Re: [Cal_Boats] audio hum

Peter, Glad to help with your alternator issue, moving on; In another life I was musician/studio engineer, now I am doing boat electrical systems. So, this kind of calls for a bit of both areas, I think. You are correct in that the square wave is part of the problem, power supplies for audio circuits filter well when dealing with the ripple waveform coming out of a sine wave powered unit. The problem with square waves is that there are higher even order harmonics being generated along with the fundamental 60 Hz waveform. Without getting into a technical discussion of how the filters work at various multiples of the fundamental frequencies, let's just say square waves and audio can be problematic... The other issue is that the ground on the laptop and the stereo system ground are at differing potentials...Plus, chassis ground in the laptop may not be the same as the audio ground is in the laptop. You can try running a separate wire from the laptop chassis to the stereo chassis, may help, may not... Another option would be to obtain a couple of miniature audio isolation transformers (one for left and one for right channel) and electrically isolate the laptop from the stereo. In the pro stuff I was doing we used a lot of low impedance (impedance being referred to as "Z")and high to low Z transformers for matching input and output impedances. We used to carry cases of various plug-in transformers for microphone and guitar cables to recording consoles. Low Z typically being 600 ohms and high Z anywhere from 10K ohms to 50K ohms. I am not a car audio expert, but I would guess a 1 to 1 ratio low Z would work...they are cheap, (you don't need the entire adaptors like we used, just the transformer itself) usually a couple of bucks each if I recall, and you can cut a standard audio cable and wire them in the middle. If it works, make up a little case for them, if not, you are not out much money. If you are totally confused, then I have done my job... What is the old saying? "If you can't impress 'em with your knowledge, dazzle 'em with your BS"? Anyway, hope this is at least somewhat helpful... Best-Mark Mark and Terry Rogers "Seirenia", 1966 Cal 36 hull #22 Newburyport, Ma Mobile Marine Electrical Services/MMES Custom Panels www.wewireboats.com ya… [at] aol.com ************************************** See what's new at http://www.aol.com

Chris, I thought of that, and actually, Charlie and I had discussed it off-line earlier today...I don't think (that's why I own a boat) there is a common ground between the Boat DC through the inverter then through the switching DC power supply for the computer to the computer. Now, if there is another cable plugged into the laptop, like for instrumentation data or maybe GPS input via the serial port, maybe that could cause it... He could try using one of those 3 prong to 2 prong ground lift adaptors at the AC plug on the laptop power supply, and then if there are any other cables beside the audio one, unplug those as well and see if it stops the hum. If it does, then it probably is a loop problem...or was. Best-Mark Mark and Terry Rogers "Seirenia", 1966 Cal 36 hull #22 Newburyport, Ma Mobile Marine Electrical Services/MMES Custom Panels www.wewireboats.com ya… [at] aol.com ************************************** See what's new at http://www.aol.com