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"Small form factor" modular multichannel TPA3255 amp build

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MCH

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Of course they do, but I seriously doubt it would make a measurable difference if they were shared in this application.
I see you are not fond of the current limiter solution :p
 

mcdn

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I see you are not fond of the current limiter solution :p
Not true, it’s a great device! I just think since you need an 800W PSU it’s going to have to be internal, and it’s going to have inrush limiting already
 
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Not true, it’s a great device! I just think since you need an 800W PSU it’s going to have to be internal, and it’s going to have inrush limiting already
Will do like this: leave the footprints for a power input that goes through the current limiter IC. If necessary, I can populate and use it. If not, I can ignore it and use any of the other connectors that are directly connected bypassing the current limiter.
(Now that I've read the datasheet I want to have to use it)
 
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Made some advances, and i think i am almost ready to lay down the traces (changes will be more difficult after that).

I implemented the in-rush current limiter circuit discussed above. It will be optional: will leave the footprints at the bottom side that i will populate only if necessary. In the case i need to use it, i just need to solder the necessary parts and use the new connector (below the 5 others) for the power input.

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I have also decided that the amp PCBs will be printed with 2oz copper but the control board will be 1oz only, to reduce cost. This PCB will be the one carrying more current, but only in one single trace that is very accessible and with no components on top, so i decided to leave the power bus trace without solder mask and tin it manually. Will be the first time i do this, i hope it works. I don't know if there is a problem with this approach

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Another thing i have been dealing with is how to connect the heat pipe to ground:
TI explains that the heat sink (in my case heat pipe) must have a good connection to the PCB ground, the reason being it is a noisy device and could increase the noise in the amp. It is not enough to leave the connection to ground depending on the contact of the heat sink with the pad on the chip (that is also connected to ground). Their recommendation is to use heat paste between chip and heat sink + screws between heat sink and PCB ground.
Obviously, i cannot do that with a heat pipe.
At first i thought that i could use liquid metal instead of heat paste between chip and heat sink to grant a good electrical contact (I would have to coat the the copper pipe, but that should be no problem). But i could not find any information about the material/compatibility of the pad on the chip nor its exact shape, so i abandoned the idea.
But this morning i thought about using a couple of spring load contacts. The ones indicated with arrows in the picture below have a height of 1.5mm when free and are aimed to work at 1.2 mm when pressed, that is the exact height of the chip. Let's see how they work.

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Another detail i need to deal with the is, potentially, impedance matching of the oscillator pins. I will be connecting the clock of the master board to the other boards through cables. I will have to find out what will be the best way to avoid potential issues beforehand. The frequency is not very high, but the distance between chips yes. TI does not make any provision about this in their evaluation board, maybe it is a non issue. I am not sure if the 3E audio boards (that expose the connectors to set their boards in master/slave mode) have some sort impedance matching. Will have to see what i can find out online....
 
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hey @mcdn i need your help.

As mentioned in the previous post, i want to find out what is the best way to connect the oscillator pins (pins 9 and 10) between chips for master/slave mode. This is something that the TI documentation does not cover (or maybe they are suggesting not to use any resistor in between?).

In any case, there are a bunch of boards out there that have more than one chip, and are wired in master/slave mode, but the only one i could find complete pictures without heat sink is the one you measured here in ASR (i believe it is an Aiyima).
According to the pictures, following the traces, the oscillator pins seem to be connected between chips through 33R resistors (see picture below), but i would love to have it confirmed. If you happen to have the board accesible, is this something that you could check? If you don't fancy messing there no worries, thank you anyways!

the two 33R resistors indicated with a red arrow:
1713294418405.png
 

mcdn

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The board I measured is a single chip design with 4xSE outputs. See section 10.2.1.2.4 of the datasheet for details on using a master clock and inter-channel delays. There's no mention of a resistor, but if used maybe it's for current limiting to protect the output from the master clock in case of short circuit? Resistance of the input would be multiple kOhms, so adding 33R in series only makes sense for protection.
 
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mcdn

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Dear god I am an idiot. I'll have to redo the review of the 4CH module because it does indeed have two chips in BTL, not one in SE. Even more of a bargain!
 
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Too many inductors for a single chip :)
And did you have the chance to follow those two resistors when you looked inside?
 
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Interesting piece of information that answers the questions I had not covered by TI eval board documentation: the service manual of the Denon Heos AVR that uses 3 TPA3251 for LR, C and rear channels. In between other things one can find in the schematics:

How to wire the master slave pins - what i was looling for:
Nothing special at the master side. At the slave side, two 10 ohm resistors and two 10pF caps, all this to be placed close to the input pins. Exactly what i wanted to know

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How to add relays to the speaker outputs to avoid on/off pops. I don't plan to use relays in my boards, i am going to trust the anti-pop system of the eval board, but I see this approach in most aiyima/fosi/etc. TPA3255 little amps. Good to know how to do it correctly, and with part numbers to make things even easier. The MCU signal switches a transistor that cascades two other transistors that energize the relays using PVDD (red line).
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i decided that i am going to go for the relays to avoid power off pop, the reason is the following:

On the contrary to the Denon AVR, none of the popular recent TPA3255 that I know uses a relay to mute the speakers, not even the TI eval board uses one, and many uses report a more or less noticeable pop, most commonly at power off. this is what we have:

fosi V3 stereo; fosi ZA3; Aiyima A07 max; 3E audio board stereo (this one the worst offender (note that one of the reasons wiim claims for not having implemented PFFB in their AMP is precisely the pop on reseting the chip. No idea if it is related, but i believe worth mentioning)
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But the new Fosi V3 mono is the first of the breed to use a relay to mute the speaker (the bigger black relay near the thermistor). The implementation is a bit different to the Denon, as it only mutes one of the speakers terminals (i believe the other contact bypasses the thermistor, don't ask me why)

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and look at the result, perfect!!
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Still has a spike at "on", but that is not so bad on my book as it feels much worse when your listening session has finished, everything is in silence, and a few minutes later, when the amp goes to sleep, "POP!" I hate that.
I can imagine they needed to do it as a result of having the the audio sense option, but anyways, well done Fosi!
I only wonder why the mechanism does not apply on power on, but maybe it is a matter of programming the microcontroller differently, who knows.
 
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