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Class D Switching Power Amplifiers and Power D/A conversion

Dual Hypex UcD180 amplifier build
Dual Hypex UcD180 amplifier build
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Old 18th April 2018, 01:28 PM   #21
pierrepierrepierre is offline pierrepierrepierre
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Join Date: Sep 2014
Quote:
Originally Posted by ghitus View Post
The psu, as told me by Hypex, doesn’t need to be grounded to the chassis because their psu (SMPS1200A700) is “class 2 safety” product (they told me that the electrical contact between the chassis and the metal plate underneath the psu is enough) and so also the amp modules. [...]

So I would advice you to try to go this way first, then if you don’t obtain a satisfying result, try otherwise. But I think you will raech you target, cause I think Hypex products are very reliable.
Thanks Gaetano, this is reassuring and I will no doubt be pleased in the end: they are a very reputable manufacturer after all. The SMPS400Axx0 datasheet (rev 14) – my PSU – states that:
When mounting the module in an enclosure, a minimum safety distance of 6mm from the SMPS to all possible conducting parts must be ensured to keep compliance with Safety Class 2. This includes parts on the top, the bottom and all sides of the board. [...]
If the enclosure is limited in height
[...] one shall need to use smaller spacers and provide a layer of insulation both above and below the SMPS with a minimum thickness of 0.4 mm to comply with the Class 2 Safety directive.
So if I understand correctly, this “class 2 safety products don't need to be grounded” thing is only re safety & shunt to earth ground (green wire). On the other hand, the chassis-PSU contact made by the metalic spacer specified on the UcD180+SMPS400A180 schema likely implements the PSU ground to chassis connection recommended in AES48.

Stephen Macatee's excellent Rane Note 151 explains that:
Many audio manufacturers, consciously or unconsciously, connect balanced shields to audio signal ground [...]. Any currents induced into the shield modulate the ground where the shield is terminated. This also modulates the signal referenced to that ground. [...] Amazingly enough, acceptable performance in some systems is achievable, further providing confidence for the manufacturer to continue this improper practice. [...]
Signal-grounded shields on balanced equipment create ground loops in the audio path and modulate the audio ground, wrecking havoc in most systems.
[...] Again, some systems with signal-grounded shields work acceptably, causing further and future bewilderment.

My concern (here) is not so much safety but rather hum/buzz prevention, and as part of a whole sound system where most of the equipment is indeed balanced.

Gaetano, I could be wrong but so far I believe the wiring scheme that you implemented introduces such a loop in the audio path. (More on this below.) It might not cause problems in your particular setup but may in mine. Again, Hypex themselves advocate the implementation of AES48:

Quote:
Originally Posted by pierrepierrepierre View Post
[Hypex's Legacy pin 1 problem application note] strongly advocates the implementation of AES48... This is in contradiction with their recommended wiring scheme (as of schema 1 & schema 2) and confused me further.
But actually I'm not so sure that there is a plain contradiction here anymore, rather possibly an oversight. Rane Note 151 further explains that:
It is easy to confuse chassis ground and signal ground since they are usually connected together, [...] let's discuss how to connect them. [...]
The shield noise currents flow through the chassis and shunt to earth ground on units with 3-conductor line cords. The key issue is that these noise currents do not flow through a path shared by any audio currents. [...] The hard part is implementing the proper layout scheme.
Connecting signal ground to the chassis can only be done in
one place in each unit. If done twice, one leaves the possibility open that the noise currents will flow through a path shared by audio.
There are two schools of thought on where to connect the signal ground to the chassis. They are both versions of the star ground scheme [...]. The first connects a trace (or wire) directly from the audio power supply ground terminal and connects to the chassis ground point (see Figure 4). It is important, in both "schools", that no other signal currents be allowed to flow through this trace. Do not allow this trace to share any other return currents from other signal-grounded circuit points, such as the input or output circuit's ground. This keeps chassis noise currents from flowing through the same trace which is a return path for an audio signal. [...] Another school of thought on where to connect signal ground to the chassis simply moves the center of the star ground to the input jack's ground.
Both wiring schematics (links above) look like Hypex advise to connect the star ground center at the audio input connector (second school of though). However as I said above, I believe that the chassis-PSU electrical contact made by the metallic spacer specified on the UcD180+SMPS400 wiring schema implements a PSU ground to chassis connection and thus also connects the audio ground to the chassis, via the PSU board.
But as Macatee explains, connecting signal ground to the chassis can only be done in one place in each unit[; if] done twice, [it] leaves the possibility open that the noise currents will flow through a path shared by audio and cause equipment compatibility issues within a whole audio system.

Is anybody able to confirm if I'm right or wrong, and why?
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Old 20th April 2018, 01:44 PM   #22
ghitus is offline ghitus
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Join Date: Nov 2017
All you have to do IMHO is just to try all the possibilities. If I were in you, I couldn’t wait to build and hear my new power amp! ;-)

Cheers, Gaetano
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Old 12th June 2018, 02:24 PM   #23
pierrepierrepierre is offline pierrepierrepierre
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Join Date: Sep 2014
It has been 1˝month since I emailed Hypex but they have not got back to me.

I don't actually believe that the Amplifier signal input wiring Application Note contradicts AES48 per-se anymore. Assuming that the PSU ground is only connected to the chassis via the UcD board, the system should indeed be ground loop free. However that scheme does not mention the metallic spacer specified on the Wiring UcD180HG SMPS400A180 schematics. Moreover, the Wiring UcD180HG SMPS400A180 schematics only shows a 2-conductor mains.


I just pinged them again, specifically asking to (1) clarify the purpose of the metallic spacer which makes a contact between the PSU board and the chassis, and (2) confirm where to tie the earth shunt of a 3-conductor line cord to the chassis in order to respect AES-48.
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