Brad Board acting like a mic........hum problems too

[nisios]

Hi every one. I was breadboarding a Cmos state variable filter and i had to amplify a lot the signal after filtering. i heard lots of hum noise. Particularly when the breadboard is sitting on the table.....raising it eliminates 90 percent of the hum but not all. Then i also realized i could hear my fingers touch the breadboard. What's this all about? Board acting like a condenser?.....but what about rising the board and the hum desapearing? Is this condenser effect related too? because im affraid this hum keeps in the circuit even after making a pcb.

[goodrevdoc]

Try connecting your circuit ground to the ground post on the breadboard. It is connected to the metal plate, which would conceivably reduce hum. Breadboards are notorious for having random capacitance issues. Sounds like thats the problem here.
-justin

[nisios]

i dont understand what you are saying. The ground is alreaddy connected........or the circuit wouldnt work. Are you talking about the same ground? or the actuall ground that goes to earth (ground) on the house plugs? im confused

[nisios]

as the board has a lot more hum when sitting in the table, i think it is accumulating change in the bottom of the sticker layer of foam...so i thought about sticking a metal plate under the board and connecting it to ground .... il try this......maybe it will do.....or maybe ill cause some sort of high pass filtering in the whole thing....dont know.....lets see....or better.....hear.

[Coriolis]

As long as you don't have your circuit in a metal enclosure, it can work as an antenna, and pick up all kinds of interference. Example: If I'm recording my electric guitar (a stratocaster) with my computer, I sometimes turn the monitor off, because it interferes with the singlecoil pickups on the guitar, and I can hear a buzz in the headphones. Sometimes it helps to simpply turn away from the monitor, or in a different angle. Just like you sometimes have to move the antenna to get a good signal on the radio.

When I build guitar effects, they're allways noisy untill I put them in a metal box - try googling "Faraday cage" and you'll see why that works, if you dont allready know.

The other thing - stray capacitance - means that if you have for an example two jumper wires going close to each other on the breadboard, the can work like a capacitor (condensor, Kondensator in some languages), and that can cause strange things, like signals getting weaker, hum, and so on. In other words: don't worry if a circuit doesn't work too good on breadboard. Try moving components around if it's very bad, try using shorter wires or try putting it on a pcb or veroboard.

Hope this helps, and good luck.

C

[Scott Stites]

Another thing that can result in hum is if your circuit is oscillating supersonically. I know 'supersonic' implies beyond the range of hearing, but the end result can be humming, especially the type you describe. IE, you can change the position of the breadboard, touch it and make a difference in the hum, etc. As an example, the interefence to Coriolis' Strat by his monitor is likely caused by the ultrasonic oscillator in the monitor's power supply. Because this type of oscillation is parasitic, the capacitance of your body can change the characteristics of the oscillation, especially while it's on breadboard.

Best way to tell if that's the problem is to look at it with an oscilloscope. With horizontal sweep set so that you can see the audio waveform, an ultrasonic oscillation will make the trace look 'thick' - zoom in on the trace, and you'll see that's made up of a very high freq sine wave - that's the high freq oscillation. This type of oscillation happens very easily with high gain circuits as you describe. If you can, insert a small value cap in the negative feedback loop of the amplifier - this allows any very high frequency to pass straight through the cap and cancel itself. This will often get rid of the problem.

You'll notice Ray Wilson often puts a small value cap in the feedback of his inverting op amp amplifiers. This is why. I believe he even mentions, as a pracitical approach, that even if there is not an oscillation on breadboard, he'll often put a cap in the feedback when putting the circuit on PCB - sometimes the stray capacitance of a breadboard will actually work in favor of the circuit and naturally provide that capacitive path. Transfer it to a PCB where such a capacitance does not exist, and you end up scratching your head wondering why it worked on breadboard but not on PCB.

Cheers,
Scott

[toppobrillo]

i agree that its a good idea to put a small cap [4.7pF orso] in with any op-amp you're asking anything above gain=10, maybe even less, to limit their bandwidth. also, i always bypass the PSupps on the breadboard with 10uF/.01uF paralell combos, and add a .01 per every other chip or so.