Hi! I just finished building my own 5.1 surround amplifier, And I got my new surround receiver yesterday, a Marantz SR5008. I'm using the pre out from the Marantz, into my LM3886 based amplifier. Sounds great on the paper, right? That's what I thought too. When I finally got the Marantz in the mail, I hooked up everything, and there; that awful 50Hz hum.
First, a little bit about the amp I made. I got a 600VA 2x24v AC Toroid transformer. The amplifier is based on 5 separate LM3886's, one for each speaker in the room. The subwoofer got it's own PCB, 3 LM3886's in parallel. All the inputs and outputs are mounted on a laser cut steel back plate, and each connector is isolated from each other with plastic sleeves. The back plate itself is not grounded.
I've been testing a lot, tried to hook up a wire from the chassis on the Marantz and into the ground rail in the amplifier, but no difference. BUT, I discovered that I get rid of the hum if I connect all the speakers, and the signal wires, except for the subwoofer. If I connect the subwoofer signal wire, I get the humming noise back.
I've tried to connect just the center of the subwoofer phono signal wire , but the subwoofer starts creating the humming noise.
How can I get rid of this noise?
I'll attach some pictures so you guys know what we're dealing with
First, a little bit about the amp I made. I got a 600VA 2x24v AC Toroid transformer. The amplifier is based on 5 separate LM3886's, one for each speaker in the room. The subwoofer got it's own PCB, 3 LM3886's in parallel. All the inputs and outputs are mounted on a laser cut steel back plate, and each connector is isolated from each other with plastic sleeves. The back plate itself is not grounded.
I've been testing a lot, tried to hook up a wire from the chassis on the Marantz and into the ground rail in the amplifier, but no difference. BUT, I discovered that I get rid of the hum if I connect all the speakers, and the signal wires, except for the subwoofer. If I connect the subwoofer signal wire, I get the humming noise back.
I've tried to connect just the center of the subwoofer phono signal wire , but the subwoofer starts creating the humming noise.
How can I get rid of this noise?
I'll attach some pictures so you guys know what we're dealing with
Attachments
So you're running input and output wires in tightly coupled parallel pairs? Bad idea, you will get lots of crosstalk. I would separate them as much as possible.
Also, you're running power wires under the board, near all the sensitive parts of the circuitry? Bad idea number 2. I normally turn the PCBs 90 degrees so that the pads face each other, and use stiff wire to connect them all together (no terminal block). This would also enable to run the speaker wire close to the chassis floor and the signal wire above all of this, closer to the case top. This creates a decent amount of separation between input and output.
Also for such kind of implementation I would use screened wire for the input signals. Also I would use a ground common at the input, and another one at the the output, and join them all together. I have done something like this before and had no hum problems. In addition, in that case I had two different power supplies but making sure all the ground had a common reference helped a lot.
There was another member here who had the exact same problem and solved it using input transformers, That is a good solution, though €xpensive.
Also, you're running power wires under the board, near all the sensitive parts of the circuitry? Bad idea number 2. I normally turn the PCBs 90 degrees so that the pads face each other, and use stiff wire to connect them all together (no terminal block). This would also enable to run the speaker wire close to the chassis floor and the signal wire above all of this, closer to the case top. This creates a decent amount of separation between input and output.
Also for such kind of implementation I would use screened wire for the input signals. Also I would use a ground common at the input, and another one at the the output, and join them all together. I have done something like this before and had no hum problems. In addition, in that case I had two different power supplies but making sure all the ground had a common reference helped a lot.
There was another member here who had the exact same problem and solved it using input transformers, That is a good solution, though €xpensive.
I've tried to connect the subwoofer amplifier board right on the other LM3886 power terminals; No difference. But I tried to move the internal signal wires away from the transformer, and now there is a little bit less hum.
I also discovered something weird. The hum is completly gone if I unplug the mains, and let the amp bleed out. The rectifier capacitors are quite big, so I still got power for about 5 seconds before they cut. Can this problem be related to the transformer?
I also discovered something weird. The hum is completly gone if I unplug the mains, and let the amp bleed out. The rectifier capacitors are quite big, so I still got power for about 5 seconds before they cut. Can this problem be related to the transformer?
Unplugging the amp also removes its connection to the mains EARTH/GROUND. Does unplugging the chromebox reduce noise?
Look into using a "ground loop breaker" between the chassis (rear panel) and PSU ground.
When you understand how it works and how it should be constructed, post the layout and components that you want to use.
Look into using a "ground loop breaker" between the chassis (rear panel) and PSU ground.
When you understand how it works and how it should be constructed, post the layout and components that you want to use.
move all signal cables, red-black, on one side and keep it together. First try simple test. connect only one signal from receiver. If you have 50Hz buzz problem is in Earth ground, you have ground loop. If you have buzz with two or more connected signals problem is in ground loop inside amp made with ground of signal cables. In that case buzz is probably 100Hz
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I've tried to disconnect everything that's connected to the surround receiver, except the pre out signal wires that goes to the amplifier. Sadly, there is no difference. If I only connect one signal from the receiver, I don't get any hum. The problem appears when the subwoofer amplifier and the other amplifiers are connected at the same time.
The hum is a bit "sharp", and not a sine wave. I'm quite sure it's 50Hz, since it got the exact same hum as the transformer does.
Thank you for all your time and patience! I really want to get this hum away, and I've never had any issues like this before.
The hum is a bit "sharp", and not a sine wave. I'm quite sure it's 50Hz, since it got the exact same hum as the transformer does.
Thank you for all your time and patience! I really want to get this hum away, and I've never had any issues like this before.
You must all input signal cables move to the right side and put it together closest you can. Surface made with ground wires must be the smallest can be.
https://www.dropbox.com/s/84wc54po6adsjaq/2015-01-11 02.22.03.jpg?dl=0
https://www.dropbox.com/s/84wc54po6adsjaq/2015-01-11 02.22.03.jpg?dl=0
I still got the same problems when using my phone, so there is no way I can blame the Marantz for this
I've tried to move all cables together, but still no difference. I got some new signal cables today, and I discovered that I actually got hum if I connect more that one signal wire. I got a tiny bit of hum if I connect the two front speakers (subwoofer excluded), but I got much more hum if I connect the right front speaker and the left back speaker. I've tested this with and without applying voltage the subwoofer amplifier. The hum is actually lower when the sub amp gets voltage!
My guess is that the 5xLM3886 circuits are the problem. The power coords don't have a huge amount of insulation, and are actually fit underneath the PCBs. The power wires are all connected in series, and I guess that also may be a problem.
I think I should rewire all the five LM3886 amplifier boards, but this time, I want to do it without any shortcuts! I'm just a young student, so I need to learn how to wire amplifiers like this without getting that 50Hz hum!
I've tried to move all cables together, but still no difference. I got some new signal cables today, and I discovered that I actually got hum if I connect more that one signal wire. I got a tiny bit of hum if I connect the two front speakers (subwoofer excluded), but I got much more hum if I connect the right front speaker and the left back speaker. I've tested this with and without applying voltage the subwoofer amplifier. The hum is actually lower when the sub amp gets voltage!
My guess is that the 5xLM3886 circuits are the problem. The power coords don't have a huge amount of insulation, and are actually fit underneath the PCBs. The power wires are all connected in series, and I guess that also may be a problem.
I think I should rewire all the five LM3886 amplifier boards, but this time, I want to do it without any shortcuts! I'm just a young student, so I need to learn how to wire amplifiers like this without getting that 50Hz hum!
Try wiring up ONE power amplifier and measuring the Hum+Noise as you change the input conditions.
Get it measuring 0.0mVac on a 199.9mVac multimeter. Not 0.3mVac and certainly not 0.9mVac even if you can't hear that on your speaker !
Don't use your ears to measure H+N ! Dozens of other Builder/Members do this. They are deluded.
Then wire up the second channel to match the H+N of the first for all input conditions.
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I connected all the input ground wires with short wires on the PCBs, and all the hum is almost gone! I still got a little bit of hum in one of the speakers, but it's much better! I tried measuring with my Fluke 179, but I couldn't get the V AC lower than 2.4mV, even when I shorted the two probes!
the problem is stray AC current loops returning through your amps input cables.
trying to shunt it down with lower grounding wires is useful to a point.
the best is lifting the inputs ground from system ground ( power ground ). Fighting to reduce milliohms is difficult versus inserting 10-100 ohms on a high impedance ground input.
your individual PCB is probably where this connection is made?
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