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Old 2nd January 2013, 06:13 PM   #31
Marc Vi is offline Marc Vi  Netherlands
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This is the thread:
LM3886 without electrolytic feedback capacitor and (hardly) no DC offset voltage.
You have to change the placing of the components on the board a little bit.
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Old 2nd January 2013, 11:26 PM   #32
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Default Schematic provided by seller.

Hi:

Here is the schematics that the seller of the PCB sent me via email.
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Old 3rd January 2013, 05:16 PM   #33
Marc Vi is offline Marc Vi  Netherlands
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Hello,

I know this schematics. I don't know if you also get - with the board - building instructions and instructions how to tune the offset voltages by the three adjustable resistors of 50 K. If so, please send it to me. If not, I think I can help you.

I have build myself a single non-inverting LM3886 amp. And I did measurements on that amplifier. With this experience and my experience of amplifiers in general, I gave some proposals for improvements. Let me hear if you are interested.

Marc.
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Old 3rd January 2013, 07:16 PM   #34
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Default Thank you, Marc.

Hi Marc:

No, the seller did not provide any building instructions, only the schematic posted and a bill of materials.

Yesterday I ordered all the stuff I need to start working on these PCB's except the terminal connectors, I'm still looking for those.

I have no clue on how to adjust the DC Offset with the three variable 50K resistors, so yes, I need your help in that department, and also with everything else. I need to know what toroidal I will need for this, I believe I have a 28V and 24V at home, also what kind of power supply, and if I will need that balancing PCB.

I also built a single non-inverted LM3886 amplifier with the help of many people here, specially AndrewT and Pacificblue, but I see this parallel amp is a totally different thing. I love the sound from the single one I built, that's what motivated me to build this.

Some of the parts I ordered from Mouser are back ordered and I requested to ship them all together so it will take a while until I get them.

And yes, again, I'm new to all this, I need ALL the help I can get, thank you very much.
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Old 3rd January 2013, 08:21 PM   #35
bcmbob is online now bcmbob  United States
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Here is the note I received back a year or so ago from Stanton at Jim's Audio on eBay:


"The DC offset voltage can be measured by using voltmeter, one probe to the output point, and another probe to the ground. The signal ground and the power ground are the same. If there is a voltage, then adjust the trimmer to reduce the voltage. If the output DC is so low ( say below 5mv) then there is no need to adjust. For 'one by one' it really means connect the 0.22 ohm resistors one by one to ensure each LM3886 has the minimum offset before hook all three together."

Output point meaning pin 3 on the LM3886 if that's what you choose to use. With the single resistor installed you can catch it at the output pad on the board.

It's the method I used and hopefully will be helpful to some.
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"Arrange Whatever Pieces Come Your Way."

Last edited by bcmbob; 3rd January 2013 at 08:26 PM.
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Old 3rd January 2013, 08:39 PM   #36
Marc Vi is offline Marc Vi  Netherlands
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Oké,

If you have the board and all the components, you assembly the board except for the resistors of 0,2 ohm: R7, R9 and R10. You do not place them.

You make a bridging wire over R7 but not the others. There is no input signal. You measure the offset voltage of the first amplifier and you tune the offset voltage to 0,0 mV by R4. Then you connect a speaker and put a signal (for instance music) on the input. You boost the input signal so the amplifier will be warm (but not too hot) for let's say ten minutes. You then shut down the input signal, wait a little while, measure the offset voltage and tune it to 0,0 mV by adjusting R4. You repeat this procedure several times.

You remove the bridging wire over R7 and make a bridging wire over R9. Then you repeat the same procedure for the second LM3886.

At last you do this for the third LM3886.

After doing this for the three amplifiers individually, you do a final measurement at each output to be sure that the offset voltages will still be around zero mV. Then you place the resistors of 0,2 ohm: R7, R9 and R10.

Now the amplifier is ready for playing music. If you have a working amplifier, I maybe can give you more advises for impovements.

Concerning the toroidal: both will work but what current do these toriodals give? That is more important than the voltage. So give me more details.

Marc.
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Old 10th January 2013, 04:21 AM   #37
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Default Power Supply.

Hi guys:

I already ordered the components I need for the two LM3886 X3 PCBs from Mouser, some of the stuff I ordered was backordered and I requested the shipment of the complete order to avoid additional shipping charges so is going to take longer than normal for me to get it, but hopefully it will be here soon so we can start putting this thing together.

Right now I've been working on finding a PCB board to put the rectifier and the resistors and snubber capacitors so I can use bigger smoother caps mounted separately in the chassis, but I haven't found anything.

I was thinking that perhaps I can use one of the PSU board I used in my single build, but instead of soldering the smoother caps to the PCB like I did there, I could instead solder some wires to it and connect the bigger caps that way. I have the PCB, rectifier, resistors and snubber cap, I want to get 4 Kendeil 47000uf 50V or perhaps 4 Kendeil 68000uf 50V caps with screw terminals. I have attached pictures of the PCB board I already have and the Kendeil caps I would like to use in this build.

I am planning to use a 24V toroidal with 500VA with this power supply.

Is this possible?.

Thanks.
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