Star Ground vs Bus Bar (newb)

[Hambamble]

Hi guys,
I understand that the point of a star ground is to minimize hum? I was wondering if using a bus bar has the same effect, as I like the idea better. I'm building a matchless clone point to point, and it looks like the original matchless amps were built with bus bars. From what i've read star ground is the best way to reduce hum for the home builder, and a lot of companies try a few different locations before the best way to run grounding is found.

[Phil_S]

This has been much discussed, probably at every guitar amp forum. Search for "star ground" on the search page: http://ampgarage.com/forum/search.php and click "search for all terms".

In general, I think you'll see the buss ground used. Star, rather infrequent if at all. Buss can be quiet as a mouse, too, if you know what you are doing.

[martin manning]

There is good reading on this subject on R Aiken's site (link is in the Reading Material on Steroids sticky), and on the Valve Wizard site. Many ground schemes are hybrids of bus and star, and often with multiple stars.

[Hambamble]

Thanks Phil and Martin, Ill check out the website. I had a search but didn't find anything too useful. I'll just keep looking! Logically i couldn't see the difference, but theres no logical difference between lots of things in the world of amps, but the sound still changes.

[M Fowler]

I've read all those grounding tech stuff and it doesn't jive with real life amp building to me.

I've tried so many different types of schemes and the results vary.

Separate the low level, middle level, and high level, ripple currents which can be 3 separate stars each with their own wire going back to the main ground point near the PT.

Mark

[Milkmansound]

I'm with Mark

In real world amp building, all bets are off on what will work best. Just gotta jump in and try one and tweak it until its quiet

[katopan]

Different way to describe the same thing is group grounds by the common filter caps their current returns to. If your filter caps are all at the power end of the amp this means seperate power and ground wires to each amp section from your cap stack. If your caps are distributed through the amp then each section is very localised.

[martin manning]

There's no voodoo in this. As long as a ground scheme follows the basic principles as described by Blencowe, e.g., it will work.

[xtian]

I'm with Mark

In real world amp building, all bets are off on what will work best. Just gotta jump in and try one and tweak it until its quiet

This is so true. You'd think this is an experiment-and-collect-useful-data situation, but it's not. I've had sloppy amps that are really quiet, and carefully grounded amps that hum. This is the least predictable and most frustrating part of amp building.

[vibratoking]

There's no voodoo in this. As long as a ground scheme follows the basic principles as described by Blencowe, e.g., it will work.

I agree, it is very well understood by some, but very misunderstood by many others. You can learn a lot from this book. I consider the first edition to be the bible on the subject of noise in systems:

http://www.amazon.com/Reduction-Techniq ... -7749700#_

The second edition appears to include a lot more digitally related ideas.

The fact that different ground schemes give different results could be confusing. Sloppy wiring, does not always equal sloppy grounding and vice versa. You have to know what the pertinent issues are and consider those in detail.

[cbass]

There's no voodoo in this. As long as a ground scheme follows the basic principles as described by Blencowe, e.g., it will work.

I agree .I haven't built hundreds of amps like some of you guys but I've never had a ground hum issue.

[Reeltarded]

Until recently I was kind of perplexed about all the little loops and why I don't have trouble and then Martin let on to something that turned the light on for me.

I think (wrongly?) that you treat the ground as if it's lightning. It likes to hit the closest thing, sorta. Ehh

[Phil_S]

Some years ago, someone explained it to me like this...the "I" reference is to the person giving advice, not me:

You don't have to solder the chassis to do a ground bus. Placing several terminal lugs under one of the power transformer nuts will do just fine for a star. I use mouser part #534-7313 http://www.mouser.com/index.cfm?handler ... deid=53413

The best grounding scheme for a simple amp involves two separate grounds...a high current ground and a low current ground. The low current ground is insulated from the chassis. Here's how I usually do it:

Put 5 lugs under one power tranny bolt (you can also apply NoAlOx if you're worried about electyrolytic corrosion). Since each lug has 2 holes, you'll end up with 10 potential attachment points at your main star...more than enough. This star will serve as your "high current" ground for the first filter cap, output tube cathodes, PT-CT and filament center tap, and bias supply if you have one. Also this is where you connect the low current ground bus (see below) I usually ground the AC cord to another terminal lug mounted on a tranny bolt closer to the cordset's entry point. No solder necessary, but you need to tighten it up once every couple of years during normal amp maintenence. Loctite helps, but it's important to keep it limited to the screw and nut 'cause a big mess can act as an insulator. Also, if you are using an aluminum chassis, you'll need to use NoAlOx on the conductive surfaces.

For a low current ground, you can bridge some #18 or fatter bus wire across the negative legs of the 2nd and 3rd filter caps. Attach all your low current grounds (jacks, controls, preamp cathodes AND speaker jack) to this low current bus. Run one and only one lead from this bus back to your main star.

Oh yeah, you'll also need to isolate the speaker jack from the chassis....especially if there's negative feedback.

That's it. Do it this way and you'll end up with a pretty quiet amp. For the low current ground bus to have structural integrity, you'll need good physical attachment of the filter caps. A term strip will work well for this. So will eyelet board. A multisection can cap won't work in this scheme since they all share a common ground.

Thought For The Day: A chassis should be thought of as an electrostatic shield and not as a circuit element!!!! Don't use it as a conductor.


The concept that drove it home for me is gaining an understanding of where each section begins and ends. Though exceptions will be made for a variety of reasons, each section ends with a cathode to ground. I found this advice easy to understand and follow. I've had good luck with it.

There is more than one way to do it. This is one way.

If I knew more, I might be willing to suggest that all ways are a variation on some basic theme.

[VacuumVoodoo]

Above sounds ok except for using transformer fixing bolt to also serve as star ground point. Just don't. If a transformer is ever removed/replaced there's no guarantee that the ground lugs will be fixed correctly. Always use a separate bolt for the ground connection. Same goes for safety ground wire.

[Phil_S]

Yeah, I forgot that was in there. I agree about not using a transformer bolt. Thanks for pointing it out!