Briangt LM3875 wiring question

[ransom peek]

Am starting final wiring and testing now. Will try lay out wiring as logical as possible.

Here is what I think I know. Is it all correct?

1. Use black wire for AC hot, white for AC neutral, green for AC ground.

2. AC ground connects to a point on aluminum chassis we call star ground.

3. Twist each of the two pairs of wires from rectifier board to amp boards.

Questions:

What else gets connected to star ground? One leg of each of the speaker connections?

The stereo rca jacks for audio in, are not well insulated from the aluminum enclosure on which they are mounted. Chances are they are in electrical contact, but perhaps intermittently. Is this OK, or should they be isolated, or should they have a definitive connection?

Any other advice?

So far I have only applied ac to the 330 va toroid and confirmed presence of 27.3 vac on one side and 27.35 vac on the other.

[samborambo]

If it's a single ended amplifier (ie, not bridged) then yes, one speaker terminal goes to ground. You're quite correct in having a "star" earth point on your chassis. All ground connections between devices (including AC Earth) should also be made to that star point.

This is to ensure that no volt drops appear on any ground point as you start passing higher currents through the ground wires. Every piece of wire has resistance.

Just remember with AC mains that Neutral is connected to Earth back at your house switch board. If you wire it wrong it may appear to work fine but as soon as you get a phase fault it could be very hazardous to your health. Keep AC Neutral isolated from AC Earth in your appliance.

Other than that, you should be fine.

Hope this helps.

Sam.

[ransom peek]

OK, so are 1,2 and 3 correct?

Here is what I think gets connected to star ground (one point on aluminum chassis)

One wire from each of two speaker pairs

Line cord ground (center round pin)

One wire from each of the two input signals (the rca jack outer case which is on eo fthe two leads


Is this all correct?

What else gets connected to star ground?

Any issue about the input signal rca jacks intermittnetly touching aluminum cabinet?

[edjosh23]

Sorry for the blurry picture, but this should be somewhat helpful.

Good luck,

Josh

[samborambo]

Also the centre tap of the secondaries from the toroid.

What you're trying to achieve by having a star point is to ensure there is no voltage offset on any part of the ground system, right from the power supply to the output stage. I believe you should make all ground connections between each stage as per normal and ALSO to star ground point - essentially two current return paths.

For more reading:

http://www.aikenamps.com/StarGround.html

[ransom peek]

But there are two secondaries, no centertap!

The article on star grounding is excellent!

[ransom peek]

Wiring check please!


Here is my post with a picture of the wiring so far. And , a few questions...

[samborambo]

Your secondaries should be connected in series to give you +V > 0V > -V.

Sounds like you need to brush up on centre tap linear power supply set ups. The point where the two secondaries are connected in series is your 0V ground reference point. Expect high currents to pass from the speaker ground terminal back to this point - therefore, don't place any sensitive signal grounds between the speaker ground and this point.

Here's an example. Say your preamp ground is connected close to your speaker ground. Both signal "return paths" are through a piece of wire back to the power supply ground. This wire has a resistance of, say, 0.01 ohms. At the peak of an output waveform, you speaker is passing, say, 5 amps (100W RMS into 8 ohms). At this given current, the wire will develop a potential of:

5A * 0.01 ohms = 50mV peak or 35mV RMS

With respect to the ground of your power supply. This is not good. The idea is that every "device" in your system should have it's own current return path back to a selected ground point. With a linear power supply, this should be the first of your reservoir caps as this would be the most stable reference. You'll see quite a lot of ripple potential developing between the secondary 0V and your reservoir caps as they tend to draw current in 100Hz (120Hz for you) spikes at the peaks of the AC supply waveform.

That link on star grounding points is good, but is written on tube amplifiers. I'm not too up with the play on tube amps (and quite frankly don't want to know) so I'm not sure where it might lead you astray. I haven't bothered reading through the whole thing.

Hope this helps.

[ransom peek]

To achieve the + and - rails, it would make sense to put the secondaries in series. But, the briangt instructions have you connect the secondaries directly to the rectifier board, not wire them in series, and on the rectifier board there are not traces connecting them in series.

Does anyone have a schematic for one of the briangt lm3875 circuits? Mine is rev 3 board dated 0705.

[Greg Erskine]

hi ransom peek,

I beleive, on the BrianGT/Peter Daniel LM3875 gainclones the earth connection is done at the amp PCB. So each rail has it's own earth wire up to the amp pcb if you follow the standard instructions.

Peter Daniel has posted the "correct" schematic on a few occassions. The last time I looked the schematic in the manual it was "slightly" wrong but it may have been fixed by now.

regards