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Old 6th April 2003, 07:35 PM   #1
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Default please help a student out

ok, i have another thread open that i invite you to look at in addition to this one. i think different questions are posed in each.

link: all my questions, any help?

ok as for this, i am a college student and am working on my first major electronics project. I had wanted to build an amplifer using lm1875 and lm3875 chips. it is for a quasi-hi-fi stereo setup. i plan to put an amplifer channel on the 2 tweeters, 2 midranges, and 1 subwoofer (i mean 1 each, so 5 channels). i had planned on a bridge amplifer for the subwoofer to get 100 watts. from the graphs, the 1875 seems not to like voltages above +-30V, and 100watts into the woofer isn't possible unless the voltage is kept near +-30, however, the 3875 charts show (i guess protection circutry...) that the power output will drop off if the supply is brought above +-35V or so.

so this brings me to quesitons about all of this. I am assuming i can't get the +-30V without regualtion.

so this brought me to a different idea, building a different amplfier for the subwoofer. i'd like to get about 100watt-300watts into my 8ohm subwoofer, and i'd like to do so without generating a huge amount of distortion or cash. i'm already over budget (by a lot). in any case it would be ashame if i had to stop the project because of funding.

so i was wondering if there was any kinda cheap amplifer that would work nice with my subwoofer. i figured i'd ask here as any design i find online i'd no doubt ask your help on.

ok. as for transformers. i have these questions. is there any way to use 2 transformers as 1 transformer, like take 2 220V primaries and wire them in series (or parellel) to allow a 110V input?

also any opinions on this: http://sound.westhost.com/project15.htm
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Old 6th April 2003, 09:10 PM   #2
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Well, if you're looking at the National chips because of your budget and had a design in mind, why not just build 1 more channel and run your sub bridged?
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Old 6th April 2003, 09:41 PM   #3
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Quote:
Originally posted by leadbelly
Well, if you're looking at the National chips because of your budget and had a design in mind, why not just build 1 more channel and run your sub bridged?

He is running the sub bridged... lol
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Old 6th April 2003, 10:05 PM   #4
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Duh, thanks SkinnyBoy, Leadbelly not too swift today...

OK, then I would say your best bet is to build all channels out of 3875's, running 2 bridged for the sub. If there is a good reason you chose 1875's, then just make your PS for the lower voltage.

Don't build a regulated supply, that goes against a great feature of these chips which is the PS noise rejection. The money spent on a regulated supply is better spent on a properly sized transformer, ultrafast dioes, PE or PP caps, etc.
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Old 6th April 2003, 10:08 PM   #5
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but, if your sub is 4 ohm (which "car" subs generally are) then bridging could be a bit of a problem....
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Old 7th April 2003, 04:36 AM   #6
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no, it's 8ohms. isn't running an amplifer bridged like playing each into half the impedance?

ok, so you say no regualtion, ok.

i'm thinking of buying 2 lm3886s so the power supply doens't have to be so close in order to put out power.

BTW, i've looked at that spike protection and it looks like the amplifer would sound really bad if it was allowed to clip even a little.
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Old 7th April 2003, 05:29 AM   #7
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i did kinda want to use that capacitive multiplier circuts because it just looks neat and since this is primarily for learing... except for reducing voltage (it's not a regulator after all) would it have any negative sonic impact? and besides, the psychoacoustic of over-building something should be pretty nice!

as for caps, i bought what look like polystyrene caps. they were 25 cents each locally, and labeled "poly". in any case the leads are nonmagnetic and all that good stuff. i haven't tried the TC test on them yet, but i may. actually i may not. i'm thinking the psychoacoustics of thinking polystyrene (and not having a frame of referance) would be pretty good. the disadvantage is the size, 1.3 inches long per cap (125V rating).

BTW, for RF filtering silver-mica is ok right?
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Old 7th April 2003, 06:41 AM   #8
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Hey, if the capacitance multiplier circuit turns your crank, then go for it. If there's 1 hard rule in DIY, its that you should do whatever makes you enjoy the end result most.

As for caps, just use what your budget supports. Stay away from tantalum & ceramic, and you'll be fine.
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Old 7th April 2003, 09:18 AM   #9
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Consider using a Class-T or class-D digital amp for the sub - that way you can get good power from a LV PSU. Unfortunately I think the circuits currently available are for car sub use, so they run off a 12V supply or batteries, and I'm not sure about their sound quality. Price should be right though, but I'm not sure.

The easy way to drop voltage for the 1875s is using a regulator, if you are using only one transformer. Obviously the best way out is to have a separate transformer but I guess for tweeter amps a regulator will not have to deal with very heavy loads. The problem will be one of transients though, so you'll have to have very fast transistors in the regulator.

Another way to get more power is to parallel chips - but you are not looking at driving a very low impedance load so maybe that is not a good way to approach this.

I think the best way is to look at separate transformers. Lowest possible cost of implementation...

Or look at ICs that will go to 35 volt rails but use feedback or attenuators to limit the power output...
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Old 7th April 2003, 07:48 PM   #10
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i'll likely look into classD sometime, but i mainly was using the lm chips for ease of use.
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