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.
Regards
Bowdown
) Any info would be a HUGE help,thanx in advance to all the gurus out there.Regards
Bowdown
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.
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.
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.
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.
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.
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.
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.
Regards
Bowdown
Regards
Bowdown
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.
I would think it would be easier and more productive to just buy or build another amplifier circuit and cut the bandwidth so that each amp has its own bandwidth and correct type of speaker attached... Lows, mids, and highs. This will be more effecient for recreating full range bandwidth for certain. or just lows and highs.
Use the one you have for lows, and build a smaller more simple circuit for the highs and tweeters. You can then push up the gain and still have low distortion overall.
As was mentioned before, it would require an almost complete redesign of the circuit to make it more powerful, and then you would have a completly different amplifier afterwards.
Use the one you have for lows, and build a smaller more simple circuit for the highs and tweeters. You can then push up the gain and still have low distortion overall. As was mentioned before, it would require an almost complete redesign of the circuit to make it more powerful, and then you would have a completly different amplifier afterwards.
Ok thanx for your info, i might just see which way i go.This is the chassis that im in the process of finishing off to house all the circuitry let me know what you guys think.I should have more pics later on, oh by the way the pic quality isnt too good as it was taken by a camera phone.Cheers.
Regards
Bowdown
Regards
Bowdown
Attachments
Bowdown,
Here are some power dissipation estimates.
Power rails are +/-90V
Assumes one transistor per half, class B
8-ohm resistive speaker, single-ended (not bridged)
max avg output power = 500W approx.
avg transistor power = 102W @ 57V Vo peak
peak transistor power = 253W @ 45V Vce
peak Ic = 7A
The MJ21193 is rated at 250W average at 25C and 16A average current. But the power limit reduces with increasing case temperature. You sort of have to put a stake in the ground for the case temperature and then try to find enough heatsinking to maintain it. A good start is to set the case temperature at 70C - where the MJ21193 is rated at 186W.
You need to divide the avg transistor power by the transistor rating to get the number of transistors per output half. In this case 102/186 = 1. The Ic and peak power are ok so your amp needs 2 transistors in total (1 npn, 1 pnp). The total dissipation of all transistors will be 204W - your heatsink must remove 204W and keep the case temperatures less than 70C. For a 25C air temperature this means a heatsink rated at 0.2W/C or less.
When you bridge two amps everything basically doubles, except the output power which quadruples:
8-ohm resistive, bridged
max average output power = 2000W approx.
avg device power = 203W @ 57V Vo peak
peak device power = 506W @ 45V Vce
peak device Ic = 14A
Now you need 2 transistors per half per amp to meet the 203W average power and you need to check the peak power is ok - two devices at 506W (254W each) is just about ok since it is a short duration event. So you should use 4 transistors per amp (2 npn, 2 pnp). The total power is now 406W requiring a heatsink for each amp rated at 0.1W/C or less.
Now you have to be really carefull with speakers. They are not resistors and can exhibit wildly varying impedances at different frequencies. Therefore, for high reliability, a good rule of thumb is to design for half the nominal speaker impedance. So for an 8-ohm speaker design the amp to work ok into a 4-ohm resistor. You use less transistors and design special power limiting circuitry that dynamically measures the transistor power but this is quite complex.
When the load is halved you basically double all the numbers. In bridged mode with 4-ohms resistive load, the avg device power is 406W, thus requiring 406/186 = 3 (rounded up) transistors per half. Peak power is 1012W which is too high among 3 devices, so I would use 4. Each amp should have 8 transistors (4 npn, 4 pnp). Each amp could dissipate 812W so the heatsinking must be rated at 0.05W/C or less.
In summary, for high reliability, for an 8-ohm nominal speaker you should use 4 transistors (2 npn, 2 pnp) and a 0.1W/C heatsink. If you will operate in bridge mode, you should use 8 transistors (4 npn, 4 pnp) and 0.05W/C heatsink per amp.
I would think your 5kVA transformer is ample and 200,000uF ('u' is for micro, not 'm' which is for milli) capacitors are ample.
Here are some power dissipation estimates.
Power rails are +/-90V
Assumes one transistor per half, class B
8-ohm resistive speaker, single-ended (not bridged)
max avg output power = 500W approx.
avg transistor power = 102W @ 57V Vo peak
peak transistor power = 253W @ 45V Vce
peak Ic = 7A
The MJ21193 is rated at 250W average at 25C and 16A average current. But the power limit reduces with increasing case temperature. You sort of have to put a stake in the ground for the case temperature and then try to find enough heatsinking to maintain it. A good start is to set the case temperature at 70C - where the MJ21193 is rated at 186W.
You need to divide the avg transistor power by the transistor rating to get the number of transistors per output half. In this case 102/186 = 1. The Ic and peak power are ok so your amp needs 2 transistors in total (1 npn, 1 pnp). The total dissipation of all transistors will be 204W - your heatsink must remove 204W and keep the case temperatures less than 70C. For a 25C air temperature this means a heatsink rated at 0.2W/C or less.
When you bridge two amps everything basically doubles, except the output power which quadruples:
8-ohm resistive, bridged
max average output power = 2000W approx.
avg device power = 203W @ 57V Vo peak
peak device power = 506W @ 45V Vce
peak device Ic = 14A
Now you need 2 transistors per half per amp to meet the 203W average power and you need to check the peak power is ok - two devices at 506W (254W each) is just about ok since it is a short duration event. So you should use 4 transistors per amp (2 npn, 2 pnp). The total power is now 406W requiring a heatsink for each amp rated at 0.1W/C or less.
Now you have to be really carefull with speakers. They are not resistors and can exhibit wildly varying impedances at different frequencies. Therefore, for high reliability, a good rule of thumb is to design for half the nominal speaker impedance. So for an 8-ohm speaker design the amp to work ok into a 4-ohm resistor. You use less transistors and design special power limiting circuitry that dynamically measures the transistor power but this is quite complex.
When the load is halved you basically double all the numbers. In bridged mode with 4-ohms resistive load, the avg device power is 406W, thus requiring 406/186 = 3 (rounded up) transistors per half. Peak power is 1012W which is too high among 3 devices, so I would use 4. Each amp should have 8 transistors (4 npn, 4 pnp). Each amp could dissipate 812W so the heatsinking must be rated at 0.05W/C or less.
In summary, for high reliability, for an 8-ohm nominal speaker you should use 4 transistors (2 npn, 2 pnp) and a 0.1W/C heatsink. If you will operate in bridge mode, you should use 8 transistors (4 npn, 4 pnp) and 0.05W/C heatsink per amp.
I would think your 5kVA transformer is ample and 200,000uF ('u' is for micro, not 'm' which is for milli) capacitors are ample.
Thanx heaps for helping me out guys .I was thinking of using 8 pairs of output devices per channel (8 x mjl21193 + 8 x mjl21194) what load can i use this down to???Im still trying to get the hang of working out what type of outputs to use on amplifiers, ive been trying to understand the data sheets for the mjl21193/4 and im not sure if im right but i think they are only good to about 1 amp @ 100v, now is this correct? I really want to use to247 packages as it is easier to fit them.Please help me understand this stuff.
Regards
Bowdown
.I was thinking of using 8 pairs of output devices per channel (8 x mjl21193 + 8 x mjl21194) what load can i use this down to???Im still trying to get the hang of working out what type of outputs to use on amplifiers, ive been trying to understand the data sheets for the mjl21193/4 and im not sure if im right but i think they are only good to about 1 amp @ 100v, now is this correct? I really want to use to247 packages as it is easier to fit them.Please help me understand this stuff.Regards
Bowdown
I have used active crossovers in the past for automotive to achieve what cunningham stated and limited input bandwidth each amp had to deal with.
I have been running multi amps (mostly resurrected receivers with bass and treble controls) on my home system but am in the process of trying to upgrade quality of the components. I have been using the bass and treble as well as the volumes on the amps to achieve a fairly flat response. I am now planing on building some chip amps to replace the weaker amps at first. Eventually having mostly diy amps. I am using a Sony sdp e800 surround sound processor for my pre amp (I don't think it is capable effectively balancing the system on its own. I also don't want to add variable bass and treble to the new power amps. Does anyone have suggestions to help limit added circuitry but still give some tonal control/filtering to the inputs or outputs?
I have been running multi amps (mostly resurrected receivers with bass and treble controls) on my home system but am in the process of trying to upgrade quality of the components. I have been using the bass and treble as well as the volumes on the amps to achieve a fairly flat response. I am now planing on building some chip amps to replace the weaker amps at first. Eventually having mostly diy amps. I am using a Sony sdp e800 surround sound processor for my pre amp (I don't think it is capable effectively balancing the system on its own. I also don't want to add variable bass and treble to the new power amps. Does anyone have suggestions to help limit added circuitry but still give some tonal control/filtering to the inputs or outputs?
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