, which also encompasses output device and power supply failure if you go too low with the load impedance. Were it not so we could use 0.4 ohm speakers. That would be a very good thing as far as the efficiency of the speakers is concerned, but would require an entire retool of the amp/speaker relationship from low voltage high current to high voltage low current. Unfortunately Tesla wasn't involved when the dynamic loudspeaker came about. Quote Link to comment Share on other sites More sharing options...
phil_the_bassist Posted June 23, 2008 Share Posted June 23, 2008 ok ok ok...even tho (several years ago) I did my physics A-level I still have a nightmare getting my head around this whole wattage/ohms thing! Broken down: Low ohms = more watts get through = louder. I get that bit, maybe not the why or how, but I get that it happens. Now, the big question...I've got a Hartke HA5500 (so that's max output 500w@4ohms but it's currently running 250w@8ohms) and I've got a VX215 cab thats rated upto 500w@4ohms. So, they're matched, it should all work OK but on the speaker outputs on my head it's saying '8ohms'. Does anyone know how I can get the full 500w@4ohms? Will I need to send it back to the factory, or is there a secret switch I need to flick? Any Ideas? Quote Link to comment Share on other sites More sharing options...
obbm Posted June 23, 2008 Share Posted June 23, 2008 [quote name='phil_the_bassist' post='224864' date='Jun 23 2008, 11:33 AM']ok ok ok...even tho (several years ago) I did my physics A-level I still have a nightmare getting my head around this whole wattage/ohms thing! Broken down: Low ohms = more watts get through = louder. I get that bit, maybe not the why or how, but I get that it happens. Now, the big question...I've got a Hartke HA5500 (so that's max output 500w@4ohms but it's currently running 250w@8ohms) and I've got a VX215 cab thats rated upto 500w@4ohms. So, they're matched, it should all work OK but on the speaker outputs on my head it's saying '8ohms'. Does anyone know how I can get the full 500w@4ohms? Will I need to send it back to the factory, or is there a secret switch I need to flick? Any Ideas?[/quote] I strongly recommend that you read paragraph 2 on page 8 of the Hartke 5500 User Manual. Quote Link to comment Share on other sites More sharing options...
Merton Posted June 23, 2008 Share Posted June 23, 2008 (edited) [quote name='phil_the_bassist' post='224864' date='Jun 23 2008, 11:33 AM']ok ok ok...even tho (several years ago) I did my physics A-level I still have a nightmare getting my head around this whole wattage/ohms thing! Broken down: Low ohms = more watts get through = louder. I get that bit, maybe not the why or how, but I get that it happens. Now, the big question...I've got a Hartke HA5500 (so that's max output 500w@4ohms but it's currently running 250w@8ohms) and I've got a VX215 cab thats rated upto 500w@4ohms. So, they're matched, it should all work OK but on the speaker outputs on my head it's saying '8ohms'. Does anyone know how I can get the full 500w@4ohms? Will I need to send it back to the factory, or is there a secret switch I need to flick? Any Ideas?[/quote] EDIT: what OBBM said Edited June 23, 2008 by Merton Quote Link to comment Share on other sites More sharing options...
Musky Posted June 23, 2008 Author Share Posted June 23, 2008 [quote name='phil_the_bassist' post='224864' date='Jun 23 2008, 11:33 AM']Now, the big question...I've got a Hartke HA5500 (so that's max output 500w@4ohms but it's currently running 250w@8ohms) and I've got a VX215 cab thats rated upto 500w@4ohms. So, they're matched, it should all work OK but on the speaker outputs on my head it's saying '8ohms'. Does anyone know how I can get the full 500w@4ohms? Will I need to send it back to the factory, or is there a secret switch I need to flick? Any Ideas?[/quote] If your VX215 is rated at 4 ohms you're already running your amp at 500W - you don't have to adjust anything on a solid state amp. You shouldn't add an extra cab to that set up, as the amp's already running at it's minimum impedance. [quote name='obbm']I strongly recommend that you read paragraph 2 on page 8 of the Hartke 5500 User Manual.[/quote] Only if all else fails! Quote Link to comment Share on other sites More sharing options...
phil_the_bassist Posted June 23, 2008 Share Posted June 23, 2008 I'm bein a prat...I didn't think to check the manual, just thought I'd throw the question out there whilst I was reading that topic! Cheers guys, I'll let you know how I get on! Quote Link to comment Share on other sites More sharing options...
Bill Fitzmaurice Posted June 23, 2008 Share Posted June 23, 2008 [quote name='obbm' post='224877' date='Jun 23 2008, 06:43 AM']I strongly recommend that you read paragraph 2 on page 8 of the Hartke 5500 User Manual.[/quote]The technical acronym for this proceedure is RTFM. Quote Link to comment Share on other sites More sharing options...
obbm Posted June 23, 2008 Share Posted June 23, 2008 [quote name='Bill Fitzmaurice' post='225034' date='Jun 23 2008, 02:09 PM']The technical acronym for this proceedure is RTFM. [/quote] Indeed it is but I was just being polite and diplomatic. Quote Link to comment Share on other sites More sharing options...
alexclaber Posted July 4, 2008 Share Posted July 4, 2008 [quote name='BB2000' post='223181' date='Jun 20 2008, 07:22 PM']For instance the Genz Benz shuttle 6.0 (which uses a 500W rated class-D module) will go into thermal shut down if asked to deliver a continuous load of greater than 70W into 4 ohm, or 200W into 8 ohm.[/quote] This is very interesting! Where did you find this info? It certainly highlights why chasing lower impedance cabs for allegedly more power can be futile. Would be interesting to see how other lightweight amps compare, presumably similarly... Alex Quote Link to comment Share on other sites More sharing options...
Count Bassy Posted July 7, 2008 Share Posted July 7, 2008 [quote name='Bill Fitzmaurice' post='223680' date='Jun 21 2008, 04:57 PM']Were it not so we could use 0.4 ohm speakers. That would be a very good thing as far as the efficiency of the speakers is concerned, but would require an entire retool of the amp/speaker relationship from low voltage high current to high voltage low current.[/quote] The other way round surely? Quote Link to comment Share on other sites More sharing options...
Count Bassy Posted July 7, 2008 Share Posted July 7, 2008 IIRC: If you have a valve amplifier with an output transformer then you get the most efficient power transfer from the transformer to the speaker when the output impedance of the transformer is the same as the impedance of the speaker. So although lowering speaker impedance increases the internal power of the amp more of rtaht power is lost in the amp output stages, so you don't get double the power at the speaker. Mind you its about 30 years since I studied this. Quote Link to comment Share on other sites More sharing options...
male33lancs Posted July 7, 2008 Share Posted July 7, 2008 (edited) with a valve amp, impedance mis match will give reduced power and bandwidth solid state using lower than recommended impedance will (in most modern gear) initiate some kind of thermal protection as the output devices will try to provide more power than the power rating of the devices, so the protection circuit kicks in... otherwise you destroy your amp Edited July 7, 2008 by male33lancs Quote Link to comment Share on other sites More sharing options...
bass_ferret Posted July 12, 2008 Share Posted July 12, 2008 Added some of this to the wiki - hope you dont mind. Quote Link to comment Share on other sites More sharing options...
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