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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
OK, that design is nice and simple. The main limiting factor is the BD139 and BD140 driver transistors, these see the sum of the rail voltages, which is what limits it to +/-40V as they're 80V parts. The MJE15034/5 recommended alternatives would remove this limitation. So would the MJE340/350, which are the ones I use in most of my amps.
The BC546 is also 80V rated, but they only see one rail.
So now we come up against the other limits I mentioned. Rod is worried that the output transistors he specified might be overstressed even at 100W/8 ohms with 40V rails. You're going up to +/-53, and the output transistors you chose are weaker than the ones he specifies. So, I'd definitely recommend adding another pair of output transistors, even if your speakers are 8 ohm.
Maybe move the 4 trannies you already have to one channel, and buy a new set of 4 for the other channel.
Finally, the circuit has no current limiting, and an amp of that power with no current limiting makes me uneasy. For goodness' sake don't short it out. Fuse everything.
It doesn't have catch diodes either, I don't know why Rod left them out.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Ok thanks for the advice. To double up the output transistors, I can parallel the collectors and bases and add another .33 ohm resistor from the emitter to the output, right?
Would it be too hard to add current limiting in there? Could I just hook something up that would cut the power if the draw from the supply gets too high?
Same question as the current limiting but with the catch diodes. What are they, how to add them if I can at all etc.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, paralleling the output devices is not as simple as that, because he used the Sziklai pair output stage, where the emitters connect to the rails and the collectors drive the speaker.
Really you should add 0.1 ohm ballast resistors between the emitters of the transistors and the rails, as well as the 0.33 ohm ones that go to the speaker. But Douglas Self just parallels devices in that configuration without even bothering, he simply connects their bases, emitters and collectors together. And he's 100x more of an audio guru than me, so maybe you can follow his advice and just tack the extra transistors on without changing the circuit in any other way.
Or change to the regular emitter follower output stage. I never liked the Sziklai pair one myself.
This page explains how current limiters work, and also shows the catch diodes and explains why they are needed. They're just diodes from the speaker output to the rails, to allow a path for inductive load current. Same as the schottky diodes we put on our SSTC gate drivers.
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Buck Converter and Audio Amp Design Advice
