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Chip Amps Amplifiers based on integrated circuits

LM3886 Layout ok?
LM3886 Layout ok?
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Old 27th September 2009, 11:36 PM   #21
jaycee is offline jaycee  United Kingdom
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The coil and resistor are there to isolate the amp from the antenna effect of a long piece of speaker wire. This is found on pretty much all amps, however the LM3886 is often used without it. If your speaker wires will be quite short, you can probably leave this out.
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Old 28th September 2009, 08:47 AM   #22
akis is offline akis  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
separate the dirty (power) from the audio grounds, separate the high current audio from the low current audio grounds.
Then connect them at a single point.

the L of the L//R can go in the route from PCB to output terminals.
The final R+C can go across the terminals.
The first R+C must be located very close to the chipamp, to minimise the L of the interconnecting traces.
I did exactly that, ended up having lots of "GND"s wires running back to the PSU.

I used the L/R but my speaker wires are only about 2 feet long. Perhaps I should not have bothered.

I removed the R+C because it was attenuating high frequencies by as much as 50%, I remember, at 20 KHz.
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Old 28th September 2009, 10:20 AM   #23
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by akis View Post
I did exactly that, ended up having lots of "GND"s wires running back to the PSU.
no! the PSU should have ONE zero volts wire going to a centrally located STAR/AUDIO Ground. The Audio Ground can be located on the PCB IF the amp is a monoblock. If you intend building a multichannel amplifier then the Audio Ground is best off the PCBs. That will require ~ 3 ground wires from each amplifier PCB and a wire from the speaker return (now 8 for a 2channel amp) and one wire from the PSU. The PSU will also be connected to the Protective Earth (PE or Safety Earth).
Quote:
Originally Posted by akis View Post
I removed the R+C because it was attenuating high frequencies by as much as 50%, I remember, at 20 KHz.
Then you chose the wrong Zobel component values.
100nF + 2r7 should have no audible effect if the amplifier is operating correctly.
100nF + 2r7 has F-3dB @ ~600kHz. i.e. an impedance of ~5.7ohms at 600kHz. Now consider what effect adding 5.7ohms in parallel to the speaker for a 600kHz signal. Will you hear that?
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Old 28th September 2009, 10:52 AM   #24
akis is offline akis  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
no! the PSU should have ONE zero volts wire going to a centrally located STAR/AUDIO Ground. The Audio Ground can be located on the PCB IF the amp is a monoblock. If you intend building a multichannel amplifier then the Audio Ground is best off the PCBs. That will require ~ 3 ground wires from each amplifier PCB and a wire from the speaker return (now 8 for a 2channel amp) and one wire from the PSU. The PSU will also be connected to the Protective Earth (PE or Safety Earth).
Then you chose the wrong Zobel component values.
100nF + 2r7 should have no audible effect if the amplifier is operating correctly.
100nF + 2r7 has F-3dB @ ~600kHz. i.e. an impedance of ~5.7ohms at 600kHz. Now consider what effect adding 5.7ohms in parallel to the speaker for a 600kHz signal. Will you hear that?
What is the difference where the "star" ground is located? Mine is on the PSU PCB, I cannot see why putting it somewhere else and then running a wire to the PSU should make a beneficial change. Actually running a wire to the PSU means generation of voltages across that wire and separation from the transformer centre tap and caps. Why is that better? Actually leaving it on the PSU is quite convenient, everything converges there naturally and the PSU ground forms the absolute ground of my device. It is a very thick track which binds all the ground wires and the centre tranformer tap together. Please explain :-)

Your filter has its pole at the 600KHz, mine was at 800KHz if I remember correctly. And yet it screwed up....
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Old 28th September 2009, 11:10 AM   #25
AndrewT is offline AndrewT  Scotland
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if the main audio ground is located on the single amp PCB and you now connect two channels inside the box, where do you connect the common audio ground. It is impossible.
Where do you take the PSU zero volts? to one or the other.
Where do you take the RCA signal grounds?

The PSU zero volts connection has massive charging current pulses running back to the transformer. This is not a good place for the main audio ground.
Quote:
leaving it on the PSU is quite convenient, everything converges there naturally and the PSU ground forms the absolute ground
no, it's not an absolute ground.
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Old 28th September 2009, 11:46 AM   #26
akis is offline akis  United Kingdom
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There is no "main audio ground". Every PCB has individual components and stages and they have their own ground if they are carrying different currents, there are no big planes of "ground" anywhere on any PCB. All those grounds meet up separately at the PSU, everything is referenced at the PSUs (pre-amp PSU and post-amp PSU). There is no other "common" ground.

Forgot to say , on the PSUs speakers and high currents are closer to the tansformer, lower levels are further out - you could conceive this as a start arrangement where the star is 1 inch further than the caps+transformer and connected by one thick wire.

Last edited by akis; 28th September 2009 at 11:48 AM.
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Old 28th September 2009, 12:15 PM   #27
Maxxtr0 is offline Maxxtr0  Germany
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So NOW Im having problems understanding everything youre writing here...

However, this is what my PCB now looks like:
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Old 28th September 2009, 02:08 PM   #28
Maxxtr0 is offline Maxxtr0  Germany
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I cant find the Edit-Button?! So sorry for double post!

Now with R/L an the two caps on board... what do you think? Is the "GND-Point" ok?

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Old 28th September 2009, 03:06 PM   #29
AndrewT is offline AndrewT  Scotland
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Hi,
give pin7 it's own connection to that central PCB GND.
Remove the speaker return from the dirty Power ground, move it to the central PCB GND.

Is there a way to move C2 & -18V to the other end of the PCB?
That will very much shorten the length of Power Ground trace between the two decoupling caps.
It also allows a twisted triplet of power wires from the PSU to the amp PCB thus minimising loop area of the dirty radiation from the power wires.

Consider removing the N/C pins. It allows more flexibility in placing/routing traces/pads from adjacent used pins.
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Old 28th September 2009, 03:09 PM   #30
pacificblue is offline pacificblue  Germany
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You should really use a bigger PCB. All components should have some air around them for several reasons. One is cooling. Another is to avoid mechanical noises that ocurr, when adjacent components vibrate and touch each other. Then there are possible shorts not only from soldering, but also from mechanical movements. Later there is the question, how to service the amplifier. E. g. you need to remove the LM3886 to replace Rm, if you find out that it does not have the right value.

It is always a good thing to run wires of the same function together, and wires of different functions separate. E. g. the power supply wires should run side-by-side and therefore have their connections on the PCB next to each other. But they should be as far away from the signal wires as possible.

When you use big copper areas, you should use thermals. Without them soldering can become very tedious, because all the heat is drawn away from the place, where you want to solder. That means, you need more power and more soldering time, both of which is bad and can even lead to component destruction.


Quote:
Originally Posted by jaycee View Post
The coil and resistor are there to isolate the amp from the antenna effect of a long piece of speaker wire.
The coil and resistor are there to protect the amplifier, when the load is capacitive. A highly capacitive load will have a very low impedance at high frequencies and can appear like a short-circuit. The resistor provides a higher impedance there and the coil shorts the resistor at lower frequencies, so that it does not affect the signal in the audio range.
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