mattcalf wrote:[*]Optional (well it's all optional I guess, but this one especially?): Reducing the gain to 7 by changing the global feedback resistors to 1.21KΩ and 200Ω (from 2KΩ and 100Ω) and increasing the feedback compensation cap value from the suggested 33pF.
With the lower gain I observed a peaking of the frequency response in the ultrasonic range, which means that the amp is borderline unstable. For those who don't have a wideband function generator, an oscilloscope and a wide selection of compensation cap values to tune the amp with, I recommend staying with the default high gain. This of course means that the dynahi would be suitable for high-impedance headphones only.
In a Dynamite setting, what VA rating on a transformer would I need for dual Sigma22s (heatsinked by conrad) to comfortably drive my speakers (a 6 ohm multiway speaker and has dips down to 4.5 ohm and is less than 5 ohm from 100Hz to 600Hz)?
While the dynahi has enough voltage swing and could be equipped with large heatsinks, I don't really recommend it as a speaker power amp at all. The output stage is four paralleled pairs of BJTs, but those output devices are rated only 1A max each, for a total theoretical max current of 4A. They are driven directly from the VAS (voltage amplification stage). Since BJTs are current-current amplifiers, the VAS has to supply the output current of the amp, divided by the Hfe of the output stage BJTs.
Assuming an 8 ohm load with a maximum output voltage swing of 21V peak, that means 2.6A of peak output current. This is 65% of the total rating of the output devices. Also each output branch has 10 ohm emitter resistors (two 20 ohms in parallel, and there are four such pairs, so the equivalent is 2.5 ohms), and they will severely reduce the output swing (at 2.6A there will be 6.5V drop on each resistor). This is not to mention that at such currents each resistor would have to dissipate over 2W of power, but the specified RN60D resistors are only rated at 1/4W, and could realistically be called 0.5W resistors.
OK, so let's say if we used much lower value output resistors (e.g., 0.47 ohm) to get around the voltage drop issue (which, in reality could lead to BJT thermal runaway but let's ignore that for now), and assume the output transistors' Hfe is 150, then its base drive must be 2.6A / 150 = 17mA. But the total current flow through the VAS is only around 30mA or so, so more than half of that current will have to go into driving the output BJTs, clearly not ideal for high performance.
If your speakers have lower impedance than 8 ohms, then the situation becomes worse. Especially in a "dynamight" configuration where the output voltage is doubled, so the output current is doubled as well.
The dynahi was designed to be a headphone amp, not a power amp. It could be used to drive speakers as is, and it would probably even achieve reasonable volume, but as a speaker power amp, it needs larger output devices, an addition of a pre-driver stage before the output stage to offload the VAS, reduced output emitter resistor values and a proper output stage biasing circuit with thermal-feedback.
