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Old 22nd April 2005, 05:48 AM   #1
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Default Power supply for 5 LM1875's

Hi,

I plan on making a small 5.1 surround sound amp for the PC (who isn't these days! ). For the power supply would a normal 35A 400V Bridge rectifier be able to handle the load of 5 LM1875's? I plan on using a 300VA 20-0-20 (or 18-0-18) toroidal transformer to power the 5 amps and around 33,000uF of total capacitance.
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Old 22nd April 2005, 06:08 AM   #2
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Default Re: Power supply for 5 LM1875's

Quote:
Originally posted by mangrovejack
For the power supply would a normal 35A 400V Bridge rectifier be able to handle the load of 5 LM1875's?
As the transformer you've selected can only support about 16 amps (8A/rail) the bridge wont be the limiting component. I'd heatsink it for longer life or maybe use a toroid with two separate secondaries and two bridges - one for each rail.
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Old 22nd April 2005, 09:00 AM   #3
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Thanks for the reply Jeff. So I guess what you wrote now begs the question, is 300VA enough for normal listening levels for 5 LM1875's? And by the way I'll be running 4 ohm speakers too.
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Old 22nd April 2005, 02:09 PM   #4
macboy is offline macboy  Canada
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For 4 ohm speakers, I'd recommend going for the 18-0-18 transformer rather than the 20-0-20. The little 1875's will run a lot cooler with the lower voltage.

300 VA is overkill, IMHO. Remember that a transformer's rating is the continuous output it can deliver, not the maximum. Even five 1875's can't suck up 300 VA of power, especially when playing actual music, which can be very peaky, so you'd be OK with a 200 VA model. Or less. But that's just my opinion, and I'm sure someone will chime in and recommend even bigger than 300.
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Old 22nd April 2005, 04:41 PM   #5
nina is offline nina  United Kingdom
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Just share my expeirnce.

I built 5 channel amp for a "home theater" application (50W via 25-0-25 Vac. with only one 300VA tranformaer) using LM3875 (not 1875). It works well for 5.1 movie system.

(For audio only: I feed my 25 classic CG with one 180 VA).
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Old 22nd April 2005, 04:44 PM   #6
nina is offline nina  United Kingdom
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(Cont.)

Forgot to mention: The shared PS is with 10,000x2 per rails. Each chip has 100UF directly connected to the pin. Relatively big heatsink with silicon attached.
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Old 22nd April 2005, 11:28 PM   #7
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Thanks for the replies guys, I think I'll try a 160VA 18-0-18 toroidal at this stage then and I'll see how I go. I would have liked a 200VA, but Jaycar only have 160 or 300 and no in-between unfortunately.

That should be enough though as its rare that I'd be turning the volume up to extreme levels.
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Old 22nd April 2005, 11:28 PM   #8
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Oh and I almost forgot, I'll be highpassing the LM1875's at around 100Hz too so there should be even less requirement for an extremely powerful power supply right?

Is 100Hz an ideal number or too high/low?
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Old 23rd April 2005, 01:17 AM   #9
nina is offline nina  United Kingdom
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Quote:
Originally posted by mangrovejack
Oh and I almost forgot, I'll be highpassing the LM1875's at around 100Hz too so there should be even less requirement for an extremely powerful power supply right?

Is 100Hz an ideal number or too high/low?
Depend on your speaker as well. BTW, If I were you I would do so as well. In case deep bass neeed, I would go for a sub woofer (rather thanexpecting under-100Hz-sound from the small 1875).

Cheers,
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Old 23rd April 2005, 11:27 PM   #10
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Thanks for the reply Nina. I didn't mention the .1 part of the 5.1 system, but that will be taken care of by either an LM3875, LM3886 or (most likely) an OPA549. So it will be fed a signal from the low-pass filter to cover all the low frequencies.
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