Cleaning the lab and setting up an amplifier connected to my computer as a source of music. I have four big speakers that usually reproduce sound just a little on the bass side. Nevertheless, when I plug my electronic piano in to play I get this hissing noise. When I turn the piano off the hissing goes away.

I'm plugged into the headphones port (the only place to plug in). Should I build something to filter out the hiss? If so - what's your recommendation?

 

Do you have a scope? If you can let us know what frequency and shape the interference is we can probably help.
Keith

 

Yeah, have a scope. But it's all the way on the other side of the bench. Getting a signal will take some work; and I've been trying to clean up my work mess - I mean my work bench.

Happens worst when I have very low volume on the keyboard (piano). If I turn the amp down and turn the piano up I can mask it. It's a white noise hiss. Annoying at low volumes. And I have tinnitus from loud music and rock & roll drums. Had to sell the drums. I want to be able to hear what you all have to say.

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I also have a hearing problem - too many heavy metal concerts. You could try a passive low pass filter for around 12 to 15 KHz. It may help. Do you have any inductors laying around? you will get a steeper slope using L and C.
Keith

 

Thankyou for specifying who should answer.
I am not familiar with music audio standards used to design pianos.
It is possible to go further in tuning. Using a microphone and smartphone free audio spectrum analyzer; android app
you can sample keyboard view the audio spectrum identify what the microphone can detect including sour notes.
If you can find out what a good keyboard response should look like, there are a lot of specialized details that a good music shop might know.
The manufacturers specifications keyboard and sound card otherwise a measured estimate might be necessary, not sure.

In general for a hiss only fix
audio hisstorians recommend getting familiar and determining if your line level is correct. (What is the output, what is the best input.)
On the computer side what is the input tolerance. Then use software to determine LP filter level. like an audio editor having various filters.
then an appropriate line level and low pass filter criteria for keyboard interfacing.

 

As so often is the case - the one asking a question fails to provide sufficient information. The jack on the back of the keyboard is for headphones. The jack where I'm plugged into the amplifier is for a microphone. There are four microphone jacks. Four of them are 1/4 inch mono plug types and there are four more that are the more traditional microphone connection with three wires.

Knowing that the keyboard headphone jack is an amplified signal I have to keep it set VERY low on volume. Otherwise I will overdrive the mic input. When the keyboard is active - wall wart powered OR battery powered - I get the hiss. So it's coming from the keyboard. Obviously the amp is picking up something headphones wouldn't detect. There's definitely an audio mismatch causing the interference. It's not an outlet problem and it's not a tuning problem. I've obviously got something hooked up wrong. Impedance matching is probably where things are connected the wrong way.

The amp is a RadioShack 250 watt stereo PA amplifier. Has four mic inputs. The fourth input can also be connected to a "Phono". There's a fifth input "CD/AUX" but that's where the computer is connected.

It should be obvious I don't know what I'm doing (or doing wrong). I just know I need some direction. The keyboard output is way too much for the mic input, but that's the only place I can make a connection. I have some extra audio cables so hacking one with some sort of line matching or filtering would probably be the right way to go. I'd like to be able to use 100% of the volume control on the keyboard, but the way it's connected I can use maybe about 5%. Any higher and it gets really distorted. Maybe @Audioguru again would have some good advice. After all, a name like audio guru makes me think he's probably pretty smart with audio stuff.

 

That extra information is very helpful. The hiss is noise coming from the keyboard. You have it turned down low because you are using the headphone output, but lowering the music volume is not reducing the background noise, which you are amplifying.
The answer is to put a simple attenuator ( potentiometer? ) between the keyboard and amplifier and turn up the volume on the keyboard. This will give you a larger signal from the keyboard for the same noise level. The attenuator will then reduce the amplitude of both.
Regards,
Keith

 

You can hear the hiss and want to reduce it but when it is reduced then high frequencies of sounds will also be reduced like an old telephone or AM radio.
Make the attenuator to reduce both the noise and the signal.

 

The answer is to put a simple attenuator ( potentiometer? ) between the keyboard and amplifier and turn up the volume on the keyboard.

Yeah, I can do that. But what about shielding? Should that be a bother? Maybe get an Altoids tin?

 

Use shielded audio cables and connect the metal case of the pot to the shields.

 

It appears that the noise is coming from the keyboard device. First question is do you hear it with earphones plugged into the keyboard? It is probably basic amplifier noise from the headphone amplifier output stage.
If your PA amplifier has a "line input" connector that is a much better input to use for an earphone level output. Dynamic microphones have a much lower level output, often 20 to 50 millivolts, which is a noise output you would not hear with earphones. A line input may go up to one volt, at which point the low hiss level would be much less noticeable.
OR, if you fell comfortable doing it, open the keyboard and locate a point before the headphone amplifier to tap in to the music.

 

Yeah, I can do that. But what about shielding? Should that be a bother? Maybe get an Altoids tin?

If you use a small metal box of any kind, mount the pot and some input and output RCA connectors on it. Connect the shielding side of the connector to the box. If you only have an insulated box, connect the shields to the pot case as Audioguru suggested.
Keith

 

What ohmage pot should I use?

Pin's 1 & 3, I'm guessing with fair confidence, goes to the output from the keyboard. Pins 1 & 2 or 2 & 3 to to the amplifier. Should I put an impedance matching transformer in there anywhere?

 

If you use a small metal box of any kind, mount the pot and some input and output RCA connectors on it. Connect the shielding side of the connector to the box. If you only have an insulated box, connect the shields to the pot case as Audioguru suggested.
Keith

I like that idea. I have plenty of RCA's, PCB mounted type, so good for the tin can approach. Also have a quarter inch female jack somewhere.

The keyboard output is stereo. The amp is mono. I have no problem accepting that the keyboard is not playing stereo, though I could use the altoid can to break that out into stereo inputs and then connect to two of the mic inputs.

 

What ohmage pot should I use?

Pin's 1 & 3, I'm guessing with fair confidence, goes to the output from the keyboard. Pins 1 & 2 or 2 & 3 to to the amplifier. Should I put an impedance matching transformer in there anywhere?

If you are connecting it to the earphone output the pot should be at least 100 ohms but not over 5K ohms. That gives you a lot of leeway.

 

What ohmage pot should I use?

Pin's 1 & 3, I'm guessing with fair confidence, goes to the output from the keyboard. Pins 1 & 2 or 2 & 3 to to the amplifier. Should I put an impedance matching transformer in there anywhere?

Use a log pot with a value anywhere form 10 to 100 Kohm. No transformer needed.
Keith

 

 

Having to merge left and right balances, maybe I should go this route? The 10K to get the voltage way down. Maybe the 50K is too big? I COULD mess with various pots.

@MisterBill2 suggests not over 5KΩ.
@KeithWalker suggests log pot. Is that an "Audio Taper" pot?

 

I didn't realize that the piano output was stereo. You don't need the balance pot - just use a couple of 1K resistors. Yes the "Log" pot is "Audio Taper". A 10K pot will be fine. The input to the amplifier will see much less resistance than that at low volume setting. If you want to load the output from the piano, connect a 33 ohm resistor from each output channel from the piano to ground.
Keith

 

"Log taper" and "Audio Taper" are nominally the same, although calling it audio allows more freedom.