TAC Magnum - DC current draw

[Siegfried Meier]

Hey all!

So we purchased a new Blue Dog supply for our Magnum and I’ve noticed it’s drawing 6 amps of DC current. The transformer is resonating, and the unit gets quite hot after a few hours of use. I’m told it shouldn’t be drawing this much, but the question is why…? I’ve fully recapped all the channels, so unless the meter bridge is causing something I don’t really know what else to look for?

Hoping someone has a suggestion…thanks!

Sig

 

[radardoug]

It would have been smart to pull one channel and measure its current draw before buying. How many channels does it have?

 

[jensenmann]

The TAC 850 PSU is rated at 10A per rail. So 6A are within the capabilities of the original PSU.
The blue dog PSU is a SMPS. There should be no transformer ringing since there´s no transformer inside. A friend who bought my TAC Matchless replaced the original supply with a blue dog to get rid of the fan noise. It worked flawlessly ever since with no noise, neither in the audiosignal nor mechanically. That raises the question if your blue dog supply is able to deliver enough current for your desk. How much current is it rated for?
Did you swap chips in your desk? E.g. changing all TL072 for OPA2134 easily doubles the currentdraw of a single channel.

The meterbridge has it´s own voltage regulators. IIRC there are tantalum caps in the regulator circuit, across the rails and in the timeconstant circuit of every meter. Before completely failing they will draw significant current since they turn into a short. Have a look in there first.

 

[gyraf]

..pull one channel and check that it drops 1/n'th part of the current? That will approximate an answer for "is something wrong"

 

[Siegfried Meier]

The TAC 850 PSU is rated at 10A per rail. So 6A are within the capabilities of the original PSU.
The blue dog PSU is a SMPS. There should be no transformer ringing since there´s no transformer inside. A friend who bought my TAC Matchless replaced the original supply with a blue dog to get rid of the fan noise. It worked flawlessly ever since with no noise, neither in the audiosignal nor mechanically. That raises the question if your blue dog supply is able to deliver enough current for your desk. How much current is it rated for?
Did you swap chips in your desk? E.g. changing all TL072 for OPA2134 easily doubles the currentdraw of a single channel.

The meterbridge has it´s own voltage regulators. IIRC there are tantalum caps in the regulator circuit, across the rails and in the timeconstant circuit of every meter. Before completely failing they will draw significant current since they turn into a short. Have a look in there first.

So I get 6 amps DC draw on the positive side (which is the +17v rail for the audio and the meter bridge, I believe) and 3.3 amps DC on the negative rail...is it possible the light meter bridge draws the same amount that the console would draw? It would make sense...there's a heck of a lot of components and massive chips driving these meters...

This is only the 26 channel console - everything was recapped a few years back (and not used much), except for the meter bridge. I'm being told the 6 amp draw is too much for this size of Magnum, but maybe the tantalums you mention are the problem...

Yes, Tim's PSU's are SMPS but it's definitely being overloaded, and there are small transformers in them (these are resonating and making a hum sound physically, as a result of overload - 6 amps is too much for his 100 watt modules...although he says he can provide 150 watt if need be)...

 

[radardoug]

So now you know the currents, you need to buy a supply that can do the job, preferably with some reserve.

 

[jensenmann]

Those metering modules look different from what I remembered. So I´m more guessing than knowing. But if you can measure the current draw on the + rail, then unplug the meterbridge and see how much current the metering pulls, ideally with the metering regulator plugged in and then plugged out. That way you can find out which part of the circuits pulls how much current.

Additionally not only the meterbridge runs off the + rail, but also the logic circuits in every channel. This could be another source for eating current. Jakobs advice might be the right way to check that out. Measure the current draw of the + rail and pull one channel after another and compare their currentdraw.

 

[Siegfried Meier]

Those metering modules look different from what I remembered. So I´m more guessing than knowing. But if you can measure the current draw on the + rail, then unplug the meterbridge and see how much current the metering pulls, ideally with the metering regulator plugged in and then plugged out. That way you can find out which part of the circuits pulls how much current.

Additionally not only the meterbridge runs off the + rail, but also the logic circuits in every channel. This could be another source for eating current. Jakobs advice might be the right way to check that out. Measure the current draw of the + rail and pull one channel after another and compare their currentdraw.

That makes a lot of of sense man, appreciate it!! We have a higher current Blue Dog coming with 150 watt modules - should provide over 8 amps of current he says...the smaller Blue Dog, I'll just keep and use to power some 500 racks or another console, nothing lost. Thanks everyone!!
Sig