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Old 10th December 2012, 05:45 AM   #11
pra3718 is offline pra3718  India
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Quote:
actually this is too loud for a room,& you can shake the room
@Joseph1, Is this your first thread & post ? I am not going to disappoint you, but LM1875 is not too good for subwoofer. or you are cheated. So I would request you to build LM1875x2 / LM1876x1 + LM3886x1.

Elektro Jiwaku: Hi End Home Theater 2.1

Or, Build 2.0 Bi-amp.

Best Regards.
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Old 10th December 2012, 07:45 AM   #12
Joseph1 is offline Joseph1  Sri Lanka
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Hi pra thanks for the link.Do you have any schematic of it?
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Old 10th December 2012, 07:57 AM   #13
pra3718 is offline pra3718  India
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The silk-screen is there in provided link. You should read and understand whether it is fit to your requirements or not. This is just for idea of 2.1. What is your planning ? Let me know.
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Old 11th December 2012, 10:23 PM   #14
Joseph1 is offline Joseph1  Sri Lanka
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Ok.After I completed making my LM1875 2.1 system I'm going to build a stereo amp using two TDA7294.

Actually I already made this LM1875 & all I need to do is fine tune to get maximum performance from it.

I have a problem.I need to measure my source outputs sensitivity or input sensitivity.

This is the procedure.

Play a track on the MP3 player & hook an oscilloscope on the output of the MP3 player.Read the peak value shown on the oscilloscope.

Is this correct?Do we use peak values or rms values to determine input sensitivity?
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Old 12th December 2012, 09:45 AM   #15
AndrewT is offline AndrewT  Scotland
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no.
record a constant voltage sinewave to a medium that your source can replay.

This sinewave should be related to the maximum that the medium can record.
say you record a 0dBfs signal, then the output of the player will be at full scale.

If you record at -6dBfs then the output of the player will be at half the maximum.

Pano did a file for a pair of -9dBfs signals in his what power Thread.
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Old 12th December 2012, 10:07 AM   #16
sreten is offline sreten  United Kingdom
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Hi,

Overall gain is too high to match well with a normal amplifier (high pass
filtered with a cap on its input) well. There is no reason to use S&K
type filters with their intrinsic gain, use a unity gain topology.

rgds, sreten.
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Old 12th December 2012, 10:13 AM   #17
AndrewT is offline AndrewT  Scotland
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or MFB filters where the gain can be selected.
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Old 12th December 2012, 10:23 AM   #18
Joseph1 is offline Joseph1  Sri Lanka
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Hi Andrew thanks

Please don't angry with me I'm new to this decibel figures.I cannot understand decibal stuff.

I actually don't know how to send a 0dB signal & -6dB signals.
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Old 12th December 2012, 01:24 PM   #19
Joseph1 is offline Joseph1  Sri Lanka
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Quote:
Originally Posted by sreten View Post
Hi,

Overall gain is too high to match well with a normal amplifier (high pass
filtered with a cap on its input) well. There is no reason to use S&K
type filters with their intrinsic gain, use a unity gain topology.

rgds, sreten.
Your point also good.

LM1875 gain = 22.6
pre amp (Filter) gain = 12.1
Supply voltage = 25V dc

My overall gain is = 22.6 X 12.1 =273

Its pretty big but the sound level is nice as a professional subwoofer system.Also with 250mV input sensitivity I can volume it to maximum level.

What you mean by unity gain topology?

Preamp = sallenkey filter + gain stage

You mean disconnect gain stage & leave only sallen key part & feed to LM1875 amp?
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