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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
Noisy wall wart PSU
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hedefalk



Joined: Aug 29, 2017
Posts: 49
Location: Stockholm, Sweden

PostPosted: Wed Nov 01, 2017 2:35 pm    Post subject: Noisy wall wart PSU Reply with quote  Mark this post and the followings unread

I've built a +/-12V wall wart PSU using the MFOS schematics to do some breadboarding away from my main eurorack case.

There are only two differences against the MFOS schematic due to what I had in stock

* I have used four 4700uF electrolyts instead of six 3300uF ones.
* I have used 1N4007's instead of 1N4004 diodes.


I thought everything was fine and dandy until today when I used it to actually listen to a headphone amplifier. There's a very noticeable 50Hz humming. I tried to add around 10 470uF caps directly on my breadboard power rails and that cut it to around half.

So my questions are:

* Which is better, 12V vs 9V wall wart to drive the LM7X12's?

* If I am to add more caps, is it better to add them on the input side of the regulators or should I add more on the regulated output? Since there's only 1uF caps on the output side of the schematic it seems to me that the design suggests that the output should be really clean coming from the regulators? Can my regulators be low quality? (sourced from aliexpress)

* Can the 4007 instead of 4004 be an issue? I thought the only difference was the higher voltage rating.


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Blue Hell
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PostPosted: Wed Nov 01, 2017 2:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

9V (nominal) just is not enough .. you'd end up with like 11V2 before the regulators (under average load).

12 V would give about 15V5 so the regulators have about 3V5 to regulate with .. not very much either but would be within specs at least.

Then things also depend on the current being available from the transformer and the current needed from the circuits attached .. when you draw too much current for the transformer you will get hum too - and it wont be filterable.

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Blue Hell
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PostPosted: Wed Nov 01, 2017 2:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ow oops .. add 0.7 V to the before regulator voltages .. i had assumed full wave bridge rectification .. you use single diodes tho. Still 9BV is too low then though.
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JovianPyx



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PostPosted: Thu Nov 02, 2017 8:34 am    Post subject: Reply with quote  Mark this post and the followings unread

yeah, 9v AC is too low for a regulated 12v output.

As a rule of thumb, I choose a transformer's AC voltage to be the same as the regulated DC output I need. This provides a regulator input a few volts above the regulator's output. Most regulators will want 2 or 3 volts above the output to have the least amount of heat dissipation. And of course what Blue Hell said - current draw will cause the transformer's output to drop, so the transformer needs to be beefy enough to supply the needed current. Too wimpy a transformer will cause complete dropout or nasty mains frequency hum that gets through the regulator and cannot be filtered.

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hedefalk



Joined: Aug 29, 2017
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PostPosted: Thu Nov 02, 2017 10:45 am    Post subject: Reply with quote  Mark this post and the followings unread

Oh yeah, was going to check before I posted because I wasn't actually entirely sure, but yes, I was using a 9V transformer. Gonna try a 12V one, thanks!
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Sven



Joined: Mar 10, 2017
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PostPosted: Mon Nov 27, 2017 1:20 am    Post subject: Reply with quote  Mark this post and the followings unread

Hei,

i highly recommend to use fullwave rectification. Its more efficient and stable. Half wave rectification can lead to a significant voltage drop when you draw enough current. The 7812 regulator has a typical dropout voltage of 2V. This means you need at least 14V at the input for the regulator to work correctly. A 12V transformer may work when you draw little current, a 14V or 15V one would be better.

Greetings form Norway, Sven
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hedefalk



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PostPosted: Mon Nov 27, 2017 7:48 am    Post subject: Reply with quote  Mark this post and the followings unread

Sven wrote:
Hei,

i highly recommend to use fullwave rectification.


Can I really do that with a two pole wall wart if I need bi-polar outs?
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Sven



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PostPosted: Mon Nov 27, 2017 12:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hmm now I see. I misunderstood your schematic and thought you are using a transformer with 2 secondary windings. Your solution is OK I think, I dont like it and would rather build a proper symmetric supply with a transformer with symmetrical output or use to 9V batteries in series Smile

Are you sure you are using AC wall warts?
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hedefalk



Joined: Aug 29, 2017
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Location: Stockholm, Sweden

PostPosted: Mon Nov 27, 2017 1:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

Sven wrote:


Are you sure you are using AC wall warts?


Yes, but I was using a 9V AC wall wart so that was most likely the problem. I haven't had the time to try it out with a 12V ac wall wart yet.

The design is Ray Wilson's (MFOS) as is the name of this forum section, I only made small modifications for the components I had at hand.

Batteries are not an option - this is to be a lab psu for breadboarding eurorack modules and I need +-12V.
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