Cmoy noise problem

[IdiotTech]

Hope you guys can help. So i just built a cmoy and i noticed that whenever i unplug my source from the cmoy i get a hum/buzzing sound, but not when a source is plugged in. I read through tangents tutorial and was able to figure out that its being caused by my power supply because i get no noise when i use a battery. However im not sure how to go about figuring it out. Does this sound like a grounding problem? Or what could it be? The power supply is a linear regulated power supply that i built.

 

[IdiotTech]

Anybody have any ideas?

 

[Robfow]

Maybe you would like to show us some pictures

 

[lcp12345]

Most likely, it is the grounding issue.

 

[Pars]

Quote:

Originally Posted by IdiotTech /img/forum/go_quote.gif Hope you guys can help. So i just built a cmoy and i noticed that whenever i unplug my source from the cmoy i get a hum/buzzing sound, but not when a source is plugged in. I read through tangents tutorial and was able to figure out that its being caused by my power supply because i get no noise when i use a battery. However im not sure how to go about figuring it out. Does this sound like a grounding problem? Or what could it be? The power supply is a linear regulated power supply that i built.

What kind of case do you have this in? Metal? Is your power jack isolated? Realize that the virtual ground of the Cmoy is not the same as what the powersupply (wallwart) will think of as ground (the wallwart's "ground" on the sleeve (probably) of the plug is actually the Cmoy's V-).

 

[IdiotTech]

Quote:

Originally Posted by Pars /img/forum/go_quote.gif What kind of case do you have this in? Metal? Is your power jack isolated? Realize that the virtual ground of the Cmoy is not the same as what the powersupply (wallwart) will think of as ground (the wallwart's "ground" on the sleeve (probably) of the plug is actually the Cmoy's V-).

So the case i have is one of those hammond aluminum ones with plasctic ends. The power supply is not a wallwart. I put the power suplly together, in other words i took the power cable, attached it to a transformer and rectified it and what not. So the cord is the three prong one with the pos, neg and ground prongs. I attached the pos and negative to the transformers leads and i attached the ground to the metal casing of the transformer, which im not so sure if it is correct. And i should clarify that the amp works well when I have a source plugged in and it is on, I get no noise. However when it is unplugged or i just turn the source off then i get the loud buzzing, which does not happen under any circumstance with the battery. I am just curious if anybody has any ideas Ive been assuming that it is a grounding problem with the power supply but havent been able to get rid of it.

Again thanks for the comments.

 

[amb]

Assuming your cmoy has no volume control pot at the input, when your source is unplugged, the input to the opamp is left "floating" (actually, there is a resistor to ground but it's 100K ohms, nowhere near the low output impedance of your source). A floating input will pick up hum and noise. That's life.

If your cmoy has a volume control, turn it all the way down (so that the input to the opamp is grounded) before unplugging your source, and there shouldn't be any audible hum/noise.

 

[IdiotTech]

Quote:

Originally Posted by amb /img/forum/go_quote.gif Assuming your cmoy has no volume control pot at the input, when your source is unplugged, the input to the opamp is left "floating" (actually, there is a resistor to ground but it's 100K ohms, nowhere near the low output impedance of your source). A floating input will pick up hum and noise. That's life. If your cmoy has a volume control, turn it all the way down (so that the input to the opamp is grounded) before unplugging your source, and there shouldn't be any audible hum/noise.

Well i have a pot, but i just havent wired it cause i was trying to sort this out. Hmm, never thought about that, I guess it is the floating input. The only thing i wonder about then is why it does not do that when I use the battery?

Well, thanks for the response. Ill just finish putting it together and learn to live with it then.

Thanks

 

[FallenAngel]

Gotta ask, what kind of power supply is this? It does sound like a grounding issue, but it could also be because of a power supply less than perfect (aka battery).

 

[IdiotTech]

Quote:

Originally Posted by FallenAngel /img/forum/go_quote.gif Gotta ask, what kind of power supply is this? It does sound like a grounding issue, but it could also be because of a power supply less than perfect (aka battery).

This is a powersupply i put together. Its basically a fullwave rectifier which uses a zener diode to regulate the voltage and has mosfet so that it doesnt load the circuit. I put it together a while ago for a lab, but i lost the schematic for it, its supposed to get rid of loading, voltage fluctuations and its supposed to filter the ripples fairly well. We actually did some pretty good testing on it. I had it just sitting there and thought itd be good to use for the cmoy, it seems to be fine when playing. So i just wanna get rid of the buzzing. I figure it is more convenient than using batteries.

 

[IdiotTech]

Fyi what i mentioned is in the schematic is only part of what it has, its got some other stuff that i cant quite remember about. I wanna get ahold of the lab sheets i had about it, ill try to find them when i go home next month, cause its killing me not remembering. If anything ill have to reverse engineer the schematic to see if i can remember all the small nuances.