Noisy 3886

jumpus · 27th June 2007, 05:58 PM

I'm having problems with a recently constructed 3886 amp. There's substantial hum when power is on, and even more if nothing is connected to to the inputs. Also, noise reduces dramatically when power is cut to the transformers, but the amp is still running (on power stored in the supply capacitors?).

I'm thinking the noise is from the transformers, and is making it's way into the signal path through the signal's ground somehow, but I'm just a beginner with this stuff.

What else could the problem be/ how can I fix it?

Thanks!

gootee · 27th June 2007, 06:21 PM

Hi Kenneth,

Hum is one of those things that usually depends on exactly how everything is connected, and layed out.

So you would probably get better advice, sooner, if you could post a photo of your setup, and a schematic, and as much other relevant information as you can.

There are a lot of people here who will like to try to help you, if they are given enough information.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

jumpus · 27th June 2007, 07:25 PM

I will put a picture or two up when I get a chance. I just wanted to see if my idea makes any sense. I'm at work right now, so the picture(s) won't be around for a while. Could the symptoms that I described point to anything? I understand if a picture is necessary for any concrete determinations of the problem, but I'm running low on things to do here (at work) and I could really use something to think about.

gootee · 27th June 2007, 08:03 PM

There are some good threads in this forum about solving hum and grounding problems, which are often related.

I don't know what kind of power supply topology you have used. So it's very difficult to make any specific statements, or even guesses, about it. But I can make a couple of general statements, that might give you some food for thought.

You should use what is called a "star" ground-return setup, in which all major ground current-return paths are run in separate conductors, which all meet at only ONE point (often the ground side of a main filter capacitor in the power supply). This is done because wires and PCB traces have parasitic resistance and capacitance, across which voltages are induced (back at the end of the conductor that's farther from the power supply, in the case of ground returns), by currents, which makes sharing of ground return paths "a bad thing".

Also, typically, the power supply and audio grounds, which usually are connected to each other only at the star ground point, are either not connected to the "earth ground" from the power cord, at all, or, are connected to it through a "disconnect network", which might include (all in parallel) a >= 10 Ohms resistor, a 0.1uF or so cap, and two opposite-facing diodes.

gootee · 27th June 2007, 09:11 PM

You might want to first check how your input connectors are grounded. If they are grounded to the case, that could be the problem.

Also, AC wiring should be kept well-away from signal and DC wiring (and components). If that's not possible, they should cross only at right angles to each other. It might also help to twist any pairs of wires you might have.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

jumpus · 27th June 2007, 09:20 PM

Thanks for the advice. I'm going to re-assess the situation when I get home...12 minutes to go...I'll get a picture up for you then.

gootee · 27th June 2007, 09:45 PM

Quote:

Originally posted by gootee

You should use what is called a "star" ground-return setup, in which all major ground current-return paths are run in separate conductors, which all meet at only ONE point (often the ground side of a main filter capacitor in the power supply). This is done because wires and PCB traces have parasitic resistance and ***capacitance*** , ...

I MEANT to type *** inductance*** (although there is also some capacitance between any two conductors, which depends on the materials and geometries involved).

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

jumpus · 27th June 2007, 10:26 PM

Pictures, as promised. Kinda big files, but good resolution so you can zoom in real good. Let me know if a smaller file would be better...

jumpus · 27th June 2007, 10:45 PM

Trying this again....

jumpus · 27th June 2007, 11:05 PM

a bit higher quality

27th June 2007, 05:58 PM	#1
jumpus diyAudio Member Join Date: May 2007	Noisy 3886 I'm having problems with a recently constructed 3886 amp. There's substantial hum when power is on, and even more if nothing is connected to to the inputs. Also, noise reduces dramatically when power is cut to the transformers, but the amp is still running (on power stored in the supply capacitors?). I'm thinking the noise is from the transformers, and is making it's way into the signal path through the signal's ground somehow, but I'm just a beginner with this stuff. What else could the problem be/ how can I fix it? Thanks!

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27th June 2007, 06:21 PM	#2
gootee diyAudio Member Join Date: Nov 2006 Location: Indiana Blog Entries: 1	Hi Kenneth, Hum is one of those things that usually depends on exactly how everything is connected, and layed out. So you would probably get better advice, sooner, if you could post a photo of your setup, and a schematic, and as much other relevant information as you can. There are a lot of people here who will like to try to help you, if they are given enough information. - Tom Gootee http://www.fullnet.com/~tomg/index.html

27th June 2007, 07:25 PM	#3
jumpus diyAudio Member Join Date: May 2007	I will put a picture or two up when I get a chance. I just wanted to see if my idea makes any sense. I'm at work right now, so the picture(s) won't be around for a while. Could the symptoms that I described point to anything? I understand if a picture is necessary for any concrete determinations of the problem, but I'm running low on things to do here (at work) and I could really use something to think about.

27th June 2007, 08:03 PM	#4
gootee diyAudio Member Join Date: Nov 2006 Location: Indiana Blog Entries: 1	There are some good threads in this forum about solving hum and grounding problems, which are often related. I don't know what kind of power supply topology you have used. So it's very difficult to make any specific statements, or even guesses, about it. But I can make a couple of general statements, that might give you some food for thought. You should use what is called a "star" ground-return setup, in which all major ground current-return paths are run in separate conductors, which all meet at only ONE point (often the ground side of a main filter capacitor in the power supply). This is done because wires and PCB traces have parasitic resistance and capacitance, across which voltages are induced (back at the end of the conductor that's farther from the power supply, in the case of ground returns), by currents, which makes sharing of ground return paths "a bad thing". Also, typically, the power supply and audio grounds, which usually are connected to each other only at the star ground point, are either not connected to the "earth ground" from the power cord, at all, or, are connected to it through a "disconnect network", which might include (all in parallel) a >= 10 Ohms resistor, a 0.1uF or so cap, and two opposite-facing diodes.

27th June 2007, 09:11 PM	#5
gootee diyAudio Member Join Date: Nov 2006 Location: Indiana Blog Entries: 1	You might want to first check how your input connectors are grounded. If they are grounded to the case, that could be the problem. Also, AC wiring should be kept well-away from signal and DC wiring (and components). If that's not possible, they should cross only at right angles to each other. It might also help to twist any pairs of wires you might have. - Tom Gootee http://www.fullnet.com/~tomg/index.html

27th June 2007, 09:20 PM	#6
jumpus diyAudio Member Join Date: May 2007	Thanks for the advice. I'm going to re-assess the situation when I get home...12 minutes to go...I'll get a picture up for you then.

27th June 2007, 10:26 PM	#8
jumpus diyAudio Member Join Date: May 2007	Pictures, as promised. Kinda big files, but good resolution so you can zoom in real good. Let me know if a smaller file would be better...

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