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Old 5th November 2013, 02:43 AM   #1
mortron is offline mortron  Canada
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Default Tracing Hum in Passive Line Level Filter

I recently built a filter for my Visaton B200 that I had found in a thread. From what I gather, it compensates for some of the dips and peaks in the response of the driver.

What I hear during playback, the circuit appears to do what is intended, but in one channel, there is a slight 60hz hum or something. It sounds just like the annoying hum of the air conditioner outside, which is probably also humming at 60hz, yes??

I don't know a lot about hum and grounding, but I know that it is present in only one channel. Is it possible there is a bad resistor or cap? How can I test them? I checked all resistors with the multimeter and they were within 5%... Thanks in advance.

Sorry should be in other Analog forum.
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Last edited by mortron; 5th November 2013 at 03:26 AM.
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Old 7th November 2013, 10:56 PM   #2
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Is it in a metal chassis?
Do all the jack shell's make good connections to the chassis?
Are you using normal coax interconnects?
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Old 8th November 2013, 01:39 AM   #3
gootee is offline gootee  United States
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The hum is far more likely to be caused by the layout, but possibly also the construction techniques.

It turns out that if you let the ground and the signal paths get physically separated from each other, it makes a type of antenna for AC hum (See Faraday's Law.).

So one problem is that the schematic is drawn "wrong" , since it's drawn the normal way and gives no clue about how to lay out the circuit that gets built.

If you are using a printed circuit board (PCB) , and it has one sie that mostly all copper, for the ground (a ground plane ) , then that will probably automatically keep the signal and ground conductors close together. But you would still need to make sure they don't separate, even a little, between the board and the jacks. If you use individual wires, there, then they should be very tightly twisted together, ALL the way to each end.

If you are not using a printed circuit board, or are using one without a ground plance, then you would want to lay out the conductor paths in a manner that resembles the attached schematic, so that there are no gaps between the signal and ground conductors or paths, anywhere.

I could be wrong but I don't think that you would want the input or output jack's ground to be connected to a metal chassis or box, if the circuit is housed in one. But you could try it both ways.

Another way to get hum is to have a ground-return current, that contains hum frequencies, sharing a ground-return conductor with a ground-return for, say, the amplifier's input resistor's ground reference point. The currents induce voltages across the inductance (and resistance) of the conductors themselves, and the voltages can appear back at the non-PSU-ground end of the conductors, such as at the "ground" end of the input resistor. Since the amplifier input has no concept of ground and merely amplifies the diference of the voltages at the two ends of the input resistor, the hum voltage is effectively summed with the signal voltage, which would be "a BAD thing" . For the solution, look up Star Grounding. I doubt that is the problem in this case, though.

Click the image to open in full size.

Just for fun, I plotted the frequency response, for two values of C1:

Click the image to open in full size.
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Last edited by gootee; 8th November 2013 at 01:53 AM.
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Old 8th November 2013, 10:54 PM   #4
kevinkr is offline kevinkr  United States
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Strikes me that the impedances in this circuit are pretty high, should be built in a metal box and the two channels including their grounds should be kept independent of each other to avoid a ground loop. One channel ground should be connected to the case to ground it. The box should be placed very close to the power amp and the cables should be short and well shielded to the amp.

Frankly if using a solid state pre-amp I would scale the resistors to 1/5 their current values and multiply the cap values by 5X, and add a 25.5K resistor across the output of the network for an amp with an input impedance of 100K - this will reduce susceptibility to external electro-static pick up and also reduce gaussian noise generation in the network.
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Old 8th November 2013, 11:04 PM   #5
kevinkr is offline kevinkr  United States
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I'll move it to analog line level.. (Moved)
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Old 10th November 2013, 06:21 PM   #6
gootee is offline gootee  United States
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Quote:
Originally Posted by kevinkr View Post
Strikes me that the impedances in this circuit are pretty high, should be built in a metal box and the two channels including their grounds should be kept independent of each other to avoid a ground loop. One channel ground should be connected to the case to ground it. The box should be placed very close to the power amp and the cables should be short and well shielded to the amp.

Frankly if using a solid state pre-amp I would scale the resistors to 1/5 their current values and multiply the cap values by 5X, and add a 25.5K resistor across the output of the network for an amp with an input impedance of 100K - this will reduce susceptibility to external electro-static pick up and also reduce gaussian noise generation in the network.
Concur. The high impedances make the loop-area problem worse, since for a given time-varying magnetic field that might be present, the induced current depends only on the loop area. So higher impedances would have larger hum voltages induced across them.
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