ChrisBogg Posted January 5, 2008 Share Posted January 5, 2008 Hi Guys Just after some opinions on a rack that im planning to build. I have a Crest CA18 to drive my TRS218 subs and does so quite nicely into 4 ohms. I am planning on adding another CA18 to the rack to drive 4 stacks of TRS so this will be 4 channels of CA18 each with two 4ohm boxes on IE 2 ohms load. Crest say that these amps will drive happily into 2 ohms however I have heard that they get very warm and have been known to go into protect. Does anyone have any experience of these in a live music situation? Also Im wondering if it would be best to have one channel (channel a) driving the top box and channel b driving the sub on both amps in order to reduce heat build up slightly or to stick with the standard tops on one amp and subs on another? Any thoughts Chris Link to comment Share on other sites More sharing options...
Rob_Beech Posted January 5, 2008 Share Posted January 5, 2008 Hi Chris. The CA series are more than happy with a 2 ohm load but as with any amp, it's less friendly than a 4 ohm load. I'd like to suggest more amps and drive them at 4ohms, but having been a CA18 owner I know how much they weigh and wouldn't expect you to rack 3 or 4 up together and expect to move it around easily. Just remember that more expensive "named" amps always get driven harder than cheaper amps because people trust them more, they WILL get warm when driven hard at 2ohms and suitable ventilation is a must. Rob Link to comment Share on other sites More sharing options...
robertnisbet Posted January 5, 2008 Share Posted January 5, 2008 I like to run three drivers off an amp channel for a 2.67ohm load; I think this is the best compromise between power and distortion. Distortion increases in any amp when the load is lessened. I appreciate here though that you are using 4ohm cabs and so this isn't an option, so I'd say just go for it. As a CA18 owner, user and, well, lover, I've had these doing 2ohm loads on more than a few occasions and they've handle it happily. Link to comment Share on other sites More sharing options...
Lamplighter Posted January 5, 2008 Share Posted January 5, 2008 Distortion increases in any amp when the load is lessened. Robert Can you enlighten us why this is so? Brian Link to comment Share on other sites More sharing options...
robertnisbet Posted January 5, 2008 Share Posted January 5, 2008 Brian, Off the top of my head, not having any notes to hand, caveat emptor, YMMV etc etc... Lowered impedance causes a higher current to be induced. If the voltage is to remain constant then it follows that a higher current will cause increased stress on the components. Is this not so? Link to comment Share on other sites More sharing options...
Shez Posted January 5, 2008 Share Posted January 5, 2008 Distortion increases in any amp when the load is lessened. Lowered impedance causes a higher current to be induced. If the voltage is to remain constant then it follows that a higher current will cause increased stress on the components. I think we may have a language issue here... Less of a load for an amp is generally considered to be a higher impedance load - less power / current required. Link to comment Share on other sites More sharing options...
Trunker Posted January 5, 2008 Share Posted January 5, 2008 If you have 4 x 8 ohm driver that should bring it down to 2ohm in parallel. What about the resistance of the speaker wire? Surely that affects the resistance? Link to comment Share on other sites More sharing options...
Simon Lewis Posted January 5, 2008 Share Posted January 5, 2008 What about the resistance of the speaker wire? Surely that affects the resistance? Yes it does, and if you download the CA18 manual, there's even a chart showing the user how much power is lost through cable resistance. Furthermore, increased cable resistance will affect the damping factor - the amplifier's ability to accurately control the loudspeaker cone movement. Lastly (as mentioned in another thread) the impedance of a loudspeaker is nominal - it varies with frequency. It's possible for an 8 Ohm drive unit to drop to nearly 4 Ohms. Therefore, for a given frequency, the amp may be driving as low as a 1 Ohm load. This is another reason why running at 2 Ohms is so stressful. Simon Link to comment Share on other sites More sharing options...
robertnisbet Posted January 6, 2008 Share Posted January 6, 2008 Distortion increases in any amp when the load is lessened. Lowered impedance causes a higher current to be induced. If the voltage is to remain constant then it follows that a higher current will cause increased stress on the components. I think we may have a language issue here... Less of a load for an amp is generally considered to be a higher impedance load - less power / current required. Yes, that probably should have said lowered, as I was talking numerically. The load itself is lower, but the load/stress on the amplifier is increased. I think you'll agree that in any RLC network, or however you want to model your loudspeaker, a 4ohm load is less than an 8ohm load is less than a several megohm load. Furthermore, increased cable resistance will affect the damping factor - the amplifier's ability to accurately control the loudspeaker cone movement. There is an excellent article/White Paper on LS damping factor in the Autumn 2006 (!) issue of Radio World's 'Engineering Extra', which I finally got round to reading last night after it sitting under my TV for, well, over a year! Link to comment Share on other sites More sharing options...
ChrisBogg Posted January 6, 2008 Author Share Posted January 6, 2008 Thanks for all the replies guys its very helpful. regarding the second part of me post would having a top channel and a sob channel on each amp help to reduce heat build up as I would think that they will be working hard at different times? or will the inbalance between channels cause more stress in the long run? Chris Link to comment Share on other sites More sharing options...
robertnisbet Posted January 6, 2008 Share Posted January 6, 2008 Hi Chris, I've found all the decent amplifiers I've run, and in fact most of the cheap ones as well, to be quite happy with a mismatched load. In fact it can be quite a good way to spread your current draw across multiple connectors. As you rightly said the two channels are less likely to be working to the same capacity if they are sub and top than if they are sub and sub, for instance. The amplifier driving two channels of bass is likely to be doing more work than the amplifier doing two channels of tops. If you spread the load, although your overall current draw will be the same as if one amp was on tops and one on subs, the amp that would have been powering tops will draw more current, and the amp that would have been powering subs will draw less. Hope this is reasonably clear... Link to comment Share on other sites More sharing options...
SteveAATW Posted January 6, 2008 Share Posted January 6, 2008 Just to add to what has already been said. I ran a CA18 on a stack of 4 x F218 at 2R per side for ages, kicked the arse out of them every Friday and Saturday for 7 hours a time and it never skipped a beat, the air coming off the fans got a bit hot during summer but it never went into protect. Just a shame the thing was so dammed heavy. Link to comment Share on other sites More sharing options...
The Magus Posted January 11, 2008 Share Posted January 11, 2008 I used to have 4 CA18s. They were superb amps in everything but their weight! I generally used them to drive a 4-Ohm load, but on many occasions I drove them into a 2-ohm load (a pair of EAW SB600s on each side), and they coped magnificently. Keep the cable runs to your speakers as short as possible, and use a heavy-duty cable - 4mm2 per conductor is ideal. Link to comment Share on other sites More sharing options...
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