Cleaning up signal on a cheapo SMPS and using for audio

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brazilianwonderboy

Active member
Joined
Mar 16, 2009
Messages
30
Location
England
Ok, I've set myself the challenge of building a budget mic preamp that should cost me no more that £40 in parts. The actual preamp itself will be based around an OEP mic input traffo and an LME49710 Op-Amp, there will be phantom power as well. The power supply, however, is the thing that will pose the problems I think.

In past projects I have always built a linear supply based on the Greenpre PSU, but the cost of the transformer will test my budget too much. My intended solution is to get a cheapo 48V SMPS intended for IT equipment (The sort of thing you can get for laptops and that cost a tenner or so).

The question is, is it possible to add any circuitry on the power input of my preamp that will clean up any signal noise that comes from the switching supply? Any ideas will be welcome. Cheers.

Dave
 
http://www.google.com/search?&q=capacitance+multiplier

KISS & Just WorksTM
 
I second the LT application note; a capacitance multiplier is unlikely to work particularly well as it is hardly effective in the upper MHz region. There is however also the issue with magnetic induction from the switcher. This is not as easily addressed as voltage ripple.

Samuel
 
tv said:
http://www.google.com/search?&q=capacitance+multiplier

KISS & Just WorksTM

I came across this page searching for something else:

http://www.acoustica.org.uk/t/3pin_reg_notes2.html
 
I thought I might revive this thread since I'm interested in doing the same thing.

I'm wondering if it would be okay to use a voltage regulator circuit (like an LM317) to clean up a cheap SMPS. Would it be possible to use an SMPS in place of the power transformer for a linear supply?

Something like this...?
SMPS > rectifier > filter caps > voltage regulator

Thanks!

 
Unless you need precision voltage that the SMPS doesn't deliver it seems all you need is effective filtering.  Of course easier said than done, but not rocket science.

JR
 
> use a voltage regulator circuit (like an LM317)

Which has better crap rejection in the typical SMPS 20KHz-1MHz crap range, LM317 or an R-C network?

83y0b.gif
 
I see... Compared to the voltage regulator, the RC filter performs better for the high frequency ripple of an SMPS.

Thanks to PRR for posting the example circuit - I'll give this a try and post my results. I'm assuming the 5K resistor is a dummy load. Would the 100ohm + 100uF RC filter remain the same values for all voltages?

Also, I'm still confused about this... if the "crap" is in the 20kHz to 1MHz range, why does it become audible when used to power something like a preamp circuit?

Thanks for your help!


 
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