Grounding question - connectors isolated from case?

[Dominique]

Hi,

I want to build the simple rjm phono preamp.

The other day I read sth. about a star ground and ground loops which was new to me.
I'm pretty much in a hurry to command some electronic parts, so I tried to find texts on the web about correct grounding, but unfortunately had difficulties to understand the theory!

If I understand the texts about grounding right, every part of the circuit should better be grounded only in one point, so the audio connectors of my preamp may better be isolated from the case, which is grounded to the circuit ground...? Or well, I could also ground the case exclusively through the cable to the connectors which are very close to each other anyway...

Err, am I right here?

Thanks for helping!
Dominique

[markp]

It is better to use isolated connectors and only one ground point for the whole amp. A 'star ground' is just one bolt with a lot of washers that have solder points on them attached together. All grounding occurs at that point so there are no ground paths to cause a ground loop or inconsistent grounding.

[SY]

There are a few ways to handle it but, yes, it's best to isolate the connectors from the case, then run their ground leads back to the star ground.

[Dominique]

Wow, those were fast answers! So I'll command those connectors with isolation... I'll try to read some more until the parts arrive

Thank you both a lot!
Dominique

[commuteman]

Isolationg the connectors from the chassis is the right thing to do, with one caveat:

If you isolate the input connectors from the local ground you will probably have a problem with RF pickup. This will sound like either added noise or loud clicks and pops when other equipment switches on or off. Worst case, you may even pick up a radio station...

The solution is to connect a small ( e.g. 0.01uF) cap between the ground terminal of each connector and the chassis as close as possible to the connector.

This gives you an isolated ground at LF (so no hum) and a local ground at HF (so the input cable is not a radio antenna)

Peter

[sam9]

I agree in principal that the connectors (RCA I presume) should be isolated. However, in fairness I have to say I've been remiss on this point sometimes and have yet to suffer for it. Since I use a grounding scheme where the RCA's are closer to the chasis eathing point than any other grounding lines, this could be a a factor.

[crippledchicken]

hi there, i too would like to get a better understanding of star ground points. if i have ps with dual filter caps, and center tap from transformer, connecting to point between the two filter caps, which = plus side of one cap, and minus side of other cap would this be considered the star ground point? and also is it better to sometimes run a separete grounding wire from points within a stereo component straight to the star ground instead of depending on circuit traces for grounding? thanks for any advice, crippledchicken

[SY]

The star ground is wherever you put it. If you make that junction in the PS your star ground, return all ground leads to it. And return one single wire from that point to the chassis. The basic idea is that you want all grounds to be at the same potential and you want to eliminate circulating currents in the chassis.

[crippledchicken]

hey thanks SY, that was a quick reply. i think my rca input grounds on my adcom gfp 565 preamp are at the star ground potential and, thought about adding a heavy gauge copper wire from star ground directly to the chasis also but not sure if needed, or be of any benefit or not thanks again, crippledchicken

[sam9]

My understanding from at least a couple of sources is that the one place the star grount should **NOT** be is at the connection between the PS filter caps as that is an extraordinarly noisy point. Rather a few inches away is preferred.