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Old 13th October 2007, 03:10 PM   #1
eddog is offline eddog  United States
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Default Band PA Question

Hello all, i have been a reader and builder on the chip amp forum for a few years and have built 2 chip amps for my bands use (we gig rarely as our guitar player has moved 250 miles away,but we push on anyway). I've built all the speakers and electronics(except crossover) and have had good success using my LM4780 amp to power my Folded Horn Subs (courtesy of Bill Fitzmaurice designs T-39's) and am using a LM3886 design for the mains(which consist of 18 4" drivers arrayed in a vertical tower design on each side. Very loud with not much power put to them). At louder volume levels I am finding out that the 3886 amp can't keep up with the Subs. This got me to thinking about instead of building another 4780 maybe I should try a solid state amp with 3 channels of available power so I cold power both the mains and the subs with one amp. The subs would be run mono and present a 5 ohm load when parallel and the tops would be run stereo with each side a 6 ohm load. Can someone suggest an amp design(point me in the right direction) that would provide 300 watts to the sub channel and about 200 watts to the mains that is solid enough to withstand a bit of road abuse. Thanks for any help getting me started on the right path.

Mark
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Old 14th October 2007, 01:07 AM   #2
jaycee is offline jaycee  United Kingdom
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For this sort of power, you definitely want a discrete design. Chipamps in parallel/bridge just won't be reliable for this sort of use.

I would probably recommend looking at Marcus Gunnarson's M250 amplifier - http://marcusgun.awardspace.com/m250.html

There is also The Leach Amp - http://users.ece.gatech.edu/~mleach/lowtim - however that is more designed with domestic use in mind, whereas Marcus's was designed for PA use (and has more pairs of output transistors with that in mind).

The only thing I will say here, is that if you are doing this to save money, then that is the wrong idea. Unless you can get a good deal of parts (big heatsinks, big capacitors, big transformer) cheap or second-hand, building an amp this powerful may well be more expensive than aquiring a suitable commercially made amplifier second hand. You would also have the peace of mind that the commercial unit is more likely to work right away.
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Old 14th October 2007, 05:42 AM   #3
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Quote:
Originally posted by jaycee
The only thing I will say here, is that if you are doing this to save money, then that is the wrong idea. Unless you can get a good deal of parts (big heatsinks, big capacitors, big transformer) cheap or second-hand, building an amp this powerful may well be more expensive than aquiring a suitable commercially made amplifier second hand. You would also have the peace of mind that the commercial unit is more likely to work right away.
That's what I was going to say. There are a heap of older mains freq power supply pro poweramps on the market secondhand at good prices as many people are moving to lighter SMPS and / or class D power.
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Old 14th October 2007, 09:22 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
if you want to save money then buy a commercial with a warranty for PA work.

If you want to learn, then either Quasi or join the Leach group buy for the 5pair Jens' board. The 3pair will not do it.
Even better would be 5pair 21193/4 To3 devices on a fanned sink. But that involves a lot of design and make, since there are no PCBs to suit.
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Old 15th October 2007, 10:27 AM   #5
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You can't build as cheap as you can buy a used piece. Pieces like the Altec 9444 or EV2600 can be purchased for less than $100 and will provide years of valuable service. I highly recommend these pieces of "commercial'' grade amplification.
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Old 15th October 2007, 01:44 PM   #6
eddog is offline eddog  United States
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Thanks for the suggestions people. I have an X-former out of a crown K1 amp that would work with Marcus Gunnarson's M250 amplifier , with a bit of winding to get up to 38V. I also have lots of Caps, chassis and other components to choose from so it might be worthwhile to pursue this amp. I realize that the Behringer Euroamps are hard to beat for price/power and would probably go that route if I didn't have this affliction to build things myself, just because ya can you know. Thanks again for the advice! Time to learn about Discrete amps!
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Old 15th October 2007, 02:15 PM   #7
AndrewT is offline AndrewT  Scotland
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38Vac or 38Vdc?
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Old 15th October 2007, 03:38 PM   #8
eddog is offline eddog  United States
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I was incorrect about the x-formers output. It's AC output is21v so I can expect about 30 vdc. This is probably not a good choice?? I have just started to read about some of the designs at DIYaudio so I have much to read up on.
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Old 15th October 2007, 04:53 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
if you need 200W into 8r0, then a 300VA, 45+45Vac transformer will do the job.
If 200W into 4r0 is your target then 300VA, 35+35Vac is your transformer spec.

If you want two channels at that power then double the VA spec.
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