PSU: switching or linear one?
Old 28th May 2014
  #1
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PSU: switching or linear one?

Good day, geaksluts members.

I have a simple question about PSUs.
Is it still really better to use classic linear power supply for audio-related circuits (say, good mic preamps) versus using good SMPS? I can see no technical reasons or benefits, but i'm not an engineer, i'm a technician, maybe i cannot see the whole picture, the small but important details.

I know a lot of good examples of using SMPS in audio, even some top-class poweramps uses them, and some good mastering engineers use them, i know a lot of nice working AD/DA converters powered by SMPS...and still i heard a lot of sceptical comments on this topic from some people.

I think we all need summarised technical advice on problems and advantages of using both PS types.

Thank you.
Old 28th May 2014
  #2
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Hi
A switchmode supply can be good but you may have to put considerable effort into making sure the switching frequency and harmonics do not appear on ANY wiring to or from the supply. This often means a basic 'switcher' module might have to be put into a larger metal enclosure with EFFECTIVE filtering on all terminals. To do this properly can make it expensive and you have to know what you are doing.
Linear supplies tend to be easier to 'tame' as the possibility of high frequency 'noise' radiation is less, although not guaranteed. As the power transformer is larger than for a switchmode design, it is possible for this to radiate mains frequency 'hum' into nearby circuitry.
You have noted that there are good and bad designs and you have to look at the various merits of both technologies and weigh up the costs for the WHOLE setup.
Matt S
Old 31st May 2014
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Thank you very much for your answer, Matt.

I have a plan to change a dead PSU for 8 mic preamps (opamps+THAT1510)...nothing special, classic +15V, -15V rated at 3A, and 48V @0.2A.
Instead of trying to get trafo, 317/337 and trying to place it in new box, i thought i could get 3 SMPS - one for each voltage, and filter HF noise with big chokes and caps. Less space, less heat, same price.

I've heard good stories of using Meanwell SMPSs in similar config, but only for power amps, not mic pres... Anyhow, PSU is in a separate box, so i thought it is worth a try.

What do you think about this idea?
Thank you.
Old 31st May 2014
  #4
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I prefer a linear regulated supply because it's simple, easy to trouble shoot, and potentially more reliable. But switching also works well.

Pros of Switching Supplies:

- Smaller and lighter.
- The switching frequency (typically 30KHz-150KHz) is above the audio band and should have no effect on audio, as long as it's designed correctly to minimize RF radiation that in rare cases can cause fundamental overload issues in adjacent equipment. Conventional supplies work at 60Hz, which is an audio frequency and therefore can be picked up as hum.
- Switching supplies can be self regulating using feedback that changes the length of time that each switching MOSFET is turned on in order to keep the output voltage constant.
- Switching supplies are more efficient. Self regulation makes them about 90% efficient. A conventional linear regulated supply is only about 50% efficient.

Cons:

- More complicated.
- Perhaps less reliable because of more delicate parts (MOSFETS).
- Slightly more ripple. But the ripple frequency is above audio, so it's not an issue.
- Doesn't respond as well to fast changing loads. But audio loads are relatively fixed, so a non-issue.
- Switching supplies generate harmonics that fall in the RF spectrum that need to be filtered according to FCC rules to keep them from radiating many miles where they can interfere with radio communications. RF radiates, audio doesn't. That's the problem. Because of this, a switching supply needs good shielding and filtering.

Both types can deliver adequate voltage and current. The self regulation feature is nice for efficiency reasons, but also makes for a more complicated circuit.

When I say FCC, I'm referring to the gvt agency that regulates use of the RF spectrum in the US. You probably have a similar organization in your country.
Old 31st May 2014
  #5
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Thread Starter
That is nice pros and cons list!
Off the shelf SMPS are well shielded, AFAIK, at least those in metal enclosures, so the only thing we should do is output filtering to suit audio needs. And using available SMPSs is much simpler than searching for adequate trafo and heatsinks, especially when high power is needed, especially in our country...the only way i can get 317/337 is to buy them from Farnell UK.

BTW, we have no problems with RF spectrum regulations, we simply do not have any kind of FCC here, free to use any lavalier transmitters and other stuff...i think RF pollution is pretty high here. )
Old 31st May 2014
  #6
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Quote:
Originally Posted by zvukofor View Post
TBTW, we have no problems with RF spectrum regulations, we simply do not have any kind of FCC here, free to use any lavalier transmitters and other stuff...i think RF pollution is pretty high here. )
LOL.

But you still don't want your switching supply to be coming over your neighbor's TV set. It's rare, but it can happen if harmonics are high enough in frequency. But it sounds like you have that covered. Sincerely, good luck.
Old 31st May 2014
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3Amps for just 8 THAT mic preamps?
That's 90Watts @2x15v!!
I think 8 THAT preamps plus balanced output drivers are using more like 300mA.
Unless the the unit has more gadgets, like LED metering, headphone amps etc.
And 8 phantom mics =<80mA. 112mA when all XLRs are shorted..
I would use a simple lineair supply.
7815/7915 if you can't get 317/337.
Or does the rabbit hole go deeper...
http://zvukofor.blogspot.co.nz/2013/...6-modding.html
Leo..
Old 31st May 2014
  #8
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Thread Starter
Quote:
Originally Posted by LeeYoo View Post
3Amps for just 8 THAT mic preamps?
That's 90Watts @2x15v!!
I think 8 THAT preamps plus balanced output drivers are using more like 300mA.
Unless the the unit has more gadgets, like LED metering, headphone amps etc.
And 8 phantom mics =<80mA. 112mA when all XLRs are shorted..
I would use a simple lineair supply.
7815/7915 if you can't get 317/337.
Or does the rabbit hole go deeper...
Profire 2626 Modding ~ zvukofor sound service
Leo..
Yes, Leo, you've got it. I'm making another one Profire 2626 mod, and i do not want to bother with linear PSU. ))

8 THATs, plus about 40 ne5532 biased in class A (+10mA for one 5532), pretty hot. And yes, plus some extra power, i know it is not good for anything to work close to limits, especially true for PSU. Phantom power via linear PSU, i think it is much more acceptible to any noises, so i need no risk here.

SMPS are not cheaper comparing to linear ones, as i calculated, but much smaller and runs cool. I think i'll use 1 rack case for SMPS, so the client can install it near his furmans, away from any audio cables.
Old 31st May 2014
  #9
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You might be able to mod two laptop supplies (19v/3-4A) to 15v, by changing the feedback resistor.
Or use lineair regulators and filtering behind them, to drop the voltage to 15v.
But two separate 15v SMPSs to make a +15/-15 supply could get you in trouble if the supplies are not starting up at the same rate.
Leo..
Old 1st June 2014
  #10
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Well i could use laptop PSUs, but they are more expensive than those from Meanwell, also less robust and IMO less RF protected. These Meanwells are in steel boxes, with good screwing contacts, have working range from -20° to +70° Celsius.

Hmm, i did not knew that they can have problems with starting up, specs says less than 200mS startup time... could this be a problem?
Maybe it is more appropriate to use one SMPS for both +15 and -15V, but is there any off the shelf ones with such specs?...
Old 1st June 2014
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Damn, if using two separate SMPS for 2rails is not good, i think i'll try to get low-profile toroidal trafo, and use some LM2596T-based regulators, they are still more easy to get and they have less problems with heatsinks.

How about this idea?
Old 2nd June 2014
  #12
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Only Meanwell dual rail model I could find is the RD-3513.
+/- 13.5 volt, but adjustable from 11.5v-15.5v.
Max 35W (just over 1A/rail), but that might be just enough for your application.
RD-3513 - Mean Well - TRC Electronics
Page has a link to the datasheet.
Leo..
Old 2nd June 2014
  #13
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Wow! How could i miss this one?
Thank you Leo, i think i'll get one and try, 1.5A should be enough really.

BTW, just installed Meanwell SMPS for 5V@5A in my own rig, replacing dead USB hub PSU. Well, i can charge 2 iPads now (with some mods to hub), it stays cool (old one was really hot even without heavy duty charging), and it is made really well.
Old 2nd June 2014
  #14
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Hi
Although the operating frequency of a 'switcher ' may be high (say 150KHz) and not cause problems with audio, if you have another unit that happens to run at 149KHz then you may get the 'sum' and 'difference' frequencies appearing on you audio as they can be conducted and radiated by ANY wiring into the 'switcher' enclosure.
149KHz you can't hear, 1KHz you can!
Be aware of the 'problems' and design them out and you will be fine.
NOTE that if you need 'split' or twin rail operation you MUST take precautions that BOTH rails are present ALL the time as some devices 'crowbar' if you miss one rail for even a short amount of time.
Matt S
Old 3rd June 2014
  #15
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Whilst we be nerding on psu's

Does anyone have any comments or experience with this psu or range or brand in general?

SRP-40A-3002 - Integrated Power Designs - TRC Electronics

If negative any suggestions? Bear in mind I will be intending to be using the +/- 15 volt AND + 5v version.

Thank you.

Gareth
Old 3rd June 2014
  #16
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Note the 700mA limit of the negative rail.
That makes it a 21Watt supply for +/-15 audio.
Unequal rails, unequal rail capacitance.
Leo..
Old 12th June 2014
  #17
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Right yes , good point.

Sorry for late reply.

Thank you.
Old 14th June 2014
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Switchers are the Spawn Of Satan. A "hit" from the power line (surge, lightning, etc) and they die. They are complex circuits, and none of the manufacturers publish circuit diagrams.

So, you spend time reverse engineering the circuit only to discover an odd-ball transformer (with multiple windings) has crapped out...or an oddball MOSFET has died ....and the manufacturer has discontinued support.

I have been working through a 1990's Neve Capricorn desk which uses multiple switchers. None of the manufacturers exist anymore/or no longer support the PSU's.

Bah...switchers are total and absolute junk. PTUI! Only good for throw-away consumer gear (wall warts) with a 30 day warranty.

There ARE specialty folks who work on these turds. I had a couple of quotes for the Capricorn supplies, running upwards towards $1000 each.

JUNK!!! Run Away!!!!

Bri
Old 14th June 2014
  #19
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I wouldn't say they are junk, Brain. I'm working on a project now that uses at least 20 of them in an automated test system. It was the only way to get the power density into the chassis.

If you do it right, you can set ramp rates, sequence the rails, monitor internal temperature, set current limits...

What was killing us, prior to fixing the filtering, is the ringing between the FETs and the magnetics. It's all up in the 200-800MHz range, so it gets onto grounds and such. Putting these things in a remote box and cabling them to the unit is probably a good idea if you can do it.

...that also makes dead supplies easier to deal with...

These are the types of regulator we are using (mostly):
Eighth-Brick | Power-One

Our biggest noise headache, however, was the 48v off-line bulk supply. I'd replace that one with a linear if possible.



-tINY

Old 14th June 2014
  #20
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I agree with Brian 100%, linear for me any day...
Old 14th June 2014
  #21
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For DIY'er and products that have plenty of room and few power restrictions, linears are the right choice, generally.



-tINY

Old 19th June 2014
  #22
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Should your circuit, whatever it is, ask the power supply for an extra-big moment of power, a linear would probably behave better than a switcher. I replaced a large switcher running a power amp with a smaller linear and it performed better.
Old 20th June 2014
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While I stick with my "Spawn of Satan" observations, I must also comment that switchers can pack a lot of power in a small package since they don't require large hunks of iron (power transformers operating at 50/60 Hz). I've been going through a pile of dead switchers used in the peripheral racks of the Capricorn desk, and these are rated to provide +5 VDC at 10 Amps and +/- 16 VDC at 8 Amps per rail. All that juice is in a package measuring a mere 12" x 7.5" x 2.75". A linear unit providing similar wattage would be MUCH larger.

But, the mere fact that I have been accumulating such a pile of dead switchers to attempt to restore to service goes back to my original statement....poor reliability over the long run. I have personally experienced this in my own computer equipment as well.

Bri

PS...sidebar...I question the need for 8 Amps on the analog rails in this situation, and guess these supplies were chosen by the AMS design engineers as an off-the shelf solution.
Old 20th June 2014
  #24
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Hi
Switchers CAN be good but to do it well and RELIABLY costs many pennies once you get above 50 Watts power capability.
AMEK flirted with switchmode supplies and they were not entirely plain sailing. The designs were 'bought in' incidentally. The 'standard' modules used in some supplies were OK.
Solar panel 'inverters' use switch mode designs and when you are dealing with up to 1000 Volts (DC input) and several Killowatts in units that get stuffed into any old corner, they do just have to work, and they get subjected to incoming line spikes and a host of 'nasties'.
Matt S
Old 24th June 2014
  #25
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Quote:
Originally Posted by brianroth View Post
While I stick with my "Spawn of Satan" observations, I must also comment that switchers can pack a lot of power in a small package since they don't require large hunks of iron (power transformers operating at 50/60 Hz). I've been going through a pile of dead switchers used in the peripheral racks of the Capricorn desk, and these are rated to provide +5 VDC at 10 Amps and +/- 16 VDC at 8 Amps per rail. All that juice is in a package measuring a mere 12" x 7.5" x 2.75". A linear unit providing similar wattage would be MUCH larger.

But, the mere fact that I have been accumulating such a pile of dead switchers to attempt to restore to service goes back to my original statement....poor reliability over the long run. I have personally experienced this in my own computer equipment as well.

Bri

PS...sidebar...I question the need for 8 Amps on the analog rails in this situation, and guess these supplies were chosen by the AMS design engineers as an off-the shelf solution.
We had a Capricorn at Sound on Sound around 2000. Those switching supplies were flaking out when that thing was "New". I don't really believe it was ever new. It must have been one of the last ones sold and I think they pieced ours together from parts lying around the office.
Brad
Old 24th June 2014
  #26
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Hi
There has been progress in the last 15 years. At that time mobile phones were the size of a brick.
Matt S
Old 24th June 2014
  #27
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Quote:
Originally Posted by Matt Syson View Post
Hi
There has been progress in the last 15 years. At that time mobile phones were the size of a brick.
Matt S
I wasn't attacking switchers as a whole, was just commenting the ones used in the Cap were never very good. My 2 SSL 9k's use switching supplies, they are 16 years old and still running. I will say once they flake out they are damn near impossible to trouble shoot. Usually it's a wholesale module swap if you can find one.
B
Old 24th June 2014
  #28
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Hi
They are very good as door stops or boat anchors.
Actually no, they are not heavy enough!
Matt S
Old 24th June 2014
  #29
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I would go for linear!
First of all it sounds better (just my opinion FWW) and secondly it is servicable!
Old 24th June 2014
  #30
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Hi
If you can 'hear' a power supply then the circuit is faulty.
A power supply is required to provide sufficient power for all defined signal conditions without varying.
If it can achieve this then it doesn't matter what you use.
Matt S
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