Hmmmm Star ground

[mark31385]

A question about star ground, is it preferred or not preferred in valve amps?

I asked the question today and was told it causes too much noise and hum, having not worked with valves before this confuses me. If I was working with digital packets could be lost with poor grounding. Why does this not apply with valves?

 

[JonSnell Electronic]

The current that causes problems with solid state amplifiers are with respect to ground. Valve amplifiers have an output transformer, so no high current to worry about. Use twisted pair for heater wiring and centre tap to ground.

 

[mark31385]

So they have a floating ground?

If you twist the cables together are you not introducing more eddy currents and potentially more noise and hum?

I don't see the logic in the "grounding" that I am doing at the moment. It doesn't seem clean, all PCBs are daisy chained.

Would it not be more sensible to have the earth from mains mounted on a PCB with various points to connect the relevant PCBs to ground?

 

[famousmockingbird]

AC current flowing through a conductor emits a field that can be inducrd into nearby sensitive stages. In tube amps the largest AC current pollutants are the power transformer and the heater/filament supply wiring. When the wires that have the most current through them are twisted together the field around them cancels each other out. This is why you can't measure AC current with a Clamp meter when they are around both hot and neutral conductors, you need to isolate one from the other to get a reading through the transducer.

My advice is make a diagram of all the different currents in your amp, their feed and return paths. Then keep the sensitive small signal currents away from large ones like the power supply. Make their respective paths happy and free from pollution.

 

[MrCurwen]

I try to do local grounds, and then connect them all together in a star ground style. Some items are directly grounded to the star.

Do a build completely point to point, it'll have you thinking about this issue differently.

 

[azazello]

Do You sure hmmm is caused by grounding? Maybe You have bad filtering 60 Hz without chock?

 

[mark31385]

Do You sure hmmm is caused by grounding? Maybe You have bad filtering 60 Hz without chock?

This is a hypothetical question, I have been told that a ground nut star will not work and I fail to see why It will not work.

If you have one main anchor point for ground surely this would be better if you tie all points to this. I don't understand the logic in daisy chaining 0v it just seems wrong.

 

[rayma]

If you have one main anchor point for ground surely this would be better if you tie all points to this.

Using a star ground increases loop areas and trace/wiring inductances.
The local current loops should be compact, and then they should be referenced to ground.
In many circuits, a well designed pcb with a ground plane can do both.

 

[DF96]

Star ground is not a magic bullet to solve all grounding problems, especially when it is poorly implemented (as it often is). One problem is that a 'star' is not a true point, but actually a very short bus often connected to the chassis at one end (and not necessarily the right end). A well-thought-out ground bus may be better for audio. RF and digital circuitry have different problems and so use different solutions. The important thing is to understand grounding and design it, not follow a recipe. Then questions about 'bus vs. star' etc. just fade away.

 

[azazello]

This is a hypothetical question, I have been told that a ground nut star will not work and I fail to see why It will not work.

If you have one main anchor point for ground surely this would be better if you tie all points to this. I don't understand the logic in daisy chaining 0v it just seems wrong.

I use thick wire /1-2 sq. mm/ about 2-3 inch in place between areas of output and pretubes, that middle is connected to main ground. I connect grounded elements to it. I never had have a problem with hmm and bzz....

 

[ericj]

I build with the chassis earthed, and other grounds are referenced to it through ground loop breakers of whichever type i decided to use that day.

I started doing that when i was working in an environment with a lot of RFI and EMI and my home made headphone amp was picking up a ton of noise.

The downside of keeping the chassis ground separate from other grounds is the added complexity of not being able to locally ground things by connecting them to the chassis.

No grounding scheme is perfect for all designs, but many grounding schemes are adequate for most designs. *shrug*

 

[mark31385]

Okay, got the reason why it is not preferred now. I just thought it would be ideal (and yes I know how to do them lol).

How does this one float then, Mains in and earth to chassis then going to a separate tag PCB to act as a "bus bar" then link of individualy to PSU, Bias etc? Have a rather good design that I tested with my headphones and am currently designing chassis lay out etc.

 

[Shoog]

I always build with a single bolt grounded star onto the chassis, and this bolt then tied to safety earth. My systems run extremely sensitive speakers of +100db drivers. I do point to point and pay only slight attention to cable routine (they usually look like a spaghetti mess). I run AC heaters but always use tightly twisted pairs.
None of my builds hum in the slightest - not even a tiny trace. Take that how you will but I would never build any project that didn't implement star grounding.

Shoog

 

[mark31385]

Interesting, my thoughts exactly I would have thought star grounding would be ideal.

Think I will experiment with this, but the headphone amp is going to be using 6V6 output tube and a ECC83 preamp tube that will deliver upto 2 clean watts think this might deafen me haha.

 

[Shoog]

There is one caveat to what I said, I like to think of currents in the system as water flowing to the source (safety earth) and the slight cable resistances as been slope on the flow towards the sea. As such I will often make a local small signal star ground on or next to the valve socket to take care of input references and treat these as tributaries to the main flow path. I then take these sub-stars by a single cable to the main star ground. It can save on multiple cables from the same physical space in the chassis.

It just helps me to have an easily understandable conceptual map of what is going on, even though the reality is somewhat more complex (negative currents flowing uphill as it where).

Shoog

 

[DF96]

Interesting, my thoughts exactly I would have thought star grounding would be ideal.

Star grounding can only be ideal when the star is ideal (i.e. a single point) but it never is. A real star is always extended so the order of connections becomes important. Once you realise you are actually dealing with a short bus you might as well use a longer bus.

The real solution to grounding is to understand the difference between safety ground and signal ground, then think about where the currents flow. One thing to always avoid is thinking about safety ground as some sort of infinite 'sink' for dumping currents.

 

[rayma]

Okay, got the reason why it is not preferred now. How does this one float then, Mains in and earth to
chassis then going to a separate tag PCB to act as a "bus bar" then link of individualy to PSU,

This is a quite good write-up on the subject. http://www.valvewizard.co.uk/Grounding.pdf

 

[Jay]

Interesting, my thoughts exactly I would have thought star grounding would be ideal.

You don't give up easily, do you

I know that you wont change your opinion in the next ten years

 

[mark31385]

This is a quite good write-up on the subject. http://www.valvewizard.co.uk/Grounding.pdf

Good shout will have a read over that.

And to the next person I will let it go once I understand the grounding in Valve amps. As I said I understand star grounding and isolating grounds. I just wasn't sure why it was not preferred for valves.

 

[Shoog]

You don't give up easily, do you

I know that you wont change your opinion in the next ten years

The problem with this response is that star grounding approached sensibly is just about the easiest and most consistent way for a novice builder to achieve a hum free project. It requires less experience to get a good result.

That has got to be a recommendation for its use.

Shoog