Hello.
It's a simple push-pull circuit, it's bridgeable.
It's a simple push-pull circuit, it's bridgeable.
An externally hosted image should be here but it was not working when we last tested it.
Insofar as all commercial amplifiers must be rated to drive reactive loads, we have to say yes. If you refer to particular loudspeakers having unusually reactive or crazy roller-coaster impedance plots, dipping below 40% of their nominal impedance in the bass region, then any conventionally specified amplifier used near its maximum power rating will be in peril there.
@Jempie
When bridged, the risk of amplifier failure is greater because of likelihood that users will continue with their low impedance speakers or assume that the nominal 8R ratings are correct, when today's loudspeakers are over-rated by about 25%. However, the NAP200 is fitted with dual-slope VI limiter circuits which should protect the output stage, though enough foolish experiments with how loud it can play could probably still break it. For the value of these gems, I'd be looking at something cheaper to tinker with or just build 200W NAP monoblocks from kits, if that's what you really want.
Bridging amps rated to drive 8 ohm loads, requires each half to be able to cope with a 2-ohm load: because bridging doubles the voltage swing into the load, and therefore, potentially draws 4 x the current. I=V^2/R
If a power amp isn't enough on it's own, bridging it's channels to drive one speaker ...is rarely a good answer, at all. It isn't likely to have the heatsinking capacity, let alone the SOA capacity to deal with the imposition (you've just more than doubled the thermal loading on the only output pairs it already had - this does not end well) Trying this no factory NAPs will go nowhere useful.
And ye, while you can build a nap clone of nominally '200w' capacity on voltage rails alone - to actually swing enough useful current takes more than one output pair; and you'll find no such clones out there: so you are into well-informed homebrew anyway....
If a power amp isn't enough on it's own, bridging it's channels to drive one speaker ...is rarely a good answer, at all. It isn't likely to have the heatsinking capacity, let alone the SOA capacity to deal with the imposition (you've just more than doubled the thermal loading on the only output pairs it already had - this does not end well) Trying this no factory NAPs will go nowhere useful.
And ye, while you can build a nap clone of nominally '200w' capacity on voltage rails alone - to actually swing enough useful current takes more than one output pair; and you'll find no such clones out there: so you are into well-informed homebrew anyway....
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I appreciate your experience and comments there, Martin.
There are some ways to parallel a second pair of output transistors that can be done with the earlier "NAP140" clone PCBs but as you say, it's in home-brew territory and requires drilling a few extra holes in the PCB and mounting all 4 power transistors on either the machined rear surface of a substantial heatsink or 2 each on top and bottom faces of a thick bracket that is mounted to one. I doubt that Naim's traditional heat spreader mounted on the aluminium chassis for cooling would be anywhere near adequate at full power there unless the chassis was heavily built with welded seams.
A bit of creativity is needed with mounting locations for the power transistors, such that the leads of pairs are in fair alignment, though unless you plan it right, the leads of each parallel pair will probably wind up in reverse order. Advantage can taken with using the additional "emitter" resistors as a 'jumper' link there. It remains to find an appropriate location for the Vbe multiplier (aka thermal sensor) if the case isn't constructed in the compact NAP thermal design manner that provides a steady, if very sluggish bias.
I've done a high power version this way quite some years ago, using MJL3281 transistors. It was quite an adventure; learning the quirks of NAP designs, biasing and sorting some of the many myths from the facts about them. I only pushed my build to about 160W/4R with the power supply I had to hand - too chicken to go further when the sound emitted from the PCB became unnerving as output was increased much above 100W
. It's normal in large amplifiers but I don't often need to do full load tests.
There are some ways to parallel a second pair of output transistors that can be done with the earlier "NAP140" clone PCBs but as you say, it's in home-brew territory and requires drilling a few extra holes in the PCB and mounting all 4 power transistors on either the machined rear surface of a substantial heatsink or 2 each on top and bottom faces of a thick bracket that is mounted to one. I doubt that Naim's traditional heat spreader mounted on the aluminium chassis for cooling would be anywhere near adequate at full power there unless the chassis was heavily built with welded seams.
A bit of creativity is needed with mounting locations for the power transistors, such that the leads of pairs are in fair alignment, though unless you plan it right, the leads of each parallel pair will probably wind up in reverse order. Advantage can taken with using the additional "emitter" resistors as a 'jumper' link there. It remains to find an appropriate location for the Vbe multiplier (aka thermal sensor) if the case isn't constructed in the compact NAP thermal design manner that provides a steady, if very sluggish bias.
I've done a high power version this way quite some years ago, using MJL3281 transistors. It was quite an adventure; learning the quirks of NAP designs, biasing and sorting some of the many myths from the facts about them. I only pushed my build to about 160W/4R with the power supply I had to hand - too chicken to go further when the sound emitted from the PCB became unnerving as output was increased much above 100W
. It's normal in large amplifiers but I don't often need to do full load tests.
Flattering, Ian, but utterly unwarranted - you've clearly pushed the idea far further that I!
And your concerns are right: such mods quickly become a study in determining, even re-establishing unconditional stability: it is not a given.
One error I made in my post above - each half in a bridge has to deal with half nominal load impedance, not 1/4: but that does mean the typical use of bridging an amp (hey! I want to drive >>100w into a 4ohm load!) means - each side of the bridge 'sees' 1/2 the target load impedance - back to 2ohms. Which is brutal, and the reason 'bridging' rarely helps - for any amp architecture, not just the NAP-alike.
Best
M.
And your concerns are right: such mods quickly become a study in determining, even re-establishing unconditional stability: it is not a given.
One error I made in my post above - each half in a bridge has to deal with half nominal load impedance, not 1/4: but that does mean the typical use of bridging an amp (hey! I want to drive >>100w into a 4ohm load!) means - each side of the bridge 'sees' 1/2 the target load impedance - back to 2ohms. Which is brutal, and the reason 'bridging' rarely helps - for any amp architecture, not just the NAP-alike.
Best
M.
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