first time builder. Need help grounding this phono preamp

bimbom

Member
2016-03-04 7:59 pm
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:
3XrYOf1.png

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.
 
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?
 
Last edited:
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.
 
Last edited:

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
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 :)

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.

2. Should the phono input/outputs be isolated from the case? (the power socket is currently as it has a plastic enclosure).

Yes, isolated.

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.


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.

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.

6. If I do use shielding, how do I connect the shield (on both ends?) ?

Many thanks for all your help.

Yes, both ends.
 

bimbom

Member
2016-03-04 7:59 pm
@AndrewT
@Mooly
@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."
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
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.
 

bimbom

Member
2016-03-04 7:59 pm
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.

Yep, that's what I'm using.

Apologies for my late reply - I finally had a chance to test everything. Unfortunately I got no signal on the output. I tested the op amps, and the voltages are correct at the various pins. I'm wondering if I completely messed up the grounding. The signal ground and power ground I think I may have mixed in the wrong way.

Here is a picture of the board in the case:


On the top is a glob of solder on top of a penny - this is the case ground.

The G on the signal output pins is connected to the penny. The shields from the output socket connections are soldered to each and other and the penny.

The grounding post (red circle thing on the right) is connected to the penny.

The input sockets (on the bottom) are supposed to be floating, but my meter is showing they’re connected to the case (penny) as well.

[IMGHTTPSDEAD]https://dl.dropboxusercontent.com/u/706356/IMG_1538.jpeg[/IMGHTTPSDEAD]
 

bimbom

Member
2016-03-04 7:59 pm
To be really honest, how should I put this :) well the soldering looks a bit crusty. Does a penny actually accept solder ?

Yes, not the best job I did there :mad:

It is a penny from 1970, so 95% copper I believe (after 1982 pennies are mostly zinc and then copper plated). I scratched off the face of it to get better conductivity, so I don't think there'll be any problems there.
 

bimbom

Member
2016-03-04 7:59 pm
I realize looking at the diagram may be easier than seeing the actual board.

Here's an updated diagram of what's going on:
[IMGDEAD]https://dl.dropboxusercontent.com/u/706356/phono-preamp.png[/IMGDEAD]

- Penny is grounded to the case.
- grounding post (indicated by the green rectangle on top) grounded to penny (case)
- OUTPUT shields grounded to the penny (case) as well as the output G on the board.
- INPUT shields grounded to the signal Gs on input side, and supposed to be floating from case

I think I've inadvertently mixed signal ground and power ground and messed something up. I'm getting no signal on the output.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Grounding errors (unless missing or shorting something) wont cause loss of audio.

Be methodical.

1/ Confirm that the supplies are correct on pins 4 and 8 of each opamp.

2/ Confirm that pins 1,2 and 3 and pins 5,6 and 7 of each opamp have zero volts on them.

3/ With it switched off, do a quick resistance check across the input sockets and the output sockets (centre pin to outer) to make sure you haven't accidently shorted the feeds.

4/ Use a metallic screwdriver tip and carefully touch pin 2 of U2. You should hear at least a click and a bit of a buzz on the audio of one channel. Do the same for pin 6 of the other channel.

If that is no good then you have a fundamental problem around U2 and or its feed to the sockets.

If you get the buzz then repeat the procedure but this time on U1. It should be similar but a lot louder.
 

bimbom

Member
2016-03-04 7:59 pm
Grounding errors (unless missing or shorting something) wont cause loss of audio.

Be methodical.

1/ Confirm that the supplies are correct on pins 4 and 8 of each opamp.

2/ Confirm that pins 1,2 and 3 and pins 5,6 and 7 of each opamp have zero volts on them.

3/ With it switched off, do a quick resistance check across the input sockets and the output sockets (centre pin to outer) to make sure you haven't accidently shorted the feeds.

4/ Use a metallic screwdriver tip and carefully touch pin 2 of U2. You should hear at least a click and a bit of a buzz on the audio of one channel. Do the same for pin 6 of the other channel.

If that is no good then you have a fundamental problem around U2 and or its feed to the sockets.

If you get the buzz then repeat the procedure but this time on U1. It should be similar but a lot louder.

Thanks Mooly for taking the time to write this. Here's what I found:

1/ Pins 4 and 8 are 15v and -15v
2/ The rest of the pins are 0 volts.
3/ Didn't find a short between center pin and out on either of the sockets
4/ Not hearing anything when I touch the pins on U2 (or U1). This should happen regardless of if there is input connected? I am just testing it with the outputs going to my amp/speakers.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Supplies sound OK (Pin 8 is the positive, pin 4 the negative... yes).

It should go crazy if you buzz the inputs to the opamps so something really fundamental is wrong here. Does the second opamp (U2) output pins (1 and 7) alter in voltage when you touch the input pins of either U2 or U1. So that's touching pins 2 and 6 as before.

Any chance you might have ground and the output wires crossed at the sockets ?
 

bimbom

Member
2016-03-04 7:59 pm
Supplies sound OK (Pin 8 is the positive, pin 4 the negative... yes).

It should go crazy if you buzz the inputs to the opamps so something really fundamental is wrong here. Does the second opamp (U2) output pins (1 and 7) alter in voltage when you touch the input pins of either U2 or U1. So that's touching pins 2 and 6 as before.

Any chance you might have ground and the output wires crossed at the sockets ?


Ok Mooly did some testing and here's what I found:

- Without any probes on any pins, if I touch pin 3 of U1 with metallic tip, I hear a buzz in the speakers.
- With probes on pin 1 of U2 and ground, if I touch pin 3 of U1, I hear a LOUD buzz in the speakers. I also see the voltage fluctuate wildly on pin 1 at this point (seemed to go up to 7v but not with any consistency)
- With probes on pin 1 of U2 and ground, if I touch pin 2 of U1, I get ~.5 V on the meter, though it fluctuates and is not consistent.


- With probes on pin 7 of U2 and ground, if I touch pin 6 of U2, I get 5-10mV on the meter.


All this is with just the outputs connected (nothing connected to input).


If it sounds like something is fundamentally wrong here, perhaps it is capacitor C5L? Maybe you can see from the picture, when I installed the grounding post, I knocked it over and broke the pins. I soldered the pins back together, but maybe it broke on the inside?
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Lets concentrate on one channel. Pin 3 of U1 is the main input, and you say it buzzes LOUDLY when you touch it with say your meter probe. You are also seeing the voltage at the output of U2 fluctuate when you do this. That's normal and seems to suggest its basically working on that channel.

So if you now connect that channels input to the turntable do you get audio at the correct level ? If not, then check for correct continuity and wiring of the leads to the input socket.

Look at the circuit. If it buzzes loudly when you touch pin 3 of U1 then it should do the same if you touch the appropriate end of R1L (47k) and finally the actual centre pin of the input socket itself.

C5L seems to be OK. If there were something amiss there then the buzz would not get through to the output sockets.