GND in a circuit

[PsychMan]

Hi Folks,

Im quite a novice with electronics and am learning as I go. A little while back somebody (NickyLyons) was kind enough to post a circuit diagram for an impedance matching pre-amp I could use to connect my record player to a more modern amplifier.

I've had an initial go at building it on a breadboard with not much luck. Im going to go back and check everything (as Id expect to), but thought Id make sure I had something right first. Sometime ago I had a go at building a guitar fuzz pedal, and could never get it to work in the slightest, despite going over the schematics time and again and trying different parts. So Im wondering if I fundamentally misunderstood something.

Generally speaking if I have a DC powered audio circuit, using for example a 9v battery, when the diagram shows a component going to earth, should this be the negative terminal of the 9v battery?

I've always assumed this to be the case, but based on my poor luck with a few circuits I've tried I wonder if Im wrong

Adam

[Nickthedentist]

Originally Posted by PsychMan

Generally speaking if I have a DC powered audio circuit, using for example a 9v battery, when the diagram shows a component going to earth, should this be the negative terminal of the 9v battery?

Yes, Adam, unless it's a very old (1950s/1960s) circuit with PNP transistors, in which case ground/chassis was often positive.

Have you got a multimeter to measure volts and milliamps, by the way? This would be hugely helpful for fault finding.

Good luck with it,

Nick.

[PsychMan]

Originally Posted by Nickthedentist

Yes, Adam, unless it's a very old (1950s/1960s) circuit with PNP transistors, in which case ground/chassis was often positive.

Have you got a multimeter to measure volts and milliamps, by the way? This would be hugely helpful for fault finding.

Good luck with it,

Nick.

Ok, maybe just me being a muppet then!

Yes, my dad gave me a few of his old multimeters so I have lots to play with

[Herald1360]

If you have a circuit of what you think you built, post it up and we can all chuck in our two penn'orth!

[G6Tanuki]

Usually, GND is -ve.

But, as has been mentioned above, this fails if you're dealing with PNP-centred transistor-circuitry from the 1950s/1960s, in which case the +ve supply-rail is "ground".

The same applies in vehicle electrics: up until the mid-1960s it was quite common to find on European cars the +ve terminal of the battery to be connected to the chassis - there being a strange delusion that doing so rendered the vehicle less susceptible to electrolytically-induced rusting.

Fortunately it was relatively easy to 'reverse the polarity' of dynamos on such cars. Series-wound motors [starter, heater-fan, wipers] didn't care about polarity - and the ignition coil merely needed the two connections reversed. The fun started with mid/upper-class cars with revcounters. At least Jaguars had revcounters driven by a little AC generator attached to one of the camshafts...

[ricard]

Originally Posted by Herald1360

If you have a circuit of what you think you built, post it up and we can all chuck in our two penn'orth!

And perhaps a picture or two which would make it easier to spot any glaring mistakes in the actual construction.

[PsychMan]

OK, I'll take another look at it to make sure I'm not being a complete tit then post a pic of what I've done.

Cheers
Adam

[Diabolical Artificer]

Adam, I've made some mistakes myself on very simple circuits. For instance I put a 100 ohm resistor on the PCB where it should have been a 100k resistor. The thing is,I checked and rechecked and re-built the circuit numerous times and still missed it.

If you're building on veroboard get a magnifying glass, re check all your holes/breaks where you cut the the track. IE, I use a small drill to sever mine. And check if there are any blobs of solder connecting/shorting out between tracks etc. Check all your resistor and capacitor values. Check you have the polarity of electrolytic capacitors the right way. Then re check again.

One other mistake I made was a mis translation of a schematic of a fuzzbox. I put a capacitor between the + supply and the circuit. Capacitors DONT conduct DC, so I had no power. The schematics of input jacks of guitar pedals can be confusing as they usually have an on/off switch which is switched by the insertion off the jack plug.

Hope that helps and as Nick says get your meter out and check you have power to the legs of any transistors/IC,s. Dont assume. Check. Good luck,let us know how you get on,Andy.

[Herald1360]

Originally Posted by G6Tanuki

Usually, GND is -ve.

But, as has been mentioned above, this fails if you're dealing with PNP-centred transistor-circuitry from the 1950s/1960s, in which case the +ve supply-rail is "ground".

Except of course some of Bush's offerings like the VTR103 which is "interesting" with its negative chassis grounding arrangements.

[ColinTheAmpMan1]

Originally Posted by Nickthedentist

Yes, Adam, unless it's a very old (1950s/1960s) circuit with PNP transistors, in which case ground/chassis was often positive.

Nick.

It is always possible, if it's an old circuit from the 50s-60s, that it's not that good. I built a few circuits in those years using OC71s and surplus "red-spot" transistors, lots of which were horrid things. How about a guitar tremolo circuit that used a two-transistor multivibrator (square-wave oscillator) which was then filtered to hell and back in order to get something like a sine-wave out of it? This was in the days when things like Wien Bridge circuits were still a twinkle in someone's eye.

Having said that, I also remember a valve echo/reverb unit where the delay was using a home-constructed spring-line. The reverb might not have been that bad (obviously professionally-manufactured spring-lines would have been far better but weren't that available...) but the "echo" was achieved by volume modulation with a circuit like a tremolo. The "intensity" of the modulation had to be pretty heavy to get anything near, but it probably wasn't that nice. I never made this thing (I tried making my own Watkins Copicat clone) but others with access to archives of "Practical Wireless" and/or "Practical Electronics" from the sixties might come up with the darned things.....

With regard to the battery polarity and earth question, it is not a bad idea to imagine that the thick line at the bottom of the circuit diagram is the case or chassis of the thing. You could also easily imagine it as a thick tinned-copper wire busbar attached at some point to the chassis. With some circuits, it can be a critical as to where on this "busbar" a component is physically attached. Heavy currents can cause problems in high-gain stages near the front end of an audio circuit. It should be said that this is likely only to be of concern in valve-powered equipment, not solid-state stuff.

Colin.

[julie_m]

Originally Posted by ColinTheAmpMan1

With some circuits, it can be a critical as to where on this "busbar" a component is physically attached. Heavy currents can cause problems in high-gain stages near the front end of an audio circuit. It should be said that this is likely only to be of concern in valve-powered equipment, not solid-state stuff.

In a remote control circuit for a gas fire that I did once, a trace between the ignition transformer and the triac that discharged a capacitor into its primary winding ran close to the 0V line with which it was electrically common. Production Engineering berated me for not filling in the intentional space I had left between the two (and in fairness, it did create a bit of an etch hazard; but there was no other sensible way to do it. And a person who could have found one would never have done it for what that place were paying me) but I knew my reasons. I filled in the break for the prototype, and it didn't work -- the hefty current pulses when the ignition spark fired were now getting induced into the 0V rail, upsetting other circuitry. Restoring the break for the next prototype (so the triac-to-transformer segment was now effectively on a short side branch from the main 0V rail) cured this.

Oh, and the board etched fine using equipment no better than we had in my secondary school; so I had no worries about a professional outfit being able to do it. And as there was already routing on the board to form a spark gap (used as a crude VDR on the earthy end of the transformer secondary, so the same electrode could be used for flame sensing and ignition) another bit wouldn't have hurt.