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Old 27th August 2006, 08:30 PM   #1
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Default transformerless amps?

I am noticing in audiophile magazines and in catalogs, more and more transformerless amps.

Of course for size and cost, this approach is a dream come true.

But I have questions and reservations about such.

There is no isolation from the power mains for the driver!

These amps are being used in powered speakers. Most are class D, or some class G, and real high power. So the rectified line voltage could be about right.

But I still have reservations. You would have to provide isolation for the input signal.

But drivers, as I know, are not made to provide line isolation. I guess the enclosure could be build for isolation, and with a non-removable grill cloth?

Does anyone have feelings about this? Anyone seen what parts are used for the input isolation? Anyone looked real close at how such a enclosure is designed? Anyone know about the saftey legalities?

Anyone have links to any articles about this phenomenon?
Old 27th August 2006, 10:10 PM   #2
anatech is offline anatech  Canada
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Hi zenmasterbrian,
It's very simple. They lied. Otherwise this equipment would never get by UL and CSA.

They use a high frequency switching supply to reduce the weight (and cost) of a normal power transformer. They can improve line regulation this way as well. But there is still a power transformer in there. It's a smaller high frequency model.

This also saves on shipping costs.

The down side is that as your supply voltage goes down, the current draw goes up. That and EMI if it's improperly designed.

Old 28th August 2006, 05:23 AM   #3
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That is very interesting. I hope what you say is true.

I've seen a parts express plate amp.

I've also seen a brand of tower speakers that has its own little circuit board in with the subwoofer. The biggest stuff on it is a few capacitors, and two TO-3 devices on small heat sinks. It is about 3" by 5", 500W

I don't see enough there for a switching power supply of the type you describe.

Maybe you are right. If you have any links to articles or product adds or anything, I'd love to see it.

If anyone else has had any contact with such, I'd like to know about it.

I hope it is like you say, that the stuff is isolated.

( By the way, I have always felt that power transformers for isolation could be eliminated if we went to strictly polarized AC plugs, and made the neutral ground. Any thoughts? )
Old 28th August 2006, 05:33 AM   #4
ssanmor is offline ssanmor  Spain
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anatech is probably right, what you are looking at is an amplifier with switching power supply. As an example, look at the one we manufacture:
Old 28th August 2006, 05:52 AM   #5
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That looks real neat!

But I can clearly see the high frequency transformer.

The one I saw in a magazine looked about half the board size, no transformer, and 500W.

It was built into the subwoofer.

Could they really be unisolated?
Old 28th August 2006, 06:00 AM   #6
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By the way, I have always felt that power transformers for isolation could be eliminated if we went to strictly polarized AC plugs, and made the neutral ground. Any thoughts
And what if anything goes wrong ????? There doesn't habve to occur a very exotic failure and your in- and outputs are carring full AC. Just imagine what happens if the device is switchend on and the ground-line is inrterrupted.
Audio devices without proper mains isolation are and remaind a no-no !!!

There would be one solution that would make a seperate isolation transformer obsolete: If the PSU itself is an (isolated of course !) amplifier, like the "ampliverter".


Old 28th August 2006, 06:12 AM   #7
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Phase, this is kind of secondary, the idea that everything could eliminate isolation transformers if we used polarized plugs.

But say I have a standard amp with a 200w transformer.

Say sold state stuff shorts. So now the speaker outputs connect to this transformer. How much less serious is that than being connected to the power mains directly?? My power transformer could have a center tap grouned. The chassis could be grounded.

So I could get zapped of my amplifier transformer, just like of the mains transformer.

If what you mean is it is less serious because the appliance transformer is much smaller, so its core would saturate sooner. Then I follow you.
Old 28th August 2006, 07:18 AM   #8
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In the case of rectified 110 Volts AC this would be more than 150 volts DC ! Definitely more than your average Hi-Fi equipment rail voltages !
Now keep in mind that there are countries that don't use wimpy 110 Volt rails but 230 Volts .......

Apart from that, the situation that you described would cause DC voltage on an output. The situation that I described could make the chassis (and whatever is connected to it) carry mains voltage.

The small inconvenince of a transformer in terms of weight, cost and space is nothing compared to suffering, lwsuits and the like IMO.

Transformerless mains-operated audio equipment is a NO-NO !


Old 28th August 2006, 08:44 AM   #9
Eva is offline Eva  Spain
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I can't think of a reason why a big powered PA subwoofer could not use a class D amplifier fed directly with rectified mains and still get all the required safety approvals, provided that adequate isolation techniques are employed.

We have things such as "linear" optocouplers with feedback, signal transformers and differential amplifiers for level shifting of the input signals. Also, now we can manufacture wooferts with very stiff and inherently isolating plastic spiders. And furthermore, PA systems may be horn loaded, thus physically preventing any user access to the driver (even when the grill is removed, which should be already considered a "service" condition).

BTW: As long as we have proper fuses built into the system, they will blow when 350V DC are applied either to a faulty amplifier or to a 8 ohm voice coil.

Note that my latest project (now working and reliable) is just a full bridge class D amplifier powered directly from rectified 230V mains (350V DC) and capable of +-30A peak output (>8KW peak), alhough it's not intended for audio but for driving mains powered motors and the like.
I use to feel like the small child in The Emperor's New Clothes tale
Old 28th August 2006, 12:06 PM   #10
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big powered PA subwoofer
This one could be tolerated as an exception. But keep in mind that large PA equipment is usually operated by professionals. You'll never know what happens with consumer stuff operated by Joe Average. Keep in mind that it isn't possible to make any design foolproof since most fools are much too inventive.

I am of course aware that there was once consumer equipment that used rectified remains. Old tube radios for instance that used constant current filaments instead of 6.3 Volts (or 4 Volts or ....) that could be connected in series. These radios had the advantage that they could be operated from AC mains and DC mains. The latter was once quite common in earlier days.
Also old TVs used the same PSU principle. Especially the radios were responsible for some lethal accidents though often provoked by misuse.
I seriously doubt whether equipment like that would pass any certification nowadays.
And I think it isn't bad practice to build equipment in a way of which could reasonably be assumed that it would pass certification - even when it is just for personal use.



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