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Old 5th June 2014, 01:03 AM   #1
teleman is offline teleman  Norway
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Default SMPS for small signal analog circuits

I've not seen any small signal analog circuits like preamps, guitar effects etc driven by a SMPS units! Why is that? Are there any special reasons for this?
Is this safe & viable alternative to linear supplies, especially running these small analog audio units?

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Old 5th June 2014, 02:09 AM   #2
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For low wattage applications, most of the advantages such as size and efficiency that switch mode supplies have over linear supplies pretty much disappear, especially if you need a dual polarity source for opamps. For hobbyists the added complexity and expense generally aren't worth it.

Mike
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Old 5th June 2014, 02:29 AM   #3
teleman is offline teleman  Norway
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Thank you! I was thinking in terms of using a 24v/1.2A SMPS by splitting the output using BUF364 or similar voltage splitter to obtain +12v-0 -12v virtual earth supply! The maximum current draw of the circuit would be around 100mV/ rail! A dumb idea, is it not?
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Old 5th June 2014, 03:18 AM   #4
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Quote:
A dumb idea, is it not?
Who am I to judge?

If you already have the switch mode supply in hand and everything else you need, it might work OK. But in my opinion, it's usually better to use a "real" split supply where possible rather than one derived from a single supply.

Mike
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Old 5th June 2014, 06:19 AM   #5
Calvin is offline Calvin  Germany
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Hi,

have You thought about a single lowvoltage DC supply that feeds into PointOfLoad (POL) DC-DC converter modules?
Its generally good to make the supply lines short.
Isolated converter modules allow fast and short, galvanically isolated supplies in close proximity to the Point of Load, hence where the power is needed.

There are numerous modules in wattage classes from 0.5W to >>100W from many manufacturers and in different makes.
For manufacturers see CUI-inc, TDK Lambda, Wuerth, TI, Maxim, XP Power etc. etc.
Modules with single or Dual oututs, unregulated or regulated, isolated or non isolated, with reduced transformer coupling capacitance, shielded casings et al.
For small signal circuits 1-5W is often fully sufficient.
See for example the TI DCP02 series, or CUI-Inc's PBQ3 and other P series.
Such modules need only a few external parts, mostly for pre and post filtering.
Its absolutely easy for Hobbyists to work with and any of the well known distributors offer those at reasonable cost.
As main DC supply You may use any lowvoltage type of sufficiently high wattage.
A external wallwart supply also spares You all the hassle of having to deal with high voltage power line voltages and associated safety issues.

jauu
Calvin
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Old 5th June 2014, 11:06 AM   #6
teleman is offline teleman  Norway
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Quote:
Originally Posted by Michael Bean View Post
Who am I to judge?

If you already have the switch mode supply in hand and everything else you need, it might work OK. But in my opinion, it's usually better to use a "real" split supply where possible rather than one derived from a single supply.

Mike
Thank you Calvin,
Ha ha..I wish there were more of you in this world...! Seriously, you've said it more eloquently in a few words than what most of these experts in most"commercial" websites have been trying convince! I suspect they have vested interests in promoting their own products, ie:selling SMPS?

Have You thought about a single lowvoltage DC supply that feeds into PointOfLoad (POL) DC-DC converter modules Its generally good to make the supply lines short.

Yes, I've actually tried using a single supply with a DC-DC converter/ splitter, but for some strange reason when the load was connected the voltage dropped to very low level! I in fact have built a few good linear dual supplies which work very well & are almost completely noise free, but this "eureka" moment came to me w.r.t using SMPS instead as it seemed to offer a cheaper alternative! I'do have all the parts neccessary to try this.....however it's not a life & death issue right now.

My linear psu is based on the drawing below using LM317T/337T regulators. So i thought I'd omit the main stage & do it this way to minimize the high output ripple smps have! It's ready, but I'm a bit worried connecting a load to it!! black charred face? Lol!

I think I got carried away by this article!

AC DC Power Supplies, DC DC Converters, Wall Adaptors, External Power Supply - Future Electronics
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Old 5th June 2014, 12:38 PM   #7
marce is offline marce  United Kingdom
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SMPS for lower power circuitry like this is more prevalent in commercial designs, where either battery operation, low power requirement, space limitations and cost are factors.
I second what Calvin has said, and most controllers from the likes of Texas have layouts that you copy exactly, so very DIYable.
If laid out and designed correctly SMPS's do =not have to have high output ripple.....

Last edited by marce; 5th June 2014 at 12:46 PM.
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Old 5th June 2014, 01:29 PM   #8
teleman is offline teleman  Norway
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Thanks Marce,
I may have not explained myself properly....I'm not attempting to DIY a SMPS as it is way above my capabilities & knowledge! I already have a SMPS 24v/1.2A DC supply unit which I bought on e-Bay, hence thought of splitting it to give a virtual earth supply.

W.r.t SMPS not having high ripple, again I may have read or understood it wrongly. In any case I will refer to the Texas implementation notes. Thanks for the lead on this.

Cheers
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Old 5th June 2014, 01:35 PM   #9
marce is offline marce  United Kingdom
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One problem I have seen with SMPS's and analogue circuitry is power matching, using an over powered SMPS can cause the supply to go into burst more when there is not enough power drawn. This quite often causes the noise spectrum to drop dramatically in frequency often into the audible range. So it is best to match the power output to the circuit requirements.
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Old 5th June 2014, 01:52 PM   #10
teleman is offline teleman  Norway
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Really?...thank you indeed for your timely warning! I'm so glad that you mentioned it! Ok, so this is what I want to do.

SMPS 1.2 A.......to split supply......powering a circuit drawing < than 100mA/ rail

Do you then reckon that this may cause the problem you've mentioned?
What would you recommend then; 250mA/400mA/500mA?

Thanks
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