Audio Output Buzzing on 3.5mm  - Page 1
Forum section: electronics / projects

author: algorithm
author_rank: Regular Contributor
timestamp: December 15, 2020, 03:44:12 pm
content: Ive tried to add a headphone output from an speaker/amp ic output on a toy device lacking any documentation from the 80s.The 8Ω speaker is directly connected to the outputs on the IC. IC =https://www.mouser.com/datasheet/2/294/NJM2073_E-275459.pdfIt is a "BTL" configuration according to schematic which Im unsure what even the abbreviation is?But the speaker is not grounded, A + B output are the speaker connections.I connected my headphone jack ground to the audio amp IC ground pin. Without even powering the device, just plugged into the wall, I get buzzing on headphone output. So would that mean its a ground loop? Should I just disconnect the GND from my jack?Looking at the amp IC schematic, it mentions "parasitic oscillation preventing circuit" involving a 0.22µf and 1Ω cap/resistor.I tried to find said components on the original circuit and didnt find those values.Made me wonder could there be an improvement in audio quality if so?I connected the speaker on the disconnect pins on the headphone jack so when anything is plugged in it disconnects the speaker.But I guess its not disconnected GND?Would be easy to add those parts but would like to know Im not wasting my time before I do.Any advice or confirmation would be appreciated from the knowledgable members here. Thanks in advance.BTW the device is a kawai ms30. Anybody know where a service manual is for that?

author: bob91343
author_rank: Super Contributor
timestamp: December 15, 2020, 06:44:11 pm
content: Apparently what you have is an audio output connection that is ungrounded, driven in push pull by two transistors. So connecting the headphones to ground isn't really getting to the amplifier, but to one of the amplifiers and ground. What you are hearing is power supply ripple.Not a good candidate for adding headphones.

author: algorithm
author_rank: Regular Contributor
timestamp: December 15, 2020, 08:09:22 pm
content: Well I could add another speaker and wire both speakers to ground for a stereo configuration.Could also remove the speaker. Or source ground from somewhere else?The speaker is ungrounded as far as I can tell, but the IC the speaker is getting sound from is grounded? Or is that not right?There is also an LC7881M and TA-75358CP. Would including either of those help? the TA-75358 is just between the LC7881m and 2073d it appears.LC7881M pinout is1=Ch1 Out2=Vref H13=Vref H24=VCC (+5v)5=WCLK 26=LRCK7=WCLK 18=Data9=BCLK10=VCC (+5v)11=Out test12=Tst 113=Tst 214=IF (Interface terminal select)15=GND16=Vref L117=GND18=Vref L219=NC20=Out Ch2I saw a long time ago on youtube 8-bit keys added an output onto 1 of his toy keyboards similarly, he tried to source output from the IC but the speaker was what he ended up wiring. He just added a capacitor between the jack and speaker, but did not elaborate on value of cap If I recall.I also found examples like this?But since my speaker is in BTL configuration...Well, what if I put 2 jacks for each sides signal?

author: drvtech
author_rank: Regular Contributor
timestamp: December 15, 2020, 10:15:03 pm
content: The chip is working in bridge mode so neither side of the speaker is grounded. To get a single-ended signal from that circuit you need to connect the shield of your 3.5mm jack to the ground pin of the chip and the tip of the jack to one or other of the speaker pins. However, as there will be a D.C. offset of half the supply voltage on that pin you need to put a capacitor between the speaker pin and the jack. The value depends on the input impedance of whatever you're feeding into, but a starting point is 10uF. If it sounds very tinny then increase it to improve the low frequency response. Hope that helps

author: fcb
author_rank: Super Contributor
timestamp: December 15, 2020, 10:28:40 pm
content: BTL stands for Bridge Tied Load. I'd add a small 600:600 style transformer in-place of or in parallel with the speaker. Take your audio from the transformer.If you can access the audio before it hits the audio amplifier, then that would also work.

author: algorithm
author_rank: Regular Contributor
timestamp: December 15, 2020, 11:00:21 pm
content: Thank you very much fcb. So sourcing it from the D/A converter would work? I dont think I have a transformer like that handy or in the right size. I worry I would mess up winding it myself. Also would have to mount it. But Its good to know that option still.Thank you very much drvtech. I dont think I have a way to find out the input impedance, but I would like to be able to connect it to my computer to record with. I guess that would be around 30-50? The speaker the device uses is 8Ω not sure if that would be what to go by?Now I know what BTL means, thanksEDIT: Some quick googling had this to offer courtesy of TI.https://www.ti.com/lit/an/sloa043/sloa043.pdf"Connecting the headphone directly to the amplifier output without any capacitor in the pathwould likely damage the headphone due to dc current flow through the voice coil. Therefore, acoupling capacitor is required to block the dc offset voltage from reaching the load. Thesecapacitors can be quite large (approximately 33 µF to 1000 µF), so they tend to be expensive,heavy, and occupy valuable circuit board area. They have the additional drawback of limiting thelow-frequency performance of the system. This frequency-limiting effect is due to the high passfilter network created with the speaker impedance and the coupling capacitance.For a 33-µF coupling capacitor (CC) and a 32-Ω speaker (RL), frequencies below 150 Hz areattenuated. Since the load impedance is typically quite small, larger values of CC are required topass low frequencies into the load. Table 1 summarizes the frequency response characteristicsof such a configuration with a CC of 330 µF and loads of 4 Ω, 8 Ω, 16Ω, and 32 "They claim at 32Ω with 330 µf results with the lowest frequency at 15hz. I guess that would be what I should go for. But the TI document is suggesting only using single coupling. I can use the typical traditional arrangement, an extra capacitor isnt a big deal. But would that mean I should use a value other than 330? It claims a drawback of crosstalk between channels, so I would like to avoid that if possible.Sorry for my ignorance lol.Thanks again.

author: drvtech
author_rank: Regular Contributor
timestamp: December 15, 2020, 11:53:36 pm
content: Crosstalk between channels is not relevant is it? Even though the chip is capable of being used as a stereo amp it is being used as a mono amp here. As you're feeding a mono signal into the computer you would, presumably feed it into both channels in parallel.As for the coupling capacitor, the input impedance of the computer is likely to be a lot higher than 50ohms so you would probably be fine with 10uF but I'd say just try it.

author: algorithm
author_rank: Regular Contributor
timestamp: December 16, 2020, 12:03:34 am
content: Well maybe I am misunderstanding how its working, because I thought both left and right channels are being sent to the speaker, but both channels might differ to create the keyboards voice? At least thats what the WCLK pins on LC7881M led me to believe?Cool, thanks again. I will give it a try and report back.

author: drvtech
author_rank: Regular Contributor
timestamp: December 16, 2020, 12:33:58 am
content: I had assumed, perhaps wrongly, that there was a mono signal feeding the NJM2073. If in fact there are two different signals feeding the two channels then it's a bit different. Do you have a schematic of the input to the 2073?

author: algorithm
author_rank: Regular Contributor
timestamp: December 16, 2020, 12:50:20 am
content: I wish I had a schematic, but from what I could assess visually its basically just going from the D/A converter of LC7881M to the TA75358CP dual op amp then to 2073D then speaker, some resistors and caps between them. There is also a proprietary MCU I think (K014-FP (RF5A37). No service manual so not sure, but I did find a similar device in a different machines manual. K016-FP in the K5000 service manual. Theres also a mask rom.Oh, does it matter what kind of capacitor I use? I presume I can just use whatever, but better safe than sorry.

author: fcb
author_rank: Super Contributor
timestamp: December 16, 2020, 09:27:49 am
content: If you take audio from the DAC, then you'll probably have to buffer it to correctly drive a headphone amplifier IC. This can be almost any IC or circuit you choose from the interweb. You'll likely want to AC couple the DAC to the headphone amplifier to remove DC.It's possible that the existing design puts out AUDIO+ and AUDIO- on the two DAC's, this way the designers could probably lower the cost/complexity of the BTL speaker driver - probably worth reverse engineering the circuit around the amplifier/DAC.You have a whole bunch of options - depends on how much time you have?

author: algorithm
author_rank: Regular Contributor
timestamp: December 16, 2020, 11:43:26 am
content: Well there is not much to reverse engineer on the keyboard, just a 6-9V DC wall wart powering it, Im using a 10v but I imagine with load its right about 9v, figured as long as it wasnt too much over the transistors could handle it. But the rating of capacitors doesnt leave any headroom.There is in fact + and - audio inputs/outputs. on 2 IC's from what I could gather.Here is the 8bitkeys video example I mentioned earlier in thread.EDIT: oh, after rewatching years later, I was able to see the value of cap he used. 1µf 50v.

author: algorithm
author_rank: Regular Contributor
timestamp: December 16, 2020, 09:30:52 pm
content: Well I installed a capacitor. After rewatching that video a few times I noticed he wired it mono and grounded 1 side of what connected to speaker.I looked at the speaker and saw it was stamped + and -So I wired like 8bitkeys did, a cap between + and 3.5 jack ground to -.The first cap I tried was 1µf, the hum was noticeably decreased, but still barely there. Got full sound, much fuller than the 8Ω speaker.I tried a 22µf cap and got lots of distortion when a note was played. I tried tapping a 10µf before and after the 1µf but still got the hum.I also was so stupid I couldnt get the speaker to work somehow.Not sure where I went wrong, but I guess I should give up on this lolI am thankful for the responses.

author: algorithm
author_rank: Regular Contributor
timestamp: December 18, 2020, 01:49:03 pm
content: Yesterday I figured out what I did wrong I think.This is how I wired it to remove buzz and retain speaker functionality. Although the speaker seems more muted than before, not a big deal. I imagine the capacitor is why the speaker sounds a bit weaker. Or the extra cable length.While I poorly cobbled this drawing I noticed I probably should have just gone from GND pin on jack to the speaker through connection on Left. Anyway.A small cap between + and R on jack, and a resistor between GND and +, quieter than a church mouse on christmas now.Also with BOOMING lows. Kinda impressive for a crappy toy lol. Some highs did sound tinny, especially polyphonically, but Im satisfied.Thanks again for all the helpful input.

author: bson
author_rank: Unknown
timestamp: December 19, 2020, 07:55:45 pm
content: Quote from: algorithm on December 15, 2020, 08:09:22 pmThis won't work with bridged speaker outputs; they have no common ground, but a TRS headphone jack has a common ground for the two channels, with single-ended signals on T and R. It's also likely that when used for line as shown the two speaker negative drivers will short since the line grounds are likely connected at the far end - resulting in the power amp overload protection kicking in (or smoke from the downstream PCB).