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Old 24th January 2013, 07:02 PM   #11
Mooly is online now Mooly  United Kingdom
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Hmmm... you can't get that output from a 50 va tranny.

OK, here's an 85 by 41mm that is 100va. The figures 50va and two times 50 watts RMS just don't add up. Voltage wise it could reach the required swing for 50 watts RMS into 4 ohms on an 18 volt tranny.

MCTA100/18 - MULTICOMP - 100VA TOROIDAL 2X18V | CPC

I'm pretty much out of time for tonight

(One thing you can check. Is that 50 watts RMS quoted as being into 4 or 8 ohms... or any other figure)
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Old 24th January 2013, 08:30 PM   #12
sreten is offline sreten  United Kingdom
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Hi,

An active speaker never runs at 50W plus 50W continuous at clipping
with music, I wouldn't be surprised to see a 50VA used for a claimed
50W+50W active speaker, sadly also claiming 100W+100W (peak).

rgds, sreten.
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Old 25th January 2013, 09:35 AM   #13
AndrewT is offline AndrewT  Scotland
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A 50W amplifier will run satisfactorily from a 50VA to 100VA transformer.

Music systems that run at output powers well below maximum can take advantage of the amplifier's ability to work with a very wide range of transformer sizes.
This applies very much so with all domestic listening equipment, where average output power is usually well below 1/10th of the maximum and often less then 1/100th of the maximum output.
This would lead to "adequate" music reproduction from a pair of 50W amplifiers running from one 50VA transformer.
But turn the volume up such that the average power drawn from the amplifier is more than 1/10th of the maximum capability and the transformer will begin to heat up. There's the problem.
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Old 25th January 2013, 07:10 PM   #14
jake01 is offline jake01  United States
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Originally Posted by AndrewT View Post
A 50W amplifier will run satisfactorily from a 50VA to 100VA transformer.

Music systems that run at output powers well below maximum can take advantage of the amplifier's ability to work with a very wide range of transformer sizes.
This applies very much so with all domestic listening equipment, where average output power is usually well below 1/10th of the maximum and often less then 1/100th of the maximum output.
This would lead to "adequate" music reproduction from a pair of 50W amplifiers running from one 50VA transformer.
But turn the volume up such that the average power drawn from the amplifier is more than 1/10th of the maximum capability and the transformer will begin to heat up. There's the problem.
This makes me wander if I should go with more than 50 VA on both units, upgrade the working one as well, since I will have both open and ordering parts anyway. On the other hand, each of the LM3886's are directly connected to either the tweeter or the mid-bass, with an active XO before the amp, so perhaps going beyond the 50 VA is overkill for this application?

I suppose ultimately I am constrained by the size of the bracket to which the transformer is bolted, the design of the entire unit is somewhat compact..

By the way, Dynaudio offered to send me a replacement part for about $100 plus shipping, and will take only 3 weeks or so, and if I am lucky, it will last as long as a normal transformer. I love the sound of their drivers, but this high rate of transformer failures smacks of design compromise.
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Old 26th January 2013, 06:23 AM   #15
Mooly is online now Mooly  United Kingdom
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As you have have access to a working unit then you really should measure the AC voltage from the tranny as 100% confirmation.

We've made a best guesstimate... and there's no problems with the figures we've used, but it would be nice to know for sure, particularly as you actually have a working one. For example, did they really push to the absolute limits and use a less common 22-0-22 tranny.
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Old 26th January 2013, 07:16 AM   #16
Baldin is offline Baldin  Denmark
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Yes, open op the other unit and do a measurement of the AC voltage.
Best is actually to disconnect the transformer on the secondary side, to get the non loaded voltage rating.
If you end up with needing a voltage which is not available, you can use one with a little less voltage rating, and make a few turns yourself, connecting these ins series with the secondaries to get the desired voltage (that is of course if it's a toroid )

What Dynaudio monitor are we talking about??
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Old 30th January 2013, 05:33 AM   #17
jake01 is offline jake01  United States
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Finally got a chance to open up the working unit. After disconnecting the secondaries, each reads 20.5 to 20.6 V. Does this mean a replacement with 20 x 2 will work? What about 22 x 2? Thanks!!!
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Old 30th January 2013, 06:16 AM   #18
jake01 is offline jake01  United States
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What Dynaudio monitor are we talking about??
It is BM5A mkII. I have been using B&W 604 with a large discreet amp, and the Dynaudio's sound a lot better to me. My room might have something to do with it, but I have taken then B&W's apart, and the tweeters used there are just a joke.
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Old 30th January 2013, 06:16 AM   #19
Mooly is online now Mooly  United Kingdom
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That sounds like an 18 volt tranny. Offload and the voltage rises by the "regulation factor" of the particular transformer. The one in my link was 8% so that would give around 20 volts off load. And that was a 100va tranny. Smaller one have worse (higher percentage) regulation.

Its an 18-0-18
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Old 30th January 2013, 06:20 AM   #20
jake01 is offline jake01  United States
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Originally Posted by Mooly View Post
Hmmm... you can't get that output from a 50 va tranny.

OK, here's an 85 by 41mm that is 100va. The figures 50va and two times 50 watts RMS just don't add up. Voltage wise it could reach the required swing for 50 watts RMS into 4 ohms on an 18 volt tranny.
Of course the stated specs were wrong, after measuring it my self, it is actually 85 x 25 mm, so that sounds about right.
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