Lightbox circuit

[splitdog]

I built an online 5 channel lightbox. It works, but the input sensitivity control (pot) is scratchy when you turn it. I replaced it, and it is the same. The lights are affected, and the scratchy sound goes back up through the input into the stereo. The music crackles.
Also, if I leave the pot alone, it still sends crackling noise up through the input. There are 2 boards, the main, and the smaller input. The pot is on the input board. I am using the pre-amp outs as inputs. (line level). Any help would be greatly appreciated.

 

[sagor1]

Noise going back to the stereo may indicate improper grounding. Make sure all grounds are connected together between the lightbox and stereo. Don't rely on the LINE out shielding as grounding. I assume you did NOT install the MIC element, as that would pick up noise and just re-amplify it back into the lightbox. Also, make sure you have the AC Neutral properly identified and wired, along with a proper 3 prong plug that has the ground pin tied to the chassis ground.
Otherwise, your stereo's "line out" may not be well isolated. Try the lightbox with another music box of some sort (or laptop line out port), see if the noise follows the lightbox, or only happens with one of the stereo or other music device.
Otherwise, it may be hard to track this problem down. A good scope looking at the incoming signals through various stages would help to find the source of the noise.

 

[splitdog]

Thank you for your prompt reply.
I made sure to ground everything proper. I did not use the mic, or speaker options. Straight line-out to inputs. Neutral identified. When there is no music and I turn the input pot, I get the scratchy sound and the lights are effected. If I leave it, the crackling sound goes back up the input and I can hear it with the music. The crackling sound seems to 'go with the music'. So this is telling me something, I just don't know what. The lights are effected, too.

 

[splitdog]

"Make sure all grounds are connected together between the lightbox and stereo. "

The lightbox is grounded, and the stereo is grounded. They both share a common plug receptacle for AC.

 

[sagor1]

Try a different source of music, like your line out from a computer instead of a stereo. Try to isolate if the scratchy sound is due only by being connected to the stereo. If scratchy sound with the laptop as well, then something may not be wired right with the lightbox. If laptop is clear audio, then something is wrong with the stereo or its preamp output levels

 

[Nigel Goodwin]

The crude sensitivity control WILL be noisy as you alter it, as it has DC flowing through it - you might try fitting blocking capacitors on the two inputs, in case the source isn't AC coupled as well.

 

[splitdog]

I unplugged the input completely and it still produced the 'noise', as the lights still are affected.

 

[splitdog]

Nigel, could you elaborate?

 

[GCook]

The drawing doesn't show the pot connected to GND. The pot may need to be across SIG and GND, allowing the signal to take alternate paths to GND or the input slider tap of the pot (to R1).

 

[splitdog]

Thanks! I will check for continuity between grnd and the pot. Also signal and the pot on the other leg. After breakfast.

 

[GCook]

The more I look at it, that's not going to be the answer. Without an additional resistor between the pot and GND, the line-level or speaker-level inputs would go straight to ground when attenuated completely. I'm thinking that a resistor is missing, that should be connected between GND and the 50K pot.

 

[GCook]

You're getting noise, because it's acting like a radio receiver antenna. Without the input pulled down, it is extremely sensitive. It needs a proper input to silence it. Try placing a 50K resistor or something close to that size (with no other connections), directly across SIG and GND, and all the noise should disappear, if I'm correct.

 

[splitdog]

On that schematic, I'm not running the mic, or transformer speaker inputs. So I imagine that's what the 2 33k resistors are for?

 

[Nigel Goodwin]

Nigel, could you elaborate?

Putting DC through a volume control (although this isn't really a 'volume' control) cause crackling when you turn it, it's a VERY well known effect going back to the very early days of electronics - and it's why volume controls are capacitor coupled.

In the case of this example though, I 'presume' the idea is to alter the load/voltage for the mike, and will have less effect without the mike. This is why the pot doesn't go to ground.

 

[GCook]

The 33K resistors are probably to keep the input from being over-driven (distorted). I don't see the DC voltages that Nigel is talking about. I only see three possibilities of AC at the input. The SIG input appears to be just hanging out there like an antenna, with no reference to ground. That's the reasoning behind my two previous replies.

 

[GCook]

Does one of the outer pins of the pot connect to GND? You'll have to look at the trace side of the input board.

 

[Nigel Goodwin]

I don't see the DC voltages that Nigel is talking about.

Try looking at the circuit posted above - specifically look at R45

 

[GCook]

Okay, I see it on the other schematic. I guess you figured out I was just focusing on the input board. I'd like to know why the circuit needs it. I'd say (after looking at the main input circuitry), that C20 is used to block the DC voltage. I still believe the pot on the input board needs a GND connection. I'm waiting for Splitdog to answer my question about the outer pins of the pot.

 

[splitdog]

I have to remove it to get an accurate reading. 10 mins.....it's a double sided board. Can't see any traces.

 

[splitdog]

No. I am not getting continuity between either out side pin, and grnd.