Yet another LM3886 PCB post!

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Hello,

I appologise if I have totally missed threads about this, however I have searched but to no avail!

I am trying to sort out a PCB layout for and LM3886, I've looked at various pcb layouts on the net, however I am getting somewhat confused :scratch:

I was taking the circuit from the LM3886 datasheet, this shows 4 caps, namely:

Cm
Cs
Cs
Ci

I have seen designs with 6 caps, where have these come from!

All I want to do is sort out a high end layout, then etch it myself!

So is the standard schematic from the datasheet ok, or a bit pants to use as the cct?

Many thanks, once again appologies if I've annoyed anyone!
 
Each Cs is a pair of caps.
RF attenuation filter cap.
Zobel output load.
DC blocking cap on input.
DC blocking cap on NFB lower leg.
Muting cap is optional.
Input RCA RF filter cap is optional.

That's 8 to 10 caps and does not include the PSU.
I have missed one there should be 11 caps.

Read the sections about what National refer to as "optional components". For a reliable and consistent build/performance all these "optional components" are required.
 
Ok,

I've read the datasheet and am looking at making some additions.
I've also started to read up about Zorbel, however I can't seem to find any info on it relating to the amp, i.e (sorry for being really basic) the configuration for this app!

I've tried looking for a thread on the subject, however to no avail!
 
All sorted!(wrt Zorbel etc).

PCB should be finished in next couple of days.

Only thing is that I've gone for SS board, so if people want to it could be made at home.
Has anyone done a comparrision between 2 layer and ss boards?

I assume that 2 layers may sound better hence why they are used?
 
Decoupling is located too far from the power pins. The HF section of Cs1&2 can be 0.1" or 0.2" pin pitch. The smaller can be located right on top of the power and ground pins. Or even underneath if that suits you and aesthetics.

Although very commonly used as a solution, I do not like distance between the Vdd & Vcc lines. I recommend that the Power Lines and the Power Return be fed in, very close to each other.
The speaker output trace could be doubled or even tripled in width.
Add a bigger pad around Pin1 (Vcc). Is Pin5 also Vcc?

You have not implemented the unmute function. This will never allow the amp to turn on!!!

You have no specific connection for Signal Return/Ground. It is shared with Power Ground and Speaker Return. Isolate the Signal Returns on the PCB and give them a dedicated Signal Ground. Signal Hot and Signal Return connect directly to the input signal hot and cold at the chassis input socket

Rm seems redundant. It is normally there before the DC blocking cap to ensure the PCB input pin is always close to Zero Volts. But you have chosen to omit the DC blocking cap, so Rm should be omitted as well.

Spkr Return on the PCB works for a single channel monoblock amplifier.

When you have more than one channel inside the same chassis, all the Returns should meet at the Main Audio Ground (MAG). This cannot be on one of the amplifier PCBs.

That's all I can see, I hope the list does not get any longer,
 
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Ok,

So I've made some changes, however I have couple of more questions.

Regarding the grounding issue, CHG is the “star” ground that I then take to a central point on the chassis which I was going to use as my MAG.

You have no specific connection for Signal Return/Ground. It is shared with Power Ground and Speaker Return. Isolate the Signal Returns on the PCB and give them a dedicated Signal Ground. Signal Hot and Signal Return connect directly to the input signal hot and cold at the chassis input socket
Do you mean that Vcc and Vdd stay as they are, with Speaker, IP and OP ground becoming an Isolated signal ground. Does this then earth via the chassis input?


Many thanks.
 

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  • Layout_2.pdf
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That looks decent. You could add a corner in the output trace so it doesn't sit so close to the one connecting R3 and Ci.

All grounds must always be connected together, it's the routing that matters. Power ground should have a separate trace from input ground, that's what everyone says, but let me elaborate on it. Chassis ground should always be connected to input ground, right where the input wire connects to the board. If you connect the chassis ground to the ground of the filter caps (common mistake) you will get hum.

On the other hand, you have created a board with planes. You don't need to separate anything because you basically have solid planes of ground and power. I've yet to have a board made like that exhibit any kind of hum. So don't waste anymore time and build it. One thing i would do would be to route that output trace to the left where it meets Rz, to allow more of the ground plane to connect both sides. It's looking good otherwise.
 
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I can't follow this layout as easily as your first.

That looks like an electrolytic as the input DC blocking cap.
Use a plastic film type if you can afford the cost. Somewhere from 330nF to 4u7F depending on what Low Bass frequency response you want.

Something seems wrong or omitted in the RF attenuator. But I'm not sure.

The HF decoupling caps (nF between + & G and nF between - & G are too far away from the chipamp power pins.
 
AndrewT, thanks again for your input.

I am going to put the nf caps on the underside of the pcb, directly on the pins as per your previous post.

Relating to the DC blocking cap, I used the electrolytic as I was drawing from effects pedals, as you advise I will replace with no polarised.

Relating to the RF attenuator, which part is that?

Also would you agree, that as power planes are used, the requiremnt for isolated signal ground is delt with?

Once again, many thanks for all advice!
 
RF attenuator is a lowpass filter with a high cutoff frequency (some 100s of khz), placed at the input. Basically a small capacitor (some 10s of picofarads) across the input resistor. It isn't needed if you're driving the amp from a low impedance source such as an opamp.

In other words, if you also have an input stage for your amplifier you don't need it. If the amp input is accessible directly to the outside world it is recommended to fit it.
 
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