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Aguilabrava 29th December 2012 05:03 AM

HELP with LM3886 X3 in parallel.
Dear Friends:

I'm back.

I need your help again. I have decided to build an amplifier using some PCB's I got from the auction site, they use three LM3886 chips connected in parallel to produce 150W per channel. I am so happy with the amp I built with your help using one LM3886 per channel that I got really motivated to work on this new project.

I have read something about some amplifiers built by Rowland that use this same design. I'm pretty sure that these PCB's will need modifications, that's why I came here, you guys were super helpful the first time and I am more than satisfied with the quality of sound obtained from the first single chip amp project.

I hope you will help me with this one the same way you did with the other one. My financial situation has improved a little bit since I did the first one, but now I'm facing some bad health issues, I need to keep myself busy now.

Here is the link for the PCB's I got:

LM3886 x3 150W Amplifier PCB Reliable Design | eBay

Your help is greatly appreciated.

svon 29th December 2012 11:17 AM

All resistors in the feedback should be matched up 0.1%

bcmbob 29th December 2012 04:40 PM

1 Attachment(s)
I built two of those from the same vendor. They are still stock as I'v been distracted with other projects. I think it important to better define what improvements you are seeking.

I found the sound very, very close to a single chip design. My initial plan was to use them on the bottom of a bi-amped system thinking the bottom would be improved. There was a little of that but the major feature is their ability to handle 4 ohm speakers.

Please supply additional information describing where you plan to use the amps an what other equipment will be in the chain.

arthur 29th December 2012 05:28 PM

Please do not quote the false "I have read something about some amplifiers built by Rowland that use this same design"

Budgie 29th December 2012 05:34 PM

Whats false about that claim? Rowland did use LM3886 in at least one model.

arthur 29th December 2012 06:41 PM

yes, jrdg used lm3886 before,
but totally different circuitry

Aguilabrava 29th December 2012 08:50 PM

Thank you!!!!

Thank you all for your prompt responses.

The claim about this being a Rowland design is not mine, again, I'm new to all this, I've read it in some posts on the Internet and the seller of the PCB's I purchased says it on the description.

I want to build another LM3886 based amplifier, if you check here in this site I have another thread, "Bought a XY LM3886....", that was the first project I have ever done, it came out great, the sound quality is excellent, I love my amp, the idea now is to build this one one with the six LM3886, three per channel. I want to see how it sounds, I guess it must sound better since it will be more powerful. In the first project I got help from AndrewT, Pacificblue and others that were very nice and generous to share their knowledge, without their help it would have been impossible.

The XY LM3886 single chip PCB's needed few modifications, the input signal ground had to be separated from the main ground, a capacitor had to be connected in parallel to R4 to filter RF, and a Zobel and Thiele networks were added to the outputs, also some resistor and capacitor values were changed to improve the quality of the sound. All these were suggestions made by AndrewT, Pacificblue and all the other experts that helped me, I couldn't be happier with the final result, the sound is rich and crystal clear.

Regarding this parallel implementation, I've seen some posts here that suggest there is a problem with DC offset, I want to know what to do to avoid that issue. Also, in my previous project I used a single power supply and one toroidal transformer. In this one, should I use a single power supply with one transformer or should I use two power supplies and two transformers?. What's better?.

Also, the builds that I have seen use an additional PCB board to balance the inputs of each of the PCB's, is this mandatory?. If it is, can you suggest the right PCB board for that purpose?.

I'm going to use this amp to drive 4 OHM and 8 OHM speakers, I only need one input, as a source I will use a QLS QA350 Wave Player.

The other issue is that I didn't realize these boards use some SMD resistors, will I be able to solder those myself with a regular solder gun?.

Thank you very much.

bcmbob 29th December 2012 09:26 PM

The SMD on this board are simply straight forward. Here are a couple video aids, The first one was all I needed to get good results. The second has also been recommended by several builders.

Surface Mount Soldering

Pro SMT Soldering

nigelwright7557 29th December 2012 09:26 PM

I wouldn't recommend using 3 LM3886 in parallel.
I would buy a good class AB amp or a class D amp.

It just seems dodgy wiring 3 outputs together where there might be a DC offset or some slight dissimilarities in the resistors.

bcmbob 29th December 2012 09:39 PM

The amps do function well as designed, but there is a rather detailed and involved process to match all the DC offsets. There is a good post about it and I will put it up when I find it. For now please look at this site.

This is the base design.

Since it has been mentioned twice on one page, it appears I need to revisit mine after buying some closely matched resistors. You can see in the picture I just used some generics purchased on eBay.

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