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Mooly 18th December 2012 07:50 AM

Speaker impedance... I would say you'll be OK with the LM1875. Its a complex subject, the amp will have a max current it can deliver and a max voltage and a max total power dissipation. So the quoted 4 ohms "covers all bases" for the published spec. Use the amp at full voltage swing into a below than 4 ohms load and it will limit. Abuse it too much and its reliability may suffer despite the inbuilt protection.

In the real world I would say 3.2 minimum (that figure will be at some specific frequency, other frequencies will have much higher impedance) will be OK.

Philfr 18th December 2012 08:26 AM

impedance curve :
An externally hosted image should be here but it no longer works. Please upload images instead of linking to them to prevent this.

Mooly 18th December 2012 09:39 AM

Where's the graph come from ? :)

It seems odd seeing the impedance drop to "zero" around 4K.

As to the LM1875, it's rated for 4 ohm which is as "good" as any small chip can drive. I still think it would be OK given the low supply voltage but that leads to another question. Are you going to be able to get the levels you need with the power available ?

Philfr 18th December 2012 11:12 AM

first curve is with PLLXO (passive line level xover)..
This one is without :

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Philfr 18th December 2012 11:18 AM

More details about sub, it's a MLTL Bipole with two 5' drivers in phase and wiring in parallel, efficiency is about 95 db (at 1 meter and taking into account effect walls)..
For my new small listening room, 5/10 watts are good enough..

johnr66 18th December 2012 03:23 PM

At +/- 12v supply. The peak voltage swing from the amplifier with 3 ohm load is 10v or less or about 7 volts rms max. 10v/3ohms= 3.33a peak or 2.33a rms. Well within the safe limit of the 1875. The 2050 has 5a peak current so there's more wiggle room. Another benefit of the 2050 is that it is able to output a bit higher voltage swing than the 1875 at a given supply voltage so this means a bit more output power. 1 to 1.5w, I'd guess.

Philfr 18th December 2012 03:42 PM

Hi John,
Thank's, I had seen TDA2050 here :
DIY TDA2050 Hi-Fi Chip Amplifier (chipamp)
So you think it's better in my case (3 Ohm)
Problem is I am not found it in my country..
TDA2050 vs LM1875 :confused:

Philfr 18th December 2012 04:22 PM

Good news :), I found TDA2050 in France :
Mixelec. TDA2050V

johnr66 18th December 2012 05:06 PM

Is that company reliable? Price seems low. There are probably more fakes 2050s out there than authentic ones. Nothing wrong with the 1875 if you have a better source.

Mooly 18th December 2012 05:42 PM

Thanks for the graph details.I still don't see how it can fall to zero ohms impedance (which is a short at those frequencies)... wouldn't the voice coil set a minimum value... not my thing speaker theory and design... it just doesn't sound right though :)

No matter :D

So are we going for the TDA2050 ?


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