I am entertaining the idea of building a tube phono preamp and are looking for others experiences with trials and error on phono circuit grounding.
My thoughts so far are to run separate ground traces for L/R channels such that signal and supply currents are not shared on a common path and can be kept channel independent (each channel will have its own linear B+ regulator and subsequent filtering). This should help control supply noise and crosstalk in the ground path. The channel grounds would then meet at a single point back at the main power supply filter which is shared between the L/R B+ channels. Im thinking this is where the chassis ground connection is made perhaps with a soft ground lift if needed.
However, upon further thought it seems that this would case a massive loop if the L and R output RCA jack shields were both connected to each other in a power amplifier or other pice of equipment down stream from the phono stage.
So my questions....
1. Has anyone experimented with a phono preamp design using separate ground paths for each channel or is large common bus / ground plane a better approach to remove a potential loop?
2. How do you control signal ground vs supply current grounds if using a large ground bus?
3. Where is the best place to make the circuit ground connection to the chassis?
4. Is is recommended to do copper pour around all PCB traces and tie this pour to chassis ground to act as a shield?
I appreciate any input!
-Matt
My thoughts so far are to run separate ground traces for L/R channels such that signal and supply currents are not shared on a common path and can be kept channel independent (each channel will have its own linear B+ regulator and subsequent filtering). This should help control supply noise and crosstalk in the ground path. The channel grounds would then meet at a single point back at the main power supply filter which is shared between the L/R B+ channels. Im thinking this is where the chassis ground connection is made perhaps with a soft ground lift if needed.
However, upon further thought it seems that this would case a massive loop if the L and R output RCA jack shields were both connected to each other in a power amplifier or other pice of equipment down stream from the phono stage.
So my questions....
1. Has anyone experimented with a phono preamp design using separate ground paths for each channel or is large common bus / ground plane a better approach to remove a potential loop?
2. How do you control signal ground vs supply current grounds if using a large ground bus?
3. Where is the best place to make the circuit ground connection to the chassis?
4. Is is recommended to do copper pour around all PCB traces and tie this pour to chassis ground to act as a shield?
I appreciate any input!
-Matt
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In my phono amp builds I used to keep the B- (if I may call it that) separate from Ground except at the Ground terminal where it was connected to the metal enclosure which was also linked to the input RCA jack screens. Some people noticed hum loop problems so I now keep the B- of both channels away from ground within the phono amp enclosure. The Ground terminal is still linked to the RCA input jack Grounds which are expected to be completely isolated from one another in the cartridge wiring. The TT Safety Ground should be connected to the supply Ground but any separate tonearm ground (my Lenco with Mission Arm has one) can be applied to the phono amp Ground Terminal. The actual B- lines which I said are floating are in fact connected to supply Ground inside the remote power supply via a 100 Ohm 7W resistor. This resistor however is not expected to carry any fault current as the TT Ground should do this task. The power supplies I use provide double insulation (dual split bobbin transformers) from house supply to output, making the chance of electric shock virtually impossible. The house supply Ground of course is solidly bolted to the PSU enclosure to cater for something like a hot wire chafe inside the PSU and carried through to the phono amp enclosure for mainly screening. The most worrying concern is what happens if the B- connection is lost between PSU and Phono Amp? If it is bonded to the Phono Amp case then this could rise to B+ and give a nasty shock if both cases are touched at the same time. All units I make for friends incorporate a B+ driven relay in the PSU that is looking for a B- return from the phono amp to operate. If it does not see a B- then it isolates the B+ from the phono amp. This may seem extreme but I believe mandatory for anything you are building for someone else, it does give you piece of mind that a simple break in a wire will not produce a hazard for the user.
The main problem you need to avoid is power supply smoothing capacitor charging impulses getting in to the audio ground.
Do the power supply separate and then just join b+ and b- once to power supply from audio circuit.
Try to keep ground as short as possible and if using a pcb use copper pours.
Do the power supply separate and then just join b+ and b- once to power supply from audio circuit.
Try to keep ground as short as possible and if using a pcb use copper pours.
Thanks Hi_Q for the response.
To clarify, you share a B- bus for both channels or do you have a separate B- for L and R channels?
It sounds like your B- line get tied to the chassis (w/ 100R resistor) close to the first filter in the power supply chain, correct?
I assume by ground here you mean tonearm ground? In this case, tonearm ground isolated from the chassis?
Thanks,
Matt
To clarify, you share a B- bus for both channels or do you have a separate B- for L and R channels?
It sounds like your B- line get tied to the chassis (w/ 100R resistor) close to the first filter in the power supply chain, correct?
I assume by ground here you mean tonearm ground? In this case, tonearm ground isolated from the chassis?
Thanks,
Matt
Hi Matt,
There are references to a lot of different sorts of ground in the above postings.
If you could provide a wiring diagram, that would probably tell quite a bit more, helping in getting the right answers to your questions.
Hans
There are references to a lot of different sorts of ground in the above postings.
If you could provide a wiring diagram, that would probably tell quite a bit more, helping in getting the right answers to your questions.
Hans
Hi Hans,
Attached is an image of what I was thinking.
Only own amplifier section is shown.
The issue I can see is the making of a large internal ground loop if the L & R output RCA jack shields are tied together at another piece of equipment upstream.
Therefore, is sharing B- between channels a better approach?
Regards,
Matt
Attached is an image of what I was thinking.
Only own amplifier section is shown.
The issue I can see is the making of a large internal ground loop if the L & R output RCA jack shields are tied together at another piece of equipment upstream.
Therefore, is sharing B- between channels a better approach?
Regards,
Matt
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Hi Matt,
To start with, connecting the RCA-Input ground to the mains ground via the chassis, gives the best immunity to preventing groundloops.
The RCA cable connecting Cart and Amp should have its shield connected to one end only, the input side of the amp.
Use a separate wire to connect the tonearm to the chassis of your preamp and make sure no other connections are made between the signal wires within your Pick-Up.
Your amp, between RCA-In and RCA-Out is of course much more complex as in your drawing.
Following the way as used in the example with a starpoint, would lead to a large amount of connections to this starpoint, which is not the best approach to keep the integrity of the signal intact.
A nice article on the subject can be read here:
http://www.hypex.nl/docs/papers/The G Word.pdf
I do not see a direct need of sharing B-, but it depends on the realisation, will it be with PCB's or will everything be wired.
Hans
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