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Topic: Audio noise caused by a micro servo (Read 2605 times) previous topic - next topic

Sikter

Hello,
I have built a wireless pedal board. I can now control my amp and my effects from switch panels taped on my guitars.
Effects are located in a box and connected to my amp by short patch cables (mono 1/4 jack).


So I tried to add a new function, kind of programmable volume control by using a micro servo. 
I have chosen this el-mechanical solution because I didn't want to modify my amplifier.
I was afraid that two sided tape wouldn't be strong enough to hold servo in position and worried about hundred other things.
See video clip: http://youtu.be/yxiDniBHDig
Both mechanical transmition and arduino sketch turned out to work perfectly but micro servo caused another problem:
Audio noise, a lot of audio noise, booming, buzzing and popping noise.
Not only from my amp but it affected PA and few other amps in the room.
(You can't hear the noise because I don't have a speaker cabinet at home so amp was off while I was recording video)

I wonder is there any way to prevent this noise... Maybe micro servo and audio equipment just don't work together.
















Grumpy_Mike

Quote
I wonder is there any way to prevent this noise

Well if it electrical noise getting into the amplifier then good power decoupling will help suppress it.
Lots of different  techniques, you might have to employ them all.
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

MarkT

And I'd say start with shielding the cable to the servo and keeping the Arduino and associated components in a metal box/enclosure and connect the case to ground.

There's likely to be RF pickup in the amplifiers from the logic speed and PWM signals flying around in unshielded cables.   Adding more
RF suppression on the audio amp inputs can help (100pF across audio input might help, though you really need to know the audio
input impedance to know what value capacitance is best.).
[ I won't respond to messages, use the forum please ]

Sikter

Thank you guys,
I spent most of last two days reading about de-coupling. It makes sense but I never figured out how
to do it in my case.
If it was only my amplifier it would be clearer to me, but how to prevent noise in PA?
PA has nothing common with my amplifier and my pedal board. But it looks like the noise affects PA through main 240V AC power supply.
The building is old and probably bad electrical system but still...

I think I'll try to run my servo and arduino on battery and see what happens.
Then I'll try to experiment with metal box and shielded servo cable.

What do you guys think about battery as power supply for servo and arduino?

There are guitar tuning systems on the marked which run on the battery.
They're probably controlled by a microcontroller and driven by special servos. They don't affect amplifiers arround them.


Another critical question:
Let's say I run arduino on 9V DC buttery and my pedals are powered by 9V DC switch adaptor.
Can I still connect grounds together?

thanks











Grumpy_Mike

You apply the decoupling to the noise generating circuits, that is the arduino's power supply and the servo's supply, not the PA.
A battery powered solution will help but you will still need the decoupling components.


Grumpy_Mike

Quote
Let's say I run arduino on 9V DC buttery and my pedals are powered by 9V DC switch adaptor.
Can I still connect grounds together?


I once tried to make a common power supply for a collection of five peddles for my son, to replace all the batteries. The result was that two of then went up in smoke.
It turns out that they have what is known as earth lift resistors in place to cope with common grounds on the signals. There was not a common power ground. In the end I had to make five small mains powered isolated supplies, one for each peddle.

So beware trying to power two things off the same circuit. Test first to see that there is a common ground on the supplies.

vasquo

Quote
I wonder is there any way to prevent this noise... Maybe micro servo and audio equipment just don't work together.


Electric guitars/amps are Hi-Z input and it's very easy for noise to couple into it.  (example: AM radio, RF buzz, fluorescent lamps, etc)....

I suspect the micro servo motor (unshielded even!) butt up against the metal case of the amp is a sure fire way of transmitting the EMI/RF noise of the servo to your amp.

You can try "mechanical shielding", see faraday cage, and then grounding that metal case. But the fact that it's supposed to be butted up/pressed against your guitar amp (to be able to turn the knob) I don't know if that will still be effective. 




Sikter

#8
Dec 10, 2012, 02:47 pm Last Edit: Dec 13, 2012, 03:34 pm by Sikter Reason: 1

Quote
Let's say I run arduino on 9V DC buttery and my pedals are powered by 9V DC switch adaptor.
Can I still connect grounds together?


I once tried to make a common power supply for a collection of five peddles for my son, to replace all the batteries. The result was that two of then went up in smoke.
It turns out that they have what is known as earth lift resistors in place to cope with common grounds on the signals. There was not a common power ground. In the end I had to make five small mains powered isolated supplies, one for each peddle.

So beware trying to power two things off the same circuit. Test first to see that there is a common ground on the supplies.


Hello Grumpy_Mike,
Thank you for your input. I've been connecting several pedals to very same power supply in many years ( 30+). So it works.
There are many pedal boards ( Like Boss BCB60 or BCB30)on the marked  where 3 or 6 pedals share common none isolated power supply.
http://www.bossus.com/gear/productdetails.php?ProductId=610
Of course I too think isolated power supply is best.
Still when all those pedals are connected in series they share common ground by their screen wire. Aren't they?

Rolland Boss power supplies unlike many others have +9V and ground swapped in the pin. If you connect wrong pedal to it that has no protection diodes it could result with the smoke.


Sikter


Quote
I wonder is there any way to prevent this noise... Maybe micro servo and audio equipment just don't work together.


Electric guitars/amps are Hi-Z input and it's very easy for noise to couple into it.  (example: AM radio, RF buzz, fluorescent lamps, etc)....

I suspect the micro servo motor (unshielded even!) butt up against the metal case of the amp is a sure fire way of transmitting the EMI/RF noise of the servo to your amp.

You can try "mechanical shielding", see faraday cage, and then grounding that metal case. But the fact that it's supposed to be butted up/pressed against your guitar amp (to be able to turn the knob) I don't know if that will still be effective. 


Thank you,
I was thinking of faraday cage and installing servo on volume pedal not direct on the amp but ... it wouldn't be exact what I had in my mind.
I would like to use this device on GAIN knobs mostly not on the volume master.
Maybe I'll try test it with de-coupling first and take it from there.


Sikter

Just a little update:
I connected arduino and microservo to battery so
they had no common wires with my other circuits.
It worked perfectly without any noise. I'll run a couple more
tests to find out what caused problems first time.
This time I had all our rig in different building where we played the gig.
I'm going to test it again in old building where we do our rehearsals.
I'll try with DC adaptor too.


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