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Old 14th November 2004, 12:19 PM   #1
TroelsM is offline TroelsM  Denmark
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A major problm with all class A/B amps is the powersupply-transformer:
1) The output voltage is fixed and that means the output-power is fixed *** well. So if you load the amp with 8Ohm's the transformer will probably be too "big" and if the amp is loaded with 2 ohm's the transformer is probably to small.
2) The regulation is very bad and you will need big capacitors to fix' this..
3)If the transformer is overloaded there is no way to "turn it down" as it cools down, -it will have to be switched off.

My solution is:
Take a normal transformer with a output voltage thats to small for the amplifier but with a decent power-rating. After the transformer you put a switchmode boost converter and convert the voltage to the right level. The conversion could be done with 90% efficiency and with a pretty simple cicuit.

The advantage would be that the power-supply is now fully regulated (+/- 3% or so) and there is i possibility to control the output voltage.

The output voltage could be controlled with a standard SMPS-chip and a PIC-controller. PIC's with 8 A/D-channels are very cheap and could be used to monitor all the relevant things in the amp: temperature, current from the supply, output voltage, Peak-Mean-ratio of the signal and so on. On the basis of all the measurements the PIC could "decide" the correct output voltage from the boost-regulator, the fan-speed and so on. Furthermore the PIC could control a LCD that could display all the measurements and give the user the possibility to alter the different setting.

If you include a VCA (controlled from the PIC) before the power amp the whole amp would be extremely adaptive and fail-safe.

I know this approach probably would be most usefull with big PA amp's but I think it could be used with Hifi as well.

So, what do you think of my idea? - Has it been done before, have you tried it and why should it not work?

(It would of course be "easier" to make a SMPS that works directly of the mains, but as I understand the rules we are not allowed to discuss it here. - I would also ending up killing my selfe so maybe that's ok.... )

TroelsM
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(as usual you will have to excuse my poor english)
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Old 14th November 2004, 12:43 PM   #2
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All this similar solutions have one big problem : delayed reaction time of delta voltage. You can do it only with digitaly delayed incoming signal, while direct signal are " preparing " supply voltage.
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Old 14th November 2004, 12:52 PM   #3
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Default Can you put schematic please, if possible, of course.

Thank you guys, with schematic things are easier to understand....hehe...and to copy to!

Carlos
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Old 14th November 2004, 12:54 PM   #4
TroelsM is offline TroelsM  Denmark
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I would not alter the voltage when ever there was a shift in the input-signal, but only when the transformer or the amp was getting to hot.

That way the rail-voltage could change very slow and you would not get nasty switching spikes on the rails.

The amp could work with +/-80V when power was applied and only when it was loaded very hard it could turn down to say +/- 50V.

TroelsM
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Old 14th November 2004, 12:58 PM   #5
TroelsM is offline TroelsM  Denmark
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I have not drawn a schematic at this time but it would probably all be very "standard"

Try googling: "Boost converter SPMS schematic" or similar.
The PIC-part is pretty easy and very flexible so people could make it just as they like.

If it seems to be a "do-able" solution I will work out a plan one of theese days.

TroelsM
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Old 14th November 2004, 12:59 PM   #6
joensd is offline joensd  Germany
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Quote:
when it was loaded very hard it could turn down to say +/- 50V.
But where´s the point in supplying +-80V to the amp in the first place then?
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Old 14th November 2004, 01:27 PM   #7
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Default OK! TroelsM, i will search.

Thanks.

Carlos
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Old 14th November 2004, 03:25 PM   #8
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by joensd

But where´s the point in supplying +-80V to the amp in the first place then?
I think the point is to allow a single PSU to be suitable fro a wide range of load impedances. For instance a high voltage, low current PSU might be great for an 8 Ohm load, but it would be overloaded with a 2 Ohm load. If it could be turned down so it's a low voltage, high current PSU then it would work great with a 2 Ohm load.
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Old 14th November 2004, 03:44 PM   #9
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One option would be to buy a ready-made off-line SMPS, that gets you around the safety issues and lets you start to built the system right away .
These generally allow setting the output voltage to say 110% - 30% of nominal. You can control that with a PIC, reacting on the *average* load of the amp, a quick reaction is therefor not needed.

One drawback could be that SMPS's are generally not constant power but have a max current that is constant with output voltage. So you would say buy one for 60V 5 amps, and then vary it between 40V and 60V, but even at 40V you can only get 5 amps as well..

It is not as bad as it looks, in fact an often overlooked advantage of SMPS's is that they beat almost any 'normal' transformer-input suply for current capacity.
A transformer is effectively disconnected from the supply for 80-90% of the time (except when the diodes conduct at the top of the mains sine wave) so for that 80-90% of the time the capacitors have to supply ALL the current to the load. With a SMPS the 5 amps (in this example) is available continuously 100% of the time, and now the caps only have to supply the occasional peak.

Jan Didden
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Old 14th November 2004, 09:04 PM   #10
Tim__x is offline Tim__x  Canada
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Lab Gruppen does something like this, just instead of automatically adjusting voltage, you set some DIP switches on the back according to your load.
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