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Two-point star ground Q’s – long

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1/5/2000 3:33 PM

Two-point star ground Q’s – long

In order to improve the SN-ratio in a DIY amp project, I’ve been gathering info on grounding principles. Until reading the tech-paper on Randall Aikens excellent site, most sources I’ve read, refers to a one-point starground scheme.

In the article (www.aikenamps.com/StarGround) Randall Aiken states:

"Even better is a two-point star....."

The article is pretty detailed – but yet there’s a couple of things I’m not sure about,
so I thought I’d better seek some advise before redesigning my grounding scheme.
The layout off my amp is with the PT in one end off the chassis and 3 radial double cap cans mounted in equal distance (4 cm) round the PT. Not in a straight line arrangement.

My doubts is regarding the physically placement off the two grounding points.

"The ground lugs for the capacitors can then be connected to the chassis in such a manner as to keep the later stage ground currents from interacting with earlier stages."

If I’ve got it right, the first ground point should be a lug ,bolted to the chassis, close to the PT – and preferably on the edge off the chassis.
The ground wires from PT center tap, first filter cap, the output tube cathodes (fixed bias), filament center tap and the OT secondary ground connects to this first ground point.

The second ground point connects with all the local common preamp stage grounds and
the remaining filter caps – the ones after the choke. (Input and output jacks isolated)

BUT - is the second ground point another lug bolted to the chassis – and where should I place it. In the preamp end off the chassis - as far from the first ground point as possible?
Or close to the second filter cap – which, in my case, is also close to the PT- and the first ground point?

What influence, if any, will the placement off the choke and OT have?
I’m also not sure where to connect the bias circuit grounds!

Any info, advise or comments will be highly appreciated. Thanks.

regards
Lion

1/5/2000 5:07 PM

quote:
"BUT - is the second ground point another lug bolted to the chassis"

It can be confusing because all the wires are called "ground" but only the first star point is bolted to the chassis. This is the "star ground". The second star point isn’t bolted to the chassis. It’s connected to the "star ground" by a wire.

Dave

1/5/2000 5:45 PM

Pardon my butting in, but while we're on the topic of grounding....I've got a 5E3 chassis from ClarkParts and it came with a "brass grounding plate" with holes to match the 3 pots and 4 input jacks on the control panel. What's the purpose of this plate? Is it to allow soldering to the chassis since aluminum won't take a solder joint? This is my first DIY and all the ins and outs of grounding are only slowly coming to me.

Any help appreciated.

thanks,
Mike

1/5/2000 6:23 PM
DavidB	Yeah,I think you would bolt the preamp & pot grounds to the chassis away from the power supply.use a gator clip to find the best location.can't remember where I read that, maybe VG. On my amps I just use one chassis ground for everything off the preamp filtercap ground,least noise for whatever r reason oh it,s a JTM45 clone. goodluck db

1/6/2000 1:15 PM

Re:Still confused – maybe Randall Aiken?

Thank for the replies Dave and David!

"It can be confusing because all the wires are called "ground" but only the first star point is bolted to the chassis. This is the "star ground". The second star point isn’t bolted to the chassis. It’s connected to the "star ground" by a wire."

Well, yes – maybe. But isn’t that just a "normal" one-point star ground with local common preamp stage ground homeruns??? The way I read it – this is not what RA describes.

So I’m still confused – maybe Randall Aiken could chime in and set things straight. That would be nice!

regards
Lion

1/6/2000 2:58 PM

Think about it this way - in your mind, replace every run of "ground" wire with a resistor. The currents through these resistors make voltages proportional to the current through the resistor. The whole trick in grounding is to keep the "ground" voltages from affecting the signal.

The most important technique is ground wire separation. If you have two stages that are "grounded" through the same ground wire/resistor, the ground currents mix in the wire/resistor and BOTH stages see their grounds wiggling around with the sum of the two currents. If each stage has its own separate ground wire/resistor, no mixing is possible, and the ground wire/resistor developed voltage only acts to reduce the stage gain, not to introduce interference. This is pure star grounding - every single thing that connects to "ground" is connected through a separate ground wire/resistor.

I haven't read Randall's sendup on grounding yet, so I'm only guessing about the two level stuff, but I'd guess he means something like the following.

Within one stage or even sometimes two stages, the ground currents are related. Certainly, the grid leak resistors and cathode connections can be connected together because there is almost no grid current anyway, and the cathode ground wire/resistor voltage is proportional to and opposes the grid leak ground wire/resistor voltage, so no interference is possible, just a slight lowering of gain. These can be connected together. A volume control from the plate has the same signal voltage across it, so its ground wire/resistor current could also be added to the grid leak/cathode current without interference, just a change in gain (depending on the physical location of the volume control, we might get 60Hz hum pickup, so we might not want to do this, but then the devil is always in the details.)

If there is a second stage that immediately follows the first, and neither stage clips, then the second stage ground current is out of phase with the signal in the first stage, so there is actual cancellation of the ground current AC signals, and combining the two ground currents can make the local collection of grounds even quieter. IF this stage clips, it also introduced hash into the first stage ground, so this must be done carefully.

A third stage is a no-no - the gain is too high, and the ground components are certain to be in-phase with something at the first input, so we introduce the possibility of oscillation from ground currents. An unrelated stage is also verboten - the currents are unrelated and don't cancel, so all you're doing is cross-feeding signals between the two sections.

What we quickly get to is that you can collect certain grounds locally to a local star-ground if you do this thoughtfully. The local stars can then be collected to the global star ground.

A special case is the first power filter capacitor. It's negative side carries the pulsed currents that charge the capacitor. If you ground the transformer CT (or - output of the rectifiers in a bridge) and then connect the other star ground wires there, the capacitor charge pulses appear on the ground wire/resistors as 120 hz buzz. You must take the CT to the - side of the cap and a *separate wire* which then only carries the DC power to the system star ground. It's in general a bad idea to ground the CT and run a wire to the cap, leaving that as your chassis ground connection, because then the charging pulses appear on the chassis. Instead, isolate the CT, wire it to the first filter cap, run a wire to the star ground *without connecting to the chassis* and once all the star grounds are properly connected, run one and only one wire to the chassis. This absolutely prevents rectifier hum from getting into things through the chassis.

You wind up with a two level star grounding system - Local stars for one or two stages where the currents are related and compatible, and a global star point where the local stars are connected. One of the "locals" is the (-) side of the first filter cap, because it's locally noisy, and one is the chassis, because it's an interference shield and you want it to short out any induced currents within the chassis, not convey them into the circuits.

The whole trick in grounding is to mentally think of every ground wire as a resistor and then to follow where the *currents* go and what they do in those resistors.

1/6/2000 3:09 PM
DavidB	the preamp ground is separate on the chassis,go down to the bottom of Aikin's page "where should the star points go" your right Lion,cool page!

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