2 Vpp out, OK
But a 200 mV sinewave at about 350 kHz too, also when silent but paired.
I do get an annoying ringing in my ears after having listened to some music with that 350 kHz background.
I know I am no bat, but then again...
Using Sennheiser HD201 headphones or small earplugs.
liz@poppyrecords.invalid.invalid (Liz Tuddenham)wrote:
Jan Panteltje <alien@comet.invalid> wrote:
[...]
2 Vpp out, OK
But a 200 mV sinewave at about 350 kHz too, also when silent but paired.
[...]
I do get an annoying ringing in my ears after having listened to some music >> with that 350 kHz background.
I know I am no bat, but then again...
Using Sennheiser HD201 headphones or small earplugs.
Could you be heaing other tones as the result of intermodulation between
the signal and the spurious frequency?
Question about low-pass output filters for Bluetooth receivers
I wanted to replace my very old Bluetooth receiver (used for TV mostly)
and bought some cheap Bluetooth receivers with a 3.5 mm phone jack.
Now it sounded a bit ..? not my idea of HiFi
So scope on output.
2 Vpp out, OK
But a 200 mV sinewave at about 350 kHz too, also when silent but paired.
Some modules more than others.
My old one is OK...
Lasse Langwadt <llc@fonz.dk>wrote:
On 9/2/26 12:16, Jan Panteltje wrote:
Question about low-pass output filters for Bluetooth receivers
I wanted to replace my very old Bluetooth receiver (used for TV mostly)
and bought some cheap Bluetooth receivers with a 3.5 mm phone jack.
Now it sounded a bit ..? not my idea of HiFi
So scope on output.
2 Vpp out, OK
But a 200 mV sinewave at about 350 kHz too, also when silent but paired.
Some modules more than others.
My old one is OK...
pwm/class-d headphone output amplifer/ADC ?
Lasse Langwadt <llc@fonz.dk>wrote:PS
pwm/class-d headphone output amplifer/ADC ?
Lasse Langwadt <llc@fonz.dk>wrote:
On 9/2/26 12:16, Jan Panteltje wrote:
Question about low-pass output filters for Bluetooth receivers
I wanted to replace my very old Bluetooth receiver (used for TV mostly)
and bought some cheap Bluetooth receivers with a 3.5 mm phone jack.
Now it sounded a bit ..? not my idea of HiFi
So scope on output.
2 Vpp out, OK
But a 200 mV sinewave at about 350 kHz too, also when silent but paired. >>> Some modules more than others.
My old one is OK...
pwm/class-d headphone output amplifer/ADC ?
No it is not PWM output
The 350 kHz is a steady nice sinewave,
unfortunalely by the square model about 2 V pp added with audio that is also 2 Vpp maximum.
I am going to try some parallel LC tuned circuit at 350 kHz in series with the headphone,
scope what is then left of the 350 kHz at the headphone side.
I am also going to listen an my AM radio for the third harmonic of the 350 kHz,
can use SSB to hear if it is frequency stable :-)
But I noticed a 24 MHz (I think it was) crystal in the small bluetooth PCBs, some chip, and some Rs and Cs, so the 350 kHz is likely very stable, looks stable on the scope.
You have a buetooth receiver? Scope it!
Who knows, you may see the same crap.
Lasse Langwadt <llc@fonz.dk>wrote:
On 9/2/26 12:16, Jan Panteltje wrote:
Question about low-pass output filters for Bluetooth receivers
I wanted to replace my very old Bluetooth receiver (used for TV mostly)
and bought some cheap Bluetooth receivers with a 3.5 mm phone jack.
Now it sounded a bit ..? not my idea of HiFi
So scope on output.
2 Vpp out, OK
But a 200 mV sinewave at about 350 kHz too, also when silent but paired. >>> Some modules more than others.
My old one is OK...
pwm/class-d headphone output amplifer/ADC ?
No it is not PWM output
The 350 kHz is a steady nice sinewave,
unfortunalely by the square model about 2 V pp added with audio that is also 2 Vpp maximum.
I am going to try some parallel LC tuned circuit at 350 kHz in series with the headphone,
scope what is then left of the 350 kHz at the headphone side.
I am also going to listen an my AM radio for the third harmonic of the 350 kHz,
can use SSB to hear if it is frequency stable :-)
But I noticed a 24 MHz (I think it was) crystal in the small bluetooth PCBs, >some chip, and some Rs and Cs, so the 350 kHz is likely very stable, looks stable on the scope.
You have a buetooth receiver? Scope it!
Who knows, you may see the same crap.
john larkin <jl@glen--canyon.com>wrote:
On Thu, 03 Sep 2026 05:05:52 GMT, Jan Panteltje <alien@comet.invalid>wrote:
Lasse Langwadt <llc@fonz.dk>wrote:
On 9/2/26 12:16, Jan Panteltje wrote:
Question about low-pass output filters for Bluetooth receivers
I wanted to replace my very old Bluetooth receiver (used for TV mostly) >>>> and bought some cheap Bluetooth receivers with a 3.5 mm phone jack.
Now it sounded a bit ..? not my idea of HiFi
So scope on output.
2 Vpp out, OK
But a 200 mV sinewave at about 350 kHz too, also when silent but paired. >>>> Some modules more than others.
My old one is OK...
pwm/class-d headphone output amplifer/ADC ?
No it is not PWM output
The 350 kHz is a steady nice sinewave,
unfortunalely by the square model about 2 V pp added with audio that is also 2 Vpp maximum.
I am going to try some parallel LC tuned circuit at 350 kHz in series with the headphone,
scope what is then left of the 350 kHz at the headphone side.
I am also going to listen an my AM radio for the third harmonic of the 350 kHz,
can use SSB to hear if it is frequency stable :-)
But I noticed a 24 MHz (I think it was) crystal in the small bluetooth PCBs, >>some chip, and some Rs and Cs, so the 350 kHz is likely very stable, looks stable on the scope.
You have a buetooth receiver? Scope it!
Who knows, you may see the same crap.
Try a cap across the headphones.
I certainly do not want 370 kHz into my big 180 W audio power amp and
maybe melt the tweeters?
john larkin <jl@glen--canyon.com>wrote:
On Thu, 03 Sep 2026 05:05:52 GMT, Jan Panteltje <alien@comet.invalid> >>wrote:
Lasse Langwadt <llc@fonz.dk>wrote:
On 9/2/26 12:16, Jan Panteltje wrote:
Question about low-pass output filters for Bluetooth receivers
I wanted to replace my very old Bluetooth receiver (used for TV mostly) >>>>> and bought some cheap Bluetooth receivers with a 3.5 mm phone jack.
Now it sounded a bit ..? not my idea of HiFi
So scope on output.
2 Vpp out, OK
But a 200 mV sinewave at about 350 kHz too, also when silent but paired. >>>>> Some modules more than others.
My old one is OK...
pwm/class-d headphone output amplifer/ADC ?
No it is not PWM output
The 350 kHz is a steady nice sinewave,
unfortunalely by the square model about 2 V pp added with audio that is also 2 Vpp maximum.
I am going to try some parallel LC tuned circuit at 350 kHz in series with the headphone,
scope what is then left of the 350 kHz at the headphone side.
I am also going to listen an my AM radio for the third harmonic of the 350 kHz,
can use SSB to hear if it is frequency stable :-)
But I noticed a 24 MHz (I think it was) crystal in the small bluetooth PCBs,
some chip, and some Rs and Cs, so the 350 kHz is likely very stable, looks stable on the scope.
You have a buetooth receiver? Scope it!
Who knows, you may see the same crap.
Try a cap across the headphones.
I tried some 370 kHz parallel LC circuits - so high impedance at that frequency - in series with
the headphone today, not working as I was hoping for, testing not finished yet,
but a bit of interruption happened,
had to go to my mains power backup as the power company was replacing the electricity as well as the gas
meters with new electronic ones that report back the use via some wireless link.
They are changing everything as many people feed back solar power into the grid,
but they no longer pay much for the feedback, claiming it 'overloads' the wires / cables.
That leaves people the choice of using batteries to store solar power themselves.
Anyways the Bluetooth alternative is a low pass and a separate power chip, easy to do with the small boards I have
some 5 V supply audio power amps?
I certainly do not want 370 kHz into my big 180 W audio power amp and maybe melt the tweeters?
So a simple filter board added to the small Bluetooth ones I have.
Might have designed the whole thing from scratch, Maybe look for a filter board on ebay..
Lasse Langwadt <llc@fonz.dk>wrote:PS
pwm/class-d headphone output amplifer/ADC ?
No it is not PWM output
I just listened to the bluetooth output with my Tecson PL600 radio,
it has long wave, no need to look for harmonics and it found that RF at 370 kHz,
clearly detected, and rock solid on SSB, no AM, no FM.
john larkin <jl@htigct.com>wrote:
On Thu, 03 Sep 2026 16:18:22 GMT, Jan Panteltje <alien@comet.invalid>wrote:
So a simple filter board added to the small Bluetooth ones I have.
Might have designed the whole thing from scratch, Maybe look for a filter board on ebay..
A simple 1st order RC should be enough. -3 dB at 15K maybe.
Use an RLC to be compulsive.
liz@poppyrecords.invalid.invalid (Liz Tuddenham)wrote:
Jan Panteltje <alien@comet.invalid> wrote:
I certainly do not want 370 kHz into my big 180 W audio power amp and >>maybe melt the tweeters?
I designed my own loudspeakers with a separate amplifier for each drive
unit. The tweeters have a timing mechanism which protects them in the
event of sustained overload.
john larkin <jl@htigct.com>wrote:
On Thu, 03 Sep 2026 06:45:34 GMT, Jan Panteltje <alien@comet.invalid>wrote:
Lasse Langwadt <llc@fonz.dk>wrote:PS
pwm/class-d headphone output amplifer/ADC ?
No it is not PWM output
I just listened to the bluetooth output with my Tecson PL600 radio,
it has long wave, no need to look for harmonics and it found that RF at 370 kHz,
clearly detected, and rock solid on SSB, no AM, no FM.
I bet the gadget has all of the FCC/UL/CE/CSA stickers.
john larkin <jl@htigct.com>wrote:
On Thu, 03 Sep 2026 16:18:22 GMT, Jan Panteltje <alien@comet.invalid> >>wrote:
So a simple filter board added to the small Bluetooth ones I have.
Might have designed the whole thing from scratch, Maybe look for a filter board on ebay..
A simple 1st order RC should be enough. -3 dB at 15K maybe.
Use an RLC to be compulsive.
Yes, but then an output amplifier / headphone driver is needed after the filter.
I just found a small ebay headphone ampolifier I already have and forgot about.
So RC lowpass, I also have a RC lowpass in the output of my Raspberry Pi audio >too, build into the connector cable.
It has a lot of RF on the 3.5 mm output jack.
Did you check yout big Pi's audio out?
I certainly do not want 370 kHz into my big 180 W audio power amp and
maybe melt the tweeters?
john larkin <jl@glen--canyon.com>wrote:
On Fri, 04 Sep 2026 08:03:43 GMT, Jan Panteltje <alien@comet.invalid>wrote:
john larkin <jl@htigct.com>wrote:
On Thu, 03 Sep 2026 16:18:22 GMT, Jan Panteltje <alien@comet.invalid> >>>wrote:
So a simple filter board added to the small Bluetooth ones I have.
Might have designed the whole thing from scratch, Maybe look for a filter board on ebay..
A simple 1st order RC should be enough. -3 dB at 15K maybe.
Use an RLC to be compulsive.
Yes, but then an output amplifier / headphone driver is needed after the filter.
No, just keep the impedances low. What's the headphone impedance?
I just found a small ebay headphone ampolifier I already have and forgot about.
So RC lowpass, I also have a RC lowpass in the output of my Raspberry Pi audio
too, build into the connector cable.
It has a lot of RF on the 3.5 mm output jack.
Did you check yout big Pi's audio out?
We use the Pi400 keyboard thing for development but don't use the
audio out. I personally design with the RP2040 chip.
https://www.dropbox.com/scl/fi/7wsg50ddqvddw653bk3sq/RPI_Dev_Sys.jpg?rlkey=qbbofpsmf66a3itee4pchbiji&raw=1
Jan Panteltje <alien@comet.invalid>wrote:
john larkin <jl@glen--canyon.com>wrote:
On Fri, 04 Sep 2026 08:03:43 GMT, Jan Panteltje <alien@comet.invalid> >>wrote:
john larkin <jl@glen--canyon.com>wrote:
On Fri, 04 Sep 2026 08:03:43 GMT, Jan Panteltje <alien@comet.invalid> >>wrote:
john larkin <jl@htigct.com>wrote:
On Thu, 03 Sep 2026 16:18:22 GMT, Jan Panteltje <alien@comet.invalid> >>>>wrote:
So a simple filter board added to the small Bluetooth ones I have.
Might have designed the whole thing from scratch, Maybe look for a filter board on ebay..
A simple 1st order RC should be enough. -3 dB at 15K maybe.
Use an RLC to be compulsive.
Yes, but then an output amplifier / headphone driver is needed after the filter.
No, just keep the impedances low. What's the headphone impedance?
That is the point, could be my Sennheiser headphones, or Chinse earplugs
or whatever, or my audio power amp,
I just found a small ebay headphone ampolifier I already have and forgot about.
So RC lowpass, I also have a RC lowpass in the output of my Raspberry Pi audio
too, build into the connector cable.
It has a lot of RF on the 3.5 mm output jack.
Did you check yout big Pi's audio out?
We use the Pi400 keyboard thing for development but don't use the
audio out. I personally design with the RP2040 chip.
https://www.dropbox.com/scl/fi/7wsg50ddqvddw653bk3sq/RPI_Dev_Sys.jpg?rlkey=qbbofpsmf66a3itee4pchbiji&raw=1
I used the Pi4 today to make some test tones, some here may like this:
# sweep audio 20 Hz to 20 kHz fr 30 seconds:
play -V -r 44100 -n synth 30 sin 20+20000
# single tones
play -V -r 44100 -n synth 30 sin 20000
play -V -r 44100 -n synth 30 sin 2000
play -V -r 44100 -n synth 30 sin 10000
play -V -r 44100 -n synth 30 sin 12000
play -V -r 44100 -n synth 30 sin 15000
play -V -r 44100 -n synth 30 sin 20000
You can also use an audio USB stick like I have,
it then gets a dfferent device number assigned by the Alsa sound system:
Generate a single 20 kHz tone for 30 seconds:
AUDIODEV=hw:3,0 play -V -r 44100 -n synth 30 sin 20000
# Sweep USB stick with audio out / audio in:
AUDIODEV=hw:3,0 play -V -r 44100 -n synth 30 sin 20+20000
# sweep goes 'over the top'
AUDIODEV=hw:3,0 play -V -r 44100 -n synth 30 sin 20+30000
Interesting, it starts again after about 20 kHz with distortion at the
sample speed point !
Also mixer to set the volume
for the intern audio volume:
alsamixer
for the extenal USB audio device:
alsamixer -c 3
Just had a lot of fun with it today, playing with the good Trio analog scope >and noticed the little audio amplifier I have does indeed some lowpass.
https://panteltje.nl/pub/chinese_headphone_amplifier_build_IMG_0193.JPG
Was a kit? no idea where I got it from :-)
But works great!
Like somebody else mentioned, the output of the Bluetooth modules may be PWM. >I did see changes is the sine wave zero crossing when triggering the scope >from the RF while there is some music playing.
Next I will look up (try to read) the chip numbers in the small Bluetooth >modules I bought.
| Sysop: | Jacob Catayoc |
|---|---|
| Location: | Pasay City, Metro Manila, Philippines |
| Users: | 4 |
| Nodes: | 4 (0 / 4) |
| Uptime: | 497099:24:14 |
| Calls: | 182 |
| Files: | 744 |
| D/L today: |
6 files (5,132K bytes) |
| Messages: | 73,551 |
