Ground scheme for triple-mono power amp

[Audiofriik]

Hi all, I'm working on a project that includes a three channel power amp to drive my front pair plus a centre channel that can be used for home theatre sound. The rear channels of my 5.1 setup have their own discrete amp at the back of the room.I have three modified quad 405-II boards where among other things signal ground is no longer connected to the heatsink, three separate power supply boards sourced here and identical to: http://dl.dropbox.com/u/4130965/PSU.pdf and three 330VA dual secondary + centre toroidal transformers.The whole amp will be built into a single box (I might separate the toroidals out into a separate box) which in turn will be hidden in a piece of furniture (heatsink protruding).The question is what my options are with ground. As I have three centre taps, it's tempting to try to keep them fully independent as far as possible, connected only at the preamp, but I'm not sure if this is possible in practice. Chassis will be grounded to mains earth.

displays the ground connection of a TDA2030 amplifier. Is this something I could use (one such ground connection for each channel)?Many thanks for your input.Cheers!Marius

 

[Audiofriik]

To clarify: Basically a ground-loop filter, but can I do any additional decoupling between the 0-volt rails on each channel? Ie. One ground loop filter. The post filter 0-volt rail is split into 3 with caps linking:

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To clarify: Basically a ground-loop filter, but can I do any additional decoupling between the 0-volt rails on each channel? Ie. One ground loop filter. The post filter 0-volt rail is split into 3 with caps linking. I have a schematic but can't post it.

 

[mikely]

I don’t see why it wouldn’t be possible to keep 0V fully independent between the three amps and I think that would certainly be preferable. Each amplifier will have its own 0V connection at its input that will connect to the signal source from the pre-amp. Connect your speaker return to the common point of the transformer centre tap and the PSU reservoir capacitors (pad 33 of your PSU board).

I wouldn’t separate the transformers away from the power amps either. Better to keep power connections as short as possible and build the transformer, PSU and amp into one unit. Three mono blocs may be easier and more flexible than one large unit. That also lends itself to keeping grounds separate.

 

[Audiofriik]

Hi again and thanks mikely for your reply. Completely independent 0V would certainly be the ideal except that it leaves the whole amplifier vulnerable to ground faults that could potentially cause mains current to flow through my signal cable shield via the pre-amp and back to earth. I'm not even sure that completely floating ground is legal where I live. I did see a diagram for a very simple galvanic isolator which is used on boats:

The principle is similar to the diagram in the original question, but doubling up on the diodes raises the pass threshold from 0.6 to 1.2 volts and loosing the cap and resistor prevents frequencies over 100Hz from being allowed to pass.It's probably a better arrangement than my original suggestion, which I think is targeted at ground loops??

 

[felix]

Presumably you are driving these three amp boards from the same source?

The way to do it is to (1) bond the metal case (including heatsink if it is part of the case) to Mains Earth, for primary safety; and (2) link signal0v Input to the amp boards to this single Earth bonding point - link this directly from from each input socket, not from the 0v input at each amp circuit board. One link only for each channel.

While (2) sounds odd it actually minimises the 'earth loop' problem: any mains leakage current on the interconnect incoming passes to Earth and isn't made common-mode across the amplifier's circuit layout (the usual source of hum problems). And if any of the transformer windings develops a problem, again, the signal 0v is bonded to Mains Earth and the problem doesn't escape this amp box.

Do not tie the 0v centre tap of the transformers or the 0v link between each pair of reservoir caps (eg. the '0v star' for each amp) to Mains Earth instead. That's a very good way to get hum/buzz problems, even some manufacurers achieve it this way ...

If you set the amp up as I suggest and you still get buzz issues then the place to use the diode+cap breaker is in each of the three signal0v to Earth leads. Always, always leave your casework metalwork bonded to Mains Earth though. HTH.

 

[Audiofriik]

Sorry about the lack of formatting here. My browser seems to remove it:Thanks Felix. That was an unexpected solution, but I do see the logic behind it as the three signal grounds are interconnected at the preamp anyway. I have two questions:1. regarding:"If you set the amp up as I suggest and you still get buzz issues then the place to use the diode+cap breaker is in each of the three signal0v to Earth leads."You mention diode+cap breaker. Can you be more explicit regarding exactly what type of breaker to use here (values? Single, double diodes? etc)?2. regarding speaker ground. I imagine this should be connected to channel 0V and not to chassis, but worth asking just to be sure.I do have another ground challenge:I'll be using 1/12 Vellaman K470 boards for speaker protection and each of these has a common ground connection. I could obviously use three boards, dedicating one to each channel, but this would have space as well as cost implications and space will be at a premium.I read on another forum that a way to solve this is by wiring the individual 0V rails to the two channel K4700 ground via a 100R resistor. Do you have any comments given that the common channel ground to chassis point will now be at the rca inputs?Cheers!

 

[Chumpy]

Joy X 3 +++++++