Help!!! GC hum when input attached

[HawkeyeStoob]

I finally finished my first GC this morning….success!!!

First I need to thank everyone here for all of the info that I’ve been able to find, and hopefully use correctly, while I was building my amp. This was my first DIY home audio project of any sort, and I have definitely got the DIY bug now.

I have a LM3875 basic stereo kit from Brian with an AVEL 250VA 18V+18V toroid (4 ohm speakers) for the PS.

The amp is almost dead quiet with just the speakers hooked up. Once I hook up the input from my preamp there is a low level, Low frequency hum that doesn’t increase or decrease with the volume. The sound is easily masked once the volume is turned up.

I am looking for any ideas on a possible fix for the problem.

[HawkeyeStoob]

I was not completely correct in my first post. The hum does in fact increase and decrease with the volume.

Would this be a situation where running resistors on the grounds coming from the amp boards would help?

Here is a picture as well if that helps at all.

http://img.photobucket.com/albums/v1.../complete3.gif"

[HawkeyeStoob]

First I want to apologize for posting before I had gone thru all the search threads completely. I was frustrated and wanted an answer quickly. (Short attention span American) I understand why you get upset with the same question being asked again and again. But there are so many threads posted here, often wandering off and back on subject, that it is difficult to read thru them and find the one little piece of information you might need.

I am proud to announce that………. I have now “fixed” the problem.

I went back, after much more reading and rereading, and disconnected the earth ground connection. Like magic the amp became dead silent. Not an unwanted whistle, squeak, or annoying hum to be heard coming from the speakers. Just some great sounding music.

My question is now what are my options for a permanent fix of this problem?

Heavier ground wire from the star ground to earth?
Resistors on the grounds coming from the amp boards?
Leave the ground unattached?

I am still reading and searching, but would appreciate any suggestions for a solution.

Thanks Again,
Craig

[Nuuk]

Craig, you seem to be quite good at finding your own answers so have a read of the FAQ section on Decibel Dungeon. I think that the answer you seek is there.

[Shoog]

Heres one solution to connecting the circuit ground to the socket ground - safely !
Connect in parallel a 0.022uf cap, 100ohm (3Watts min) resistor and then two high power diodes - each facing in the opposite direction. Take this small bundle and insert it into the earth line to the socket.
The cap functions to bleed off radio interference. The resistor will block current flowing around in loops. The diodes are there to allow a high current fault to travel to earth in either the positive or negative direction.

This should do the trick with the minimum off effort.

Shoog

[HawkeyeStoob]

Thanks Nuuk and Shoog for your help.

I have read FAQ section of Decibel Dungeon and have tried to change the grounding arrangement to match what you have described there. While the noise did drop some it was still there.

I will have to run by my local electronics parts store to get the parts required to try your solution Shoog. So that will be a couple of days away.

As I said before I disconnected the earth ground from my power input which solved the problem. I am not all that comfortable with the thought of having a metal chassis with no ground however. I reattached the ground and went back to researching. After reading many more posts I came across a couple of threads where people were simply not connecting the chassis ground (CHG) on Brian’s boards. In fact some were saying that it might be the preferred way to set up the amp. I removed my CHG from the star ground and that also solved my problem.

This leads to a couple more questions:

Am I hurting anything by not having the boards grounded?

I am also curious as to why the boards are grounded in the first place? I suspect that it is merely to protect them the components mounted on them and the speakers on down the line.

If that is the case wouldn't running fuses in line with the outputs protect the speakers in the event of a problem with the amp? Thereby protecting the speakers at the expense of that particular amp channel if a problem occurred.

Thanks Again,
Craig

[Nuuk]

When more than one piece of equipment is connected to mains earth, this can cause the earth loops that give hum.

One solution has been to disconnect the earth in one piece of equipment although this can bea safety issue.

It's a very complicated issue and I have never recieved the difinitive answer even from experienced commercial designers who I have consulted. In my experience such problems are often overcome by empirical methods.

As regards speaker protection, the best way to is to use a speaker protection module as described on DD.