How to filter high pitch noise from toy loudspeaker ?

[kultakala]

Hi,

I am using some electronic equipment but i dont have much experience in building circuits by myself.
And as i am not a native english speaker i hope you get what i mean.

I have an old toy piano from Sega Toys (Grand Pianist).
I did some mechanical repairs and it works properly now.
The only problem is that the built in speaker is not very loud and that there is a constant high pitched noise when its powered on.
The maximum volume is intended i guess because there are some parts which already start resonating/oscillating (?) when the volume gets to the maximum.
But the high pitch noise is really annoying.

There is one small 3W speaker which is directly connected to the loudspeaker output on one of the internal boards.
Would it be enough to put a simple passive low pass RC filter in between the cables to the speaker ?
The highest piano tone is C8 which is about 4200 Hz so i guess a filter cutoff frequency of 4400 Hz or so would be good ?
I dont know the frequency of the noise but would guess its somewhere around 10 KHz.

Would a filter with fc at 4400 Hz lower the volume ?
To keep the maximum possible volume for the piano tones would be great.

And what would be a good combination of R and C value ?
According to a filter tool a 1100 Ohm resistor and a 33 nF capacitor would have 4400 Hz cutoff frequency.
Would a different combination be better ?
Or should the frequency be higher to keep maximum volume ?

Maybe something completey different would be even better ?

Hope you can give me some hints.

Thanks!

[Zipdox]

Using such a high resistance resistor will leave no power for the speaker. But filtering out the noise probably isn't the right way to do it anyway. You should fix the source of the noise.

[kultakala]

Unfortunately i think the electronics itself are generating the noise.
Guess i cant fix it on the board itself, at least i have no idea how to do that.

[kultakala]

Okay, so if the resistor needs to be low...  what about a 1 Ohm resistor and a 33 µF capacitor, would lead to 4800 Hz frequency cutoff.
I could try that out...   Otherwise i would have to use an active filter but there is not much space within the piano to fit anything.

[CaptDon]

A Low Pass rolloff filter will probably sound bad. The noise is probably centered on a very narrow range of frequencies (related to the master clock, dividers or sample rate). If you have a program like 'Spectrum' and a microphone you can plug into your computer you could see exactly where the noise frequency is located and perhaps build a notch filter? I realize you said there isn't much room inside. That limits your options. What is the value of the toy, nostalgic? Perhaps a better toy is the answer.

[kultakala]

A Low Pass rolloff filter will probably sound bad. The noise is probably centered on a very narrow range of frequencies (related to the master clock, dividers or sample rate). If you have a program like 'Spectrum' and a microphone you can plug into your computer you could see exactly where the noise frequency is located and perhaps build a notch filter? I realize you said there isn't much room inside. That limits your options. What is the value of the toy, nostalgic? Perhaps a better toy is the answer.

It is a very rare toy item which was only sold in Japan about 15 years ago.
It was not cheap, about 400 USD i guess.
In its way its unique as it has original 88 keys keyboard and can physically move the keys according to the notes played using 88 solenoids and mechanical arms to do that.
So buying a better one is no option 

A notch filter would be more complicated i assume...  dont know exactly what you mean by "rolloff".
You mean that the lowpass filter curve will drop down slowly ?
But i need only the frequencies up to 4400 Hz and the noise starts at about 10 kHz.
I can try that out easily with a resistor and a cap.
i dont have a microphone to check for the exact frequency (although its no constant single frequence) but maybe i can in a few days.

[Terry Bites]

Check the frequency with a phone app. Frequqncy analyser things.
It sounds to me like the speaker amplifier is oscillating.

[TomWinTejas]

Fascinating toy... I found a YouTube video of one playing:

Since it has a 3.5 mm output, perhaps you can add an external speaker with its own filter.  You can probably also find an audio spectrum analyzer for your mobile phone and use its microphone... might not be super precise, but it should get you in the ballpark.

[ledtester]

But i need only the frequencies up to 4400 Hz and the noise starts at about 10 kHz.

Did you mean 10 Hz?

I looked up the Sega Grand Pianist on the web. Are you powering it with the included AC adapter? How about try powering it with batteries?

[IanB]

Did you mean 10 Hz?

No, 10 kHz. It is a high pitched noise.

[tunk]

Probably doesn't matter: How do you power it? E.g. do you use the
original PSU with a step-down transformer? If you use something else,
maybe try with another PSU. I'd also guess the 4200Hz tone has some
harmonics.

[kultakala]

Check the frequency with a phone app. Frequqncy analyser things.
It sounds to me like the speaker amplifier is oscillating.

I installed an analyzer app on my phone...  shows 3 peaks at 7.4 kHz, 14 kHz and 21 kHz.
The noise is independent from the volume setting.

Fascinating toy...

Since it has a 3.5 mm output, perhaps you can add an external speaker with its own filter.  You can probably also find an audio spectrum analyzer for your mobile phone and use its microphone... might not be super precise, but it should get you in the ballpark.

Its even more fascinating in reality   I managed to get some midi file on SD card into the correct format so it plays more than the 100 built in songs.
And i added an additional switch so i am able to cut the power for the solenoids.
The mechanics are almost as loud as the speaker sound so disabling the mechanics is cool.
But then the noise is even more annoying.
Using the line out is also not an option. First i want to use the piano just by its own. And second the sound is more terrible than from the internal speaker.
The noise is more amplified and there is also 50 Hz noise from the power adapter.

But i need only the frequencies up to 4400 Hz and the noise starts at about 10 kHz.

Did you mean 10 Hz?

I looked up the Sega Grand Pianist on the web. Are you powering it with the included AC adapter? How about try powering it with batteries?

No, as i measured now the noise is high pitched, starting at 7,4 kHz.
Battery power is not possible...  and via power adapter it draws more than 1 A at 6 V, depending on how much solenoids are active at a time.
I can try to power it via a DC power supply, didnt try that yet.

So, i still guess it would be best to filter the speaker output over 5 kHz.

[kultakala]

Probably doesn't matter: How do you power it? E.g. do you use the
original PSU with a step-down transformer? If you use something else,
maybe try with another PSU. I'd also guess the 4200Hz tone has some
harmonics.

I power it by the original multi voltage wall adapter .
Will try with a separate dc power supply later.

[IanB]

It's very common to have passive crossover filters in loudspeakers that direct appropriate frequencies to different drivers (tweeters, mid-range or woofers).

So it should be very practical to create a low pass filter that blocks higher frequencies.

I don't know how to design them, but you could look at loudspeaker design handbooks for some ideas.

[RJSV]

Hey?!..
   Wait a second...88 Solenoids you say ?
   Ever try, (I don't know, a Clothes Washer Machine might have 4 solenoids), try;
   Connect '22' Washing Machines up, all running, with the internal solenoids in each machine all firing.
All within 5 feet (you'll have to stack some of them).
   
   Sorry, that was fun, but to your question, is that noise during the 'Player' mode, or just when sitting?
Because, that's a LOT of solenoids. Are they on AC power, or even (screech); SCR circuits, from like 1974?

   One basic thing you should be able to check, if it is DC driven solenoids, you would see a diode, pointing towards positive; those are for absorbing / suppressing coil sourced negative voltage spikes, on turn-off, if I remember correctly.

[RJSV]

   Actually seriously, here's what I would do:
   Considering that's a special deal, I would just skip (any) audio power output, going the route of some cheap, anywhere Boom Box, small and fine, these days.
If you really can't find just regular TONE, I don't have answer; I had those 'linear' slides, on Boom Box one each for BASS; MID; TREBLE.
Heck, I always liked that TREBLE control dialed way back, almost off.

   Considering the value, might be best to simply 'Tap' the AUDIO left and right, perhaps a 1 kOhm pot, and with 1 kOhm resistor off of the potentiometer center tap.  That resistor does nothing to your (small) ac signal, but protects from other 'accidental' connection being shorted (to ground thru the pot).

[kultakala]

   Sorry, that was fun, but to your question, is that noise during the 'Player' mode, or just when sitting?
Because, that's a LOT of solenoids. Are they on AC power, or even (screech); SCR circuits, from like 1974?

   One basic thing you should be able to check, if it is DC driven solenoids, you would see a diode, pointing towards positive; those are for absorbing / suppressing coil sourced negative voltage spikes, on turn-off, if I remember correctly.

The noise is always there, starting when switching the power on.
While playing it does not get louder but is still noticeable.

The Piano just gets DC 6.5 V, the internal circuits are running with 5 V.
Its not easily possible to check the solenoids circuit because the unit which contains the solenoids, driver board and the mechanics including the keys is internally capsuled.
Opening or disassembling it would lead to a big puzzle of 2 times 88 small plastic levers which are all different.
I had the chance to have a look at a disassembled piano, it was not possible to put it back together.

   Considering that's a special deal, I would just skip (any) audio power output, going the route of some cheap, anywhere Boom Box, small and fine, these days.
If you really can't find just regular TONE, I don't have answer; I had those 'linear' slides, on Boom Box one each for BASS; MID; TREBLE.
Heck, I always liked that TREBLE control dialed way back, almost off.

You mean to take the line output signal and go through an external boom box ?
Thats no option for me, because the piano should stand by its own.

What do you mean by "If you really cant find just regular TONE" ?

I dont have the needed parts but will order some to test if a simple RC low pass filter will help.

[Le_Bassiste]

rather than weakening the (already too silent) output even further by using a resistor in the output path, you  may have more luck and even steeper roll-off characteristics by using an LC lowpass instead of an RC lowpass.  have a look:
https://www.jobst-audio.de/tools-frequenzweiche/frequenzweiche
as for the crossover frequency of such a contraption, i would start at the frequency of the highest key on your piano, basically 440 Hz times the number of octaves above a0, give or take.

[kultakala]

Thanks for the link, i checked for frequencies 4400 Hz and higher, but the exact L and C values are not gettable.
It almost fits with L of 39µH and C of 3,3µF...  but it would mean to pay a lot of shipping cost compared to the value of those 2 parts
Maybe i can try it when i am able to get the parts locally.
I will test with a RC low pass next weekend.
I have all parts for that.

Btw, for both RC and LC filter the capacitor needs to be a bipolar one, correct ?

[RJSV]

Wow, I see what you mean, not wanting to disassemble that Solenoid Bank.
But...you've been contradicting yourself, couple times;
   Wanting a sort-of 'purely', untouched, vintage Audio Machine, yet...are asking "How can I modify things ?...to get rid of the noise ?"
BUT must be stand-alone.

   I would, then, suggest, try simply disconnect (any) tweeter or hf speaker. I'm not sure what that console has, as internal speakers.

Oh, yeah, TONE, I just meant; You could BUY that, a cheap mik mixer or 'DJ' mixer would have 'TONE', although, yes, it's also nice to learn.
A battery, IS a DC power supply, just maybe has differences, maybe noise figures are 'different', from AC Wallwart type.
-Rick