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


tv

If you sprinkle when you tinkle, please be neat and wipe the seat.

robovox

Re: Cleaning up signal on a cheapo SMPS and using for audio
« Reply #2 on: May 31, 2010, 11:35:50 PM »
Check out Linear Technology's AN-101 by Jim Williams

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1002,C1243,P1981,D11877

Good stuff on cleaning up switching supply spikes.
I have an electric grandpa bed that talks to me while I sleep.
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Samuel Groner

    Zürich, Switzerland
  • Posts: 2935
Re: Cleaning up signal on a cheapo SMPS and using for audio
« Reply #3 on: June 01, 2010, 02:46:40 AM »
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


Re: Cleaning up signal on a cheapo SMPS and using for audio
« Reply #5 on: June 03, 2010, 07:04:59 AM »
Cheers for the responses. All of that should be really helpful. I'll let you know how I get on.

Regards
Dave

earthsled

Re: Cleaning up signal on a cheapo SMPS and using for audio
« Reply #6 on: December 16, 2010, 12:00:20 PM »

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!


JohnRoberts

Re: Cleaning up signal on a cheapo SMPS and using for audio
« Reply #7 on: December 16, 2010, 12:49:20 PM »
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
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

PRR

Re: Cleaning up signal on a cheapo SMPS and using for audio
« Reply #8 on: December 16, 2010, 08:05:26 PM »
> 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?


earthsled

Re: Cleaning up signal on a cheapo SMPS and using for audio
« Reply #9 on: December 18, 2010, 02:45:15 PM »

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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