LM7809 noise on loopstation

[Yur]

Hey guys,
Here's the situation:
I have a 12v battery, a small boombox running on 12v, and mijn Loopstation RC-505, running on 9v.
I joined them all together, so that I can go out on the street with my loopstation, and make music.
So, I have the battery connected to and built into my boombox, I made the most simple reference 12-to-9v circuit(see attachment), and plugged that into my loopstation.

Then, I have a stereo 6mm to 3.5mm converter, and a 3.5mm stereo jack cable running from my loopstation back to the boombox.

It works. Sort of. My loop station gets enough power, the boombox powers on, I even have audio. But with that, there's an annoying noise in the audio. Sort of mixed "ssssshhhhhhh" and "wiiiiiiiiiiiii". It is gone when I unplug the jack cable from my loopstation. It's also not there when I plug some headphones into my loopstation. So it's only there, when there's a closed loop between my loopstation and my boombox, being power in, audio out. Interrupt one of those, and it's gone.

Do you have any suggestions on what the problem might be, and what I can do about it? After reading this thread I tried to put some 100 and later 1000uf caps on both the input and output, but it didn't help.

the data sheet provides me with another circuit diagram of a ripple reduction circuit. Is this what I'm looking for and do is need all of it?

Any help would be greatly appreciated.
Best regards,

Yur

 

[MikeMl]

I have not a clue what a "loopstation" is, but what you are describing is a "ground loop". This is caused when the audio signal common wire also acts as a DC power supply return for the audio source. The Power supply current causes a small voltage drop along the common wire, and that voltage drop appears as a common-mode noise source and contaminates the audio signal.

I'll give you an example: If I try to power an MP3,CD player, or XM receiver in my aircraft using a voltage regulator like your 7809 (to drop 14V to 3.5V), I get both alternator whine and the CD player motor buzz into the audio feed. If I run the audio source on batteries, the problem goes away. It is the act of tying the DC power ground (normally an isolated battery) into the ground of the audio amplifier that introduces the noise.

To get rid of it, for stereo, I had to use two two-winding audio transformers (sold as a ground-loop isolator) to effectively disconnect the audio ground from the source (CD or MP3 or XM) from the audio ground of the audio amplifer built-into the aircraft.

I'll make a sketch and post it when I have time...

 

[JimB]

the data sheet provides me with another circuit diagram of a ripple reduction circuit. Is this what I'm looking for and do is need all of it?

No, that is not a "Ripple Reduction Circuit".
It is simply the test circuit which the manufacturers used to test the "Supply Ripple Rejection" of the LM78xx.
There is no point in you building this, it wil not help you in any way.

JimB

 

[Yur]

JimB: Thanks, glad I can forget about that.

MikeMl: Awesome, problem probably located. From what I saw after googling, it's basically two adjacent coils per channel, thus transferring the frequency, but nog connecting the gnd, right? What kind of coils should I use and where could I get them? I'm Dutch, so if you can tell me what the parts are called, that'd be great, then I'll try to find a local supplier.

btw, this is a loopstation.

Thnx so far!

Yur

 

[audioguru]

"Close your eyes, I want to lose you". Huh?
I liked hearing it until the RAP ruined it.

 

[MikeMl]

Did you click on the "ground-loop isolator" link in my previous posting?

 

[Yur]

Yep, but my nerd-ego wanted to build one himself, but I figured it's not worth the effort for 8 bucks, and I'll probably get better audio from a Boss isolator then from a homebrew variant.
I'll just get Boss one. I'll report back when I have it installed, so this thread can be closed.

 

[MikeMl]

Yep, but my nerd-ego wanted to build one himself, but I figured it's not worth the effort for 8 bucks, and I'll probably get better audio from a Boss isolator then from a homebrew variant.
I'll just get Boss one. I'll report back when I have it installed, so this thread can be closed.

I have built my own out of two audio transformers I happened to have on hand.

For the HI-FI enthusiasts out there, I wonder how good a small (less than 2cmX2cmX2m) 1:1 audio transformer (the ones used in the commercial ground-loop isolators) are at passing all frequencies between 20Hz and 20,000Hz? I cant hear anything above about 4Khz, so I am not a good judge. I didn't frequency-response sweep the transformers I built-into my aircraft audio system, but they sound good through the ANR headsets I use in the cabin, especially at the low end where I can still hear.

I remember back in the tube amplifer HiFi days that audio transformers were huge, but that likely had to do with the power being coupled to a speaker. How small can an audio transformer be (nominal 1KOhm in to 1KOhm out) and have good audio response?

 

[audioguru]

Most cheap little transformers pass only telephone frequencies (300-3kHz). It needs a pretty big core and low resistance (thick wire) for good low frequencies into a low resistance load. If the load resistance is higher then a cheap little transformer goes lower but its high frequencies are muffled due to capacitance in the transformer and at the load..

 

[Yur]

Got the isolator finally yesterday, and it works. It's **broken link removed**.
Beautiful, noiseless audio. Thanks guys! Thread can be closed.

 

[MikeMl]

Got the isolator finally yesterday, and it works. It's **broken link removed**.
Beautiful, noiseless audio. Thanks guys! Thread can be closed.

Glad my aircraft audio wiring experience panned out...