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Old 24th February 2009, 08:13 PM   #11
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Two more comments:

1.
Note for my upper comment: Let's change wording and say Earth instead of GND. GND can be anything and is often used for the negative node of the mains rectifier. wg_ski already highlighted that this is not at all identical with Earth.
Typically this node shows one negative mains half wave vs. Earth.


2.
I guess even without considering safety standards it is making sense to have isolated the parts which are protected by Earth-Leakage-Sensing.
If not - the system will shut down whenever touched at a critical area...
And probably concert pros will rate a system shut down the second worst thing to happen. (Close to the worst thing, which probably is a spectacular system meltdown.)
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Old 24th February 2009, 09:59 PM   #12
Eva is offline Eva  Spain
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When speaker wiring and voice coils can carry 120V AC with respect to earth, they have to be considered as mains wiring and appliances anyway, like in mains powered tools (for professional use too, or maybe not, consider a hairdryer for example). One isolation layer must be there, the other is provided by the ground fault protection.
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Old 24th February 2009, 10:33 PM   #13
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...well, our beloved gold plated Bi-Amping screwing speaker terminals seem to be not adequate for insane power applications...
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Old 25th February 2009, 05:40 AM   #14
Eva is offline Eva  Spain
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Hehehe.

I never liked this stuff anyway Exposed connections to power nodes are a risk. I have always used mains style wiring for speakers

Not to mention that it's cheaper...

But most people reading this forum use those fancy binding posts
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Old 25th February 2009, 01:04 PM   #15
iand is offline iand  United Kingdom
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Quote:
Originally posted by wg_ski
The problem with non-isolated supplies is that your signal ground (which you can and will make contact with) may be at some potential that's well above "earth". The "low" side of the DC supply is the (-) side of the rectifier and that's NOT at ground no matter which way you plug it in. Earth that by accident and you blow breakers, rectifiers, or maybe your fingers. The only kind of supply that won't cause this trouble is a full wave bridge, split supply that's +/-169 wrt "earth". Even with that, just one unconnected (floating) neutral somewhere - and don't dare say it won't happen - and you can get on a world of hurt from "signal ground".
If by "signal ground" you mean the potential that the input connectors, screens and exposed metalwork connect to, then this will be 0V (earth) just like any other amplifier -- the isolation barrier is between this and the output stage.

The only places the high potentials appear is inside the amplifier output stage and at the speaker terminals; this is no different to a 5000W/4ohms amplifier (or any valve amplifier) with a high-voltage isolated supply -- in either case, poking around inside may be fatal.

As Eva points out, such an amplifier with a non-isolated supply but with ground fault detection is actually safer for the user (or somebody poking around inside) than a conventional amplifier...

Ian
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Old 25th February 2009, 02:50 PM   #16
bver100 is offline bver100  United Kingdom
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or indeed accidentally coming in to contact with the earth referenced mains isolated o/p of a mega power amp.

from an approvals point of view i am pretty sure that as long as the output was 'touch proof' and appropriately labelled that i could get approval without any isolation BUT users might be required to only connect approved speakers.

this whole area is grey, just like the forums policy. speaker manufacturers never submit their passive products for safety testing and their customers never require it. while the manufacturers of the amplifiers have to do all sorts of things to comply with the regulations while only ever being a part of the finished system. the situation arises because the world has moved on from 50W amplifiers and the regulations / mindsets of users has not.

having said that, (and not being a speaker manufacturer donít know) there may well be a safety standard that should be applied to speakers that are rated to run from a dangerous voltage and its just that said requirement is always ignored !

BTW i don't think that an RCD can count as one level of insulation ... having had experience with non isolated active speakers, i too am intrigued by what Eva is getting up to ....
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Old 25th February 2009, 04:24 PM   #17
iand is offline iand  United Kingdom
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Quote:
Originally posted by bver100
or indeed accidentally coming in to contact with the earth referenced mains isolated o/p of a mega power amp.

from an approvals point of view i am pretty sure that as long as the output was 'touch proof' and appropriately labelled that i could get approval without any isolation BUT users might be required to only connect approved speakers.

this whole area is grey, just like the forums policy. speaker manufacturers never submit their passive products for safety testing and their customers never require it. while the manufacturers of the amplifiers have to do all sorts of things to comply with the regulations while only ever being a part of the finished system. the situation arises because the world has moved on from 50W amplifiers and the regulations / mindsets of users has not.

having said that, (and not being a speaker manufacturer donít know) there may well be a safety standard that should be applied to speakers that are rated to run from a dangerous voltage and its just that said requirement is always ignored !

BTW i don't think that an RCD can count as one level of insulation ... having had experience with non isolated active speakers, i too am intrigued by what Eva is getting up to ....
If speakers would need approval for isolation from the voice coil to any parts that the use can potentially touch like the chassis, they'll still need this for either type of amplifier (non-isolated or high-power isolated) -- like I said, it doesn't matter where the volts come from...

You're absolutely correct that the mindset of users (and regulators, and some members of this forum :-) haven't moved on from the days of much lower-power amps, hence the "mains is dangerous so you must use isolated supplies" attitude.

Maybe we should all hope that the regulators don't catch on, otherwise high-power amps may have to meet much more restrictive regulations and you won't be allowed to use one without wearing rubber gloves and wellies :-)

Of course the issue isn't that mains isn't dangerous, it's that a high-power amp could kill you just as easily -- maybe more so, users expect to be able to touch speaker terminals and live...

Isn't all this just the same as the requirements for double insulated (i.e. not earthed) tools, which say the everything inside must be insulated and the (usually plastic) case provides the second layer of protection? (they have to be safe even if the case breaks)

Ian
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Old 25th February 2009, 06:21 PM   #18
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Quote:
Originally posted by iand
...
Isn't all this just the same as the requirements for double insulated (i.e. not earthed) tools, which say the everything inside must be insulated and the (usually plastic) case provides the second layer of protection? (they have to be safe even if the case breaks)

Ian
Double isolation does not mean at all that everything inside is additionally isolated.
Also it does not mean that you cannot touch anything dangerous if the plastic cover is breaking. The wording in the safety standards is not self explanatory.
For double isolation it is sufficient to ensure a certain material thickness, creepage and clearance. At the same time double isolation may require triple layers of isolation if the isolating material is very thin...

Mechanical ruggedness is of course also an important topic.
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Old 25th February 2009, 08:18 PM   #19
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And what are the technical benefits of non-isolating a high power class D amp?

I assume that the amp's outputs are close to earth at idle, hope this is undisputable.

In the end you need around +-100 to +-150V for the full bridge audio PWM stage, capable of several dozens of amperes at least. As far as I understand to achieve that you may use either:

1. boost PFC, full bridge transformer coupled H-bridge PSU, audio amp (isolated)
2. active front end (AFE) PFC for symmetrical rails, symmetic bucks, audio amp (complicated)
3. AFE PFC and duty cycle limited audio amp (you'll need 1000V switching transistors for audio)

So why bother? My feeling is that this is an idea inspired by the synchronous motor driver circuits, which use nonisolated approach for obvious reasons, one can quarrel taht a speaker actually is a motor, but...
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Old 25th February 2009, 08:46 PM   #20
Pafi is offline Pafi  Hungary
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Quote:
I assume that the amp's outputs are close to earth at idle, hope this is undisputable.
Not neccessary. Supply rectifier can be an ordinary graetz.
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