A hum problem

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Qmavam11

Member
2014-03-09 4:18 pm
I have a slightly different problem, but it concerns hum.
The setup is such; internet radio into a mechanical switch , the switch output goes to a Rolls SL33B limiter.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
The limiter output goes into an FM transmitter.
I didn't have a the hum until I installed the Limiter. It is fairly low level hum. If I touch both the FM transmitter and the limiter with my hand the hum is reduced considerably (good enough), but a clip lead shorting the two doesn't do the same as me touching them.
Each of the units internet radio, limiter and FM transmitter, have their own power supply. All are switching type power supplies. Would it help to run everything of of one supply?
Any ideas how to eliminate the hum?
 
Sounds like capacitive coupling from the mains that's messing with the floating ground (or maybe the connection between two components), and you are draining part of it to ground when touching things. There are two typical ways for a SMPS to introduce that:
1. Many supplies have a Y capacitor (1n-ish) across the transformer from secondary-side ground to mains neutral. Not a big deal if neutral is actually that, but even in countries with polarized outlets faulty wiring could still upset the order.

2. Alternatively, secondary-side ground may be tied to the midpoint of capacitors between live and neutral. This solution is polarity-agnostic, but means that you'll always have secondary-side ground floating close to half mains voltage unless tied down. Leakage current tends to be high enough to easily light up the neon bulb in a mains tester.

Time to inspect the limiter power supply. Try turning it around first. Otherwise 15 V / 0.3 A does not sound like a major challenge for a transformer-based supply, though I don't know how clean the voltage would have to be, plus one may have to solder a new plug if it's not a universal type (- inside / + outside is rather oldschool).
 

Qmavam11

Member
2014-03-09 4:18 pm
A Hum Problem.

Sounds like capacitive coupling from the mains that's messing with the floating ground.

Thanks for your response sgrossklass.
I spent some time on the problem last night and found an unrelated open wire leaving me with a mono signal, don't know how long it's been like that, but I fixed it.
Then on the hum I tried wraping the audio cables around a ferrite toroid, it didn't help. I did get the hum tamed, but it seems to just be finding a proper placement of the signal wires. Although, the Limiter has a two sets of
input connectors, RCA phono plug and 1/8" mini plug. I tried the RCA connectors and after I did that I got it tamed. It needs to be revisited, but since it works, I probably won't.
Thanks
 
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