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Old 21st March 2005, 08:33 PM   #1
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Unhappy Lm4651, Lm4652

Iím building a class D amplifier using the LM4651 and LM4652 chip set I intend to provide two full range and a subwoofer/ low pass channel. Power will be provided by three separate toroidal transformers to avoid crosstalk and something like 30 and 18 odd thousand uF on the board which I hope will stabilise it without regulators. Any comments on this are welcome.
Iíve simulated a filter system which seems to work quite well which hopefully I can post here somewhere, but how does this affect the input resistance the main amplifier is required to see or match with the ďvaluesĒ of R1, R2 (not their ratio) and RLP, CLP should they be removed with such an active filter?
Is this a good way to take a low pass from a stereo signal?
Any has anybody please got any ideas on how to implement a volume control on such a beast?

Cheers from spud
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Old 21st March 2005, 08:35 PM   #2
homer09 is offline homer09  Canada
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I suggest that you move your thread to Amplifiers - Class D forum, im sure you will find more help there than here.
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Old 21st March 2005, 08:40 PM   #3
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tried that and had absolutly no replies in ages, don't know what i'm doing wrong
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Old 21st March 2005, 09:38 PM   #4
homer09 is offline homer09  Canada
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Which i could help but i dont know anything about class D. try asking less questions at once, and more specific ones. (lazy ) people will tend to help more in those cases.
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Old 21st March 2005, 09:46 PM   #5
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cheers mate, I'll try that cool smilies eh?
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Old 21st March 2005, 09:57 PM   #6
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Default Re: reply to homer

Quote:
Originally posted by spudspud
tried that and had absolutly no replies in ages, don't know what i'm doing wrong
Your title actually has very little to do with your post. Your question is really about a subwoofer crossover and the fact you are using LM465x is irrelevant to this question.

Basically, you will need 1 volume control between the source and your filter stage. Then you will need 3 controls (you can use trimmers if you want) to vary the sound level coming out of each power amp. You adjust the 3 controls to match it so it sounds right and leave those alone, and just use the 1 single control everyday.
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Old 21st March 2005, 10:08 PM   #7
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will there not be a lot of noise on the output if i remove the signal but the gain remains the same. I thought i had to attenuate the whole lot on the OP but this would create a lot of heat and theirs where i get confused.
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Old 21st March 2005, 10:11 PM   #8
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Quote:
Originally posted by spudspud
will there not be a lot of noise on the output if i remove the signal but the gain remains the same. I thought i had to attenuate the whole lot on the OP but this would create a lot of heat and theirs where i get confused.
I don't understand. Please be more detailed. Remove signal from what, the filter stage input? Create a lot of heat where? An op amp driving against a pot will not create much heat, your standard 1/8W or what cheapo potentiometer will do.
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Old 22nd March 2005, 06:26 AM   #9
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Default Re: reply to homer

Quote:
Originally posted by spudspud
tried that and had absolutly no replies in ages, don't know what i'm doing wrong

I think it would help if you posted a legible schematic, preferrably one that's not inside a zip file. I have no idea which resistor is R1 or R2, or what RLP and CLP is.

As far as I can make out you have two low pass filters per channel, and you've connected the outputs of two of those together, presumably for the sub-woofer. That won't work. Remove one of the op amps for the sub output and bring the input 1k resistors coming from the filters together to the inverting input of the remaining op amp.

You can actually save some components by adding the channels first, and filtering the sub-woofer after the adder. Same end result.

In general, a reasonably well designed filter will produce neglible noise at power amp signal levels, so I don't know what you're worried there.

Rune
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Old 22nd March 2005, 06:20 PM   #10
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Any reason you dont want to use the examples in the datasheet?

Im probably just going to make the PCB given in the datasheet as thats the easiest...

Only problem is there are no drill holes in their pcbs.. (anyone know if they have eagle files for these examples?)

Oh and the Reference PCB would be the one for a subwoofer right?

thx
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