Paralleled Transistors


Is there any truth to the argument that many paralleled output transistors, despite strong attempts to match closely, will smear music signals as they are not identical. How about those designers. using only N channel mosfet pairs rather than complimentary P Channel devices? Just curious whether using larger more powerful Mosfets, and thus fewer pairs, is better in any way than let’s say 12 smaller pairs (24) per channel? Thanks for helping me to understand.
audiobrian
If this is true, we've never seen it born out in distortion, square wave or any other measurements. 

To the contrary, ultra simple circuits often have the most "color" added by distortion or high output impedance.
The people over here (including Nelson Pass) would probably have some relevant experience and opinions:  https://www.diyaudio.com/forums/pass-labs/
Pass is a good example, because while it has many fans, their uber simple design does not win everyone over for sound quality.

I think if the idea of transistor smearing was accurate or a convincing win no one would buy any amp that used more than one pair. Instead these types of designs, and tube equivalents, remain a niche sound.

I encourage the OP to find a FirstWatt kit and make his own, see what conclusions he comes to. :)

Best,

Erik
Not really. The more transistors paralleled results in less current through each transistor. The benefits are: less distortion and less of a beta drop, making any inherent beta mismatch not a great factor (and further reduced by emitter resistors that enable each to share current more evenly). The reduction in distortion, especially at 4-ohm loads and lower, probably negates any mismatch.

However, there is no free lunch. More transistors in parallel will make it harder for the amp to control parasitic oscillations. It will also require the voltage amplification stage bias circuitry to source more current for the multiple transistor bases, which may also require inserting a driver stage before the output. All of this drives up the cost rapidly. Considering there are excellent high power transistors available (On-Semi NJW0281G), it doesn’t make economic sense to put a lot of transistors in modest powered amplifiers.

As far as mosfets go, Nelson Pass is the one to ask. Mosfets are less linear, have higher internal capacitance (must be charged/discharged very rapidly at high frequency signals) and they require voltage swings of 5 volts (instead of 0.6) at the gate for them to conduct. Yet Pass makes them work great.
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