Can I use MANY MANY low power devices for 1 HIGH power amp? - Page 4 - diyAudio
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Old 13th August 2004, 09:32 PM   #31
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Here's a 622. Sorry I don't have a cleaner image.
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Old 13th August 2004, 11:05 PM   #32
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Thanks! does anybody get that topology... maybe it's the beer, but it's just too many crossed wires now


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Old 13th May 2013, 11:57 PM   #33
Maitchy is offline Maitchy  New Zealand
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Interesting to look at this discussion after Self's circuit for a power amplifier massively paralleling opamps! Just a couple of points to add here:

1. using many lower power transistors (in parallel, bridge-tied, etc... all with problems to solve like power sharing with thermal stability and so on) does have an advantage that can make it worthwhile... look at the hFE vs current curves for most power transistors - when you try to use most transistors at even half their maximum current the hFE is falling significantly; in some situations this might not be a big problem (but very often it is), and high-power transistor tend to have much lower hFEs at the best of times. Compared with that the AD161/162 combination has a high gain that is releatively constant with current and using many in parallel gives a more linear device than a single much higher powered device. As I say, it depends on the circumstances as to whether this a big problem or not, but most driver stages are high impedance (even in the Silicon Chip 200W MOSFET circuit I'd say the VAS/driver needs a better approach), so hFE is likely to be darned important.

2. There are many side issues with the examples given that need to be looked at in their own right, to avoid clouding the issue of paralleling many devices. Some of the options use old transistors that will have poor high frequency performance (anything like 2N3055 vintage, and unfortunately the germanium ones) that could rule them out (yet I know of beautiful-sounding 2N3055 guitar amplifiers... so it is going to depend on how they are used). And some MOSFET designs make great projects because they will take all manner of ill treatment, and within MOSFET options the cheaper devices like hexfets are not going to sound so great as laterals - where the complementary devices match better (hence lower THD) and the temperature coefficients are more suited to square-law designs like Ian Hegglun's... but they have limited current capability so really would benefit form using several in parallel.. yet doing that could spoil the square-law goal is source resistors have to be used. That said, I did come up with a (still untested) design that allows many transistors to be used in parallel without being super-matched (i.e. have some series resistors to ensure good current sharing), yet retains (and possibly enhances) square-law efficiency and distortion cancellation. I might publish it here if the co-designer of the circuit doesn't mind.
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Old 14th May 2013, 04:15 AM   #34
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Maitchy, it's an interesting topic, but did you notice the thread is from 2004?
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Old 14th May 2013, 10:46 AM   #35
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a 10pair mje15034/5 each biased to optimum ClassAB would give a high total bias for good ClassA output.
10*2*50 = 1000W of total output stage wattage.
Divide by 5 to give ~200W of maximum power capability.
Reduce this by 30% to 50% for the higher operating temperature of the high biasing current and you end up with a low power copy of a very famous Power amplifier. Is it Halcro that does similar?
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Old 8th July 2013, 10:44 PM   #36
Maitchy is offline Maitchy  New Zealand
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Default History repeats...

Quote:
Originally Posted by bear View Post
Maitchy, it's an interesting topic, but did you notice the thread is from 2004?
Yes... but notice how the idea of paralleling multiple low-power devices came to life in the relatively modern Elektor design using opamps. It is a good example of how "old" threads don't really become outdated. I bet there will be more designs for very good amplifiers using cheap-but-good devices (and have some of my own designs under way), and not just when somebody has a heap of devices sitting around without a use - look at the cost of some modern low-power high-gain good-linearity transistors when bought in quantity, compared with the price and quality of one big transistor they might replace. Heatsinking can be easier (since the heat is distributed over a large area). Matching devices becomes very easy too.

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