Audio amplifier power supply rails are +/-40V because of SELV regulations?

[treez]

Hello,
Our customer has requested a PSU to give +/-40V at 320W (for a half bridge class D amplifier supply). Why has he not just requested an 80V rail at 320W and then he could have used a full bridge class D amplifier?
Is the reason because +/-40V at 320W gives the same 80V difference but is within SELV voltage regulations?

 

[FvM]

I don't see how a +/-40 V DC supply could comply with SELV requirements. Either it's not separated or the accessible voltage is 80 V.

 

[mtwieg]

How is the speaker connected in their configuration? If one side is connected to the midpoint, then you would only get +/-40V at the terminals. But you could also get that with a 40V single supply with a full bridge, no need for 80V. In fact the 40V half bridge might even be safer.

I suppose that the client happens to have to amplifier circuit designed such that they need a midpoint, and don't want to redesign. Though if they did I'm sure the overall system would be cheaper.

 

[Dan Mills]

+-40V allows a half bridge output stage which allows more flexibility in load connection then a full bridge does, also it is a very conventional thing to request when doing a power amp.

Consider that a two channel stereo half bridge amp can be easily set up for bridging mode output if only a single channel is required (at double the power into double the impedance), you cannot easily do this with full bridge BTL outputs.

You did used to see single supply amps in old hifi with honking great elcos in series with the output, it has gone more then somewhat out of fashion.

SELV is not a consideration here, plenty of amps can put well over 100V between the speaker terminals....

REgards, Dan.

 

[mtwieg]

+-40V allows a half bridge output stage which allows more flexibility in load connection then a full bridge does, also it is a very conventional thing to request when doing a power amp.

Consider that a two channel stereo half bridge amp can be easily set up for bridging mode output if only a single channel is required (at double the power into double the impedance), you cannot easily do this with full bridge BTL outputs.

This is true if the two amplifier channels have to share the same power rails, I suppose. So maybe the client has a two channel system in mind.

 

[treez]

so you mean with a two channel system, with a half bridge amplifier for each channel, then you have the option of either coupling the two half bridges together and having a double power , single channel, or, you can leave the two half bridges separate, and have a stereo output (2 channel), but with half the single channel power in each channel?

Why would you have a stereo guitar amplifier?

I thought stereo was for having the bass and drums (say) in the left speaker, and the guitars and vox (say) in the other speaker, I didn't think stereo was relevant for single instruments such as a guitar?...wouldn't it just be the same music (guitar strums) coming out of each channel's speaker, and hence a waste of time?

 

[mtwieg]

I'm not even going to try to guess what the musicians or audio engineers are thinking. You'd best ask the client yourself, but I doubt they'll want to change the design.

 

[Dan Mills]

You didn't mention a guitar in the original post, just an amplifier....

'Stereo' amps are also useful when running with electronic crossovers (You have one amp driving the woofer and one the tweeter), the extra power amp is sometimes more cost effective then the large, low frequency inductors needed for an old school crossover network.

There could be many reasons, but a split supply is entirely conventional and most class D power amp chipsets above a few tens of watts will be designed for it.

Regards, Dan.

 

[treez]

There could be many reasons, but a split supply is entirely conventional and most class D power amp chipsets above a few tens of watts will be designed for it.

That's interesting, because a split supply (eg +/-40V) requires more effort in the power supply design, than a single supply (EG 80V). For example, a Full Bridge SMPS (in current mode) with a split rail output would typically require the output inductors to be coupled in order to get equalised voltage on each of the rail halves.

'Stereo' amps are also useful when running with electronic crossovers (You have one amp driving the woofer and one the tweeter)

...so may I presume that stereo amps are of no use for bass guitar amplifiers?, because they would just be having a woofer and no tweeter?

 

[Dan Mills]

Actully bass guitar is a surprisingly full range instrument and bass guitar cabs commonly have a tweeter fitted so 2 way active is entirely possible (if not yet commonplace).
It is bog standard electric gutar that usually gets a single speaker with no tweeter (It is usually quite 'bright' enough thank you very much).

Watch out for rail pumping if doing a switchmode supply for class D, it can be a real headache.
The usual think is a dead crude forward converter regulated on whichever rail is higher that cycle and fitted with a bucket load of output capacitor to deal with the rail pumping.

Split rail may make the power supply harder (Back in the linear days it was just a case of splitting the bus capacitor and hoking up the transformer centre tap, no big thing), but it makes the amp much easier.

Regards, Dan.