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Old 1st October 2012, 05:50 PM   #11
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Update: As WSJ suggested, I moved the orange and grey transformer wires away, which brought the hum down. I also twisted the speaker wires, which did not further improve the hum. The hum is now at a very low level - only audible with the ear right at the speaker.

I am happy now with the sound of my first amp project and Iam sure it wont be the last.

Again, thanks a lot for your input.

Michael
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Old 3rd October 2012, 03:50 AM   #12
gootee is offline gootee  United States
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Actually, you need to twist EVERY pair of wires, using several twists per inch. The input AC wiring is particularly important, as are the input signal/ground pairs. But so are the transformer secondaries, and the rectifier-to-caps pair. Also, any pairs that must approach each other should do so at a right angle.
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Old 3rd October 2012, 06:08 AM   #13
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Wooden box is the main source of the problem.

Gajanan Phadte
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Old 3rd October 2012, 05:22 PM   #14
gootee is offline gootee  United States
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Quote:
Originally Posted by gmphadte View Post
Wooden box is the main source of the problem.

Gajanan Phadte
If there were no enclosed loop areas, the wooden box would not matter much.

If using a metal box instead of wood helps to mitigate a hum problem, then there is a problem with the implementation of the circuit.

Any conductive path that forms a loop should not have any open space between the conductors. If there is any geometric area between the conductors, then any and every time-varying magnetic field in that area will induce a corresponding time-varying current in the loop, with the magnitue of the current proportional to the enclosed area (all else being equal). See Faraday's Law or Maxwell's Equations.

So don't make antennas by leaving enclosed loop area.

Priority number one should be to tightly twist ALL pairs of wires that connect to the transformer, ALL the way to each end, and to do the same for the wires from the rectifiers to the reservoir capacitors. Keeping the AC and rectified AC away from everything else is also a good idea.

Also do the same for the input signal/gnd pairs, and make sure that they stay extremely close to each other on the PCBs, ALL the way to the input pins.

Three or four turns per inch would be good.

On PCBS, use a ground plane opposite the signal traces, everywhere, or a ground trace on the opposite side that ALWAYS stays directly under the signal trace, or, as a last resort, signal and ground traces that ALWAYS stay as close to each other as possible.

A separate possibility is improper sharing of ground-return conductors. The signal ground should NOT share a conductor with power and output ground, on its way to the star ground from an amp PCB (or anywhere on the PCB). If it did, then since the time-varying currents in the "dirty" power and output ground induce voltages across the inductance and resistance of the ground-return conductor, those voltages would effectively be arithmetically summed with the input signal voltage.

Cheers,

Tom

Last edited by gootee; 3rd October 2012 at 05:30 PM.
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Old 3rd October 2012, 05:43 PM   #15
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Tom,

Thanks a lot for the tips! I will build a metal case for the amp and re-do the wires. In fact, encouraged by this first project, I have ordered a LM3875 kit from audiosector.com . I will implement the lessons learnt there. I have already ordered a second toroid to make the next one dual mono and in a more decent metal box.

Best regards

Michael
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Old 3rd October 2012, 05:45 PM   #16
Pemo is offline Pemo  Mexico
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Quote:
Originally Posted by gootee View Post
So don't make antennas by leaving enclosed loop area.
Hi Tom,

This is very interesting!!!
Could you pleas further explain this to us. This is the first time I hear about it.

RGDS

Pemo
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Old 3rd October 2012, 06:45 PM   #17
bcmbob is offline bcmbob  United States
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I would also recommend following all of Tom's guidelines. The fact that there is some noise when the BrianGT is near the computer indicates I could do better. A commercial Hefler amp in the same location is silent. I made a thin aluminum cover but that doesn't do the trick. I just ure the BGT in a different system in another room.

In the previous picture you can see there is no twist between the PS and the amp. Even as short as they are there may well be an antenna effect going on there.
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Old 3rd October 2012, 07:49 PM   #18
WSJ is offline WSJ  United States
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Here are some links about Grounding and Shielding Audio Devices.

I approve these links.
http://campuspa.com/downloads/grounding_tutorial.pdf
Grounding and Shielding Audio Devices
Ground noise - Wikipedia, the free encyclopedia
Faraday's law of induction - Wikipedia, the free encyclopedia
Star Grounding

What do Twisted Sister and Fruit Loops have in common?
When in doubt twist everything.
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Old 3rd October 2012, 09:10 PM   #19
WSJ is offline WSJ  United States
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I measured 430 uV RMS at the output of this amp. The HP 3400A true RMS voltmeter, 10 Hz to 10 MHz and a crest factor of 10.

The DC power supply wires are twisted; the 120 AC wires are twisted and shielded. The wires form the bridge to the caps were not twisted, but itís a good idea to twist them also. The RCA jacks are isolated form the chassis and are connected to the amp with shielded cable. The AC switch fuse and transformer input connections are in the same small area.

An amplifier input noise can be measured with a test setup using a gain of 1,000, the reading is divided by 1,000 to get the amplifier input noise. Using my Opamp test fixture, I measured a LM3875 input noise of 1.4 uV RMS. In audio power amplifier data sheets, such as LM3874, the amplifier has a gain of 20. Therefore, the amplifier noise will be 20 x 1.4 uV or 28 uV RMS. Any additional signal at the output of the amplifier is a result of noise or him induced by the wiring and/or layout.

Discrete Opamp test circuits
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Old 3rd October 2012, 09:14 PM   #20
bcmbob is offline bcmbob  United States
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Does that transformer introduce noise as compared to a toroid or R-core?
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