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Request for help fixing Nobsound Class D amp
http://diyaudioprojects.com/Forum/viewtopic.php?f=10&t=6368
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Author:  brimley [ 23 Oct 2018, 16:25 ]
Post subject:  Request for help fixing Nobsound Class D amp

Hi All,

I just purchased a Nobsound Bluetooth 5.0 class D amp (https://tinyurl.com/y9cudj26) and it has a noise problem
that just makes it virtually unusable for any music with quiet passages.

I'll show my age here and say it sounds like an old Sinclair computer trying to load from tape. (mp3 attached).
(The return postage costs are prohibitive, so it's fix or bin).

I found the same product (just with a different brand I think, where a guy has suggested a fix for it.
(See product feedback at https://tinyurl.com/yak8l4zq)

Picture attached and quote here for easy reference:

There is design flaw that cause audible Bluetooth noise. Can be repaired but requires soldering skills. See the picture.
The two capacitors marked yellow should be disconnected from local ground and connected to the ground of preamp
or as marked on the photo to the ground of potentiometer.


My soldering skills are ok but my electronics isn't (I do think I could get into this though.. :) )

My question is what capacitor leg should I connect to what on the potentiometer?

I don't want to take the box apart until I'm fairly sure what it is that I'm trying to do so I'd really
appreciate any thoughts/suggestions etc.

Many Thanks
Steve

Author:  Yungman [ 27 Oct 2018, 17:48 ]
Post subject:  Re: Request for help fixing Nobsound Class D amp

I listen to the MP3, is that the sound of your music or it's just the noise? That's not normal of what we called noise!!! I thought it's the beginning of a song in the jungle with tambourine or something!!!!

If you just bought it, I would return it. You don't have schematic, I cannot make out what end is what.

Author:  laurie54 [ 28 Oct 2018, 05:02 ]
Post subject:  Re: Request for help fixing Nobsound Class D amp

Hello Brimley. Not having a sch of the amp makes it hard to point to a source of your problem. However much experience with working with these Bluetooth modules has shown me that all audio signal lines must be differential or they are VERY susceptible to RF interference / injection from the BT module. I see they use an external antenna but that does not stop the RF from migrating within the cabinet. If the signal chain is at any point changed from differential to unbalanced that is where the RF will get in. And once it is in the signal chain you are hooped. At that point it is impossible to get rid of. What it looks like from the pics is the sig is unbalanced threw the volume control and then fed into one side of the differential input to the power amp part of the cct.
--An unbalanced signal line has 1 wire = signal and 1 wire = gnd. Therefore any RF, (or unwanted signal) on the board that gets injected into the sig line and amplified along with the desired sig.
--A differential line has 1 wire = sig+ (non-inverting), 1 wire = sig- (inverting), and gnd is a shield. The sig+ is fed into the non-inverting input of the differential amp and comes out as same. eg: 3v+in = 3v+out. The sig- however inverts so 3v-in = 3v+out which is then added (in parallel) to the sig+for a total, of 3v+ signal. Any noise however say 2v+ will be injected equally and with the same potential and polarity into both + and - inputs. So 2v+ = 2v+ but 2v+ on the inverting leg will be 2v+ = 2v-. Then when the 2v+ and 2v- are added the result is 0v. In other words the unwanted signal is eliminated by cancellation. It is call common mode signal rejection ratio.
Now when an unbalanced or non-differential sig is used as the source and connected to a differential input it is common practice to connect sig to the v- (inverting) input and then tying the v+ input to gnd. So, sig in still = sig out just inverted. But this means no cancellation of common mode signals. This arrangement works and is used widely but requires the NOT used v+ input to be tied to gnd at a point where the signal and gnd are pure and "noise" free. Since this gnd point of the power amp in the pic is a ways from this pure point (pot/volume conrol) in the sig chain there is a possibility of different potentials between the gnd at the pot and gnd at the power amp input gnd.

1- de-solder the cap (circled in yellow). One leg is sig+ input and the other is gnd. But it is gnd at the power amp input. What they are suggesting is to reconnect the one cap leg to the sig+ input as before but instead of connecting the gnd side of the cap to gnd at the amp connect a wire to it and solder to gnd at the pot. Remember one side of the pot is sig input and the other side is gnd. This will make the differential input not being used which is then referenced to gnd,, be referenced to gnd at a point where the signal is pure of noise and a type of sudo common mode rejection can be taken advantage of and allow maximum noise rejection of unwanted signal (RF) from getting into the signal chain. Then do the same to the other cap on the other channel.

Author:  brimley [ 28 Oct 2018, 10:06 ]
Post subject:  Re: Request for help fixing Nobsound Class D amp

re: I listen to the MP3, is that the sound of your music or it's just the noise

LOL, my taste isn't that bad, I think it's bluetooth noise.

Sadly, the return postage is prohibitively expensive, it's just not worth it.

Author:  brimley [ 28 Oct 2018, 10:14 ]
Post subject:  Re: Request for help fixing Nobsound Class D amp

Many thanks for the reply Laurie. (I haven't taken this thing apart yet).

I'm hoping the cap legs are clearly marked, but I'm still not sure which of the pot lugs I should connect to???

(It sounds a bit fragile to leave a cap on one leg, somebody suggested cutting the track and soldering to that.)

Author:  Yungman [ 28 Oct 2018, 12:09 ]
Post subject:  Re: Request for help fixing Nobsound Class D amp

If you never open the unit, you must got the picture from other sites. Give the link of the site so we can look. What you show is not even close to have enough info to say anything what to do.

Author:  brimley [ 28 Oct 2018, 15:16 ]
Post subject:  Re: Request for help fixing Nobsound Class D amp

Unfortunately, all I have is the pic and the short explanation I put in Italics.
Here's the original though: https://tinyurl.com/yak8l4zq
Click on the feedback stars and see the pic/comment by M***x from PL.
Thanks

Author:  Yungman [ 28 Oct 2018, 23:45 ]
Post subject:  Re: Request for help fixing Nobsound Class D amp

I am not comfortable to take the unit apart and mod according to one user feedback. On top, there is no enough info in the picture. To do that, you have to open your unit and take more pictures of the other side of the pcb. Then you have to HOPE the person is correct.

I don't know what to tell you, you should complain to the seller. It is defective and they should pay for the return shipping. When I buy anything from ebay or Amazon, if the product is defective, I demand them to pay for return shipping and I always get it.

You don't know electronics, it's not easy to remove the caps from the pcb that has ground plane. I think this is over your head. I'd write to the seller, send them the MP3 file to show the problem. Don't open the unit and monkey with it.

In another words, if I face with this, I would not open it even I have the knowledge and all the tools and skills to do it.

Author:  brimley [ 29 Oct 2018, 02:58 ]
Post subject:  Re: Request for help fixing Nobsound Class D amp

>It is defective and they should pay for the return shipping

I agree but it's AliExpress in China and they don't. (I have learned a lesson here!)

The postage is so expensive, it's not worth sending back for a maybe refund if they feel like it
so it's fix or bin. I may as well try.

Author:  Yungman [ 29 Oct 2018, 04:25 ]
Post subject:  Re: Request for help fixing Nobsound Class D amp

Open the unit, take pictures of the back side of the pcb, take close up pictures under the pot and the caps in question.

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