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In Reply to: RE: Your premise posted by PaulF70 on October 12, 2016 at 06:46:25:
Well, you want to match the two channels as well, so there is a minimum of 4 output transistors that have to be matched. I've read that transistor tolerances are very tight these days due to modern manufacturing methods, so maybe there isn't much to be gained from hand matching anymore. Also, if you're using paralleled BJTs in the typical arrangement, the emitter resistors inherently balance the output current across the devices, so if the devices aren't perfectly matched their variations kind of average out. So I would think that the more output pairs you connect in parallel, the less critical it is to match. One other thing I forgot to mention was noise. The self-noise from each transistor is uncorrelated with the self-noise from other transistors, so when you parallel them up their noise adds incoherently while the signal adds coherently. This improves SNR. In the best case where the output noise is dominated by the self-noise of the output transistors, you could net up to 3 dB SNR improvement by doubling the number of output devices.
My real-world experience with minimal output stages mostly comes from owning a Simaudio i-5 LE, which has a pair of BJTs per channel and is rated for 75W into 8 ohms and 120W into 4 ohms. I actually owned two of them. I ended up regretting selling the first and bought a second one on Agon, which I still have but am not currently using. I also own a Simaudio W-5 LE and Titan. At one point, I connected the W-5 LE to the preamp outputs of the i-5 LE so I could make a direct comparison of the amplifier sections. These share a similar amplifier topology and use the same devices, but the W-5 has 4 pairs of BJTs per channel vs. one and much greater power supply capacity. The little i-5 has slightly warmer and sweeter midrange tone, and seems a little faster and more rhythmically engaging, but the W-5 has greater dynamics, slightly greater transparency, and deeper tighter bass, though it's a bit leaner through the upper bass & lower midrange. I could not say the i-5 is sonically purer.
I've also heard Naim amps a number of times including a couple of auditions with my speakers. Naim uses a single pair of output transistors in their designs, but they are not among the most sonically pure solid state amplifiers I've heard. Naim amps are a bit grainy to my ears. However, they are rhythmically engaging and good at timing, so that is something they share with my Simaudio.
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Follow Ups
- RE: Your premise - Dave_K 11:54:45 10/12/16 (0)
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