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Star grounds are just as important with low level signals.
Star grounds are just as important with low level signals.
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Old 28th September 2012, 10:33 PM   #1
nigelwright7557 is online now nigelwright7557  United Kingdom
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Default Star grounds are just as important with low level signals.

I always though that star grounds were only important in power amplifiers.

I quickly threw together a 6 channel pre-amp onto a pcb and got it made.
When I got it back I powered it up only to get a lot of hum.
I tried wiring up the ground tracks to reduce impedance and this only helped slightly. I found just shorting out between some traces helped more than others.
In the end I split off thew power supply onto a seperate peice of veroboard and the hum just went away.

So I started on my next revision of pcb, this time using star grounds for the power supply and the audio IC's. When I got the pcb back it worked like a dream. I wasted a batch of pcb's because of my ignorance of grounding. I had thought just good decoupling was needed but this clearly wasnt enough.

A valuable lesson in electronics.
Murton-Pike Systems PCBCAD51 pcb design software. http://www.murtonpikesystems.co.uk
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Old 29th September 2012, 06:55 PM   #2
indianajo is offline indianajo  United States
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I second the title. I knocked some hum out of my RA88a disco mixer by floating the RCA rings (analog ground) from the steel case (safety ground and turntable arm shell). I used o-rings. The PWB is still a jumble, but it is close enough in sound to the legendary PAS2 to furlough the 120 watt tube icon to the attic.
Amazing how the production engineers can mess up a design. They had the 120 vAC power switch right next to the magnetic phono (50x gain) op amps. Eliminating the 17 VAC center tap power transformer from the steel case and buying a wall transformer helped a lot, too.
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 29th September 2012 at 07:08 PM.
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