LM1875 Maximum Gain

Joseph1 · 2012-12-08 4:52 pm

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 · 2012-12-08 5:03 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 · 2012-12-08 5:08 pm

AndrewT said:
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 · 2012-12-08 5:17 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 · 2012-12-08 6:31 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 · 2012-12-09 12:50 am

AndrewT said:
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 · 2012-12-09 10:58 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 · 2012-12-09 11:26 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 · 2012-12-09 12:21 pm

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 · 2012-12-09 4:25 pm

pra3718 said:
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

pra3718 · 2012-12-10 6:45 am

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.

Joseph1 · 2012-12-10 8:45 am

Hi pra thanks for the link.Do you have any schematic of it?

pra3718 · 2012-12-10 8:57 am

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.

Joseph1 · 2012-12-11 11:23 pm

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?

AndrewT · 2012-12-12 10:45 am

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.

sreten · 2012-12-12 11:07 am

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.

AndrewT · 2012-12-12 11:13 am

or MFB filters where the gain can be selected.

Joseph1 · 2012-12-12 11:23 am

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.

Joseph1 · 2012-12-12 2:24 pm

sreten said:
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?

LM1875 Maximum Gain

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