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On Saturday, October 14, 2017 at 10:33:23 AM UTC-4, Don Pearce wrote:
On 14 Oct 2017 09:54:25 -0400, (Scott Dorsey) wrote:

Don Pearce wrote:
On 14 Oct 2017 08:27:00 -0400, (Scott Dorsey) wrote:

A simple follower has 100% feedback and should have very very low distortion
as you note. So start changing things. Replace that current source with a
resistor. Change the source impedance using a shunt resistor. Pull the
transistor out and stick a TIP50 in (a personal favorite trick since a TIP50
has just awful linearity).

A simple follower does have 100% feedback, but unfortunately not from
the right point. The current-dependent emitter resistance fall outside
the loop, in series with the output, so it will add distortion which
gets worse as the load resistance decreases. It is this resistance
that prevents emitter followers from achieving unity gain - they are
always a bit below.

Yes, this is true. And you can reduce the emitter resistance to make
it better into a lower load impedance, but then you can't swing as much
voltage on the output.

But, if this is the issue, you can tell because the distortion character
will change with the load impedance. You can see it on a scope and you
can hear it change as you add shunt resistances to your load.
--scott

Yep. More current makes this better. But better yet is some gain in
front and an external feedback loop that comes from the emitter pin.
Op-amp anybody?

The phono cartridge preamp I made used (2) Op-Amps per channel, via Walter Jung's design. Even published in (defunct) High Fidelity Magazine. Impressive.

Jack


d

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