# An Inductance Problem

1. Sep 15, 2008

### Lancelot59

I'm trying to setup a DC filter to remove noise thats is entering an amplifier circuit. It's just standard line current noise at 60Hz. I can't figure out how on the other hand how to calculate the necessary values to pick an inductor that will do the job.

The DC power in is 16V @ 4A, DC Resistance just using Ohms Law is 4Ohms.

To pick an inductor I need to know the inductance in Henry's, and a few things I have know nothing about:

Q @ Freq
Self Resonant Frequency

Most likely I'm going to go with a Ferrite core but I'm not 100% on that yet, but I think it would work best.

What do you guys think?

2. Sep 16, 2008

### Staff: Mentor

Welcome to the PF. It would help to know more about the system that you are dealing with. Why are you finding 60Hz hum in your DC power supply? Is it from ripple from the input bridge rectifier, or some other shared impedance pickup from some 60Hz AC powered device?

There are several things that we would need to know to help you design and optimize a filter for your application. Can you tell us more, and maybe even attach a sketch of your system?

3. Sep 17, 2008

### Lancelot59

Well the 60Hz noise was just an idea. I'm actually building it as a prototype at the moment. I've been using a DC power supply, and also got the same results using a car battery. So I just kinda guessed that it was line voltage noise. I only heard the hum on the speakers. Another idea that got mentioned is a grounding issue, but I don't see how what would work.

But mentioning the ripple there made an idea pop into my head. The OPAmp has a pin labelled "voltage ripple rejection" but my teacher said that I shouldn't worry about it, so I just left the row blank. What would that normally connect to?

Here is the data sheet for the OPAmp: http://www.datasheetcatalog.org/datasheet/philips/TDA1554Q.pdf

It's kinda late right now but I'll throw up a sketch of the system tomorrow for sure.

4. Sep 17, 2008

### Staff: Mentor

If you're getting 60Hz hum when running off of a battery, the problem will not be solved with filtering. You need to figure out where the hum pickup is coming from, and address that source.

5. Sep 17, 2008

### Lancelot59

Today after some more cankering I grounded the ripple rejection, which totally killed the audio out. I then tried to connect the second speaker, the connector was damaged so I'm not sure if it was working properly. It system worked with a grounded ripple rejection pin and a 100uF capacitor. The hum was only present on one channel.

Tommorow I'm going to bring in a DC powered audio source (a pocket radio), instead the the huge AC powered radio I'm using right now. I hope that'll work out better. I think that the noise might be coming from the radio's internal DC rectifier.

I'll post a sketch when I get home.

Last edited: Sep 17, 2008