Switching Regulator for Audio Amplifier

[Fred]

I have an audio amp based on the LM3886. In order to get 50W out of
it I need to use a transformer who voltage rises too high for the
LM3886 to tolerate during no or light load times.

I have modeled a linear regulator for the amp, but is is very
wasteful, it dissipates over a hundred watts a channel. I had to
parallel four regulators on each rail in order to keep their
individual dissipations manageable.

I have started considering a switched mode power supply, and have been
looking at chips such as the LM5116. Some time ago, I heard using
SMPSs in audio amplifers in problematic because of the difficulty in
filtering out the switching noise from the output.

Any suggestions would be appreciated.

FB

 

[[email protected]]

I have an audio amp based on the LM3886.  In order to get 50W out of
it I need to use a transformer who voltage rises too high for the
LM3886 to tolerate during no or light load times.

I have modeled a linear regulator for the amp, but is is very
wasteful, it dissipates over a hundred watts a channel.  I had to
parallel four regulators on each rail in order to keep their
individual dissipations manageable.

I have started considering a switched mode power supply, and have been
looking at chips such as the LM5116.  Some time ago, I heard using
SMPSs in audio amplifers in problematic because of the difficulty in
filtering out the switching noise from the output.

Any suggestions would be appreciated.

FB

The specs on this amp aren't particularly great. You could probably
buy an off the shelf amp for less than what you are trying to build.
My recollection is Randy Sloan's amp design book takes a dim view of
regulated supplies for power amplifiers.

 

[MooseFET]

I have an audio amp based on the LM3886.  In order to get 50W out of
it I need to use a transformer who voltage rises too high for the
LM3886 to tolerate during no or light load times.

I have modeled a linear regulator for the amp, but is is very
wasteful, it dissipates over a hundred watts a channel.  I had to
parallel four regulators on each rail in order to keep their
individual dissipations manageable.

I have started considering a switched mode power supply, and have been
looking at chips such as the LM5116.  Some time ago, I heard using
SMPSs in audio amplifers in problematic because of the difficulty in
filtering out the switching noise from the output.

Any suggestions would be appreciated.

(1)
The higher the frequency of the switcher (within reason) the easier it
is to keep it out of the audio. Remember to make sure that the servo
loop of the design is very well damped. You don't want it ringing up
if a certain frequency is in the audio material

(2)
You may not need the switcher and may be able to do better than the
linear.

(2A)
Place a large MOSFET between the rectifier and the storage
capacitors. Sense the voltage on the storage capacitors and if it is
going too high turn off the MOSFET for the next cycle. If you are
careful to turn on and off the MOSFET when there won't be any current
in it you won't make much high frequency noise.

(2B)
Magnetic amplifiers can be kind of cool. A saturable reactor in
series with the transformer could knock the voltage down by 25% under
the light load case.

 

[Eeyore]

I have an audio amp based on the LM3886. In order to get 50W out of
it I need to use a transformer who voltage rises too high for the
LM3886 to tolerate during no or light load times.

You need a transformer with better regulation and probably larger filter
caps. That's the easy solution.

The loop response times of many switching regulators simply isn't good
enough for audio since you can get extraordinary step current changes.

Graham

 

[Eeyore]

Oh -- and if the switching supply works at 20kHz, yes, it'd be crappy
with a hi-fi amp.

But if you can put one together that works at 100kHz, then you don't have
to worry too much about switching noise. Transients yes; switching noise
only to the extent that it doesn't get into your front end stages and
overload things.

I've heard them cause 'spuriae'.

Graham

 

[Eeyore]

Reevaluate your requirement for a 50 watt amplifier. You
would be hard pressed to hear the difference between a 50
watt amplifier and a 40 watt one.

Near as dammit ONE dB in fact.

Graham

 

[MooseFET]

You need a transformer with better regulation and probably larger filter
caps. That's the easy solution.

The loop response times of many switching regulators simply isn't good
enough for audio since you can get extraordinary step current changes.

You can put thunderingly big capacitors on the output. He doesn't
want the voltage to follow the audio material. This makes the loop
response easy to do. You just use a large proportional gain. The
regulation won't be perfect but it would be more than good enough.

 

[Bob Eld]

I have an audio amp based on the LM3886. In order to get 50W out of
it I need to use a transformer who voltage rises too high for the
LM3886 to tolerate during no or light load times.

I have modeled a linear regulator for the amp, but is is very
wasteful, it dissipates over a hundred watts a channel. I had to
parallel four regulators on each rail in order to keep their
individual dissipations manageable.

I have started considering a switched mode power supply, and have been
looking at chips such as the LM5116. Some time ago, I heard using
SMPSs in audio amplifers in problematic because of the difficulty in
filtering out the switching noise from the output.

Any suggestions would be appreciated.

FB

I think you are putting the effort in the wrong place. Instead of screwing
with regulators, switching supplies and the like, the effort should go into
the amplifier(s) and not the power supply.

If it were me, I'd design an amplifier to replace the 3886 using discrete
components that had sufficient head room for any voltage I desired,
unregulated. Plus and minus 20 volts, plus and minus 30 volts, plus and
minus 40 volts, or more, whatever.

Remember a well designed amplifier does not need regulated power, it is in
effect a high speed (regulator) on it's own, turning raw DC into controlled
(regulated) audio voltage. adding additional regulation is redundant,
wasteful and can be noisy. Why do it? Put the effort into the amplifier not
the power supply to fix a poor amplifier.

 

[Vladimir Vassilevsky]

I have an audio amp based on the LM3886. In order to get 50W out of
it I need to use a transformer who voltage rises too high for the
LM3886 to tolerate during no or light load times.

I just tested the new 1kW HiFi amplifier with the regulated switching
power supply. This is one of the things which I design as the business.

I have modeled a linear regulator for the amp, but is is very
wasteful, it dissipates over a hundred watts a channel. I had to
parallel four regulators on each rail in order to keep their
individual dissipations manageable.

Is the global warming that you are worried about?

I have started considering a switched mode power supply, and have been
looking at chips such as the LM5116. Some time ago, I heard using
SMPSs in audio amplifers in problematic because of the difficulty in
filtering out the switching noise from the output.

Rumors, stories, fairy tales..

Any suggestions would be appreciated.

The most problematic element in that sort of SMPS is the common group
regulation choke.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[Vladimir Vassilevsky]

MooseFET wrote:

(1)
The higher the frequency of the switcher (within reason) the easier it
is to keep it out of the audio. Remember to make sure that the servo
loop of the design is very well damped. You don't want it ringing up
if a certain frequency is in the audio material

The higher is the switcher frequency, the smaller is the power supply
ripple rejection in the amplifier.

The higher is the switcher frequency, the higher are the losses.

The properly designed servo loop will have the cutoff frequency around
0.1 of the switcher frequency.

The reasonable frequency for the medium or high power audio SMPS is
somewhere in the range of 30...40kHz.

(2)
You may not need the switcher and may be able to do better than the
linear.

The OP's 50W power requirement is just nothing.

(2A)
Place a large MOSFET between the rectifier and the storage
capacitors. Sense the voltage on the storage capacitors and if it is
going too high turn off the MOSFET for the next cycle. If you are
careful to turn on and off the MOSFET when there won't be any current
in it you won't make much high frequency noise.

So, there will be a huge power ripple and the surge current through that
FET.

(2B)
Magnetic amplifiers can be kind of cool. A saturable reactor in
series with the transformer could knock the voltage down by 25% under
the light load case.

How about the 60Hz?


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[Fred]

Reevaluate your requirement for a 50 watt amplifier. You
would be hard pressed to hear the difference between a 50
watt amplifier and a 40 watt one.

I must acknowledge this is probably the wisest solution to using this
chip: Just settle for 40 watts.

This is what the audio engineers at National said. They also said
getting 50 watts out of the chip "was difficutl" without regulating
the supply, and that goes against all their engineering effort to make
the device have as high ripple rejection as it does.

They were aghast when I told them I put 50,000uF on each rail. And
you know what...

It actually sounds better without all that reserve capacity

So basically, I guess what I want is simply a more powerful amplifier.

 

[Fred]

The higher is the switcher frequency, the smaller is the power supply
ripple rejection in the amplifier.

The higher is the switcher frequency, the higher are the losses.

The properly designed servo loop will have the cutoff frequency around
0.1 of the switcher frequency.

The reasonable frequency for the medium or high power audio SMPS is
somewhere in the range of 30...40kHz.


The OP's 50W power requirement is just nothing.


So, there will be a huge power ripple and the surge current through that
FET.


How about the 60Hz?

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultanthttp://www.abvolt.com

I have been looking at thoses 1kW rack mount PA amplifiers, and
wondering how in the world do they get that power in such a small
place. I know SMPSs have something to do with it. I have also seen
the terms maginetic amplification and digital amplifaction thrown
around in advertising for those high powered PA amplifiers made by
Crown, Carver etc.

In any case, I've been wanting to get familiar with SMPSs and may be
using this amplifier problem as motivation to start at the bottom a
new learning curve

 

[Fred]

You need a transformer with better regulation and probably larger filter
caps. That's the easy solution.

The loop response times of many switching regulators simply isn't good
enough for audio since you can get extraordinary step current changes.

Graham

I've read that using a linear regulator after the SM regulator can
clean up the output from it.

 

[Jan Panteltje]

So basically, I guess what I want is simply a more powerful amplifier.

Well, mine does a bit more, never had 'regulator problems' as it has none:
http://panteltje.com/panteltje/amplifier/index.html
Of course your speaker's impedance determines how many watts you can deliver at a given supply voltage.

The advantage of a switch mode supply that I see is
1) less weight (mains transformers are heavy),
and
2) perhaps easily adapted to both 230 V / 50 Hz and European 110V / 60 Hz US supply.

If the amp has good supply rejection (it should), then a simple rectifier would be enough.

 

[Vladimir Vassilevsky]

I've read that using a linear regulator after the SM regulator can
clean up the output from it.

Generally, not. Look at the regulation vs frequency curves.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[Eeyore]

You can put thunderingly big capacitors on the output. He doesn't
want the voltage to follow the audio material.

Ah, that's another technique some claimed to have mastered although I see no
product in the market.

This makes the loop
response easy to do. You just use a large proportional gain. The
regulation won't be perfect but it would be more than good enough.

You'd have slower looop response due to the large C but I guess you're saying
it wouldn't matter because the C makes up for it ?

Graham

 

[Eeyore]

I think you are putting the effort in the wrong place. Instead of screwing
with regulators, switching supplies and the like, the effort should go into
the amplifier(s) and not the power supply.

If it were me, I'd design an amplifier to replace the 3886 using discrete
components that had sufficient head room for any voltage I desired,
unregulated. Plus and minus 20 volts, plus and minus 30 volts, plus and
minus 40 volts, or more, whatever.

Designing a discrete amp to beat the LM3886 needs the kind of skills and
experience I have. It's NOT a hobby task.

The typical 'gainclone' circuit the OP is likely usung has PUNY reservoir caps.
Around 1500uF IIRC. Make them 4700uF and use a larger transformer in
'under-rated mode' to get better supply regulation.

Remember a well designed amplifier does not need regulated power, it is in
effect a high speed (regulator) on it's own, turning raw DC into controlled
(regulated) audio voltage. adding additional regulation is redundant,
wasteful and can be noisy. Why do it? Put the effort into the amplifier not
the power supply to fix a poor amplifier.

Since the LM3886 will sustain +/-47V (obviously allow for AC line overvoltage
on top of this) where's his problem ?
http://www.national.com/mpf/LM/LM3886.html

Graham

 

[Eeyore]

The higher is the switcher frequency, the smaller is the power supply
ripple rejection in the amplifier.

The higher is the switcher frequency, the higher are the losses.

The properly designed servo loop will have the cutoff frequency around
0.1 of the switcher frequency.

The reasonable frequency for the medium or high power audio SMPS is
somewhere in the range of 30...40kHz.

Last time I looked, QSC were using 120kHz but they may have upped that since
that was in early models. It wasn't regulated either it was more of just an
chopper / rectifier type design.

Graham

 

[Eeyore]

I have been looking at thoses 1kW rack mount PA amplifiers,

Small by today's standards.

and wondering how in the world do they get that power in such a small
place. I know SMPSs have something to do with it.

Not necessarily.

I've quite happily done 1600W (conntinuous 'rms') in 2u using a perfectly
conventional PSU. It helps to understand how to design transformers though. It
did have 2 fans btw but that was just for the output devices..

It would do 2kW into 2 ohms/ch but that wasn't continuously rated.

Graham

 

[Eeyore]

I've read that using a linear regulator after the SM regulator can
clean up the output from it.

And make a lot of heat.

Graham