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Old 10th September 2011, 04:53 PM   #21
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Here is the schematic for the LM3886 "XY Power Amp" circuit board that is widely available on Ebay:

Click the image to open in full size.


Click the image to open in full size.


I couldn't find the schematic posted anywhere, so I traced the routing myself.

These are very inexpensive - I just bought 10 bare boards for about US$25, with shipping included. I plan to build up a set of these amp boards using good quality components for use in active speaker projects. If you want moderate power levels (30W@8 ohms or 50W@4 ohms) and low distortion across the audio band, this is probably the simplest and least expensive way to get it.

-Charlie

Last edited by CharlieLaub; 10th September 2011 at 04:58 PM.
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Old 10th September 2011, 05:06 PM   #22
AndrewT is offline AndrewT  Scotland
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RF attenuation is omitted.
Very easy to correct: add a parallel cap across R5 either on top of the PCB, or underneath the PCB.

Output Thiele Network is omitted.
Very easy to correct: remove the output terminal block. Connect the Zobel capacitor in the spare location.
Tap off the speaker output from the top end of R1.

The other half of the Thiele Network (L//R) can be mounted offboard in the line that joins the amp PCB to the speaker terminals fitted to the chassis.
The speaker return connects to the Main Audio Ground, not to the amp PCB.

Finally,
check to see how R5 and C4 are connected to Signal Ground. They must not connect to Power Ground on the underside of the PCB.
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Last edited by AndrewT; 10th September 2011 at 05:09 PM.
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Old 10th September 2011, 05:26 PM   #23
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Quote:
Originally Posted by AndrewT View Post
RF attenuation is omitted.
Very easy to correct: add a parallel cap across R5 either on top of the PCB, or underneath the PCB.

Output Thiele Network is omitted.
Very easy to correct: remove the output terminal block. Connect the Zobel capacitor in the spare location.
Tap off the speaker output from the top end of R1.

The other half of the Thiele Network (L//R) can be mounted offboard in the line that joins the amp PCB to the speaker terminals fitted to the chassis.
The speaker return connects to the Main Audio Ground, not to the amp PCB.
It was never quite clear to me why these "Thiele" (zobel?) networks were needed. Is this to shunt RF picked up by long speaker cable runs to ground?

Quote:
Originally Posted by AndrewT View Post
Finally,
check to see how R5 and C4 are connected to Signal Ground. They must not connect to Power Ground on the underside of the PCB.
Yes, it is as shown on the schematic, with R5 and C4 connecting to the signal ground, which is isolated from the power ground.

-Charlie
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Old 10th September 2011, 05:30 PM   #24
AndrewT is offline AndrewT  Scotland
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Hi,
the Thiele Network is there to help with stability of the amplifier depending on what is connected to the chassis speaker terminals.

A Zobel on it's own does very little to attenuate RF and back emf coming from the speaker side of the chassis.
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Old 10th September 2011, 06:13 PM   #25
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Quote:
Originally Posted by AndrewT View Post
Hi,
the Thiele Network is there to help with stability of the amplifier depending on what is connected to the chassis speaker terminals.

A Zobel on it's own does very little to attenuate RF and back emf coming from the speaker side of the chassis.
Thanks, Andrew. I did some reading on the web and found some nice explanations of each part of the output network.

But in my case, I will be directly connecting the chip amps to drivers with nothing but a short piece of wire in between. I found this old post by Svante about this scenario:
Quote:
Originally Posted by Svante View Post
If you use a normal solid state amplifier with no crossover between the amp and speaker, impedance compensation is not needed.

Impedance compensation can only be needed when the source (=amp+filter+cables) has an output impedance that is comparable with that of the driver.

Some say that a resistive load is better for the amplifier than a reactive load, but in normal cases, the load rather becomes more difficult for the amp if a compensating network is used even though it becomes resistive.
I would tend to agree with Svante - that none of the output network(s) are really needed. I don't see how the complex impedance of a single driver is going to matter all that much in this case. But I would like to get your opinion, too.

-Charlie
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Old 13th September 2011, 11:46 AM   #26
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I was planning on buying one of these XY Power Amp kits from Ebay soon - So I'd be interested on what shortcomings it has (and how one can remedy them).

It will be used to drive a small pair of DIY bookshelf speakers.
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Old 13th September 2011, 05:42 PM   #27
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Quote:
Originally Posted by undefinedza View Post
I was planning on buying one of these XY Power Amp kits from Ebay soon - So I'd be interested on what shortcomings it has (and how one can remedy them).

It will be used to drive a small pair of DIY bookshelf speakers.
Just follow AndrewT's suggestions to add on some extra networks at the input and output of the PCB.

Charlie
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Old 13th September 2011, 08:31 PM   #28
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Quote:
Originally Posted by CharlieLaub View Post
Just follow AndrewT's suggestions to add on some extra networks at the input and output of the PCB.

Charlie
I will, however unfortunately I have no idea what RF attenuation is and what value of cap to use...

Zobel/Thiele network? Purpose? Components involved?

Can you point me in the right direction please.

Thanks,
Chris
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Old 14th September 2011, 07:44 AM   #29
AndrewT is offline AndrewT  Scotland
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look at the National datasheet
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Old 15th September 2011, 07:44 PM   #30
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Hi AndrewT,

I've looked through the datasheet and see the optional components section however it doesn't mean much to me - nevermind the implementation thereof.

Is there anyway you could briefly tell me what to add and what each addition would do?

I would be grateful.

Kind Regards,
Chris
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