Grounding in a biamped buffered system

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I'm designing a biamped system based on GC and buffered ala Rasmussen with an OPA627.
My question is about the grounding, as there will be THREE power supplies.
Should I just make a power star ground common for the 3 PS, an other star gnd for the 3 stages and join in only one point?
Should the GND of the OPA power supply go to the signal GND of the buffer, or to the main GND star power gnd?
Has anyone had a problem with the turn on/off of a LM1875? I've almost completed the poit to point- PCB for both 1875 and 3875, but maybe with the 3875 there will be no problems for the tweet (yes, I know it has to be protected)
 
Raka, presumably you will be using four amplifiers to bi-amp a two-way speaker system. Will you be using seperate PSU's for each amp?

If so just build them as seperate amps.

For powering the OPA627 buffers, you could use one or two PSU's (I used one but it was only a two channel amplifier). Either way, connect the ground connections from each buffer to the signal ground star of the corresponding amplifer and you should be OK.

However, I think that if I was doing a bi-amped system, I would use an active preamp rather than four buffers!
 
I'm not sure if I've understood well your post...
I'm not going to use four buffers, just one for each channel.
After the RCA's I'll fit the opa (one ps) that feeds the high and mid-low amps (two more PS's) each amp with a passive filter in it's input. Each amp will have it's own transformer and bridge.
Then I start thinking about the grounding scheme. Starting with the buffer, I'll have a neutral for the signal and a neutral from the ps.
The woofer amp has his own supply, so that means that there are two more grounds. There is also the tweeter amp, so two more gnds to consider...
Should I join all the power supplies gnd (tweeter+woofer+opa) in a star point (and take the speaker lead from here) and do the same with the signal gnds? Or should I join the power gnd of the opa PS with the signal gnd before going to the power amps?

I have the file with the wiring, but I don't know if the software allows to export as a jpg. I'll try a hand drawing if I can
 
Thanks for the link but the document is not available!!

If I think as pre+powerX2 as separate, not integrated, I understand very well the grounding, but as integrated...

As maybe you are starting to think that actually I don't build anything, this is the speaker that is going to be biamped

Do you like it?
 

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Raka said:
I'm not sure if I've understood well your post...
I'm not going to use four buffers, just one for each channel.
After the RCA's I'll fit the opa (one ps) that feeds the high and mid-low amps (two more PS's) each amp with a passive filter in it's input. Each amp will have it's own transformer and bridge.
Then I start thinking about the grounding scheme. Starting with the buffer, I'll have a neutral for the signal and a neutral from the ps.
The woofer amp has his own supply, so that means that there are two more grounds. There is also the tweeter amp, so two more gnds to consider...
Should I join all the power supplies gnd (tweeter+woofer+opa) in a star point (and take the speaker lead from here) and do the same with the signal gnds? Or should I join the power gnd of the opa PS with the signal gnd before going to the power amps?

I have the file with the wiring, but I don't know if the software allows to export as a jpg. I'll try a hand drawing if I can

I think it will be easier to visualise if you start from the basic goal to avoid that any wire carries a (large) current that could appear as an input signal to some stage. For instance, if the speaker return wire runs to one gnd point, and the supply to another, then it matters were you ground the input stage ground or screen, because there may be a difference between the ground points seen at the input.

In the extreme, what you would want is ONE starpoint where all gnd wires come to, supply transformer midpoint, supply cap gnd leads, speaker return, feedback network gnd point (VERY sensitive!), input lead screen gnd, input stage gnd from PCB, you name it. This often is not practical, but at least all supply-related gnd wires (xformer, caps, speaker return) should go to one point.
For the low-current wires that normally are coming together on the PCB like input gnd, feedback network gnd, they can run together through a single wire to the starpoint.

This principle holds for bridge, SE, push-pull, class A whatever. You just have different numbers of wires.

Jan Didden
 
Raka said:
Should the GND of the OPA power supply go to the signal GND of the buffer, or to the main GND star power gnd?
Hi Raka,

If the opamp is configured as a buffer, it doesn't have its own signal ground connection. So, the question is should its power ground be hooked to the main power ground or to the signal ground, and I guess your question assumes that the connection to the signal ground would be somewhere close to the opamp itself.

Jans’s suggestion is generally excellent and you’ll find similar recommendation in the Analog’s famous app note 202 (page 6, figure 10). From this point of view the answer to the question is: to the main power gnd.

However, line stage opamp (and its supply) operates at low currents and it is less critical about this than the power chips are. Also, since the opamp is in unity gain mode, it won’t amplify the errors. So, I’d say, wherever you hook the line opamp’s power ground, it would be probably OK. Note that there can still be some differences soundwize.

Pedja
 
This is the layout I've thinking about, it's semi-p2p because all is mounted in a veroboard.
The actual values are still unknown, because I haven't measured the in box responses of the drivers, but I think you will recognize all but the filter modules, that are going to be switchable passive filter modules, with 3 pins: input, gnd and output.
It's a pcb express file.
What do you mean "with feedback network gnd"?

PS: Well, maybe two beers ;)
 

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Raka said:
[snip]What do you mean "with feedback network gnd"?

PS: Well, maybe two beers ;)

The feedback network ususally consist of a resistive divider (possibly with some phase correction caps) of which one leg is connected to ground. If you think about it, the feedback signal is a fraction of whatever the network is connected to (usually the amp output) and gnd. Choose your gnd point wrong, and you have a different amplifier! So, not only should you choose the output connection directly at the outgoing speaker line (and not, for instance, somewhere on the output stage emitter resistors) but the gnd should come from the common point of speaker return, input stage and input cable screen. Because of the low currents in the input and screen, ususally you can get away by combining the feedback gnd, input gnd and screen gnd on the PCB and running a single wire to the speaker return/supply start ground. But is is not optimal.

Jan Didden
 
Carlos,

What do you mean each side? are very close to the chip and I will feed with batteries (maybe the LMs also). Chip? The cheapest I can get: an OPA627 :clown:

Jan,

Thanks for your comments, but I still have some doubts. What do you mean with "outgoing speaker line"? Do you mean the connector itself? Do you have any drawing to explain those concepts?

PS. Ok, ok, three beers...
 
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