Switch mode power supplies for audio amps

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#21
I seem to remember a few Members here telling us that there are a few amplifiers with switching PSUs that are not aimed at the PA market.

SMPS generally do not tolerate high capacitance on their output.

A power amplifier has to pass the transient current demand of the speaker back to the PSU.

If there are no output capacitors to meet this transient current demand then the SMPS has to supply.
That is an onerous task.

eg.
I work with 100W into 8ohms rated amplifiers.
100W into an 8r0 dummy load requires an output current from the PSU of ONLY 5Apk
But I know that in real music/audio reproduction that many speakers will demand at least twice that very often and more rarely can demand more than three times that.
I test my 100W amplifier into a 2r7 dummy load. I expect it to deliver ~15Apk
That is the demand that my PSU has to meet.

If I used a switching PSU, I would expect it too to meet that 15Apk current.

For a 100W+100W into 8+8ohms amplifier I would have to specify +-45Vdc (since it would be regulated SMPS) and 30Apk output. That is 2700W equivalent for a pair of 100W amplifiers.
I will just carry on using a 35+35Vac transformer rated @ 300VA to 600VA.
 
#22
Reason I even think about SMPS is because I just bought a used Nakamichi PA-7 which is very similar to Threshold S/300. It is by no means a high end amplifier by any stretch. But it's already so heavy!!! I almost consider driving a few hundred miles to buying a Threshold S/500 that is close to 100lbs. I am glad I decided to go with the PA-7 instead!!! Krell are even heavier, into 140lbs!!! That make me think about SMPS.

I read more threads on SMPS after I posted this. Sounds like SMPS will work better if it is class A or heavy biased class AB. You draw heavier current so the variation of current is not as big, which is more favorable for SMPS.

I looked on ebay, 48V 10A for $50 free shipping. Two of them can make a nice medium power amplifier.
 
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#23
StephenR said:
Reason I even think about SMPS is because I just bought a used Nakamichi PA-7 which is very similar to Threshold S/300. It is by no means a high end amplifier by any stretch. But it's already so heavy!!! I almost consider driving a few hundred miles to buying a Threshold S/500 that is close to 100lbs. I am glad I decided to go with the PA-7 instead!!! Krell are even heavier, into 140lbs!!! That make me think about SMPS.

I read more threads on SMPS after I posted this. Sounds like SMPS will work better if it is class A or heavy biased class AB. You draw heavier current so the variation of current is not as big, which is more favorable for SMPS.

I looked on ebay, 48V 10A for $50 free shipping. Two of them can make a nice medium power amplifier.
Click to expand...
Do you mean two of them i series for +/-48?
 
#24
cs said:
I'm currently looking at using switch mode power supply 'bricks' for my active loudspeaker project, and have a concern about permitted load capacitance. The modules I have looked at specify no more than a few 10s of uF, otherwise they may be unstable or fail to start-up.

Does anyone have any experience of this ? I am concerned that the low reservoir capacitance will be a problem on large load current transients, and that the rapid decay of the supply on turn-off will result in loud 'pops'. (With a normal linear supply you usually have a few 1000 uF to take care of both issues).
Click to expand...

You only need small capacitors on the output due to the high frequency.

On power up the SMPS allows high current for a short period or it thinks there is a short on the output and shuts down.

Its a juggling act, not so long that it misses a short and not so short that it doesn't power up at all on power up current surge.

I designed my own LLC SMPS using a PIC and a mosfet gate driver.
I used a current sense on the driver mosfets into the PIC to monitor output current.
 
#25
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#26
thimios said:
Do you mean two of them i series for +/-48?
Click to expand...

Yes, that's the idea. 48V 10A can power up a decent power amp. You'll get about 78W into 8ohm, 153W into 4ohm with reasonable size filter cap. I'll let the more experience people here to comment whether this is good enough without using very large filter caps that it might cause problem on start up.

I guess for me, 36V 10A is plenty. That will give me about 64W into 8ohm, 120W into 4ohm and still 8A peak. Then you don't even need a huge filter cap at all to provide the peak surge current. It will work as long as you prevent output sticking when going into saturation. You can use higher bias current to get a bigger class A region.
 
#27
xrk971 said:
There's a really good deal on an excellent +/-53v 450w SMPS from Abletec for $20. You need to use a resistor to load the secondary 5v and 7v outputs for to be stable. Thread here:

http://www.diyaudio.com/forums/class-d/281698-abletec-53v-dual-rail-450w-supply-20-a.html

Premade wiring harness (pigtails) here:
http://www.diyaudio.com/forums/class-d/275915-200w-irs2092-amp-20-a-34.html#post4604297
Click to expand...

Is that a little low? Dual 53V 450W calculates to max current of 450W/(2 X 53V)=4.24A.

But I'll let the expert here to estimate the peak current and everything. Just a question.

cheers.
 
#28
StephenR said:
Is that a little low? Dual 53V 450W calculates to max current of 450W/(2 X 53V)=4.24A.

But I'll let the expert here to estimate the peak current and everything. Just a question.

cheers.
Click to expand...

Here is the data sheet. It can do 16.5 amps for 10ms, 4.13 amps for 5 minutes, 1.38 amps continuous. Granted, it's not like big toroidal transformer, but certainly less expensive and lighter weight.

http://www.parts-express.com/pedocs/specs/129-147-parts-express-specifications.pdf

There are really powerful SMPS used in telecon equipment racks. They are all 48v (but adjustable) and in the 2kW range. They are called "Rectifiers".

Here is a 35amp one with DSP control with 1700w for $90.

Emerson R48 1800 Rectifier 35A 48VDC 1700W | eBay

Two would be about $200 for +/-48vand 35amp supply. How big would a toroidal transformer and diode bridge and caps have to be to do that?
 
#29
xrk971 said:
Here is the data sheet. It can do 16.5 amps for 10ms, 4.13 amps for 5 minutes, 1.38 amps continuous. Granted, it's not like big toroidal transformer, but certainly less expensive and lighter weight.

http://www.parts-express.com/pedocs/specs/129-147-parts-express-specifications.pdf

There are really powerful SMPS used in telecon equipment racks. They are all 48v (but adjustable) and in the 2kW range. They are called "Rectifiers".

Here is a 35amp one with DSP control with 1700w for $90.

Emerson R48 1800 Rectifier 35A 48VDC 1700W | eBay

Two would be about $200 for +/-48vand 35amp supply. How big would a toroidal transformer and diode bridge and caps have to be to do that?
Click to expand...
Wow!!! That's good for a huge amp!!! I think SMPS is very promising.

Not for anything, the weight!!! I have to be a little careful picking up the PA-7. can you imagine a Threshold S/500 or the Krell KSA250?

In certain sense, it's easier to filter out 40KHz switching frequency than 120Hz rectified wave. Those big cap are expensive.
 
#30
xrk971 said:
Here is the data sheet. It can do 16.5 amps for 10ms, 4.13 amps for 5 minutes, 1.38 amps continuous. ..............
Click to expand...
That's a useful way to specify the maximum output currents.
I have looked at many smps specifications and I have NEVER seen that 10ms transient current in any of them.
Obviously looked at bad ones where they hide the information they don't want one to read.
 
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#31
I totally forgot one thing that SMPS is much better. I was just scoping the filter caps of my Nakamichi, they always have a saw tooth wave where amplitude depends on the size of the cap, but you always have it. You don't have that with the regulated SMPS.
 
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