Question about grounding line level circuits.

[freddiecas]

I'm collecting bits for an active crossover, 3 way, with +/- supply.

The circuit is effectively 3 parallel circuits per channel so each channel input is a phono to 3 phonos out.

I had thought the best way of grounding the 3 parallel circuits per channel for signal is at the input phono (which would be preamp or dac output) then the 3 outputs would go to 3 channels of independent power amp.

However what would be the best place to connect the supply gnd? at a star at the input or make the star at a single output?

The total circuit would be 18 OPA134's so supply current would be around
100mA I guess. Is the grounding critical for such low currents?

Thanks for any ideas.

[Dave Cawley]

YES! 100ma is huge, you need to measure distortion and noise and select the best pattern?

Dave

[freddiecas]

I was concerned about avoiding gnd loops with effectively 6 parallel circuits within the box. Unfortunately I don't have access to noise or distortion measuring equipment and was thus just asking for general pointers of good layout practice.

[Alex Nikitin]

I would probably go for a common reasonably massive ground plane for all inputs and outputs, connecting the P/S central point to it somewhere closer to the output. If you prefer to keep the input ground connected through the circuit then again I would recommend to connect the P/S midpoint to the common output ground for all outputs (I would at least connect the ground on all output sockets together). However all these recommendations are layout dependant and if you are using a particular layout it may be not the best way to arrange the ground connections.

Alex

[freddiecas]

thank you, I think I'll connect all outputs together with a bus of solid copper 13A mains cable and star to that point for the power supply and the circuit gnds. As a first try I'll leave the single input gnds through the circuit gnd. Without measuring equipment I'll see how noise or hum increases with and without the crossover between pre and power at full volume no signal and then try moving the input gnd around to see if it makes a difference. Thanks for all the suggestions.

[Primalsea]

Its a little more subtle than that. The main issue you will have is that the 3 power amps will very probably form a ground loop through their mains cables and the input signal cables from the XO. This gives you 2 choices really which are to put mains earth breakers into the power amps which could potentially be dangerous. Or you can break the earth loops in the XO. Initially I would think about having the presumably last Sallen Key filter stages with balanced outputs going into the final buffer stage and having that earthed via a 10 ohm resistor to the inputs of the XO. Do not connect the earths of the XO output buffer to each other though.

I cant guarantee it will defo work but its what I would do as a start and then take it from there.

[freddiecas]

Hi, thanks for the tip, it certainly is worth knowing. Fortunately the Audiolabs I'm using in the passive tri-wire at the moment (3 power amps per channel) are double insulated and don't have an earth connection to mains, so hopefully that particular loop will be avoided. Since the preamp is the same and the level of buzz/hum at full volume now with ear against drive units is very low indeed I am hoping the addition of the active crossover between pre and powers is not going to upset that if I only earth the metal box but not the signal or supply ground ie make a double insulated active crossover. That is why I was wondering about the best grounding within the active crossover not to introduce loop problems. Thanks again :)