[chadh]

I'm currently doing my best to put together my first two DIY projects:  not at all ambitious ones, but hopefully instructive.  I built a super-cheap passive volume control (of a shunt attenuator variety), and now I'm trying simply to rehouse a Sonic-T amp (without the pot) in a new enclosure with standard connectors and a 12v battery inside.

Both projects seem pretty mechanical.  But I realise that I just don't know what I'm trying to accomplish with grounding.  I mean, I have absolutely no idea.

If my component uses a standard grounded power cord, I would imagine grounding is a simple issue - everything that gets grounded goes to the third pin.  But neither the passive volume control nor the repackaged, battery powered Sonic-T has a power cord at all.  Then I hear people talking about grounding things to the chassis of a component.  Like I said, I know nothing - but this sounds dangerous to me!  Anyway, in my wisdom (and my frugality) I'm experimenting in super cheap RS plastic enclosures.  So I don't imagine I can ground anything to a chassis.

Currently, the volume control has the grounding tabs on the input jacks connected and the grounding tabs on the output jacks connected and then both of these are tied together.  Also the wires that shunt voltage to ground via the pots are tied into this mess.  I presume that this means that the volume control is actually grounded through the input and output interconnects which attach to a properly grounded amplifier or source.  But I could be wrong.

The Sonic-T repackaging:  I presume that I tie the grounding tabs of the two inputs together.  The input grounds are also attached to the circuit board at the appropriate places.  Then there is another ground wire that leaves the circuit board, which I imagine connects to the input grounds near the input jacks.  The four output wires go to the respective binding posts, so they don't get tied together, nor are they tied to anything else.  Does this sound right?

I suspect that I'm making this much more difficult than it should be.  But it seems like it should be important.  So any helpful advice would be gratefully received.  And sorry for being so vague.

Thanks.

Chad

[andyr]

Hi Chad,

Mebbe I can be of assistance?

You said: "If my component uses a standard grounded power cord, I would imagine grounding is a simple issue - everything that gets grounded goes to the third pin."

Firstly, lets first make a bedrock statement:
* a "safety earth" is often important.  This means the "earth" wire in a 3-wire power cord is connected to the chassis.  It's called a "safety earth" because this chassis mains-earth connection will save your life if, by some mishap, the "live" power wire ever touched the chassis.

I personally believe the only reason hi-fi mfrs use a 2-wire power cord and not a 3-wire power cord is to save themselves 10c - ie. I believe all components should have an earth wire coming in.  Certainly, some other people disagree with me!  

However, you said:
a)  you are using a plastic case, and
b)  the Sonic T-amp is battery powered.

So "safety earthing" is not relevant!  However, you may find when you've got it going that your amp sounds a bit harsh ... by using a plastic - not metal - case, you've lost the protection from RFI which a metal case ("Faraday cage") gives.

Secondly, in my universe (and my whole system: head amp, phono stage, pre amp, active crossovers, power amps is DIY) no part of the sound chain is connected to (mains-earthed) chassis.  IE. the "ground" on each PCB inside each component all "float" wrt the chassis.  AFAIAC, this way you minimise "ground loops" - ie. hum - yet get maximum protection against RFI.

You said: "Currently, the volume control has the grounding tabs on the input jacks connected and the grounding tabs on the output jacks connected and then both of these are tied together.  Also the wires that shunt voltage to ground via the pots are tied into this mess.

I presume this means that the volume control is actually grounded through the input and output interconnects which attach to a properly grounded amplifier or source."

Weeelll ... sort of!    Your volume control is a shunt attenuator - hence, as you described, "the wires that shunt voltage to ground via the pots are tied" to the input/output RCA ground tags.

As long as this point is attached to the "ground plane" on the Sonic-T amp, all will be well ... even though this ground plane is not "safety earthed" to chassis bcoz you have a plastic case and no power cord!

You said: "I presume that I tie the grounding tabs of the two inputs together.  The input grounds are also attached to the circuit board at the appropriate places."

YES.

"Then there is another ground wire that leaves the circuit board, which I imagine connects to the input grounds near the input jacks."

DON'T KNOW ... I can't understand why this ground wire is needed?    Mebbe this gives you the option of attaching the signal ground to chassis ground ... in some environments this can give you a lower noise floor?  In which case, you won't need to use it.

"The four output wires go to the respective binding posts, so they don't get tied together, nor are they tied to anything else."

YES

So that leaves only one Q .. which, as I don't know the Sonic-T, I can't help you with.  This is ... where does the 0v battery connection attach to?  In my battery-powered head amp (2x6v gel-cells in series = 0/+12v), the 0v battery terminal attaches to the PCB ground plane and the +12v battery terminal attaches to the '+' power rail.

I would imagine this should happen with your Sonic-T amp but, if it works off +/- power rails, it will be slightly different.

Regards,

Andy

[chadh]

Andy,

Thanks for your detailed response.  Just a couple of questions, then, that will prove that I know nothing.

1) when you refer to the Sonic-T's "ground plane", what does that mean?  Pretty basic, huh - but I warned you that I knew nothing.

2) Your question about the 0v battery terminal has shot right over my head.  This thing has connections for +12v and -12v, so the two leads from the battery go to these respective points.  Is this the distinction between having a single rail (I think I read that the Sonic-T had a single rail) and two rails?  Does this mean that the Sonic-T is "working off +/- power rails", as you put it?  Which leads me back to question 1 - what's the ground plane?

I know I'll feel much wiser once this is all sorted out.

Thanks again.

Chad

[andyr]

Andy,

1) when you refer to the Sonic-T's "ground plane", what does that mean?  Pretty basic, huh - but I warned you that I knew nothing. ...

If you have a look at the copper side of the PCB (or both sides, if it has tracks on both sides!    ), you should be able to make out where the wires to the input and output RCA ground tags connect.

If you follow this through the circuit, you should be able to see that these are con-joined ... ie. this is the "ground plane".

Some components on the PCB will have one end connected to this "ground plane" - eg. even though you say the amp has "+/- 12v" power rails, I'm pretty sure there must be a "0v" area in the middle.  You should have some electrolytic capacitors (which show the '-ve' lead as the one next to a white stripe on the can) which go between the '+ve' and '-ve' power rails and earth ... on the '+ve' power rail the white stripe will be on the ground plane; on the '-ve' rail the white stripe will be on the '-12v' rail.

Andy,

2) Your question about the 0v battery terminal has shot right over my head.  This thing has connections for +12v and -12v, so the two leads from the battery go to these respective points.  Is this the distinction between having a single rail (I think I read t ...

Sorry to ask you a very simple Qu ... but you said you knew nothing!!  

Are you sure you have the correct battery?  IE. did you buy the battery with the amp or did you just buy any old 12v battery?

As I said, I know nothing about the Sonic-T but if the amp says +12v and -12v then this normally means it has dual power rails so it might require 2 x 12v batteries?  The first has its '+ve' terminal connected to the '+12v' connection and the '-ve' on the 0v connection (ground plane); the second has its '+ve' terminal connected to the ground plane and the '-ve' connected to '-12v'.

Regards,

Andy

[chadh]

Andy,

Thanks again.  This is taking WAY more of your time than it deserves.

I think I've got the (simple) ground plane concept down now.  Thanks.

As for the battery - I'm as close to certain as I can possibly be that I have the right battery.  A single SLA 12v battery (mine is 5ah, but this doesn't seem relevant).  I think I simply made a mistake in labeling things:  now that I look at the pcb, one of the connection points for the battery is labelled "+12v" and the other is not labelled at all.  Either I dreamed up the "-12v" label, or borrowed it from some other mistaken wiring diagram.

Moreover, I made another mistake before.  I said that the grounding tabs on the input jacks were tied together  and to the board at their respective points and then later that the grounding lead from the board was tied to these input grounding tabs.  You were confused as to what the last of these grounding points was all about.  Well, the input grounding tabs are not actually connected to separate grounding points.  They only connect to the board at the single point that was described (so a wire runs from that point to the wire that joins the two grounding tabs).  Is that even vaguely clear?

Anyway, I think you've cleared up most things for me, except maybe things that are just to painfully embarrassing to ask about.  Everything is hooked up, in fact, and almost working.  The volume control is working properly.  And one channel of the amp is working too.  I'm not totally sure what's wrong with the other channel:  I hear something very faintly, and only at very loud volumes, but that could be crosstalk from the first channel.  The connections all look fine, so maybe I messed up one of the traces when doing some sloppy soldering.

Again, thanks for your explanations.

Chad

[andyr]

Andy,

Thanks again.  This is taking WAY more of your time than it deserves.

I think I've got the (simple) ground plane concept down now.  Thanks.

As for the battery - I'm as close to certain as I can possibly be that I have the right battery.  A single SLA 12v battery (mine is 5ah, but this doesn't seem relevant).  I think I simply made a mistake in labeling things:  now that I look at the pcb, one of the connection points for the battery is labelled "+12v" and the other is not labelled at all.  ...

Hi Chad ... no problem at all.  I knew even less than I know now, a few years ago ... and so learnt a lot from Forums like this one!  

Now it's making sense.  You appear to have a single +12v power rail (like my battery-powered head amp).  Sooo, the '+' terminal (red?) from the battery is connected here and the '-' battery terminal (black?) is connected to the non-labelled spot (the "ground plane").

As I said before, if you try to follow your way around the copper traces, you should find that wire you were talking about (for the input ground tags) is also connected to this ground plane.  And the output grounds.

And "5ah" means 5 ampere hours - ie. the battery will support a current of 1 amp for 5 hours ... or 1/2 an amp for 10 hours.  That's not actually a lot for a power amp ... I use 5ah batteries for my headamp, which only consumes about 30ma!

Moreover, I made another mistake before. I said that the grounding tabs on the input jacks were tied together and to the board at their respective points and then later that the grounding lead from the board was tied to these input grounding tabs.  You were confused as to what the last of these grounding points was all about.  Well, the input grounding tabs are not actually connected to separate grounding points.  They only connect to the board at the single point that was described (so a wire runs from that point to the wire that joins the two grounding tabs). ...

Great ... so now I understand what that wire is for - yes, makes perfect sense.

Everything is hooked up, in fact, and almost working. The volume control is working properly. And one channel of the amp is working too. I'm not totally sure what's wrong with the other channel: I hear something very faintly, and only at very loud volumes, but that could be crosstalk from the first channel. The connections all look fine, so maybe I messed up one of the traces when doing some sloppy soldering. ...

Mmmm, that's a pity.  If one channel is working, together with its side of the volume control then it sounds like that channel is AOK!  

The fact that you only hear a faint sound when the vol control is turned right up does suggest you're hearing crosstalk and that the actual circuit on that channel is OK (otherwise, I suspect you'd hear nothing at all).

Check the connections from the input RCAs on the "bad" channel - maybe it's broken?  Or is that channel of your attenuator stuffed?

Also swap your input RCAs and see if the "dead" channel changes to the other side ... if it does, you know the amp is OK but the IC or the previous component is stuffed.

Regards,

Andy