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Old 13th December 2014, 10:41 PM   #1
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Default toroidal mechanical noise

I am building a tube microphone preamp, schematic here: http://www.gyraf.dk/gy_pd/g9/g9_sch.gif

The PSU has two toroids (80VA Nuvotem encapsulated). Both work and measure fine when hooked to the mains power, but in the PSU configuration the 12 V makes a continuous buzzing sound. It is not from the PSU as same happens when nothing is hooked to the trafos except mains power. It is not that loud but one can hear it when its quiet and the noise doesn't sit well with the purpose of the gear.

I also tried each secondary besides the parallel, and same. Also from the other secondary of the 15V, all combos make the buzz.

Is it something wrong with the 12V trafo, or will it go away once the amp is completed with the tubes drawing (maybe adding some positive effect from the closed chassis?) I only tested them "in the air". Moving them around doesn't help.

Thanks!
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Old 14th December 2014, 01:02 AM   #2
roclite is offline roclite  United Kingdom
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Hi,

I have had a similar problem and my post may help you:

Keces DA-131 toroid (torid?) noise tamed!

You need to be clear exactly where the noise is coming from. If it is the toroid, then it could be caused by mechanical vibration of the core at your mains frequency. This is a quality issue and an encapsulated toroid will be very difficult to quieten using the method I used on my DAC. Make sure both transformers are not interacting together (keep well apart). Sorry I cant offer any more direct help. Good luck
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Old 14th December 2014, 01:10 AM   #3
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Thanks. It is coming from the toroid, and only when fed from the 15V. If I feed it from the mains it is quiet. It does it even if placed about 1 foot from the other. It is free-in the hand, trying to squeeze it doesn't help. When mounted it is going to be bolted on the top of the other with several leather gaskets in between but I have the feeling that won't help. Maybe a proper electrical load will help, or simply replacing it?
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Old 14th December 2014, 01:55 AM   #4
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Some toroids do this. In fact I might suggest that in general purpose power toroids, this is not at all uncommon. In most applications it is not an issue. For hifi gear it is.

You will find a number of people in the forums who have tried using DC blocks on the mains. This might work for you but adds complexity and is something I have not got into.

In my experience the only (other) solution is to buy an alternative transformer. Which I have done more than once.
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Old 14th December 2014, 09:28 AM   #5
JMFahey is offline JMFahey  Argentina
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I'd get another one.

Not a fan of the "line DC " theory, or to be more precise: if a transformer can't be connected to Real World mains voltage without trouble, then something's wrong, either design or construction (or both) but anyway: the buzzing one here is not directly connected to mains but to floating 15VAC supplied by another transformer, which should act as a natural high pass filter.

Just looping the loop and to leave no stone unturned: that transformer ***might*** be too tightly specified, it can stand 220V on the 220V winding but not 15V on the 12V one.

Just for kicks connect it to any 12V transformer you have available and check whether it still buzzes or not.
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Old 14th December 2014, 09:36 AM   #6
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I think this is correct, putting 15V into the 12V winding is probably saturating it.
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Old 14th December 2014, 10:52 AM   #7
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The first version of the schematic had 15V on both trafos but in many cases there wasn't enough HT voltage so the designer changed the second trafo. I might try with a 15V instead of 12 as currently I seem to have plenty of juice, without load it goes to a whopping 400V !

But first will finish the build as is and see if the noise gets in the signal path. That would be a bummer. The mechanical part can be less of a problem if I place the preamp far enough from the recording spot.
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Old 14th December 2014, 11:19 AM   #8
AndrewT is offline AndrewT  Scotland
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Buy the correct B+ transformer.
One that goes from mains 220/240Vac to your required Vac without using a stepping up back to front contraption.
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Old 14th December 2014, 11:40 AM   #9
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Thanks, good idea. This one for example. 200mA seems to be enough, and has the exact voltage I am getting from 15 to 12V: 275

Amp Maker: Guitar amp kits and parts :: Power transformers :: 0-190-275V/6.3V toroid power transformer
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