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-   -   LM1875 Maximum Gain (http://www.diyaudio.com/forums/chip-amps/225258-lm1875-maximum-gain.html)

 Joseph1 8th December 2012 03:52 PM

LM1875 Maximum Gain

Guys I use LM1875 as a subwoofer amplifier.It is fed from a sallen key Fc=100Hz low pass filter circuit.

Power supply is 25V DC dual supply.
Speaker (subwoofer) = 8 Ohms

I use gain by adjusting the feedback resister pair,(39K/0.56R)+1 = 70

Is this gain too high?What is the maximum gain it can handle?

 AndrewT 8th December 2012 04:03 PM

that gain = 70 gives a maximum input of 200mVac.

Does you low pass filter give good noise performance if the "maximum" signal is only 200mVac and the "normal" signal is <20mVac?

 Joseph1 8th December 2012 04:08 PM

Quote:
 Originally Posted by AndrewT (http://www.diyaudio.com/forums/showthread.php?p=3276247#post3276247) that gain = 70 gives a maximum input of 200mVac. Does you low pass filter give good noise performance if the "maximum" signal is only 200mVac and the "normal" signal is <20mVac?
Thanks Andrew

My low pass filter uses KIA4558 opamp.Actually I don't know is it giving good noise performance :)

Please tell how did you calculate that 200mVac? Is that gain ok?

 Joseph1 8th December 2012 04:17 PM

Hi Andrew thanks for the comment.

How did you calculate maximum input = 200mVac...!!!!

I use KIA4558 opamp for low pass filter.Is my gain too much?

 AndrewT 8th December 2012 05:31 PM

25Vdc for supply rails will probably allow an absolute maximum output of around 20Vpk.
This converts to an AC sinewave of ~14Vac
Divide 14Vac by 70 and you get an input (for absolute maximum output) of 200mVac.

But you can't play music at that level.
More likely you will listen to music at about -20dB ref maximum output.
That puts your input signal around 20mVac

Another unhelpful posts to go along with post2?

 Joseph1 8th December 2012 11:50 PM

Quote:
 Originally Posted by AndrewT (http://www.diyaudio.com/forums/showthread.php?p=3276341#post3276341) 25Vdc for supply rails will probably allow an absolute maximum output of around 20Vpk. This converts to an AC sinewave of ~14Vac Divide 14Vac by 70 and you get an input (for absolute maximum output) of 200mVac. But you can't play music at that level. More likely you will listen to music at about -20dB ref maximum output. That puts your input signal around 20mVac Another unhelpful posts to go along with post2?
Hi AndrewT now I understood the maximum input is 200mVac.

I'm new to this amplifier world.Those -20dB figures are newer to me.

You mean in my system the minimum input signal needs to be = 200mVac?But now I have 200mVac at maximum.

From how much gain do I need to reduce?What will be my new gain I have to set?

Thanks

 AndrewT 9th December 2012 09:58 AM

you need to find the specification for your source or sources.

Look up the output voltage levels and output impedance and recommended load impedance.

Do this for each source that you may connect to the amplifier.

When you have all this information then YOU can make an informed decision.

Without any information, YOU and us will be guessing.

Guess what?
I don't recommend guessing. Make informed decisions !

Another unhelpful post?

 Joseph1 9th December 2012 10:26 AM

Hi

The main idea of my 2.1 system is to get compatible to a wider range of sources.It can be a DVD player,mp3 player or desktop onboard sound card.

My mp3 player has 250mv sensitivity.

Just concentrate on a single amp LM1875,it is feeding from a sallenkey filter.This is I call as a subwoofer amplifier.

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

Now what is the minimum input sensitivity on my system?

 pra3718 9th December 2012 11:21 AM

Quote:
 Just concentrate on a single amp LM1875,it is feeding from a sallenkey filter.This is I call as a subwoofer amplifier.
Are you happy with single amp LM1875 for subwoofer amp ?

 Joseph1 9th December 2012 03:25 PM

Quote:
 Originally Posted by pra3718 (http://www.diyaudio.com/forums/showthread.php?p=3277138#post3277138) Are you happy with single amp LM1875 for subwoofer amp ?
This is good for a small 2.1 system which can place in a room.But actually this is too loud for a room,& you can shake the room.This system is more louder & quality than commercial available 2.1 systems which is in below 150\$ budget.

First I need to fine tune this system & after I'm going to make a bigger one :)

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