Quote:
Originally Posted by amb
You're being overly optimistic. You'll get four times the output power only if there is no reduction of supply voltage nor any current limiting. The fact that the dynalo's output impedance is in the order of a few ohms (which causes voltage loss and current limiting to the speaker load) and you won't see anything close to that level of linear scaling at speaker impedances.
Let's just assume you're aiming for 2Wrms output into 8 ohms. A simple calculation tells you that the output stage needs to be able to deliver need 0.5Arms or 1A peak. The dynalo is not anywhere near rail-to-rail (in non-bridged configuration it could swing just a bit over 4Vrms into 33 ohms before clipping with +/-15V rails, much less into 8 ohms or 4 ohms), so to maintain the 2W output goal you cannot really decrease the rail voltage much even though bridging would increase the swing. If you could find TO-92 transistors that could handle 250mA+ of current, the power dissipation on them would be over 2.5W each even if you decreased the rail voltage to +/-10V. Not gonna happen on those devices!
Even if you substitute some beefier output transistors and heatsink them, assuming that the their Hfe is 100, the base drive current would need to be 10mA peak which far exceeds what the Vas stage flows so it won't work either. You'd have to basically re-engineer the whole amplifier...
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The comment on four times the output power was merely to demonstrate that output power scales 4x with bridging, but dissipation only scales 2x. So, for a given setup with energy dissipation as a limiting factor, bridging should double its power output capability all else things being equal. So, if the dynalo were only capable of 1/4 Watt, bridging should bring it to 1/2 watt without making the transistors any hotter. I also made the secondary arguement that since the effective impedence is halved, you see less voltage swing so lower rail voltage is possible, meaning you could increase the power further for the same output wattage, within the limitations of the new rail voltages.
I realize that integral to that statement is the assumption that there is a solution to the problem of current limitations. The basic Dynalo transistors are capable of dissipating almost half a watt, which would probably suffice for this application. My question is if there are any with the current handling capability aswell, such that a tradeoff between rail voltage and output current could be made.
With regard to dissipation, when you are pushing one amp of current into 8 ohms, the output of the amp would be at 4 volts for a bridged amp. Suppose you knocked the output resistors to 10 ohms and decreased the quiescent current a bit to improve power handling. (250mA per transistor)*10ohms = 2.5V + 4V. The emitter of the output transistors would be at 6.5V, so if your rail voltage were, say, 10 volts, their peak dissipation would be a little under a watt. RMS, that is more like half a watt, which is not totally unreasonable as a maximum load condition with the right transistors.
As for the preamp current capability, that can always be adjusted with selection of the right resistors. Just drop the impedence of all the resistors in the gain stages and POOF! more current and approximately the same gain.
All in all, though, I am starting to agree that the dynalo, as it is, is not reasonably easily adapted to low-power speakers. I'd probably only get 1 to 1.5 Watts per channel out of it, which is less than I would ideally like for small desktop speakers. The M3 using the case as part of the heatsink looks promising though...