Hello, I've paired this amp WONDOM JAB with two 8ohm speakers (ND90) powered by a 5s battery pack for a Bluetooth speaker project. I have added a volume knob on the amp (comes as kit for this specific amp). Every time I modulate the volume using the knob I can see the speakers moving in and out (while music is not playing of course), moreover i get a fairly substantial noise coming from them when the knob is turned higher than 9-10 o'clock (noise stops after a while when there is no music/signal, i suppose the integrated Bluetooth module /amp goes into a power saver mode when it sensing that there is no signal). Power supply is common for source and amp since the Bluetooth is integrated on the amp and besides it doesn't sound like a ground loop. I suspect it might be the volume knobs cables that pick up noise (I attached a photo of the wiring you can see the map on the right hand side and the cables that run to the volume knob which are folded not looped).
Do you have any recommendations on how I should try to fix this these two issues?
Do you have any recommendations on how I should try to fix this these two issues?
The usual cause of that is a DC offset on the volume control making the aplifier follow the DC voltage and amplifying it. If the low frequency responce goes low enough the speaker cone can get damaged.
Try placing a blocking capacitor on the signal input.
Try placing a blocking capacitor on the signal input.
But how will I do that, the Bluetooth board is integrated on the Amp. Can I maybe splice two caps on those signal wires that run to the pot, what type of cap do I need to use?
Should I just remove the volume pot and use the Bluetooth volume to modulate?
Why is there dc on the signal? Bad filtering design by the manufacturer?
I felt inspired to look and the signaling is something like Lout, Lin, Gnd, Gnd, Rin, Rout. The volume control appears to have six individual wires, while each in and out should be a ground shielded coax. The "WFO" wiring may be where the noise is coming from.
If you temporarily pull the "out" wires off each channel - disconnect them in some way, such as pull the pin out of the header - and the speaker movement problem goes away, there's DC getting from the amp into your potentiometer. Block that with a series cap, as suggested above.
WONDOM is notorious (in my book) for no schematic available for this amp, anywhere. Probably dont want people to see their mistakes...
If you temporarily pull the "out" wires off each channel - disconnect them in some way, such as pull the pin out of the header - and the speaker movement problem goes away, there's DC getting from the amp into your potentiometer. Block that with a series cap, as suggested above.
WONDOM is notorious (in my book) for no schematic available for this amp, anywhere. Probably dont want people to see their mistakes...
The amp originally came with two jumpers shorting two pin pairs together (pic attached) and leaving 2 pins unconnected? I suspect the are shorting Lin-Lout/ Rin-Rout and leaving the ground pins unconnected? could i use these two jumpers to check your hypothesis?
WFO ?What's that? I did find the wiring odd, at first at thought the noise was because i hadn't twisted the wires...
If you have a DMM. Put the two jumpers back in. Measure DC from the little metal part atop / inside the jumper (shown) to either of the open, center two pins (Gnd). If you get a DC voltage - even a fraction of a volt - they must've handled that "downstream" of the volume connector.
With DC across it, your volume control is effectively a signal generator when you turn it - and your speakers are responding to that signal.
Hopefully this test cinches it, then you can add the cap as advised above for the fix!
You might get rid of the noise pickup by reconstructing the cable in a better way. (Certainly just putting the jumpers back in and using the BT source volume control would do it) I'm imagining replacing the two ground wires with ones of longer length, then twisting the new ground wires around the two "in/out" wires, separately twisting a ground wire for each channel signal pair. That would give a little bit of a shield for them. Along with shortening them all to whatever's practical to get to where the volume pot is mounted, will give it less of an "antenna" to pickup noise.
Good luck!
With DC across it, your volume control is effectively a signal generator when you turn it - and your speakers are responding to that signal.
Hopefully this test cinches it, then you can add the cap as advised above for the fix!
You might get rid of the noise pickup by reconstructing the cable in a better way. (Certainly just putting the jumpers back in and using the BT source volume control would do it) I'm imagining replacing the two ground wires with ones of longer length, then twisting the new ground wires around the two "in/out" wires, separately twisting a ground wire for each channel signal pair. That would give a little bit of a shield for them. Along with shortening them all to whatever's practical to get to where the volume pot is mounted, will give it less of an "antenna" to pickup noise.
Good luck!
Hello again, sorry for the very late reply but I couldn't find bipolar caps anywhere near me and I didn't have the time or the will to open it up and start poking around.
But I finally did and so I come back with some new information.
First of all removing the pot and replacing it with the jumpers doesn't change the noise, I also removed the input jack. I checked the dc voltage between the top part of the jumper and the ground pins, it's 1.584 volts..... That to me sounds enormous for Bluetooth output voltage. Maybe the design is truly horrendous or I might have a bad board or I might messed up something.
I connected a 10μF 50volt bipolar cap (I couldn't find 35 volt I hope that's fine) between the "out pin" and the " in pin"
The noise persist. I measured the DC voltage again. It was still 1.584 But this time it was dropping, dropping really slowly, maybe by 1mV per second. After about 3 minutes it finally almost reached 0-1mV. The was not eliminated thought.
Why is the voltage dropping this slowly?
Is any of this new info useful?
There's no load resistance to ground on the input side of the amp. Surprising, but who knows what was the thinking - maybe the volume control takes care of that. Good attempt though! Unsure why the noise persists however...
I did some even further testing. The cap completely eliminates the issue with the cone moving when you adjust the volume. So this was a success.
I turned off the Bluetooth of the board and this stopped the noise. So it's propably a bad Bluetooth integration. The line in of the board is many times more silent so maybe I will place a separate bt board inside..
The peculiar thing is that even with bt turned off, while there is no noise, I can still see the 1.584V between the audio channels and the ground. But there is no DC going to the speakers. Maybe they blocked it off somewhere else..
None the less, thanks a lot for your help.
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Where are you measuring this? I'm not clear what you mean.
I have had plenty of noise problems with BT modules. Only a few BT amp boards I've bough have been clean. Usually isolating the grouns between BT receiver and amp has killed the noise. You might no be able to if the receiver is built into the amp PCB.
I ve attached two close up pictures for you. Excuse the lighting. It's night and the board is inside the box.
The Amp board has a dedicated connector to attach a volume pot (sold separately for this specific Amp) the connector has 6 pins. Rout Rin ground, ground left in left out. Since the pot is optional, the board comes with two jumpers (not shown here, but you can see them in a previous image in this thread) shorting Rin - rout and Lin - lout. The jumpers are exposed on their top, so I measured between the exposed jumper part to the ground pin(as instructed in one of the previous posts of this thread). That's where I musured 1.584V.( for testing the blocking capacitor I have attached three jumpers the blue one is ground and the yellow, orange are on Lin-lout)
This DC potential doesn't go away if I shut off the BT( the noise does thought). I do not measure any DC across the speaker terminals. By placing a bipolar cal in between the in and out pins I can get the DC potential to drop to near zero( noise isn't affected thought)
I've also attached a Pic of the bt board (I think?), it's integrated unfortunately. The noise is extremely loud and it doesn't sound a normal hiss, I can also feel a periodic thumb if I touch the cone of the speaker. The noise volume is attenuated by the position of the potentiometer.
You think this is similar to a ground loop? I mean heck, if you where designing a Amp with an integrated bt you better take this into consideration. Even a 5 euro amp/bt board was much more quieter.
I hope I covered you.
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Thanks. 👍 If I'm understanding correctly the amp input pins have about 1.6 volts on them, as compared to ground. I don't know what the amp is, but I've worked with many that do have DC on the input pins. That means that there has to be a DC blocking cap. If not you will feed DC back into whatever source is upstream. Or also into the volume pot. Normal practice is to put DC blocking caps between the inputs and the "outside world." That will be clear on any application sheet. Maybe the designers didn't read the app sheet?
As for BT noise, it's pain in the butt. Too many boards get it wrong. But I've even had trouble with external BT modules - good ones. The only solution was to decouple the ground via transformer. Usually I did that with a DC/DC convertor. For amp boards with noise built in BT, I just gave up and either tossed the amp, or used it without the BT. There are some good boards out there, but finding them is a gamble. No one tells you about the noisy ones. 🙁
As for BT noise, it's pain in the butt. Too many boards get it wrong. But I've even had trouble with external BT modules - good ones. The only solution was to decouple the ground via transformer. Usually I did that with a DC/DC convertor. For amp boards with noise built in BT, I just gave up and either tossed the amp, or used it without the BT. There are some good boards out there, but finding them is a gamble. No one tells you about the noisy ones. 🙁
If the cap fixed the speaker moving problem (congratulations!) I'll assume the connection is ChannelSignalOut -->Cap-->VolumePotHighSide --> VolumePotLowSide --> Gnd, while the input is taken from the "VolumePotWiper" and ground. In this arrangement, the cap blocks the DC, while the resistance of the pot loads the cap to ground. The DC-free audio signal is available (at various levels) on the Pot wiper -
The BT issue makes me wonder if it's as noisy when the BT is successfully connected to something, say, in the quiet part between songs?
I wonder if all the noise is the BT module "searching" for a connection, or somehow "busy" when not satisfactorily connected?
The one photo of the volume pot audio connector placement is amusing. Looks like some kind of switching regulator just to the left, the BT just to the right - what a "happy place" for it, steeped in between those "digital, EMI generating" circuits...
The BT issue makes me wonder if it's as noisy when the BT is successfully connected to something, say, in the quiet part between songs?
I wonder if all the noise is the BT module "searching" for a connection, or somehow "busy" when not satisfactorily connected?
The one photo of the volume pot audio connector placement is amusing. Looks like some kind of switching regulator just to the left, the BT just to the right - what a "happy place" for it, steeped in between those "digital, EMI generating" circuits...
About the wiring. I am not sure if what you described is the case. I mean the pot/connector came wired in from the factory so I just assumed that I placed the cap like this: lout->cap->pot wire that normally gets connected to lout(Vcc) -> pot wire that gets connected to Lin(wiper pin) -> Lin and the ground pin goes to the pot wire that is connected to the pots ground. Then again I don't know anything about electronics, I forgot how pots work so I had to Google how to wire a potentiometer for volume control to write the previous sentence...
Regarding the bt noise. It makes the noise even after it's connected, even during a song playback, or a quiet portion or the time in between tracks. The only time it doesn't do it is if I pause the music, the it does it for a second and after that it makes a "pop" sound (accompanied by speaker movement) and then it stops. I assume this is the board going into a more power efficient mode. The noise returns immediately after resuming the music. Honestly because of the application I only hear the noise the potentiometer is turned really high but the digital audio volume of the bt device is relatively low, resulting in low playback volume while at the same time the noise is quite loud because of the high pot placement. And of course when I pause the track.
I think these characteristics (along with the weird tonality of the noise) are a dead giveaway that this noise is due a flawed design with the bt. I really hoped that the cap would fix it. Since the pot is not the culprit I won't even try to splice the cap let alone shield its cables.
Apparently whoever designed it expected the user to adjust volume with the pot, versus with the "digital volume control" on whatever device is sending the BT signal. On my system, there's a pot to control volume and on the player app, there's a choice to have a volume control in software, or just let it play "bit perfect" (unaltered). I have it in "bit perfect" mode, which means I have to get up and walk a couple steps across the room to make volume adjustments.
Now there's an OP subject. Is there any difference in sound between "bit perfect" and using a software volume control set at 100%? In which case I could turn it down some from my normal listening level without having to get up off the sofa, when the need arises.
Well, the way I understand the disadvantages of digital attenuation even in a 16 bit dac there shouldn't be any issues if you where to set it to 100% since that would mean that the output signal is attenuated by 0db. In any case, I've never managed to hear any difference in audio quality between analog and digital attenuation, and I really doubt I will in this application where there are many other limiting factors. I guess, since there is no damage done, I will just have to get used to it.