BPA300 Build

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Hi Folks,

I recently bought four of these fellas...

Assembled LM3886 X3 in Parallel 150W Pure DC Mono Power Amplifier Board HIFI | eBay


... and I am interested in building something very similar to the BPA300 by Alex.

I'll hold my hands up an admit here that I am a complete novice to the DIY audio stuff and don't really understand what I am doing. I have dabbling with electronics for the last few years and working with things like RPi and Moteino to build some home automation stuff, so whilst I have a decent basic understanding of electronics at this stage this is quite a radical departure and I will need a lot of hand holding if people are willing to help. I do learn quickly as long as small words and visual aids are used. lol.

Looking at the build information on shine7 the design and construction of this setup appears to be fairly straightforward and I assume the board I have bought from ebay is a close derivative of the PA150 design, it certainly appears to be.

What I am curious about is the power amp setup used in the BPA300 seems quite different from what is used in other gainclone builds, where exactly is the rectification taking place in his setup. I am probably missing something obvious but would like to understand it better.

Best regards, Peter.
 
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Welcome to diyAudio :)

The boards in your link require an external power supply. This would consist of a transformer with a either dual winding or a split winding, a bridge rectifier and two reservoir caps.

The board say up to -/+36 volts DC and so that places an upper limit of using a 25-0-25 volt AC transformer, however for safety and to allow for voltage rise at light loading a 20-0-20 would be more suitable. The caps should be around 6800uF to 10,000uF and the rectifier should be a 35A 400 volt type.

The transformer will need to be substantial if you are going to run four of these off one power supply, so something like a 500VA toroid in practice.
 
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The biggest concern is the fake components.

I think it's a given that some of the components came from the flea market. I'd be more concerned about the engineering that went into that amp.

For example their claim of 150 W is flat-out wrong. It is true that a single LM3886 will provide 50 W into 8 Ω on a ±35 V supply, but that doesn't mean you'll get 150 W if you put three in parallel. You'll still only get 50 W as the amp will be limited by the supply voltage.
You'll get about 100 W into 4 Ω, which is an improvement over the 68 W provided by the LM3886 by itself.

Tom
 
I think it's a given that some of the components came from the flea market. I'd be more concerned about the engineering that went into that amp.

For example their claim of 150 W is flat-out wrong. It is true that a single LM3886 will provide 50 W into 8 Ω on a ±35 V supply, but that doesn't mean you'll get 150 W if you put three in parallel. You'll still only get 50 W as the amp will be limited by the supply voltage.
You'll get about 100 W into 4 Ω, which is an improvement over the 68 W provided by the LM3886 by itself.

Tom

Tom,

Is this board/amp not basically a clone of the PA150 by Shine7, it also has the chips wired in parallel, if not what do you believe the differences are ?

BR Peter.
 
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Is this board/amp not basically a clone of the PA150 by Shine7, it also has the chips wired in parallel, if not what do you believe the differences are ?

Note the spec by Shine7: 150 W into 2.7 Ω. Amplifiers are normally specified at 4 Ω or 8 Ω. Not 2.7 Ω. 150 W into 2.7 Ω is 20.1 V RMS, which will dissipate 101.25 W into 4 Ω. Physics.

I'm not interested in evaluating the various copycat eBay products. I am very familiar with the time, effort, skill, and money it takes to develop a good circuit and a good PCB layout. I see no point in helping the copycats in their mission to drive honest businesses, such as mine, out of business. I hope you understand.

If you're interested in Shine7's circuit, how about contacting him/her directly? It looks like they respond to comments via their Facebook page. Shine7 Audio DIY Page

Tom
 
Note the spec by Shine7: 150 W into 2.7 Ω. Amplifiers are normally specified at 4 Ω or 8 Ω. Not 2.7 Ω. 150 W into 2.7 Ω is 20.1 V RMS, which will dissipate 101.25 W into 4 Ω. Physics.

Tom

Yes, you are correct at that point, I had a good read again through the information on his site.

Note the spec by Shine7: 150 W into 2.7 Ω. Amplifiers are normally specified at 4 Ω or 8 Ω. Not 2.7 Ω. 150 W into 2.7 Ω is 20.1 V RMS, which will dissipate 101.25 W into 4 Ω. Physics.

I'm not interested in evaluating the various copycat eBay products. I am very familiar with the time, effort, skill, and money it takes to develop a good circuit and a good PCB layout. I see no point in helping the copycats in their mission to drive honest businesses, such as mine, out of business. I hope you understand.

If you're interested in Shine7's circuit, how about contacting him/her directly? It looks like they respond to comments via their Facebook page. Shine7 Audio DIY Page

Tom

I kind of see where you are coming from, at the same time I don't how providing me some high level detail on what you believe makes their product inferior is helping them. I would say quite the opposite is true, I would be more likely inclined to buy from a reputable source once I had some understanding of the differences.

Having said that I am going to get some of the original PCBs made up and will source the components myself and hopefully learn a bit more along the way.

BTW - I had a look at your products, some very nice kit there, fair play!
 
To be honest, I can't see enough of the eBay board to be able to provide any meaningful critique of the layout. If it's a well done 4-layer board, it could be a good layout. I strongly doubt it's anything beyond a connect-the-dots 2-layer layout, however.
From what I can see, it looks like the eBay vendor forgot to add the local decoupling on the LM3886es - both the 2x100 nF and the 2x10-22 uF per LM3886. They could be on the bottom as is the case with Shine7's board, but I doubt they are. You rarely see boards with hacks like that for sale on eBay due to the added assembly cost of such fly-by-wire components.
Another thing is that you can't buy the components alone, even at high quantities, for the total cost of the done-up board with shipping. That should tell you something.

Looking at the Shine7 layout, I see several issues. First off, bringing the supply in from each side is about the worst you can do from a rail-induced distortion point-of-view. Douglas Self writes about this in his Power Amp Design Book.
There is no local decoupling on the LM3886es. The 2x100 nF he solders to the bottom of the board are not enough. You need a pair of small electrolytic or tantalum caps (10-22 uF) there as well. The Thiele network (L||R in series with the output) is missing in the Shine7 circuit, but strangely present in the eBay circuit. Go figure.
The passives around the LM3886 look to be 0805 size, maybe 0603. This means the feedback resistor is rated for 125 mW or maybe 62.5 mW. Granted, Shine7 wants you to use 200k/10k for the feedback network, so you'll be OK from a power dissipation perspective. Sadly, 200k/10k is 20 dB noisier than the more commonly used values of 20k/1k. Change the parts to 20k/1k and you'll bump into the dissipation limit on the 20k and even if you don't, it'll certainly get hot, which means you'll get additional distortion from its temperature coefficient.
I would also add ballast resistors on the MUTE pins of the LM3886es to make sure that they all come all the way out of the MUTE state. That's more of a CYA thing, though.
I do like that the Shine7 board includes options for using either SMD or leaded resistors for the 0.2 Ω ballast resistors. Finding low TCR resistors in SMD can be a challenge at times.

It's interesting to note that the eBay circuit carries forward some of the design flaws (lack of decoupling, for example) of the original circuit. That should tell you something about their technical skill. :)

If you skimp on the decoupling (i.e. use less than 10 uF || 100 nF), the LM3886 will oscillate at higher output currents (so as the output swings near the rail with a 4 Ω load). Ask me how I know. ;) You can see my take on LM3886 decoupling here: Taming the LM3886 - Supply Decoupling.

Best of luck with your build. I hope I've given you a few search terms to pursue and nudge you in the right direction.

First and foremost: Read the LM3886 data sheet, including the application section in the back.

Tom
 
bauderline - I think it’s fair to say that Tom understands at least as much about maximizing performance the performance of the LM3886, and engineering work-arounds to mitigate for its shortcomings as any member frequenting these forums. One only need to read the “Taming” section of his site to realize he’s put some thought into the subject.
 
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