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Amperes required by LM1875 Guitar Power Amp

Started by exztinct01, March 15, 2016, 07:01:29 AM

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phatt

What * incurably_optimistic* said :dbtu:

just adding;
The idea is to keep low current high sensitivity preamp ground returns picking up High Current of the power stage.
Think of the ground plane as a second power supply rail and needs as much attention as the voltage supply rails.
In schematics the ground path is often assumed for the sake of clarity but that can be rather confusing for the novice.

Opamps have fairly good PSR (power supply rejection) so you can get away with a bit more but the old days of discrete (both Valve and Transistor) you had to be very mindful of ground path.
Look up Valve Wizard's site for some common ground plane gotcha's,,on Valve layouts.
Phil.

exztinct01

in a power amp pcb (e.g. P27 power amp), is the signal common connected to its supply common? Mr. Fahey stated in his post they should return to star ground in different paths. P27 power amp have only one return path and I think both signal and supply commons pass there since preamp is isolated by a 10 ohms resistor from the common point.
~ Stephen

exztinct01

this is my amp layout, let's say it's high current amp. Which connection should I cut?
Power amp's pcb has signal common and supply common directly connected, same with preamp. I already found some errors like my center tap terminates after filter caps. Another, loops. May I also say that I don't have true earth there so grounds' floating. I can't make any fix yet for that floating ground since it will require redesigning my house's electrical system.
~ Stephen

phatt

Forget that 10R resistor as it is not needed,,, then ESP drawing will make more sense.
Ground path goes from centre of main filter caps to power amp then onwards to preamp then to input sockets.
That common wire can be one or split it up (Star ground)

Rod mentions the input socket wireup option but it's debatable as to which way is best. :-X

You can connect *circuit Com point* (at Main filter caps) directly to metal Chassis or you can connect the input socket to chassis *BUT NOT Both* as that will introduce a ground loop and hum may increase.

No matter how good your design even experts end up testing several ground options with a fly lead and an alligator clip testing to find what ground path gives the best hum rejection.

In Rods design the gain is not massive so I doubt you will have much issue with the ground layout. :tu:

That fancy ground lift idea is perfectly valid but as most guitar players use pedals nowadays which are often powered via a *non grounded* wallwart supply it can cause issues. No medals for guessing why I know ?? :P

And if your wallwart/plugpak is one of those cheap switchmode types they can introduce a lot of noise problems if your amp chassis is not well grounded. At some point in all those connections you have to have a ground otherwise noise and humm WILL BE a problem :'(

Frankly you are better off just grounding the chassis to circuit common or if you must have it make it a switchable ground lift.

Phil.

exztinct01

oh sorry, I missed attaching the image in my last post.
Here it is
~ Stephen

incurably_optimistic

you have to remove either a ground path from the preamp to the power amp (to achieve more of a "star grounding"), or you can remove a ground path from the preamp to the PS filter caps (to get something in the mold of "Ground bus" - you will have a return path starting at the lowest current part (input), then gradually increasing the current - preamp, then power amp, then power supply).

Like it was mentioned, you cannot keep both of those connections.

phatt

Yes I have been trying to find a clear and well defined example of how to wire up but I can't find anything so I'll use your drawing.
It's easy to get it wrong,, I've been building stuff for a long time and i still fluff it up sometimes. :-[

When you interconnect boards like this you only ground one end of the shielded cable otherwise you make a ground loop.
If the input socket is plastic then just connect both ends of shielding.

hope it helps, Phil.


exztinct01

thanks a lot to both of you  :dbtu:
illustrations are much better than text, since I honestly don't understand some ideas or terms here.
Another question, what if the two equipments have different power supply, is it still okay to cut that ground path? For example I use a pedal, is it enough to just connect the signal path from pedal to preamp, and not the common path?
~ Stephen

incurably_optimistic

In general you cannot do this - you need to have grounds of all the devices connected one way or another. The reason is that the voltage by definition is not an electric potential, but it's a difference of potentials between two points (that's what you mean when you say "voltage across a resistor" - it's a difference of potentials between two ends of a resistor). We always assume a point of "zero potential" to be the ground point of our circuit, but in reality this is rather artificial - you could say that the ground is at 100v potential, and it still wouldn't change anything because what we're measuring is a difference of potentials. It's just convenient to say that the ground has zero potential.

You want the reference points in all the circuits (by that we usually mean grounds of the circuits) to be at the same potential, so you need to connect them by a piece of wire - then you cannot say that the ground of one circuit is at 100v and the ground of another circuit is at 200v, because they are electrically connected, so they must have the same potential.

The problem with your circuit wiring was that the preamp and power amp grounds were connected not only by one path (that is good), but by TWO paths: one going straight from preamp to power amp, and another one going like this: preamp->power supply caps->power amp - which introduces a ground loop which may cause trouble (look at the attached image).

When you remove one of those two connections there is still ONE left, so all is good! Ideally you want all of ground points of all the circuits in your amp (power amp ground, preamp ground, power supply ground and what not) connected to one, chosen by you, ground point of your amp (usually the point between the filter caps) by just one path.


Back to the pedal-preamp case: of course it may happen that the pedal power supply is not "floating", which means that it's ground is in some way connected to the mains ground and also your amp common is connected to the mains ground - then you don't need the ground part of the guitar cable going from the pedal to the amp, but it's a rather rare situation so don't cut that cable  :trouble

Man, I fell like I'm really awful at explaining anything so I'll just wait for Phil or someone to clear all the confusion I surely introduced  :loco

exztinct01

~ Stephen

phatt

@ *incurably_optimistic*
Well judging by that posting you don't need my help. :dbtu:
Phil.