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Old 17th August 2014, 08:47 AM   #1
Philfr is offline Philfr  France
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Default LM3875 Premium Kit

Hi,
Planning to order LM3875 Premium Kit..
The goal is to drive 4 Ohm speakers (95 db) crossed near 200 hz, so I need best sounding 200 to 20000 hz. Source is Dac (1,9 volts output), Musical Fidelity VDac 2 with diy power supply. So I hope source is free of DC !!
Transformers will 20/0/20 volts 160 Va (cause I have it) with two separate secondary winding.

Can you check this :
1 : remove R1 and use 27 Ko potentiometer at input
2 : snubber, add R1 R2 0,1 Ohm and C3 C4 3,3 nF, change C1 C2 for 10000 uF, I read that snubber provides more clear and detailled mid and hight frequencies ?
3 : add 220 pF between pin 7 and 8, cause I have some "noises" at home.
4 : for dual power supply I do order "MUR860-KIT-DM", right ? In the kit 16 MUR860 are included.

Thank's..
Phil
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Old 23rd August 2014, 08:29 AM   #2
Philfr is offline Philfr  France
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Hi,
Nobody ?
Phil.
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Old 23rd August 2014, 09:22 AM   #3
Gopher is offline Gopher  United Kingdom
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LM3875 is not well suited to driving 4 Ohm loads. Look at the datasheet to see why.

LM3886 would be a better choice.
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Old 23rd August 2014, 10:49 AM   #4
Philfr is offline Philfr  France
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Think's,
You are right about LM3886..
But I'm confused when I read :
"I do the same test for 4ohm load (8 resistors in parallel) and initially we can see 56W power output with rails going down even further to 29.5V"
Page 36 of the LM3875 user guide from Audio Sector.
I don't understand that.
Phil.
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Old 6th September 2014, 03:26 AM   #5
ttan98 is offline ttan98  Australia
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Hi,

I built both LM3886 and LM3875, the latter sounded better though, both powered from same size transformer(26Vac). I actually referred LM3875. I used quality resistors on LM3875 though. Your speakers rated at 95dB you really don't need lots of power, 10-20 Watts/channel amp is plenty.
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Old 6th September 2014, 06:56 AM   #6
Philfr is offline Philfr  France
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Hi ttan98,
You are right but LM3875 is not well suited to driving 4 Ohm loads..
My speakers are 4 Ohm.
Phil.
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Old 6th September 2014, 07:45 AM   #7
ttan98 is offline ttan98  Australia
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Hi,

It may not be suitable to drive 4 ohms, if you push the amp very hard like pushing it to max. power output, at normal listening level which your speakers should be around 1-10 watts range, at this operating range LM3875 is ok. This is what I would expect. Even at max power output, it will not go unstable, the worse scenario is the distortion will rise very rapidly. My LM3875 drives my speakers complex load(on some freq. range around 2-4ohms) quite easily and quite loud, my speakers has sensitivity of around 88dB

If you are concern, I suggest you may like to check this with Peter from Audio Sector. After selling this chip for so many years he must have very good knowledge, he might even have conducted tests at 2-4ohms load.
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Old 7th September 2014, 10:33 PM   #8
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Agreed. My LM3875 built from Peter's kit sounds great with my 4 ohm dipole speakers (89db) played in a large space (90 m2, 5m ceilings). I have experienced no problems at all.
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Old 9th September 2014, 04:19 AM   #9
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Quote:
Originally Posted by ttan98 View Post
It may not be suitable to drive 4 ohms, if you push the amp very hard like pushing it to max. power output, at normal listening level which your speakers should be around 1-10 watts range, at this operating range LM3875 is ok. This is what I would expect. Even at max power output, it will not go unstable, the worse scenario is the distortion will rise very rapidly. My LM3875 drives my speakers complex load(on some freq. range around 2-4ohms) quite easily and quite loud, my speakers has sensitivity of around 88dB.
This is exactly how it works. I'v ealso heard from people who were successfully driving LM3875 into 2 ohm loads.
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Old 14th September 2014, 08:14 AM   #10
Philfr is offline Philfr  France
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Hi,
Thank's for reply.
For my speakers and my taste, only 5 watts is needed.
Below impedance curve :

Click the image to open in full size.

So, it will work ?

Can you check this :
1 : remove R1 and use 27 Ko potentiometer at input
2 : snubber, add R1 R2 0,1 Ohm and C3 C4 3,3 nF, change C1 C2 for 10000 uF, I read that snubber provides more clear and detailled mid and hight frequencies ?
3 : add 220 pF between pin 7 and 8, cause I have some "noises" at home.
4 : for dual power supply I do order "MUR860-KIT-DM", right ? In the kit 16 MUR860 are included.

Phil
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