hum with my 3886 GC and my iMAC !

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I've fired up my 3886 gainclone build and have been testing it out. So far, with the iMAC headphones out as the source I get a lot of hum. I have only one amp installed so far and want that hum free before proceeding. I have a star ground arrangement with safety GND and audio GND tied together at one point on chassis. The RCA jacks are isolated from chassis and the shield is tied to the amp board input (Chipamp). I've tried different GND connections and this works best so far. Here are some experimental results:

* iMac disconnected, input cable still attached = no hum, nice and quiet!
* iMAC connected = significant hum and activity noise (disc and mouse activity make more noise)
* changed power cord outlets = no effect
* connected to Sony FM tuner as source = no hum, sounds great!
* connected to my Emotiva pre as source = no hum, sounds great!
* floating iPOD as source = no hum, sounds great!

This is a classic GND loop problem and I'm convinced the iMAC audio GNDing scheme is the root cause of the problem, not my build. Eventually I want a DAC between the iMAC and amp so that should solve the problem in the long run. Still, I want this to work for awhile with iMAC analog source.

Anyone else have this problem with a MAC or PC as source??

My thought is to install a GND disconnect network to try and reduce the loop noise. I have a 2W 10ohm resistor, a 25Amp AC rectifier bridge, and the 0.01uF ceramic cap for the network. My line fuse is a 3Amp, slow-blo.

Onwards and I will report the results !!!!

I am using a laptop (13A UK main plug) as my source. I have an USB DAC. (2 pin US plug)
The USB DAC won't eliminate the hum. When the laptop is running on battery, it is quiet.

I have try the following ground config for my Lm3886 integrated amp.

1.Connect the signal ground to the main ground via a ground disconnect network.
2.Connect the RCA ground directly to the main ground.
3.Connect the RCA ground to the main ground via a ground disconnect network.

Config 1 and 2 give similar result. When I turn the volume up to about 3 o'clock position, I start hearing the hum.

Config 3, I can't hear anything when the volume is at max.

I don't have the knowledge to give you any advise, for detail get a copy of Audio power amp design handbook and see if you want to try it.
News flash!
I added the 10ohm disconnect network and it made a HUGE difference ... success!
Now I can just barely hear the hum if I push my ear against the speaker, full volume.
Definitely more than adequate! Now to test, with my other preamp as a source, to make sure it works with that gear as well.

I spent the last day reading many posts on audio GNDing techniques. Thanks for the fine work here on this forum.

So, what I finally did was build the disconnect network, tie one end to the chassis (safety earth) and for each chipamp board, I ran a wire from its CHG point to the network. The RCA shields, btw, stay tied to the signal GND on the amp boards.
I hoep to add a diagram soon.

Can you clarify your findings a bit?
By not hearing anything for Config 3, I take it you mean noisewise?

So ... for config 3, you have RCA GND to main using the network. What about the signal GND?

Config 3, my present config no hum or hiss, with laptop's main supply connected.

When I built my amp, my first amp and the only amp I built. I use the “Audio power amp design handbook 5th edition” as my main reference book. Read chapter 18, pay attention to figure 18.1.

The attached diagram is how I do it. Do not use it as a reference. It may contain errors here and there. Please read the book yourself. But since you have solve the problem already, no need to try it.

By the way, nice built.


  • LM3886.bmp
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