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Old 19th August 2004, 08:57 AM   #1
bowdown is offline bowdown  Australia
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Default getting more power from amp

Hi all. Just wandering if i want more power from an amp can i just double the output bjt's or mosfets,double the transistors on the VA stage and also increase the toroidal size, or do i have to do a whole lot more..I dont really want to build two modules and have to bridge them as i had a real bad experience bridging the amp last time.(All output stage bjts and all the driver transistors blew up ) Any info would be a HUGE help,thanx in advance to all the gurus out there.

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Bowdown
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Old 19th August 2004, 03:18 PM   #2
DrG is offline DrG  South Africa
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Nope, sorry... nice idea but fatally flawed.

Theoretically you could parallel more o/p devices and so gain more peak Iout into low impedance loads. But only if the driver circuitry could drive the extra transistors.

Paralleling VAS transistors will also not increase gain or peak voltage swing. Bias levels will almost certainly be thrown out in the process. And more distortion may result from loading the diff stage.

To increase power into a given load, say 8R, you would need more output voltage... which would mean higher rails... which probably means higher-SOAR devices... which would need bigger heatsinks... and higher rail voltages are likely to upset the bias of the small-signal and VAS stages, if not cook them... and you'd probably need to replace reservoir caps with higher-voltage versions.

So in my opinion the whole amp would need to be evaluated, with a high probability that you're going to end up with major circuit surgery. Better to start over with a new design I would say.
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Old 19th August 2004, 04:33 PM   #3
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Default More Power

I agree with DrG. The power output of an amplifier is the fudamental defining characteristic. There are so many decisions that have to be made once the desired power output is determined that to get more power required fundamental changes all through the amplifier.

Please note that anything less than doubling the power of an amp if essentially inaudible. A rule of thumb is that you need a 3db increase to appear 10% louder to your ears. Anything less than doubling the power of an amplifier is a waste of time.

Here's something like the process.

To get more power into the same load impedance, you need to raise the power supply voltage. To do this, you need a higher voltage power transformer, with higher amperage too because as you raise the voltage, you will also increase tha amperage draw. Now bear in mind that the transformer is the most expensive part in an amp. So now you have a bigger transformer.

This results in needing higher voltage filter capacitors. These are also expensive parts.

Now, you will need higher voltage output transistors, and with a higher current rating.

Once you do this, you may need more drive current.

Then since efficiency does not increase, you will also need bigger heatsinks.

To fit all this in, you will need a bigger case to put all this in.

Maybe a heavier power cord and power switch too. Heavier wiring. Maybe heavier speaker terminals.

By the time you're done, you have re-used the input socket. Everything else has been changed.


If you want more power by being able to run a amplifier capable of say 4 ohm operation into a 2 ohm load now, the same holds true except you don't need more supply voltage. However, you still need more current capability, meaning a new transformer and more output transistors, heatsinkd, case, etc. anyway.

So the answer is no. There is no way to increase an amplifier's power enough to bother with. Not without having to rethink every one of the myraid of compromises made to make the amplifier the power rating is has not.

And no, the designer of a 200 Watt amp does not build a 500W amp and put something to limit the power because the marketing dept want to have a 200W amp. A designer tries to squeeze every watt thay can and if an amp is capable of 500W, it is sold as a 500W amp, maybe even as a 1000W amp. Sometimes even higher.
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Old 19th August 2004, 04:40 PM   #4
sam9 is offline sam9  United States
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I agree with the previous post especially if you ar talking about an existing commercial amp. While the undertaking is not impossible, sucess is unlikely, There is a lot of economic pressure on a manufacturer build in the minimal acceptable capabilities. Bridging would be a lot easier though expensive.

If you were taking about moding a proposed DIY project there might be greater latitude since quite a few DIY designs are "over built" for the very reason that DIYers seem to always want a little more. If you are starting from scratch, it is much more feasible to enlarge heatsinks, toroids, etc than to retrofit them.

If you have a specific amp in mind, post it an maybe you will get more definite info. Please beware of posting copyrighted schematics, however.

In my opinion there isn't much benefit to increasing power unless it is by a factor of two or more. So if you are thinking of turning a 100W'er into a 125W'er you are usually talking about a lot effort to get a benefit that is barely percetable.
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Old 20th August 2004, 02:06 AM   #5
bowdown is offline bowdown  Australia
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Thanx guys i really appreciate all your advice.Ok so if i dont mind getting bigger heatsinks,bigger power supply and also more power supply caps and i want more power into lower loads then i pretty much all have to do is parallel more output devices, am i correct??Please let me know if i am correct or not.Im only looking at my options. thanx again guys.


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Bowdown
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Old 20th August 2004, 04:17 AM   #6
sam9 is offline sam9  United States
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Quote:
lower loads
Ah! Now you just might be taking about something a little more do-able. If your situation is that the amp in question is sufficient with (nominal) 8-ohm speakers but you want to use it with (nominal) 4-ohm speakers your endevor is a lot simpler. In this case, you don't need to change the rail voltage, you just want to be able to supply more current at the same voltage in to a lower load.

For either BJT's or MOSFET's you need to spend some time looking a datasheers and SOA charts. Depending on where you are starting from you may be able to replace existing output devices with ones that can handle more current. You will still need to increase the heatsinkage. Actually, you can get some additional capability just from larger sinks and better cooling since both BJT and MOSFETs can handle more current when kept cooler -- there is a limit to what can be done but there is definatly a potential here. Attention to how well the devices are mounted on the sink is important too. Also, if the device package is an open question, TO-3 metal cans, and "Sanken-type" packages are claimed to move heat to the sinks better than ordinary flatpacks. Can you bear the thought of a fan? Finally, if you are thinking lateral MOSFETs, check out the Exicon dual die packages.

The VA rating of the transformer may help you up to a point. It's kind of a "soft figure" since the manufacturers are not real forth-comming about how they derive it. Howeve, as a rough generalization if you take the watts you amp is intended to deliver into an 8-ohm load, multiply by the number of channels the tranformer will be expected to supply, then multiply by 2.5 -- that's about the point where diminishing returns set in -- i.e., beyond that the gains are minimal. As for caps 100uF per watt per channel seems a common figure that a lot of people treat a maximum practical. No harm is done by exceeding either of these but it is not always true that just because a lot of something is good, even more is better.

One last thought, MOSFETs don't require nearly as much current to drive them, they respond primarily to voltage. This means there is less concern about about demands placed on the VAS stage and thus parralleling output devices creates fewer difficulties for the "upstream" stages.
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Old 21st August 2004, 09:03 AM   #7
bowdown is offline bowdown  Australia
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Thanx again guys.I will try n parallel more output devices for lower loads.
Thanx

Bowdown
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Old 21st August 2004, 10:50 AM   #8
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Don't forget that when you half the load you quadruple the power, so you may need to quadruple your transistors. If you tell me what your rail voltages are and your power transistor types I can tell you how many you need for 4-ohms or whatever load you are thinking of using.
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Old 21st August 2004, 10:42 PM   #9
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more power???
how about the bootstrap with a cap and H-class?
it seems it is feasible with no major changes concerning transformer, output transistors etc., but i'm affraid not that easy.
What do you think??
cheers
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Old 22nd August 2004, 01:29 PM   #10
bowdown is offline bowdown  Australia
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Thanx Traderbam,Im using +/-90v and mjl21193/4 for outputs.Let me know how many i need.I have a 5Kva toroidal on order and was going to use 200,000+ mF per rail, Will this be enough??
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Bowdown
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