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AndrewGM 28th December 2009 01:03 PM

Earthing concern - pre amp and traffo help
I've just put together a pre amp with separate boxed power supply and experienced the usual buzzing associated with ground and earth issues.
The traffo is 18 - 0 - 18 / 160 va from which I ran a 3 pin Din plug arrangement up to the pre amp.
It all worked but the hum / buzz was awful.
I lifted the '0' v wire from the 3 pin din socket at the traffo side and it stopped all the noise and it now sounds great.
My concern is that it may be dangerous unless I connect the now floating '0' v line to chassis ground / earth point ( green / yellow wire )
Should I do this for safety puropses or doesn't it matter ?
Many thanks

Mooly 28th December 2009 01:15 PM

The mains safety earth is to prevent ANY parts you can touch becoming live... it's not relevant what you do with the secondaries.

I am puzzled over you saying you disconnect the centre zero. That means you are using a preamp that doesn't need split supplies... or do you mean a wire from the center zero to ground ?

AndrewGM 28th December 2009 01:27 PM

This could be my explanation at fault.
I took the three secondry wires ( 18-0-18 ) directly to the preamp via DIN plugs and sockets - it was noisy.
Disconnecting the ' 0 ' from this cable eliminated noise / hum completey.
However, the ' 0 ' of traffo is not connected to anything now - is this safe ?
Apologies if my terminology is not clear.
Thanks for reading my post

Mooly 28th December 2009 05:34 PM

Yes it's safe because the mains earth should be connected to the case etc. That is what makes the item "safe" in simple terms.
Without seeing the circuit etc it's hard to say. From what you describe the preamp runs on a single rail... not a split supply.

If the disconnected 0v wire touched live making the whole amp live... then the mains safety earth comes into play instantly blowing the fuse.

Nico Ras 28th December 2009 08:56 PM

You are running the pre-amp in a virtual earth mode. There seems to be enough leakage current throught the power supply capacitors to give you a virtual earth i.e. supply/2. I actually run all my audio equipment with virtual earth for the reason of iliminating hum. Only problem is you may have some swith on/off plops. You can ease this by placing a 10k resistor across each power supply cap inside the pre-amp.

jaycee 28th December 2009 09:14 PM

Are your input grounds isolated from the chassis ?

AndrewGM 28th December 2009 11:41 PM

Nico Ras - I do have the ' plops ' - nice way of describing the BOOOOOM !
I'll try the resistor suggestion - thanks - I've also lived in a virtual earth for years according to my wife !!

Jaycee - Input grounds are not isolated from chassis ground of pre amp box
( star inside pre amp case ) but are isloated from traffo box with mains earth to chassis. ( if that makes any sense )
I have two chassis going on here - one with traffo inside and another with attenuator and pre amp board + RCA's in and out etc.
Cable taking the 18v x 2 secondry voltages from traffo to pre amp box originally had the ' 0 ' volt line connected and it hummed it's head off.
When I lifted the ' 0 ' v wire it fell silent and now works beautifully.
I was concerned that the ' 0 ' volt line should be now connected to the chassis ground of the traffo box instead of just floating.
It's seems it doesn't matter, it works, is stable and I have not had a shock from any components so far.
As an improving newb to DIY I'm reluctant to just try it, close my eyes and hope - as I used to.
Hope this answers properly - but many thanks for your consideration.
I'll try and post some pics - it's the nicest thing I've done so far.
Some of my work has been really poor and unworthy of this site.
Thank you all

jaycee 29th December 2009 12:04 AM

Your signal grounds should not be connected to the preamp chassis. You will get other earth loop problems when you start connecting sources. Isolate the sockets from the chassis.

If you are using a 3 pin DIN connector for power, then ideally you should have +, 0 and - on pins, and use the shield connection to connect mains earth to the chassis. In the PSU case, you should connect 0V to mains earth via a "loop breaker", such as two diodes in reverse parallel.

For more info on this, Rod Elliot has a great article called Earthing (Grounding) Your Hi-Fi - Tricks and Techniques

jaycee 29th December 2009 12:11 AM

Another query, is the PSU box just the transformer? OR the transformer and rectifier? Any capacitance in it?

I'd recommend the rectifier and some capacitance in the PSU box, and then some more capacitance and the regulators in the Preamp box.

AndrewGM 29th December 2009 05:58 PM

Jaycee Hi
I've reconnected +, 0 and - on the din plug and I've lifted the signal grounds from pre amp chassis.
I switched it all on and the hum / buzz was back.
I have now lifted the earth wire from the mains plug of my CD Player and it's silent again = result.
Reverse parallel - is that merely two 1N 5402's or similar side by side but in reverse polarity ( sort of thing ? ) Interpretation OK ?
Only the traffo in the power supply box.
Rectification, reg's and smoothing done in pre amp box.
Is it easy to implement additional smoothing like caps and diode's on the two secondry 18 volt AC lines ?

Really appreciate you thinking about this for me.


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