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Old 30th January 2013, 07:22 AM   #21
jake01 is offline jake01  United States
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Quote:
Originally Posted by Mooly View Post
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

Thanks for your help!! The 18 V are all over the place, the 20 V was going to be hard to find, so that great to hear. Again, thanks!!
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Old 30th January 2013, 07:24 AM   #22
Mooly is offline Mooly  United Kingdom
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Your welcome

Well here's a smaller 60va with 11% regulation.
MCTA060/18 - MULTICOMP - 60VA TOROIDAL 2X18V | CPC From This Range

You'll be fine with an 18-0-18 (and that has to be what the original was). Its a case of seeing what is available in your area and that fits.
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Old 1st February 2013, 10:22 AM   #23
JMFahey is offline JMFahey  Argentina
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1) agree and add: check the datasheet for LM3886.
They are capable of driving 8 or 4 ohms but with different acceptable rail voltages.
If you know you have "50W" , now you need to check speaker ohms and you can choose which one is needed.
Of course, knowing expected rail voltage, AC value is 0.707 times that.
Just to play it safe, pick 1 VAC less per winding, if available.
You won't hear any difference and it's better to err "low" than "high".

2)
Quote:
BTW it looks to have 3 sets of primaries in a config I have not seen before:
(Lead 1, 2 or 3 selectable via jumper)
Lead 1 + Lead 4 = 230 V
Lead 2 + Lead 4 = 120 V
Lead 3 + Lead 4 = 100 V

I wander if this design is what is responsible for the high rate of failure in these transformers..
No, it means it can be used in
230V: Europe/Latin America/ 50% of Brazil/Australia/most of Asia/etc.
120V: USA/¿Canada?/50% of Brazil
100V: Japan

By itself, that's not a problem.

OEM supplier quality is the real one
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Old 1st February 2013, 12:35 PM   #24
FoMoCo is offline FoMoCo  United States
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In my opinion, I'd go with an 18 volt secondary with a higher VA rating.

The 50VA rating probably contributed to the failure. Andrew is right when he states that 50VA will power a 50W amplifier, until being overdriven.

I would assume that it's going to be overdriven to a square wave output to be safe. This might be overkill, but, then again, you've already seen a transformer failure.

A quick calculation shows that 100VA would be close if delivering a square-wave to a 4 ohm load.

Since you shouldn't be able to tolerate that amount of distortion, the 100VA transformer should give a margin of safety and allow you to select an off the shelf that's close to that rating. For instance, 90VA would also be close enough.

I would not try to push my luck with a 20 volt transformer. What about high line conditions? The transformer will live. The LM3886 will too. But, the protection circuit will most likely limit your undistorted power to less than you'd get with an 18 volt transformer.
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Old 1st February 2013, 01:44 PM   #25
Mooly is offline Mooly  United Kingdom
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The whole unit could be a compromise though.

Maybe the heatsinking is insufficient for sustained higher power if using rails that don't "droop".
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Old 1st February 2013, 03:29 PM   #26
JMFahey is offline JMFahey  Argentina
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Quote:
The whole unit could be a compromise though.

Maybe the heatsinking is insufficient for sustained higher power if using rails that don't "droop".
Sounds very reasonable to me.
But fomoco's suggestion of using a new transformer with somewhat reduced rated voltage sort of amounts to "pre-droop" to me , and should take care of it.

Otherwise it's like tossing the coin between "how will it fail today? ... burnt chipamp or burnt transformer?"

Oh well.

A classic Service Tech principle is: "Repair it, don't reegineer it" .... but I think *this* is one good exception to the rule.
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Old 1st February 2013, 09:41 PM   #27
FoMoCo is offline FoMoCo  United States
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Quote:
Originally Posted by Mooly View Post
The whole unit could be a compromise though.

Maybe the heatsinking is insufficient for sustained higher power if using rails that don't "droop".
Good point. But... If that's the case, the 3886 will just shutdown. Those things are hard to kill. Transformer lives... All he's out is the price difference between the two transformers.
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Old 7th February 2013, 03:13 AM   #28
jake01 is offline jake01  United States
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Quote:
Originally Posted by FoMoCo View Post
In my opinion, I'd go with an 18 volt secondary with a higher VA rating.

The 50VA rating probably contributed to the failure. Andrew is right when he states that 50VA will power a 50W amplifier, until being overdriven.

I would assume that it's going to be overdriven to a square wave output to be safe. This might be overkill, but, then again, you've already seen a transformer failure.

A quick calculation shows that 100VA would be close if delivering a square-wave to a 4 ohm load.

Since you shouldn't be able to tolerate that amount of distortion, the 100VA transformer should give a margin of safety and allow you to select an off the shelf that's close to that rating. For instance, 90VA would also be close enough.

I would not try to push my luck with a 20 volt transformer. What about high line conditions? The transformer will live. The LM3886 will too. But, the protection circuit will most likely limit your undistorted power to less than you'd get with an 18 volt transformer.

Good idea, I too have been thinking about upping transformer size, and although the heat sink is rather massive, I am limited here by the size of the bracket that holds the transformer inside the monitor. The spacing is quite tight and I would have to get creative with new placement, something I unfortunately don't have the time for. This is too bad, considering that the 50 VA is driving not one, but two LM3886's.

On the other hand, each chip amp is wired right into a driver with no passive XO components in between. Moreover, a switch on the integrated active XO lets me choose a low end cut off frequency (flat, 60 Hz, 80 Hz) which I always use to limit the bass going to the woofer (and the corresponding LM3886) so my hope is that 50 VA will be sufficient.

Thanks for your help everyone, this makes me want to run across more of these "broken" units.
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Old 22nd February 2013, 03:47 PM   #29
jake01 is offline jake01  United States
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The new transformer finally came in (a Triad part from DigiKey), and although it's spec'ed identical to the original part, it weighs only 75% as much (32 oz for the original, and 24 oz for the Triad), not sure why I WANT to know why this is, but I really do..
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Old 22nd February 2013, 03:51 PM   #30
jake01 is offline jake01  United States
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And a secondaries wiring question - the circuit uses each of the two secondaries per LM3886. What about combining the secondaries in parallel instead, and feeding both LM3886's from the same rails (or is it rail)? Would this not work better, given that the tweeter amp will use less current, and pooling all current together is a better way to load balance?

Thanks for your input!!

Last edited by jake01; 22nd February 2013 at 03:54 PM.
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