What should be the optimim level of Input Signal to a LM3886 based amplifier - diyAudio
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Old 9th May 2008, 09:56 AM   #1
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Question What should be the optimim level of Input Signal to a LM3886 based amplifier

How the input level signal to a gain clone amplifier is selected for the optimum use of its power?

Is this way correct -
Considering a gain of 21 (the Reference schematic given by NS in LM4780 datasheet, 1+(20/1)) for LM3886 or LM4780, and if the chip is operating at +30-30 volts. Load is 8ohm, so for power of 50 watts,
the output voltage swing is = sqrt(power*ohm) = 20 volts, and
hence input voltage level = 20 volts/gain = ~1 volt.

is it correct?
i have just finished a 3-way active filter based on this circuit,

3 Way filter

But when i connect it to my LM4780 amplifier, the sound output was quite low even at full volume

Can anyone please suggest how to raise the sound level,
( do i have to add a gain before or after the 3-way filter so that LM4780 chip gets enough input level signal).
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Old 9th May 2008, 10:06 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
firstly, +-30V won't allow 20Vac at the output.
20Vac=28.3Vpk.
loss through the chipamp ~4V.
droop of the PSU on full power ~5V.
PSU voltage ~28.3+4+5 ~=+-37.3Vdc, not +-30Vdc.

for full power you need 20Vac as stated.
Input for full power = 20/21=950mVac.
most CD players put out a maximum of around 2Vac.
usually this is attenuated by at least -6dB and often by -30dB and we still consider it loud enough.

If you cannot get sufficient volume then something is wrong.
Look first at your speakers. Do they suit a chipamp?
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Old 9th May 2008, 10:17 AM   #3
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If I bypass the 3-way filter circuit, i am getting the normal sound reproduction as it should be. I am using a My PC sound card output as source.

I have made the 3-way filter exactly as in the article(using opa4134 opamp), now i am facing this reduced output problem. All the filters in that article are at unity gain, so dues a gain stage is required at output or input).
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Old 9th May 2008, 11:39 AM   #4
AndrewT is offline AndrewT  Scotland
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There's something wrong.
1Watt usually sounds quite loud.
That needs 2.8Vac at the speaker and only 130mVac from the source.
What is the typical output from your source.
What speakers are you connecting to the chipamp?
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Old 9th May 2008, 02:23 PM   #5
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You say "based on". Can you show us your exact circuit for the crossover?
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Old 11th May 2008, 02:24 AM   #6
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here is the filter schematic
and the link for filter design - 3-way crossover

and i am using DANITY 6" 8ohm woofers, 3" mid and tweeter with it, these are runningexcellent with LM4780 with out the filter, BUT with filter the sound level is decreasing.
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File Type: jpg filter.jpg (43.6 KB, 227 views)
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Old 11th May 2008, 10:11 AM   #7
Nuuk is offline Nuuk  United Kingdom
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So it is not "based on" but an exact copy of that circuit?
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Old 11th May 2008, 03:24 PM   #8
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Now you are talking just my old english professor,
Yes, its exactly as per the published circuit diagram of filter..

but any answers to my query??
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Old 11th May 2008, 05:24 PM   #9
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Well from you tell us, it sounds like the problem is in the filter. And a possibility is that you have made a mistake somewhere and are filtering too much of the signal, or the wrong frequencies.

So check that out first. I know from experience it is very easy to make a mistake with filter circuits.
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Old 14th May 2008, 11:17 PM   #10
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If you want to find out, which input voltage level you need for nominal output power, use the "Overture Design Guide". You can download it from http://www.national.com/appinfo/audio/

Nice tool and easy to use. Also helps to determine heat sink size, gain resistors, high pass frequency, snubber component data and mute resistor, if applicable.
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