Electrical Engineering

@ThePhoton electric hum, it's a piano adapter and the hum gets to the speakers from the piano (I verified that it's the source: If I dim the piano input on the mixer, the hum disappears)

The Photon

@tohecz And the dc adapter is external from the piano itself?

tohecz

@ThePhoton yep.

I'm just annoyed because it's the original YAMAHA stuff, not a cheap Chinese crap

The Photon

You can probably put some notch filter in the wire between the adapter and the piano.

tohecz

7:03 PM

@ThePhoton some what? I think that I just found the word I needed

The Photon

@tohecz A notch filter of some kind

You have 60 Hz mains?

tohecz

@ThePhoton 50 Hz / 230V

The Photon

Search for "50 Hz notch filter"

You don't need to worry about the 230 V part, because you'll be filtering the ac-dc converter's output, which should be somewhere betwen 5 and 48 V.

tohecz

@ThePhoton ok, thanks. I just need to find the Czech translation ;)

@ThePhoton yep I know

or I'll go to my electronics shop and ask them for a high quality 12V/2A DC source ;)

The Photon

Yeah, I just did a search on google images and none of the top hits would actually work for your situation (couldn't handle enough current through them). So be careful what design you use if you build it yourself.

tohecz

7:08 PM

@ThePhoton The notch filter looks like some expensive stuff...

@ThePhoton well, I'll go to the shop and ask them

otherwise, can't I build some simple stabilizer? It seems that I need to go only like 6dB down, which is 4x down. I was thinking of just putting a reasonably large impedance in the way

The Photon

@tohecz You mean regulator?

tohecz

@ThePhoton coil? They have low resistance to DC and reasonably high to AC

The Photon

@tohecz The problem with just adding impedance is that it might make the piano misbehave when you actually try to produce a 60 Hz note.

tohecz

@ThePhoton if the impedance is put on the DC input?

The Photon

@tohecz Yes because while the dc voltage is constant, the current drawn by the piano will have frequency content at "all" frequencies.

And the point of the ac-dc converter is that it should be able to maintain a constant dc voltage, no matter how the current drawn by the load changes.

tohecz

7:20 PM

@ThePhoton ah ok. So when the piano's consumption oscillates, I quite ruin the thing...

The Photon

@tohecz Hopefully it wouldn't catch fire, but it might put the piano into reset or something.

You want a filter with high impedance at 50 Hz looking in from the power supply side, and low impedance looking in from the load (piano) side.

tohecz

so in the end, I need some regulation. Because I can't simply short the two wires by a large capacity

@ThePhoton yeah I see

it's damn complicated

The Photon

You can do it with capacitors and inductors, but the parts you need might be big and costly.

Another thing to consider is that there might already be a filter inside the piano or the converter to solve this problem, but it's gone bad somehow.

tohecz

@ThePhoton is there any website that nicely compares the methods and schematics?

The Photon

@tohecz Not one that specifically answers the question for your situation, AFAIK.

tohecz

7:24 PM

@ThePhoton could I ask such a question on the site?

The Photon

You could make a question on EE.SE. Be sure to say you're designing a filter, and include the voltage and current ratings from the converter.

tohecz

@ThePhoton ok, I'll try

The Photon

Q: 50hz noise removal from ECG power supply

We are currently working on a bio medical project that is an ECG machine.There is a problem that we are unable to solve on software side is the removal of 50 Hz noise. Now we are trying to remove 50 Hz noise using analog filters. Does anybody have an idea of removing 50 Hz noise using analog comp...

tohecz

@ThePhoton how does a passive filter look like?

The Photon

For example,

(I doubt those values work for 50 Hz)

tohecz

7:28 PM

@ThePhoton 11.9 mH looks like a lot to me

The Photon

@tohecz True. But 50 Hz is a pretty low frequency.

tohecz

@ThePhoton yep, I know

which means that it's in the end even simpler to construct an active filter?

The Photon

@tohecz But power handling is harder with an active filter. What's the current rating on your converter?

tohecz

@ThePhoton 12V/2A, the piano eats officially 13W as far as I remember (I'm not athome now)

ah so the official consumption is only 7W...

@ThePhoton I hope I don't annoy you

The Photon

@tohecz No but I do have to get some work done.

tohecz

7:43 PM

@ThePhoton ah ok. Thanks a lot. I still dunno how to solve it, but it'll come :)

jippie

8:19 PM

morning

tohecz

@jippie evening :)

jippie

did I mis anything good today?

tohecz

@jippie me being angry at YAMAHA?

jippie

@tohecz 'sup?

tohecz

@jippie their DC source is letting hum through to the audio output of my piano

jippie

8:35 PM

did it always hum or just lately?

and what is the type number, can you find the thing on google images?

tohecz

@jippie well, I only know about the hum recently because I got a gear up

it's Yamaha PA 150 with EU 2-plug

jippie

Can't really tell if it is a linear or a switched mode power supply

tohecz

@jippie just a bit more ligible photo: picture.yatego.com/images/5087d18bee9e63.9/YamahaPA150A-kqh/… But it prbbly doesn't help

jippie

Could it be a ground loop? What other cables are connected?

tohecz

@jippie what is a ground loop? There are many things connected: HiFi, two powered speakers, PCM recorder, microphone, EQ

but it's coming from the piano somehow, if I dim the piano input on the mixer (and keep all cables plugged in), it disappears

jippie

8:46 PM

Couple things you can try:

Turn the adapter the other way around in the wall socket, it could be a capacitive coupling thing.

Disconnect everything. Only connect the things really required to test the power supply, so piano and amplifier

Use a single power strip that you power all devices from.

@tohecz A ground loop ia annoying, and hard to explain.

tohecz

@jippie I do this thing

and I'll try the first two suggestions, thanks :)

jippie

It happens when you have things somehow connected in a circle, rather than a star

but it can be both power and signal leads

tohecz

@jippie seems to be only signal here

jippie

@tohecz Don't be too hasty on drawing conclusions. I don't know your configuration, maybe you are right. But many people are being bitten by ground loops, even experienced.

tohecz

@jippie ok, thanks :) I may as well try to do it another way, like, connecting a spare car battery I have at home as the DC source

jippie

8:55 PM

Car batteries are usually a bit over 12V

tohecz

@jippie I'll measure it before use, don't worry ;)

jippie

The good thing is that you cannot have a ground loop with a car battery ;o)

tohecz

@jippie that's the idea

jippie