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Old 23rd February 2009, 02:04 PM   #1
iand is offline iand  United Kingdom
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Default are isolated supplies needed for high-power class-D?

I know this subject has been "banned" as dangerous, so can anyone explain why it's any worse from a safety point of view than the very high output voltages you can get at the speaker terminals anyway from a very high-power class-D amplifier?

-- Powersoft K20 is 5200W/4ohms which is 140Vrms which puts it into the same safety/isolation class as mains (maximum peak output is quoted as 225V/102A)

Or is the worry not so much voltage as the high currents you could get with an accidental short to ground?

(but is this any worse than shorting the output of an amp that can source more than 100A?)

Ian

P.S. Not intending to build one, just trying to see whether high-power amps are just as dangerous as non-isolated SMPS...
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Old 23rd February 2009, 05:59 PM   #2
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110V (or more likely 220v as used here and in europe) is enough to kill you, but it depends muchly on where it flows through your body. If it's just between two fingers of your hand, it may hurt, but is harmless.
If you touch speaker terminals, this is the most likely conduction path.

Flowing through your heart, 10mA is a deadly current, so you don't need supplies capable of delivering many amps. Of course current depends on the ratio of voltage to skin-resistance, so a higher voltage, or moist skin is more likely to facillitate a deadly current flow.

The danger of un-isolated PSUs is in the way AC power is delivered to your home. One side of the AC line is at ground potential (the Neutral).
If you happen to touch the live, either directly - or indirectly through a conduction path through the amplifier, the current path will be through your hand, to your feet (on the earth) via your chest. Or from one hand, via your chest , to the other hand that may be resting on an earthed metal cabinet, or similar.
That is the dangerous part.
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Old 23rd February 2009, 06:47 PM   #3
bver100 is offline bver100  United Kingdom
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My take on why mains is 'banned' and high o/p voltages not is that it is a bit of a knee jerk reaction, there are plenty of places more dangerous than the mains. After all, what about the mains earth referenced DC power rails used in valve (tube) gear or high power amps ?

These are potentially much more dangerous because (ignoring the obvious technical reasons, DC for one) folk are brought up to think of mains as dangerous and might well have their guard down when working on the secondary side of their gear.

Concerning speaker outputs, their connections have to comply with all the requirements of any other connector that carries high voltages as defined by the applicable standards. This is why you tend to see Speakons used on high power pro-gear.

Before anyone mentions that as some speakers have nice uninsulated brass binding posts on them so there must be an inconsistency, I should point out that this actually is a problem. If these products were submitted for safety testing (assuming that they could handle the power required for the voltage to be an issue) they would fail !
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Old 23rd February 2009, 11:01 PM   #4
iand is offline iand  United Kingdom
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Quote:
Originally posted by Steerpike
110V (or more likely 220v as used here and in europe) is enough to kill you, but it depends muchly on where it flows through your body. If it's just between two fingers of your hand, it may hurt, but is harmless.
If you touch speaker terminals, this is the most likely conduction path.

Flowing through your heart, 10mA is a deadly current, so you don't need supplies capable of delivering many amps. Of course current depends on the ratio of voltage to skin-resistance, so a higher voltage, or moist skin is more likely to facillitate a deadly current flow.

The danger of un-isolated PSUs is in the way AC power is delivered to your home. One side of the AC line is at ground potential (the Neutral).
If you happen to touch the live, either directly - or indirectly through a conduction path through the amplifier, the current path will be through your hand, to your feet (on the earth) via your chest. Or from one hand, via your chest , to the other hand that may be resting on an earthed metal cabinet, or similar.
That is the dangerous part.
I don't see the difference. If you touch live mains current can flow through you to earth. If you touch a speaker terminal carrying 200V peak referred to earth (which it will be) current can flow through you to earth.

There's nothing magic about whether the current comes directly from the mains or a high-voltage amplifier output, they can do exactly the same damage.

Hence my question...

Ian
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Old 23rd February 2009, 11:09 PM   #5
iand is offline iand  United Kingdom
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Quote:
Originally posted by bver100
My take on why mains is 'banned' and high o/p voltages not is that it is a bit of a knee jerk reaction, there are plenty of places more dangerous than the mains. After all, what about the mains earth referenced DC power rails used in valve (tube) gear or high power amps ?

These are potentially much more dangerous because (ignoring the obvious technical reasons, DC for one) folk are brought up to think of mains as dangerous and might well have their guard down when working on the secondary side of their gear.

Concerning speaker outputs, their connections have to comply with all the requirements of any other connector that carries high voltages as defined by the applicable standards. This is why you tend to see Speakons used on high power pro-gear.

Before anyone mentions that as some speakers have nice uninsulated brass binding posts on them so there must be an inconsistency, I should point out that this actually is a problem. If these products were submitted for safety testing (assuming that they could handle the power required for the voltage to be an issue) they would fail !
I agree this is why Speakons are used, but note that they're not rated to carry "mains" voltages in a touchproof manner like (for example) Powercon connectors.

So the obvious conclusion is that a non-isolated SMPS and a high-power amp like the K20 have exactly the same safety requirements; either both are OK, or neither are -- possibly because the authorities haven't realised that audio amps nowadays can put out enough volts (and amps) to kill you or start a fire in just the same way that mains can...

Ian

P.S. Obviously any exposed parts of the amp would have to be earthed for safety; all this means is that instead of the isolation barrier being in its normal location (between mains and all the amplifier circuits) it would be between the input/case section of the amp and the output section, which would have to be "floating" inside the case.
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Old 23rd February 2009, 11:15 PM   #6
Eva is offline Eva  Spain
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In fact, there is a new generation of high power class D amplifiers with non-isolated outputs coming, it may become the standard for powered speakers within a few years. I know because I'm working in that field.

However, considering the electronics skills of most people writting here, it's quite dangerous to promote these ideas through the DIY comunity. Most people already have enough trouble making a standard isolated amplifier work reliably without things exploding, particularly if it's class D...

Also, a very reliable circuit that disconnects the amplifier from mains instantaneously when earth leakage is detected is a *must* in these cases. This approach is actually a bit safer than any isolated high power amplifier because there is no way in which the output voltage, either the audio component or the mains one, can shock you with more than 5mA for more than 25ms (USA ground fault standard). It must disconnect from mains instantly if you happen to touch anything.

Input signal must be obviously isolated (quite challenging when you want to have good SNR without going digital), and proper isolation has to be provided between isolated and non-isolated parts of the amplifier too, including the case, which should be earthed. The amplifier should not be allowed to power up when safety earth is not present either.

I can't post about most of the work that I have been doing for the past 2 years, in part due to non-disclosure agreements with customers, and in part because it involves non-isolated circuits, but this stuff is powerful, efficient and compact, and it will be on the market soon
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Old 23rd February 2009, 11:24 PM   #7
iand is offline iand  United Kingdom
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Quote:
Originally posted by Eva
In fact, there is a new generation of high power class D amplifiers with non-isolated outputs coming, it may become the standard for powered speakers within a few years. I know because I'm working in that field.

However, considering the electronics skills of most people writting here, it's quite dangerous to promote these ideas through the DIY comunity. Most people already have enough trouble making a standard isolated amplifier work reliably without things exploding, particularly if it's class D...

Also, a circuit that shuts down power to the amplifier instantaneously when earth leakage is detected is a must in these cases.
I agree that suggesting the the amateur that they should play with non-isolated SMPS is not a good idea -- actually, suggesting that they should play with *any* mains SMPS is a bad idea...

An earth leakage detector is obviously a good idea -- so I wonder if the K20 has one to detect a poor sound engineer trapped with one hand on the speaker terminals and his feet in wet grass?

The point I'm trying to make is that for very high powers mains isolation in the SMPS is no longer needed, which opens up a whole load of possibilities (especially if you've got 3-phase available).

I'm assuming that the input signal will be digital (or with ADC on the input side) and DSP processed like in the K20, in which case CMRR on the audio doesn't matter, the only thing that crosses the isolation barrier is digital data.

Ian
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Old 24th February 2009, 02:29 PM   #8
bver100 is offline bver100  United Kingdom
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Hi iand

Your comment about Powercons and Speakons prompted me to check the clearances. I have just measured them and they are the same (as was my rusty recollection) on this basis both can be considered suitable for use up to the voltages specified by Neutrik from an external touch point of view. BUT, the Powercon has large webs that separate the connection side whereas the Speakon does not (I am talking about the chassis connectors, as far as I can see the plugs are very similar).

As an aside, I am sure that somewhere I have read that Powercons are not in fact certifiable for domestic use. this I don't understand because EN60065 definitely does cover the use of gear in a domestic environment and I have got loads of products through this standard that do use powercons ... BTW the Powercon carries a VDE mark, this particular (genuine) speakon has no safety marks.

Back to the thread ...it seems quite clear the safety of a device has little to do with whether the voltages that it runs from / generates are isolated or not from the mains, rather that no such potentials can be accessed by the user - this is why what Eva is proposing poses no greater intrinsic risk than say a food mixer. NOVICES SHOULD NOT TAKE THAT COMMENT TO MEAN THAT IT IS NECESSARILY SAFE EITHER. the devil is in the implementation.

From a DIY point of view where guys and girls will be inside gear the forums well intentioned blanket ban on the mains side of things clearly makes no practical sense if the gear uses or generates ground referenced voltages higher than the mains and these discussions are 'legal'.

Having said that, what are the forum owners to do ? they are in a difficult position. Mains is clearly dangerous but so are loads of other things ! are they to ban discussions about valves etc. too ?
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Old 24th February 2009, 02:41 PM   #9
wg_ski is offline wg_ski  United States
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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".
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Old 24th February 2009, 07:54 PM   #10
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Hi Wg,
if somebody is running his powerstage without mains isolation, he will have to implement the isolation of the input signal. Demanding task in analogue high quality systems, less headache with digital signals.
The minus node of the mains rectifier of course must be not accessable, i.e. ensured by safety connectors for the speakers. This safety isolation must be consequently realized throughout speaker wiring, speaker and cabinet design.


Hi Eva,
...what's the detailed background in the safety standards for such ground leakage detection shsut down/disconnection circuits?
From what I normally see in safety standards, it is required to ensure double isolation or reinforced isolation for high voltages. The idea of this is that you need things to go double wrong until it goes lethal.
From my understanding such a ground leakage detection would bring one safety level, but not two levels. Means - I can imagine that this might open the door for special regulations that accept to use basic isolation plus GND leakage sensing instead of double/reinforced isolation.
But having high voltage directly touchable with just a GND-leakage-detection is sounding like giving a very difficult task to your colleagues in the certification department...
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