switch mode PSU's for audio.. pro's & cons, please discuss..

[Rob Flinn]

Having looked at the SSL website they state that they use switch mode psu's for everytrhing now.  In fact the whole SSL corporation has pretty good green credentials & they have an interesting page explaining their philosophy.  They state quite rightly that the switch mode supplies are less wasteful of power.

Anyhow, what got me thinking a bit more about this is that today I was asked to look at a Siemens C4 console that a friend has recently purchased.  It was supplied with a psu made buy a company naming itself Velvet Audio.  The previous owner had not actually hooked this up, & part of my work was to sort the power loom out to connect it.  I opened the PSU box (a 2U rack) to discover about 5 generic switch mode PSU modules, each of which had an additional large cap strappped across its o/p.  I power the console up & the 2 supplies that were for the + & - 15v became over stressed & collapsed to around 6v each & you could here motor boating which became worse the more channels that were installed.  The 24v relay supply also became unstable, & I could here the relays turning on & off rapidly. In the end it would only behave normally with 2 channels in the console.


So is it the case that the load in this console is just too inductive/capacitive for these switch mode supplies to deal with, which explains them collapsing under load ?  Would it be better to construct a linear supply, or is there a way to get them working with this console.  I think remember reading that SSL use switch mode supplies but they have linear regulation afterwards, & since they are using them it's obviously not impossible to use them.

Any thoughts /

 

[JohnRoberts]

Can you find any markings on the generic power supplies to determine current output or better yet make and model?

Were any of the powers supplies normalled together at their outputs? or did each rail have a single supply per voltage?

Do you have any way to load test (big power resistors) the supplies?

If there was room it would be better to add the big caps inside the console on the far end of the PS cable, while it would be good to filter out any switching noise before the cable so it doesn't re-transmit it into the room.

Console loads should not be unusually difficult, while you may want to add some extra LxC filtering to clean up a generic switcher output.

Good luck...

JR

 

[Rob Flinn]

Hi John

The relays supply was rated at 8.5A, but the rest didn't have visible ratings that I could see but looked maybe 1/2 the size of the relay supply physically, but the console isn't massive.

Basically it has 5 rails, 5v, 25v, 48v, & +&-15v.  Each has a seperate SM PSU.  The bipolar supply was made by stacking 2x 15v supplies.  The thing is the bipolar rails were seriously motorboating with on 4 i/p ch installed, & it got worse the more ch installed.  I would reckon the bipolar supplies must have been 3A & I can't see that not being enough for 4ch.  It sounded really nice with 2 ch in, no dig noise, dead quite & punchy.

 

[JohnRoberts]

Is it possible there is one or more bad modules... maybe thats why the PS is new?

JR

 

[gswan]

You also need to consider that a capacitor across the power supply output looks like a short circuit at switch on, which can cause current foldback circuits to operate and motor-boat the supply. If you hook up a load of modules and each has a bunch of filter caps on it then it does not take long for the capacitance across the supply to be quite large. Check the output of the supply with a scope and capture the voltage and current waveforms at switch on.

Also remember that a relay switching a large DC current will have its contacts buggered in a very short time, possibly eventually welding them together.

 

[Rob Flinn]

The audio bipolar supply is definitely doing something very weird because you can see the power onleds oscillating.

If you only have 2 channels in it switches on ok.  If you add 3 then it oscillates briefly before stabilising.  4 channels....forget it .

This console has a lot of caps in it for it's size.

 

[JohnRoberts]

Sounds like fold back current limiting that is cycling due to heavy capacitive load.

Several fixes come to mind, none simple.  Some switching supply chips have soft start circuits, I'm guessing you don't have access to inner workings of the switcher.

Is it fixed voltage or externally adjustable? If you have access to voltage adjustment, you could incorporate a ramped start so  the voltage ramps up slowly and doesn't hit the capacitors so hard.

Another rube goldberg option is to add a series resistance to current limit the start up, then short across the resistor after stabilized. Perhaps a dedicate inrush current limiter could be used.  Do you have any huge old PS inductors laying around (not a serious suggestion)?

If this is going squirrelly after only a handful of modules I don't feel lucky... 

Have you confirmed the PS will handle full resistive load?

Do you have a variac to try bringing it up slow?

JR

 

[sahib]

.....
Another rube goldberg option is to add a series resistance to current limit the start up, then short across the resistor after stabilized. ....

That is what came to my mind, by using a simple timer/relay.

 

[Walrus]

Do you have a variac to try bringing it up slow?

JR

Can you variac a switch mode psu?? :-\

I would have thought it was either OFF or ON as a lot of them work through a range of 90-260 volts.

 

[JohnRoberts]

Do you have a variac to try bringing it up slow?

JR

Can you variac a switch mode psu?? :-\

I would have thought it was either OFF or ON as a lot of them work through a range of 90-260 volts.

They are not all universal input.

varying the mains voltage will reduce/limit the available input power. But this could blow them up, which is another way to resolve the problem..    (sorry not that funny and in fact not a great idea. ).

JR

 

[Andy Peters]

Do you have a variac to try bringing it up slow?

JR

Can you variac a switch mode psu?? :-\

No, because the whole point of a stable DC power supply is that its output is supposed to be independent of the input voltage. That is, as long as the input voltage is within the specified range (and the loading is also within spec) then the output voltage doesn't vary.

-a

 

[JohnRoberts]

Do you have a variac to try bringing it up slow?

JR

Can you variac a switch mode psu?? :-\

No, because the whole point of a stable DC power supply is that its output is supposed to be independent of the input voltage. That is, as long as the input voltage is within the specified range (and the loading is also within spec) then the output voltage doesn't vary.

-a

Not to quibble, and I have already posted this may not be good for the health of the switcher, if you limit the unregulated input voltage below it's rated low line, you will likewise limit it's potential output power.  Unless it's using a step up voltage regulator, it will just saturate full on. If the unregulated rail before the switcher is only 10 V it won't likely put out full voltage and current. 

I repeat this is not a good idea even if it works, I was only trying to determine if the switchers can support the full load after up and stabilized. 
======
You could also hot plug in modules, one at a time another bad idea....

I'm on a roll here...  :-[

I should stop posting bad ideas.

JR

 

[ricardo]

It was supplied with a psu made buy a company naming itself Velvet Audio.  ....  5 generic switch mode PSU modules, each of which had an additional large cap strappped across its o/p.

As this isn't a Siemens PSU, there is a possibility that it has NEVER worked.  Can you contact Velvet Audio?

It might be worth removing the large caps strapped across each PSU o/p or at least on the ones which go flaky.  I'm suggesting the possibility that the PSU wasn't designed but built ad hoc with several bodges which might not meet the approval of the SMPSU makers.

Can you measure the current draw of each of the modules?  Then you could test the +/- 15V supplies with a load corresponding to your zillion modules + one to see if the voltage remains regulated.

 

[Andy Peters]

It was supplied with a psu made buy a company naming itself Velvet Audio.  ....  5 generic switch mode PSU modules, each of which had an additional large cap strappped across its o/p.

As this isn't a Siemens PSU, there is a possibility that it has NEVER worked.  Can you contact Velvet Audio?

It might be worth removing the large caps strapped across each PSU o/p or at least on the ones which go flaky.  I'm suggesting the possibility that the PSU wasn't designed but built ad hoc with several bodges which might not meet the approval of the SMPSU makers.

Now that you mention it, I suspect those big caps across the outputs are hurting a lot more than they are helping. The switcher has to charge those big caps first, and if it can't do it, perhaps because there's some kind of input current limiting, it'll fall down.

Don't ask me how I know this ... :-\

-a

 

[JohnRoberts]

+1 the caps may not be helping but I have more/other questions.  It apparently works with the caps and only a few modules. More modules and it starts motor boating.

Can the PS supply the needed current with a resistive load?

If the PS is externally adjustable, perhaps a soft start can be rigged up via that adjustment facility.

Or just blow it up and start over...

JR

 

[Rob Flinn]

It was supplied with a psu made buy a company naming itself Velvet Audio.  ....  5 generic switch mode PSU modules, each of which had an additional large cap strappped across its o/p.

As this isn't a Siemens PSU, there is a possibility that it has NEVER worked.  Can you contact Velvet Audio?

It might be worth removing the large caps strapped across each PSU o/p or at least on the ones which go flaky.  I'm suggesting the possibility that the PSU wasn't designed but built ad hoc with several bodges which might not meet the approval of the SMPSU makers.

Can you measure the current draw of each of the modules?  Then you could test the +/- 15V supplies with a load corresponding to your zillion modules + one to see if the voltage remains regulated.

I'm pretty sure that the previous owner of this console never fired it up with this PSU.  I had to make a harness to connect it to the console.  Also after a bit of detective work I realised that the master channels weren't plugged in the right slots ......    I have no idea who velvet audio are, but I believe they are probably based in Germany, since that is where the console came from. To be honest when I opened it it looked a bit home made, despite having a nicely engraved front panel.  But then I guess anyone can use front panel designer.

I did think about the big caps across the output of each supply , but as stated before it works fine with 2-3 modules installed.

I think some of the supplies have fixed voltages, but there were definitely a few with variable.

I appreciate all your suggestions, but I will have to answer them on the next visit to my friend as he lives 80 miles away.

 

[joe-electro]

I'm sure this is a stupid question, but you measured each rail of the console with an ohm meter, right? I mean, there are no obvious load problems, right?

Joe

 

[gswan]

I'm pretty sure that the previous owner of this console never fired it up with this PSU. 

Ditch it and design one that is suitable. You may be able to use some of the modules in this PSU, or maybe not.

 

[CJ]

do you play chess?

what is your rating?

1800 or above?


throw out the switcher and purchase a large transformer that will keep you and you signal warm for a long  long time,

keep it analog unless you want yo mixes to sound like a frog,

no wait, everything is MP3 with 48k so buy a switcher from Granger, these punks nowadyas won't notice a thing while doing there beer bongs,

 

[Rob Flinn]

OK guys, I hear you.   

Thing is it's a difficult call because if I say that a new linear psu is needed, & it doesn't sort the problem, (I suspect it will) then it's quite an expense, for not sorting the problem.  I don't have any supplies here that I can easily sub in, because my console is an MCI and al PSU's are + & -24v.

CJ this console actually sounds very good with the switcher........... until there are more than 2 channels in it ........................