Help with Class D amp measurements and XLR input wiring

[isophase]

My guess is you may need fan cooling.

I installed the power supply on top of x2 aluminium sheets of 2mm (couldn't find a 5mm plate at the metal shop junk box..) and added some thermal paste for good heat transfer.
I originally had bought and tried a 600Watt SE series Meanwell power supply, but this unit had really impressive fan noise! it was WAY too noisy and so i decided to change it and got an UHP series, this model uses no fan but they need to dissipate the heat on the chassis or an aluminium plate as advised on the UHP series documentation.
You can see on the photos the placement of the IEC connector was originally very close to the input connectors due to the quite big 600W PS (not very good especially with vulnerable unballanced inputs!) Paradoxially, the new power supply can deliver more power (750W) and is smaller ! so i moved the connector away from the inputs, and put a little aluminium plate to close the big ugly hole...
All my fault as I had started the chassis work before testing the power supply, lesson learned !

 

[pvision]

Very neat final result!

 

[Mr. Moscode]

Really nice clean construction. Thumbs up!

What I'm offering next depends on whether I understand the in and out scheme.
RE inputs: As I understand it there are 16 inputs ( 1/ch) each with 1 xlr input jack.
And your feeding a balanced signal from sound cards into pin 2 and 3, right?
But the board inputs are all single ended so you have an unused card output, the negative phase, which you are shorting through pin 3 to ground.

That may work just fine However! You're shorting a card output to ground which could damage or cook the neg phase driver amp in the card, or it could introduce distortion that could bleed to the + output. Those drivers will be driving a short which could cause the op amp drivers to get hot. It all depends on whether the sound card chips have short circuit protection for a continuous short.

What is the input impedance of each channels (which should be identical)? 10K maybe? If you can't find that out use 10k.
What ever that is, use that value resistor to put between pin 3 and audio ground for 2 reasons. Your sound card will be protected and your noise rejection should be better because the balanced output will see a balanced input.

I see the speaker outputs are balanced going thru XLRs?
If you have RF radiation that causes

I hope it works just like you want it to. Again nice work!
Good luck

 

[isophase]

Hi,

Thank you Mr Moscode and Pvision for the compliment on the build.
No, all pin 3 from the XLR inputs are connected to input ground on the pcb (and jumpered together by stereo pairs) but the PCBs are NOT referenced to ground, only to psu 0 volt. So pin 3 are not shorted to ground. I don't know if this is the best solution for now but it works.
The outputs are not balanced, they are normal positive/negative speaker terminals, but i used XLR connectors as requested by the artist. I did suggest using speakon connectors instead in order to avoid confusion in the future, but as i understand the artist already owns a similar amplifier (with fewer channels) and using some transducers already equipped with XLR connectors, i did this so that this system remains "compatible" with the other system.

I'm still trying to figure out the "SD" connection on the pcbs "GND/MUTE/SD" connector, I have no idea what it is, or what it means?
The amplifier does "plock" on switch on/off so i'm thinking of adding a mute switch to the unit to protect the transducers on power on/off.
I did some measurements on the 3 pin connector "GND/MUTE/SD"
GND/SD = 10VDC
MUTE/SD= 10VDC
Chassis/SD=2VAC
GND/MUTE=1,7AC
I also fed the inputs with 1Khz 300mV (max voltage on my phone's signal generator) and tried to measure on the 3 pin connector to see if i could measure any signal, but the readings keeps jumping around on my multimeter. I'm trying to figure out whether or not i could use a unique single pole switch for muting all eight pcbs at once and avoid having to using an expensive 8 pole switch to mute each cards individually. Do you know what i mean?
Signal does mute the signal when i short either "MUTE/GND" and/or "MUTE/SD" pins.
Its so frustrating that there is no schematic or even simple documentation provided with these cards...

 

[isophase]

 

[isophase]

Actually, i can hear a slight "buzz" when the inputs are left unconnected, but it goes away as soon as i connect a cable on the input from my sound card or even a cell phone running on battery. I think this is all normal. The buzz varies with the volume setting so the noise is being picked up at the inputs...
I'm more concerned about the light dimmers in the real installation... fingers crossed

 

[isophase]

Hello,
We did the first installation today and it was…… noisy.
In the end, I had to short pin 3 to ground on all the inputs for much quieter noise, better but even with that, we can still hear a little bit of noise (strange high pitch noise) the light dimmers don’t affect the noise at all so that’s good.
I did test a couple of inputs with “d.i. Box” which is really a simple 1:1 Neutrik isolation transformer rather than a D.I. And that instantly cured the noise. I don’t see a better solution?
To isolate all the XLR inputs with small transformers (is there any other good economic transformers in the market in the same price range as the neutrik?)

I’m thinking the small 4:1 neutrik transformer is a good candidate also to better match the input of the amplifier cards to +4dBu. That would provide 12dB attenuation.

Or is there any other good solution?

 

[isophase]

Also trying to find a solution for muting all sixteen channels at once before switch on and off, to protect the transducers.

 

[cyrano]

I think the "SD" pin is actually an ON/OFF.

In most of these chip designs, the chip is constantly connected to power. If the SD pin is low, it's on standby, with very low power consumption.

Nice project, BTW.

I "built" something similar, but bought the 4-channel amps ready-made. I'm up to 12 identical channels. The last 4-channel amp I added, was a different design, unfortunately.

 

[isophase]

Thank you Cyrano.
What do you mean when you say “if the SD pin is low”?

The “high pitched” noise is finally cured!
It went away by adjusting the voltage from 22VDC to 24VDC on the meanwell power supply. These amplifier cards are supposed to work with a voltage range between 10 to 26VDC. Mabe 22VDC is asking too much current? I don’t know?
Whatever it is, I’m happy the amplifier is very quiet now.

 

[cyrano]

I mean a logical low, like zero volt for logical zero and +5V for logical 1.

In this case, logical 1 could even be 10V. Check the spec sheet!

 

[isophase]

I had a look again at the spec sheet. I think « SD » stands for « shut down » most probably correspond to the « SDZ » pin on the spec sheet. A Logic input. Thank you Cyrano. Let’s see how I can implement a global mute switch now.
Till soon,

 

[Mr. Moscode]

Look up the chip spec sheet for chip functions.

 

[isophase]

Hi all,
The first installation and tests went quite well
I absolutely need to install a global mute switch as the amplifier makes a huge "plock" on all outputs on power On or Off.
Until now, i have to unplug all 16 transducers manually before i power On or Off in order to protect them, a real pain and not at all practical !

On the chip spec sheet, pin description for SDZ and MUTE functions seem to do the exact same thing? (see attached screen shot from the document)
Both SD and MUTE functions kills the output signal for protection and both work by sending an High/Low TTL logic voltage. The main difference is that Shut Down function can be configured as a "low current state" to reduce power consuption when the amplifier is not in use...

Document also says it is not recommended to leave the SD pin unconnected "because amplifier operation would be unpredictable" now this is confusing me even more :-/

-When i short either "SD" or "MUTE" pins to GND on a single card, the signal does mute/shuts down so thats good.
-But I would like to mute all x8 cards at once and trying to avoid using an expensive (and hard to find) 8PDT switch to mute all the cards individually.
> would it be safe to connect all x8 MUTE (or SD) pins permanantly together to a cheap SPDT switch ? and short all to ground at once.

Thank you in advance for any help or advice

 

[Mr. Moscode]

Use 4 double pole relays, which are pretty cheap, 1 pole/stereo amp. Apparently SDZ needs to be at 5 volts so it can't wander through leakage or RF in the box. Use a pole for each SDZ pin. Wire them so the relays send 5 volts to the chip when on and grounds the SDZ when switched off. You'll have to remember to manually mute them before powering off. You could automate that with a 555 chip timer to delay the main PS from going off and on after the mute switch. You can also leave the power on for the session and mute when not needed. If it all loses power at once run the relay power supply separately with very little filtering so the relays drop out before the power supply discharges.