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Old 4th March 2016, 08:45 PM   #1
bimbom is offline bimbom  United States
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Default first time builder. Need help grounding this phono preamp

First time poster and builder here. I am following a really great circuit digram from Rod Elliott but looking for some advice on how to properly ground things and reduce hum as much as possible.

Here's what I have wired up so far:
Click the image to open in full size.
The horribly drawn squiggly thing on the bottom right is a resistor to an LED for power on indicator.

I'll list all my questions out at once, forgive me if some seem repetitive.

1. Should the metal case itself be grounded/connected to anything?
2. Should the phono input/outputs be isolated from the case? (the power socket is currently as it has a plastic enclosure).
3. The output pins on the P06 (J1) have “RLG”. Does the “G” go to the shields of both the L and R sockets? On the input side there’s J3 and J4 which each have their own “G”, so I didn’t have that confusion there.
4. Are all “G” markings the same ground?
5. Am I ok without using shielding on the audio cables if they are short?
6. If I do use shielding, how do I connect the shield (on both ends?) ?

Many thanks for all your help.
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Old 5th March 2016, 12:31 PM   #2
AndrewT is online now AndrewT  Scotland
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Default Mains Power - Electrical SAFETY

A1.)
If the metal enclosure (or any exposed conductive parts) is mains powered, then it MUST be connected to the Protective Earth wire in the 3core mains cable.

If it is not mains powered and you can guarantee that mains can NEVER enter the enclosure, then you don't need a PE protected enclosure.

BUT,
how do you guarantee that your enclosure is safe if there is a mains powered accessory electrically attached to it?
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Last edited by AndrewT; 5th March 2016 at 12:47 PM.
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Old 5th March 2016, 12:44 PM   #3
AndrewT is online now AndrewT  Scotland
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A2.)
The RCA/Phono socket/s should be electrically isolated from the Chassis/Enclosure. But all exposed conductive parts should be connected to the protected Chassis.
A3.)
RLG seems like Right, Left, Ground (meaning signal ground/return).
All three need to be connected to the signal pairs.
R to Signal Flow/Hot (right)
L to Signal Flow/Hot (left)
G to left shield/screen AND to right shield/screen.
A4.)
All "G" may not be the same.
eg. G for a speaker is actually speaker return.
G for a line level input is signal return
G for a line level output is signal return, but on the output circuit.
G for a power supply is PSU zero volts
G for a chassis is actually Protective Earth (PE) (the Americans call PE "ground" for some reason to do with being different).
G on an amplifier PCB is usually Power Ground, but can be a mislabeled signal return or a mislabeled speaker return or a mislabeled zero volts. This all comes about because of the generations of lazy designers before us, who called everything "ground", instead of using the unique label for each connection's actual purpose.
A5.)
using a coaxial cable for the two wire signal connection is OK. BUT !!!!! do ensure that both wires of the coaxial cable actually connect the Flow and Return of the circuit. Do not break the circuit route !
A6.)
always connect both the Flow and Return of a two wire signal connection.
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Last edited by AndrewT; 5th March 2016 at 12:48 PM.
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Old 5th March 2016, 12:57 PM   #4
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by bimbom View Post
First time poster and builder here. I am following a really great circuit digram from Rod Elliott but looking for some advice on how to properly ground things and reduce hum as much as possible.

I'll list all my questions out at once, forgive me if some seem repetitive.
Welcome to diyAudio

Quote:
Originally Posted by bimbom View Post
I'll list all my questions out at once, forgive me if some seem repetitive.

1. Should the metal case itself be grounded/connected to anything?
Its not a definite requirement but typically yes, it would be grounded in some way which allows the case to shield the internal circuitry, rather than allowing the case to float and pick all kinds of stray rubbish up.

Based on the fact you show an external AC input I'm going to suggest you either ground the case to the 0 volt line on the power supply board. Typically that could be at the point the black wire exits the board at the right in your diagram.

Quote:
Originally Posted by bimbom View Post
2. Should the phono input/outputs be isolated from the case? (the power socket is currently as it has a plastic enclosure).
Yes, isolated.

Quote:
Originally Posted by bimbom View Post
3. The output pins on the P06 (J1) have “RLG”. Does the “G” go to the shields of both the L and R sockets? On the input side there’s J3 and J4 which each have their own “G”, so I didn’t have that confusion there.
I would route each output separately from the board, so ground and signal to each output socket.


Quote:
Originally Posted by bimbom View Post
4. Are all “G” markings the same ground?
Yes and no They all have continuity to each other but that doesn't mean you can connect haphazardly to them. You need to be aware that as current flows in a conductor it generates a voltage along that conductor... however small. With a sensitive amplifier you don't want to be connecting things to ground such that the tiny volt drops get amplified and treated as a 'signal'.

All that should have been taken care of by the board designed.

Quote:
Originally Posted by bimbom View Post
5. Am I ok without using shielding on the audio cables if they are short?
For the output terminals, yes. Not for the inputs. Use shielded cable.

Quote:
Originally Posted by bimbom View Post
6. If I do use shielding, how do I connect the shield (on both ends?) ?

Many thanks for all your help.
Yes, both ends.
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Old 5th March 2016, 05:47 PM   #5
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It looks like the AC connection on the board is for a centre tapped transformer (GND-AC1-AC2). You have no connection to AC2 as you have it drawn.

John
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Old 7th March 2016, 01:52 AM   #6
bimbom is offline bimbom  United States
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@john blackburn

Thanks so much for your responses. To clarify, I'm just using a plug pack for the AC IN, which in that case the instructions from Rod Elliot says I only connect to AC2 + GND:

"Note that with a single 16V AC input to the supply, you must use only terminals AC2 and GND. If you elect to use a 15-0-15 transformer, the centre tap connects to the GND pin as shown, and the two 15V leads go to AC1 and AC2."

I did hear back from Rod so I thought it would be useful to share his response here to the thread:

"The case MUST be earthed, most commonly to the input or output sockets (but not both).

That means that one set of phono sockets will be connected to the case, and the other set will be floating. All 'G' points on the PCB are joined together. With the unit in an earthed metal case, short unshielded leads will normally be fine. Make sure that no signal wire is close to an AC or DC power lead."
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Old 7th March 2016, 06:59 PM   #7
bimbom is offline bimbom  United States
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I did forget to mentioned one thing. The binding post (which I connect the turntable's ground wire to), where does this get connected to on the circuit? (or to the chassis?)

Thanks
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Old 7th March 2016, 07:09 PM   #8
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Sorry for the confusion, Rod definitely knows best!

John
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Old 7th March 2016, 07:40 PM   #9
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by bimbom View Post
I did forget to mentioned one thing. The binding post (which I connect the turntable's ground wire to), where does this get connected to on the circuit? (or to the chassis?)

Thanks
The binding post connects to the chassis, the chassis then being grounded as mentioned earlier by connecting it to the PCB, or as Rod suggests, one of the pair of sockets. None of these grounds are 'signal grounds'... and I know its confusing when they all apparently connect together.
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Old 7th March 2016, 08:48 PM   #10
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I would think that the small wall-wort transformer has Class II(2) insulation. So no Protective Earth/Safety Ground connection is required.
The DC power supply common and the circuit common should connect to the chassis at a single point and that point is near the audio input connectors.
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