Class D amp on mains voltage?

[Death By Bass]

is it possible, to rectify the mains voltage (240volts AC here in Australia) to 340volts DC, then have that going to an array of mosfets, controlled by a Class D amplifier circuit, then have the audio output from the high voltage mosfets run into a high frequency switchmode transformer, to step the voltage back down from the 340ish volts, to a usable level for a subwoofer? something like a 4:1 transformer, so the voltage is 1/4 the input...

rather than use a seperate Class D amplifier, and switchmode powersupply, that give like +-120volt rails or something...

any ideas?

 

[olly_k]

Are you after Ice power?....

**broken link removed**

 

[Death By Bass]

I was after ideas for a home made amplifier...

perhaps using the national semiconductor Class D amplifier controller IC, but switching higher voltage mosfets, connected straight to the rectified mains voltage

 

[Styx]

Yes you can but what is the point!!!!

Normal method:

You take AC-mains and use a XFMR to step the voltage down and also provide isolation center-tapped.

Big DC-cap (centre-tapped) to ensure the output stage is views the link as a Voltage-source.

Output stage is two FET's feeding an L-C filter to the speaker, the return tied to the mid-point of the centertapped Caps

Direct Mains.
You would still need a XFMR to provide mains isolation
This gives you 0-DC_High
Output stage is now made up of 4 switches in H-bridge configuration (as opose to a phase-leg).
This then needs to feed a more complex L-C filter arrangement to then drive another XFMR to step the voltage down to speaker-level


So all in all you end up with two XFMR (instead of one), two extra switches and a more complex output filter

 

[Oznog]

Yes you can but what is the point!!!!


Direct Mains.
You would still need a XFMR to provide mains isolation
This gives you 0-DC_High
Output stage is now made up of 4 switches in H-bridge configuration (as opose to a phase-leg).
This then needs to feed a more complex L-C filter arrangement to then drive another XFMR to step the voltage down to speaker-level


So all in all you end up with two XFMR (instead of one), two extra switches and a more complex output filter

I'm not familiar with this. SMPS does not isolate the mains stage, there's no point to this. The high freq transformer provides the isolation and steps down the voltage, then a rectifier and filter makes the DC.

 

[Styx]

I thought the original poster was after building a Class-D amplifier?
If so I would want a XFMR in there for isolation.

Yes SMPS can have isolation, but I didn't think that was what he was asking, I might of mis-read

 

[Death By Bass]

I thought the original poster was after building a Class-D amplifier?
If so I would want a XFMR in there for isolation.

Yes SMPS can have isolation, but I didn't think that was what he was asking, I might of mis-read

my idea, would have a high frequency transformer on the high voltage, high frequency class D output from the amp, which would provide isolation from the mains...

 

[Nigel Goodwin]

I thought the original poster was after building a Class-D amplifier?
If so I would want a XFMR in there for isolation.

Yes SMPS can have isolation, but I didn't think that was what he was asking, I might of mis-read

my idea, would have a high frequency transformer on the high voltage, high frequency class D output from the amp, which would provide isolation from the mains...

It sounds a pretty pointless exercise?, and has the HUGE disadvantage that it's not got a regulated supply, and that the entire workings will be live to mains! - don't forget, you would also have to provide isolation for your input!.

You don't have your location filled in?, but it's quite possible that you're in a country with a fairly unreliable mains supply?.

 

[Death By Bass]

I thought the original poster was after building a Class-D amplifier?
If so I would want a XFMR in there for isolation.

Yes SMPS can have isolation, but I didn't think that was what he was asking, I might of mis-read

my idea, would have a high frequency transformer on the high voltage, high frequency class D output from the amp, which would provide isolation from the mains...

It sounds a pretty pointless exercise?, and has the HUGE disadvantage that it's not got a regulated supply, and that the entire workings will be live to mains! - don't forget, you would also have to provide isolation for your input!.

You don't have your location filled in?, but it's quite possible that you're in a country with a fairly unreliable mains supply?.

most amplifiers don't have a regulated supply, I don't see how its a disadvantage at all... :? also, how is it pointless... when a normal class D house amp may have a fairly complex switchmode powersupply, followed by a fairly complex switchmode amplifier...

my idea, is to do away with the powersupply, as the output from the amplifier should just be able to be stepped down to a usable voltage level...

and, the audio input WILL be isolated... :?

 

[Dean Huster]

You don't want a regulated supply for a power amplifier because it wouldn't be able to handle transients involved with bass and percussive instruments. Supply voltage has nothing to do with output level other than the clipping level. As long as you don't drop down to the clipping point, the supply voltage is irrelevant and can be totally unregulated. The usually rule of thumb is to just have (in an old "linear" supply) a big power transformer, a bridge rectifier and lots of capacitance to handle the transients.

NOW, if you're going to have a Class D amp, what's the point of a 40-pound power transformer? There was an article in Electronics Nowmagazine back in the 1990s for a line-operated Class D amplifier. I don't remember where they got their isolation in that case, but a magazine would be foolish to publish an unsafe project.

I would think that if you wanted true isolation, building a beefy switching supply with lots of capacitance at the output to power the amp would be a good way to go. Like the big automotive power amps, the supply will be 3/4 of the circuit volume and the actual amp the other 1/4. But result would be a light box to carry around.

Dean

 

[Nigel Goodwin]

You don't want a regulated supply for a power amplifier because it wouldn't be able to handle transients involved with bass and percussive instruments.

A number of quality amplifiers have used regulated supplies over the years, Quad being one of them. The main reason for not using them is down to cost!. Interesting idea though?, claiming transient power handling as an 'advantage' of an inferior powered amplifier? - are you perhaps a marketing man? 8) perhaps you should be? :lol:

NOW, if you're going to have a Class D amp, what's the point of a 40-pound power transformer?

I don't think anyone ever suggested that? (and it would be pretty silly!), the use of a regulated SMPS is what I was thinking of! - regulated because such supplies are, as a matter of their method of working.

This also helps to remove spurious mains borne noise from the amplifier, which a line powered one would be prone to - not to mention the highly expensive (and massive) electrolytics required to reduce mains hum to an acceptable level (and hum on the supply rails will pass through the low pass speaker filters to the speaker).

 

[Dean Huster]

Nigel, that's always been the argument against using a regulated supply for an audio power amplifier. The supply voltage to an op amp or an output amp has no affect on the output voltage for a given input voltage. In only affects how high that input voltage can be before clipping the output. If you have to be able to handle the current levels of any signal using a regulated amp, the regulator has to be capable of regulating at those high current levels, meaning a supply that is at least four times larger than necessary for most program material.

Please don't insult me, as I'm not a golden-eared audiophool nor a 5th Avenue marketing man. I measure my amp power in RMS watts and also specify the distortion at that level. To tell you the truth, I've NEVER seen an audio amp with a regulated supply, whether Sansui, Pioneer, Marantz or whatever. Not to say they don't exist, but that they're plain not necessary. With an unregulated supply, an amp can supply the rated RMS power and still pop out the necessary power for transients that a regulated supply designed for the same AVERAGE current could not do unless it fed a big output capacitors -- in which case, what's the point of a good regulator?

Back in the 1970s, there were a lot of fabulous high-power amplifier designs hitting the market every year, anywhere from 50W to 500W. Here in the U.S., Southwest Technical Products was a big provider of such kits. They all used unregulated supplies and specified their power in RMS watts into 8 ohms.

So, yes, they don't do regulated supplies because of the expense, not only of the extra parts count, but because of the extra size they'd have to have because of the extra current requirements.

Dean

 

[Nigel Goodwin]

Nigel, that's always been the argument against using a regulated supply for an audio power amplifier. The supply voltage to an op amp or an output amp has no affect on the output voltage for a given input voltage. In only affects how high that input voltage can be before clipping the output. If you have to be able to handle the current levels of any signal using a regulated amp, the regulator has to be capable of regulating at those high current levels, meaning a supply that is at least four times larger than necessary for most program material.

But that doesn't apply to a class-D amplifier, changes in the power rails are transferred directly to the speaker - as will mains hum on the supply rails.

Please don't insult me, as I'm not a golden-eared audiophool nor a 5th Avenue marketing man. I measure my amp power in RMS watts and also specify the distortion at that level. To tell you the truth, I've NEVER seen an audio amp with a regulated supply, whether Sansui, Pioneer, Marantz or whatever.

You mean you've NEVER seen a Quad 303 amplifier?, still a very sort after amplifier even today! - and one that used a regulated PSU.

BTW, why did you choose those particular three manufacturers to mention?, they are generally more middle and low end gear?.

 

[Styx]

Nigel, that's always been the argument against using a regulated supply for an audio power amplifier. The supply voltage to an op amp or an output amp has no affect on the output voltage for a given input voltage. In only affects how high that input voltage can be before clipping the output. If you have to be able to handle the current levels of any signal using a regulated amp, the regulator has to be capable of regulating at those high current levels, meaning a supply that is at least four times larger than necessary for most program material.

But that doesn't apply to a class-D amplifier, changes in the power rails are transferred directly to the speaker - as will mains hum on the supply rails.

Well depends on the loop-speed.
Not talking abt the main audio loop which only needs to go upto 44kHz I am talking abt teh output PWM voltage loop, for Class-D they tend to have a switching Freq of a few hundred KHz,

Any oscillations on the Main DC-link will be regulated out by the 100kHz PWM so it shouldn't be a problem if the link has mains hum (of 100Hz) that will not be trasposed onto the output/speakers

THIS is one of the many advantages of Class-D amplifiers

 

[spuffock]

I think this argument has gone off at a tangent. We seem to be using a SMPS to power something which is basically a SMPS. Why the hell can't we just modulate the first one?

 

[Death By Bass]

I think this argument has gone off at a tangent. We seem to be using a SMPS to power something which is basically a SMPS. Why the hell can't we just modulate the first one?

thats EXACTLY the point!!!

the end audio output will be exactly the same!!! the amplifier would have half as many parts, and be more efficient...!!!

I've been meaning to try it the other way....

use 12volts, at high current, switching it (class D amp), then stepping the voltage UP!!! (bit safer to try this, than a mains powered amp)

 

[Nigel Goodwin]

use 12volts, at high current, switching it (class D amp), then stepping the voltage UP!!! (bit safer to try this, than a mains powered amp)

It's common practice, most modern car amps do just that!.

 

[Death By Bass]

use 12volts, at high current, switching it (class D amp), then stepping the voltage UP!!! (bit safer to try this, than a mains powered amp)

It's common practice, most modern car amps do just that!.

no they don't... I'm yet to see a single amplifier that does actually. :?

 

[Nigel Goodwin]

use 12volts, at high current, switching it (class D amp), then stepping the voltage UP!!! (bit safer to try this, than a mains powered amp)

It's common practice, most modern car amps do just that!.

no they don't... I'm yet to see a single amplifier that does actually. :?

Sorry, your post was rather misleading - you mean generating the audio at 12V, then increasing the speaker impedance using a transformer?. Your line "stepping the voltage UP!!! " made me think you meant the supply voltage to the amplifier.

This was done in the past, but it's not a very good method (more current means more losses), probably the largest in-car amps I saw made that way were only 50W or so? - but way back, before switch-mode supplies were used in car amps!.

 

[Death By Bass]

- but way back, before switch-mode supplies were used in car amps!.

yes, in other words, it wasn't a class D amp, with a high frequency transformer