Proper star ground for DIY Amplifier

[Nugget_]

Hello everyone,

I am building a 12-channel amplifier using the 3e PFFB modules. The concept is almost finished.

Unfortunately, I currently have very little time for this hobby - and therefore also little time to fix any problems that may arise later. That’s why I’d like to ask about best practices here before starting the build. I appreciate everyone taking the time to help very much.

So that everything is in one post, this is what the modules look like:

(image from Audiophonics)


I have an enclosure resembling the following (the dimensions in this internet image are not accurate, i just wanted to provide a viewing angle from below, with regards to the following sketch. In my case, 6 modules and the Meanwell HRP-600N3-48 fit very precisely into the enclosure, although there is very little tolerance left):

A 5 mm aluminum plate (red rectangle in sketch) will be mounted suspended from the inside top of the enclosure using a thermal pad and 8 screws in 2 rows at the long sides of the plate. The amplifier modules and the power supply will first be mounted onto this plate.

Here is my early sketch:

Specifications:

The inputs are 12 isolated Neutrik combo XLR / 1/4" TRS sockets. According to AES recommendations, these should (will) be connected to chassis ground directly at the point where they enter the enclosure.

At the Powercon inlet (right side in the sketch), there should be the central grounding point. Both the protective earth (PE) connected to the chassis and the protective earth input of the Meanwell power supply, the power supply ground output (V-), and GND (V-) of the amplifier modules (supply voltage input) should be connected there.

The enclosure has several openings (TRS inputs, wooden side panels, ventilation grille at the bottom), so my concern is not RF/HF blocking but hum and noise performance.


Questions:

1. Must or should the signal ground of the TRS inputs (which is connected to chassis ground and via the central grounding point to V- of the amplifier modules) also be connected to the signal ground input (at the 3-pin input connector) of the amplifier modules? Or should I connect the inputs only with two conductors (Signal -, Signal +)? [see “?” in the upper left of the sketch]
2. V- of the Meanwell power supply and the amp modules run physically parallel for some distance. Does it make sense to connect them directly there and then run a single cable to the central ground point? [see “?” in the middle right of the sketch]
3. All modules are screwed onto the 5 mm aluminum plate, which in turn is connected to the chassis via an insulating thermal pad and 8 non-insulated screws. Should I additionally connect this aluminum plate to the central ground point, or is the connection through the screws sufficient? This plate is also connected to the heat sinks of the amp modules via screws. The heat sinks have a resistance of 85 kΩ to amplifier signal ground. [see “?” in the middle of the sketch]
4. Do I need to take special measures in signal routing or possibly even shielding for the small-signal wiring between the input sockets and the amplifier inputs? According to my plan (see Question 1), only Signal - and Signal + would be twisted and routed to the inputs. Due to limited space, these will inevitably run close to the output filters of the amplifier modules.
5. For cable routing, it would be easier to arrange all modules so that the output filters and outputs face inward toward each other. Are there good reasons to turn them around so that the input stages face inward toward each other instead? Keep in mind that in this case the output filters would be positioned directly next to the audio input sockets.
6. My preferred cable routing (signal input shield to chassis; only Signal - and Signal + to amplifier inputs) is intended for balanced connections. Does this routing also work reasonably well for single-ended (SE) inputs? I understand that with these modules, no BTL operation occurs in that case (unmodified and SE input) and only 1/4 of the output power is available. The question is specifically about hum and noise floor.

Again, thank you very very much in advance. Any help is greatly appreciated.

Have a nice day.
Nugget

 

[Sokel]

Twelve channels in a tiny enclosure does not seem like a good idea.
Are you planning active cooling?

 

[Nugget_]

I did think about this, and want to see how hot it runs, before adding a fan (which would not be a problem, a 120mm slim computer fan at say 800 rpm is incredibly quiet, especially at the underside of the enclosure.

The PSU does have a controlled fan which would draw air in from the outside and improve circulation.

Apart from this, the aluminium plate bent into the chassis is 400mm by 500mm. If we consider both sides (I know that one side would be inside the enclosure, but the aluminium surfaces of the modules and PSU would effectively increase surface area, so i just assume the surface area of the plate x2 for easy of calculation), chatgpt calculated approx. 0,5 K/W. Correct me if thats wrong.

If we consider 60 degrees C max and 25 C ambient, that leaves about 70W that can be dissipated. With 100 mA idle per board (30W total) and some PSU losses (maybe 10W for idle) that leaves 30W for Music, or 300W Peak, considering a typical crest factor of about 10db. Sure, thats just 40W average peak power per channel, but this is a home environment with thin walls and some channels will only power tweeter or atmos height channels. That is plenty in real life. For stereo music i will implement a switch that can easily shut down some modules. Note that this is also long term calculation, and the pure aluminium mass will provide thermal mass. And if it doesn't work, the implementation of a fan would only take 30 minutes of my time .