How do I build a simple parallel (tank circuit)/notch filter?

Discussion in 'The Projects Forum' started by yeto, Jul 3, 2011.

1. yeto Thread Starter New Member

Jul 3, 2011
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0
I have a phone line that has some hum on it. The hum originates in the phone companies lines. They do not intend on replacing the phone lines in my area any time soon as they say the hum level is under the guidelines set by the government.

To make the best of a bad situation my idea is to try and attenuate the hum by inserting a simple parallel (tank circuit)/notch filter in the line going to my phone. The hum seems to be in the 60cycle/60hz range.

The only experience I have with electronics is building guitar effects stomp boxes. I have no idea as to what value components to use or even if this has a chance to work. Any and all help will be greatly appreciated.

Thank you in advance for any help,
yeto

2. #12 Expert

Nov 30, 2010
16,655
7,293
My first stab at it uses a 1uf poly cap at 200 volts (because it's cheap and non-polarized) and the inductor calculates as 7 henrys. That's not cheap and easy to find.

Here's the math: Xc=1/(2PiFC)
Xl=2PiFL
substituting 377 for the 2PiF is a shortcut I give you now.
When the Xc matches the Xl, you have a 60Hz tank circuit.
Whatever C you have, calculate its Xc
substitute that result as Xl and calculate the inductor.

3. yeto Thread Starter New Member

Jul 3, 2011
12
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Thank you for your help and taking the time to reply. I will give that a try.

Kind regards,
yeto

Jul 7, 2009
1,585
141
Note you may be able to find some motor start capacitors with capacitances in the 200-500 uF range and an appropriate voltage rating. Lowering the capacitive reactance has an analogous effect on the inductance, so you could find an inductor with 200 to 500 times less inductance, which will be easier.

Here's another approach, but it will be more work.

5. CDRIVE Senior Member

Jul 1, 2008
2,223
99
Odds are your source of hum is an unbalanced line. This will typically be caused by leakage to ground by one of the two conductors. If I remember correctly, the tel line spec is >=18MΩ to GND on either. If this is the case you should be able to re-balance the line by applying the same amount of ground leakage resistance to the conductor that's not leaking. On the other hand, if this is a heavy leak it could be problematic doing this because the balancing resistance would be low.

6. #12 Expert

Nov 30, 2010
16,655
7,293
I had a problem like this, many years ago. I measured the resistance from each side of the telephone line to earth ground and found about a million ohms on one line only. The final solution was to find a bit of corrosion from a connection to a piece of wood baseboard in a bedroom.

Just a tip about chasing down high impedance leaks.

7. yeto Thread Starter New Member

Jul 3, 2011
12
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I would like to try your idea. I have a small ohm meter. Can you give me step by step instructions on how to do this? Do I set my ohm meter on ohms, dc volts, etc.?

yeto

8. #12 Expert

Nov 30, 2010
16,655
7,293
Set the ohm meter to ohms, highest scale. Connect one lead to Earth ground, like a metal water pipe. Connect the second lead to one of the phone wires. Read what the meter says. Then connect the second lead of the ohm meter to the other phone wire. Read what the meter says.

It requires a good meter to measure in the 20 million ohms range. Make sure your meter can go as high as you need.

9. CDRIVE Senior Member

Jul 1, 2008
2,223
99
This is a real shot in the dark but it's worth checking out. After you do your measurements you may want to disconnect anything connected to the tel lines and then measure again. This is because back when Ma Bell strictly dictated what you could connect to the tel lines, which was always their products, made by Western Electric, which I think was a division of Bell, standards were maintained very tightly. Today, you may have a phone, modem, tel line networking, faxes and who knows what hanging on the tel line. If any device has ground leakage this problem will manifest itself.

You may also want to check the Jackbox on the side of your house. There could be dead insects or electrolysis tracks in there. Inside the Jackbox there are also spike/lightning protection devices connected from each conductor to ground. I believe these devices are in the family of Varistors but I'm not sure. These things can and do go bad. It is very easy to divorce your home tel lines from the line going to Ma Bell's PBX because there are heavy duty RJ11 jacks inside the box. Unplugging the Tel line from your house will aid you in determining who has the leak.. you or the telephone company line.

When you do your measurements you will want to find a good ground. At the Jackbox it's easy because you will see a heavy conductor coming out of the box going to a ground rod in the earth. Inside your house a cold water pipe is convenient but not always near a telephone jack. In this case you won't have much choice but to use the ground pin on the nearest electrical outlet. I don't like recommending this to nubes though!!!

IMO, the best place to do your tests is right at the Jackbox because a good ground is already there and you can easily divorce your house.

10. yeto Thread Starter New Member

Jul 3, 2011
12
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Kind regards,
yeto

11. yeto Thread Starter New Member

Jul 3, 2011
12
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I am definitely going to give this a shot. That hum is annoying and I am going to do everything in my power to eliminate it or at least lessen it. Thank you for taking time out of your day to write such a long post. I really appreciate the help.

Wish me luck,
yeto

12. CDRIVE Senior Member

Jul 1, 2008
2,223
99
Yeto, good luck!

13. CDRIVE Senior Member

Jul 1, 2008
2,223
99
Here's another tip... Water, mineral deposits and oxidation can create a ground leak that looks more like a semiconductor junction than a resistance. Because of this it's a good idea to measure ground leakage in both directions. In other words negative (black probe) connected to ground and the positive (red probe) connected to the tel line conductor. Then repeat with the probes swapped.

14. yeto Thread Starter New Member

Jul 3, 2011
12
0
Hi and thanks to all who have helped so far,

I just received the capacitor and the inductor for the tank circuit filter. Am I correct in that I just connect them from the red wire screw to the green wire screw inside the telephone wall jack box? How is it that the connection from the inductor not be a dead short and maybe damage something???

yeto

15. CDRIVE Senior Member

Jul 1, 2008
2,223
99
I see we're back to the tank circuit again. Have you been listening?

16. yeto Thread Starter New Member

Jul 3, 2011
12
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Yes, absolutely. I had to order a different ohm meter. The one I currently have will only go to 2M.

I thought I would try the tank circuit until it comes.

Thanks,
yeto

17. CDRIVE Senior Member

Jul 1, 2008
2,223
99
Does this mean that you measured with the meter you have and the lines measured infinity?

18. #12 Expert

Nov 30, 2010
16,655
7,293
No. You put the capacitor and inductor in parallel with each other. That makes a "tank". Then put the tank in series with one of the phone lines. A tank circuit has high impedance at the frequency it is tuned to. You use it like a resistor that only resists one frequency.

19. yeto Thread Starter New Member

Jul 3, 2011
12
0
I did try the meter that I have and it would not settle on a number. It is a digital meter. I figured it was not good enough to do the job so I order a meter that will read up to 20M. Did I do the right thing?

Thank

20. yeto Thread Starter New Member

Jul 3, 2011
12
0
I placed a .056uf Vishay/Spraque Orange Drop film capacitor in parallel with a 470uH 5% choke. According to this online calculator (link posted below) that should filter at 981KHz. I determined that the hum is around 1 KHz by going to an online hearing test site and comparing a 1 KHz sound to the hum on the phone line.

When I connected the cap/choke parallel circuit in-line it seemed to make the hum louder and a slightly higher pitch. I then connected the circuit to just the red screw wire on the telephone jack box and connected the other end of the parallel circuit to the round ground pole on a nearby receptacle. The phone then started to make an even worse hum as if an electrical motor had been connected to the line. The following two articles (links below) suggest that I may be doing something wrong. Please share if you have any other suggestions as I am really confused and I am afraid I might blow something up or electrocute myself.

http://www.daycounter.com/Calculators/LC-Resonance-Calculator.phtml (calculator)