Hi all
I have apparently successfully wired my center tap transformer to my rectifier board from my L3875 kit.
I'm obtaining plus and minus 38.2 V after rectification and without any load.
I really want to proceed very carefully here and I'm seeking advice on earthing my system. Again I have been reading through the forums and sometimes the advice is conflicting and or confusing.
From what I understand I connect the Greem/Yellow earth wire from my AC cord to my chassis. From this point I run a wire to CHG (Chassis Ground Connection) location on the amp boards.
(I should also place a fuse in series between the AC cord and the transformer.)
Ok so far?
Alan
I have apparently successfully wired my center tap transformer to my rectifier board from my L3875 kit.
I'm obtaining plus and minus 38.2 V after rectification and without any load.
I really want to proceed very carefully here and I'm seeking advice on earthing my system. Again I have been reading through the forums and sometimes the advice is conflicting and or confusing.
From what I understand I connect the Greem/Yellow earth wire from my AC cord to my chassis. From this point I run a wire to CHG (Chassis Ground Connection) location on the amp boards.
(I should also place a fuse in series between the AC cord and the transformer.)
Ok so far?
Alan
You've got it. The connection of the earth wire in your power cord to the chassis is often referred to as 'safety earth'.
One wire from that point to chassis ground on each amp board. Don't connect the chassis grounds of the two amp boards together then run a wire to your safety earth. Earth each board separately.
A fuse is definitely a must. For gainclones I generally use a 2 amp fuse, generally a 'Slo-Blo'. Put the fuse in line with the "hot" wire from the AC mains. Basically AC hot into chassis direct to the fuse, then from the fuse to the on/off switch then back to the rectifier board.
One wire from that point to chassis ground on each amp board. Don't connect the chassis grounds of the two amp boards together then run a wire to your safety earth. Earth each board separately.
A fuse is definitely a must. For gainclones I generally use a 2 amp fuse, generally a 'Slo-Blo'. Put the fuse in line with the "hot" wire from the AC mains. Basically AC hot into chassis direct to the fuse, then from the fuse to the on/off switch then back to the rectifier board.
Hi,
sorry to disagree and throw in another option.
The mains incomer should go;- Live to fuse to switch, Neutral to switch, Earth to chassis safety earth.
I think we are all agreed on this first bit.
You can use insulated terminal strips to help with these first interconnections, but try to keep the wires short and any that go wandering (to find a switch) should be double insulated.
Keep the safety earth beside the mains incomer.
Make it permanent.
NEVER dismantle the safety earth from the chassis when carrying out maintenance and/or testing and/or modification if there is ANY chance someone might plug the unit into the mains socket outlet.
Any additional connections to the safety earth must not require dismantling of the safety earth to chassis connection.
Now on to the audio ground.
I do not recommend direct connection from audio ground to safety earth.
I do recommend an indirect connection consisting of one/some/all of the following forming a parallel network. Some experimentation here may prove valuable.
The network can include:-
10r power resistor.
power bridge diode wired in inverse parallel.
high frequency capacitor (10nF to 100nF ceramic).
Ground lift switch (common in PA gear but generally not needed in domestic. Closing this switch often causes injection of hum and buzz into the amplfier)
The RCA input grounds should be isolated from the chassis.
All the amplifier grounds should be isolated from the chassis.
sorry to disagree and throw in another option.
The mains incomer should go;- Live to fuse to switch, Neutral to switch, Earth to chassis safety earth.
I think we are all agreed on this first bit.
You can use insulated terminal strips to help with these first interconnections, but try to keep the wires short and any that go wandering (to find a switch) should be double insulated.
Keep the safety earth beside the mains incomer.
Make it permanent.
NEVER dismantle the safety earth from the chassis when carrying out maintenance and/or testing and/or modification if there is ANY chance someone might plug the unit into the mains socket outlet.
Any additional connections to the safety earth must not require dismantling of the safety earth to chassis connection.
Now on to the audio ground.
I do not recommend direct connection from audio ground to safety earth.
I do recommend an indirect connection consisting of one/some/all of the following forming a parallel network. Some experimentation here may prove valuable.
The network can include:-
10r power resistor.
power bridge diode wired in inverse parallel.
high frequency capacitor (10nF to 100nF ceramic).
Ground lift switch (common in PA gear but generally not needed in domestic. Closing this switch often causes injection of hum and buzz into the amplfier)
The RCA input grounds should be isolated from the chassis.
All the amplifier grounds should be isolated from the chassis.
AndrewT said:
Andrew,
I agree a DPDT switch to switch both AC lines is the best setup.
About connecting signal ground to earth- it depends on where in the circuit we are. With chip amps it is pretty simple to have only one 'signal' ground that is floating. With tubes the concept of 'signal ground' is a bit more complex and there are areas where the signal ground will be earthed.
Guys...you'll understand now why I get confused.
If it makes it any easier I just want to make sure the box will be safe in the first instance. So I just need the simplest, safe, solution.
If there are different earthing configurations which affect sound quality I can explore those later as I learn more theory and practice.
(On a side point...in the UK we call the wires that come in on the ac mains cable Neutral, Live and Ground. In the US, where I am now, I guess it's Ground and what....Hot? For all practical purposes, being AC, do I just consider the two wires carrying the current as Hot relative to Ground?)
Regards
Alan
If it makes it any easier I just want to make sure the box will be safe in the first instance. So I just need the simplest, safe, solution.
If there are different earthing configurations which affect sound quality I can explore those later as I learn more theory and practice.
(On a side point...in the UK we call the wires that come in on the ac mains cable Neutral, Live and Ground. In the US, where I am now, I guess it's Ground and what....Hot? For all practical purposes, being AC, do I just consider the two wires carrying the current as Hot relative to Ground?)
Regards
Alan
ash_dac said:
If they aren't insulated from the chassis you have a very good to excellent chance of having hum in your amp. Unless your chassis is engergized in some way you shouldn't get damaging current through those components. You will get hum and noise. I always either use insulated RCAs or mount my RCAs on an insulated panel and attach that to the chassis.
Also both the speaker outputs both signal and ground should be completely isolated from the chassis.
I think that those two things in particular are what Andrew was referring to when he said signal ground shouldn't be connected to the chassis ground.
A plank of wood isn't a great insulator but it is usually sufficiently non-conductive that you shouldn't have problems in testing.
only for the connection between the signal ground and powerground... i.e. you want more resitance between the ground and the input ground than you do between the ground and the speaker's ground return.... this will keep the nasty earth currents away from the clean input.... real good hum killer.
For doing the mains wiring in your gainclone, just go and get some 6A 3 core flex. Strip the outer insulation off so that you have the 3 inner wires free. Use that 
Rod Elliot has a fab article on this - http://sound.westhost.com/psu-wiring.htm
Rod Elliot has a fab article on this - http://sound.westhost.com/psu-wiring.htm
If you insist on one of Rod's circuits (I love em)... then look at
Also if you your transformer has a ground tab, don't forget to connect it.
An externally hosted image should be here but it was not working when we last tested it.
Also if you your transformer has a ground tab, don't forget to connect it.
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