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Old 18th January 2007, 12:36 PM   #1
bowdown is offline bowdown  Australia
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Default Adding more transistors to output stage?

Hi i was just wandering what would happen if you add extra output tranisitors to an amplifier? I guess you would be able to run a lower load. but what is the possible maximum outputs that can be added to a current amplifer for example a leach amp or a super leach? Would you have to make any other changes to the circuit? Would you just keep on adding more and more in parrallel? Any help on this subject would be highly appreciated.


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Bowdown
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Old 18th January 2007, 01:25 PM   #2
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Default Re: Adding more transistors to output stage?

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Originally posted by bowdown
Hi i was just wandering what would happen if you add extra output tranisitors to an amplifier? I guess you would be able to run a lower load. but what is the possible maximum outputs that can be added to a current amplifer for example a leach amp or a super leach? Would you have to make any other changes to the circuit? Would you just keep on adding more and more in parrallel? Any help on this subject would be highly appreciated.


Regards


Bowdown
Mostly depend on what generation of output devices you are using (if bipolars): with modern devices as Toshiba or Sanken are, once provided tha Hfe is reasonably matched, adding output devices improve dissipation capabilities of the amplifier, *not* his power capabilities to cope with heavy loads: the drivers are just the same and, if they are not designed to soustain drive current required by single or doubled output devices when heavy loaded, then they simply don't.

With old generation of bipolars (roughly that on sale 'til the mid of '80), paralleling device may bring to a surprisingly different result in linearity of amplifier that, with the *same* driver current, may drive more heavy loads... and more linearly. That's because old power transistor (with some uncheap exceptions) start decaying in their beta very soon, often when the collector current go over a couple of ampere or so.
Nowadays this limit has been shifted up at least of 4-5 times (i.e, Hfe is reasonably soustained 'til 8-9 ampere and this make a *lot* of difference because enhance the treshold were a "good" amplifier became a "bad" amplifier from the linear viewpoint: once that threshold was comprised roughly between 5 and 10-12 Watt on 8 Ohm speakers while now is often well over 50-60 Watt on the same load impedance i.d. the output section is *really* linear on the full range of practical domestic powers.

With Mos ouput devices is obviously a totally different situation but... not so easy with paralleling devices as generally believed.

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Old 18th January 2007, 03:01 PM   #3
jaycee is offline jaycee  United Kingdom
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Adding extra devices, in itself, will not yield a higher output wattage from the amplifier. What it will do, is give more margin in each transistor, so they won't be stressed as much.

Generally increasing the output stage size is one part of increasing an amp's wattage... but you also need to increase the rail voltages and for that... you may well have to redesign a large part of the amp.

You also need to be sure that the driving stage before the outputs can handle the load. This depends largely on the transistors used for the driving stage. Most will handle about 4A.


The Super Leach uses the extra transistors in a cascode configuration to deal with the higher voltages seen from collector-emitter by each transistor.
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Old 18th January 2007, 04:11 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
the original Leach (lo tim and later upto 4.5) used just 2pair of To3 output devices.

The Leach clone on this Forum used 3pair of To247/264 plastic packaged devices.

The extra capacitance from this extra device does not seen to be a problem.

The extra gain from modern devices takes a lot of stress away from the drivers.

The extra current through each upto the droop point again reduces stress on the drivers.

Taking account of both these effects, more current in the linear region and more gain, has allowed the driver a very easy life,
I suspect that the extra capacitance from extra devices will not become an issue upto about 4pairs when using the modern high gain, higher current outputs.

The original Leach could go down to just about 4ohm moderate reactance and 8ohm severe.

3pair of MJL4302/4281 can do 4ohm severe reactance.
4pair of MJL4302/4281 can almost manage 3ohm severe.

Seems like Jans' choice of 3pair plastic was rather good value.

3pair of the 150W plastic devices will give similar load driving ability as 2pair of the To3 in the original.
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Old 19th January 2007, 11:44 AM   #5
bowdown is offline bowdown  Australia
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Thanx guys for the replies, Im not after more output, i was just curious and thought if there are significant pluses in doing it, i might as well do it. I think that the lowest Impedence i would use the amp is probably 2 Ohm, but is there any chance of running the leach and super leach down to 1 Ohm?? Just a question. Dont think i will ever run it that low but i thought if i could design it so that the board is in separate pcbs, i could just keep adding "lots of 4 pairs" as i would have a pcb up until the drivers, then the output stage on its own, then the catch diodes and zobel network and lastly the bias board with the diodes on it. What do you guys think of that?


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Bowdown
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Old 19th January 2007, 01:59 PM   #6
sss is offline sss  Israel
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more output transistors will improve the damping factor , but the capacitance seen by the driver will be increased also , and can cause an oscilation.
if u want more power , the power supply have to be changed also
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Old 19th January 2007, 07:15 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
I think that the lowest Impedence i would use the amp is probably 2 Ohm, but is there any chance of running the leach and super leach down to 1 Ohm
if
Quote:
4pair of MJL4302/4281 can almost manage 3ohm severe.
cannot quite manage 3ohms from the standard 8ohms severe, then it seems that a LOT more than 4pair is going to be required to get down to 2ohm and I doubt 1ohm is even worth wasting time considering let alone trying to implement.

Have a look at the output stages of the KSA50 and KSA100 in the respective Klone threads and see that Krell doubled up the driver AND output when beefing up the smaller amp.
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