mediatechnology wrote: ↑
Fri Feb 15, 2019 7:03 am
The Symetrix design lends itself well to a simple AC-coupled Rgain without overall NFB.
I don't like the cap coupled gain either, while I have used it more times than not...
I was thinking about the Symetrix circuit and how one would add feedback to it.
Since the op amp output isn't floating up to Ve, the (current) feedback would have to be AC-coupled into the emitters.
Keeping the resulting 2 pole response from peaking might be difficult.
That's what I was trying to describe a couple posts ago.
If you take the actual output from the top of the caps there would not be a LF bump there, just down at the op amp.
I do remember that drawing.
There sure are a lot of transistors in that level-shifting circuit.*
no more IMO than needed to perform the task well... the low impedance buffer stages could be removed and replaced with op amps but that would involve far more devices. The all discrete nature of this might appeal so some market segments, who buy with their eyes.
I have looked at replacing my 3-put circuit with real op amps and it would take at least two, and they would need to be biased up at +24V so marginal for 36v IC process. The bias point could be lowered with lower final output level perhaps requiring make up gain. A third opamp to single end the output could add gain (and noise), but then we would likely want a 2 legged output so even more additional ICs.
(Statement coming from a guy chastised by the engineering manager for using an 8 cent multi-sourced part in production for 30+ years. TL431 cough.)
I should not have to defend my design cred (and for the record I have been more than an engineering manager, that was just one of my several positions along the way), but I am glad you are so easily amused. If only 30 years old that part was not even in production, when I started working at Peavey
. It would be far from a cheap, old, trusted part to look into alternate applications for back when I was most active in executing new circuit designs. FWIW I also had to deal with a number of specialty ICs that went EOL (like dedicated meter ICs) during my time in engineering. I'm sure they all seemed like good ideas at the time.
I am not opposed to embracing new technology and used my share of new components in my drum tuner designs (like class D chips, LDO cmos regulators, and LED drivers). Back at Peavey there was a pretty high threshold to bring in new parts without some compelling advantage. Over 15 years I brought in only a handful (like a PTC fuse for my trick boosted rail power amp to power limit the boost stage) but not many. I already had a rich inventory of components to pull from. No need to bring in a low noise 737 when there was already a 786 in the system.
As I have stated many times, use whatever floats your boat, you don't need (or apparently value) my blessing.