Another discrete OPAMP

Jump to Latest
#21
The NPN/PNP cascode was used because the PNP transistors had very low Hfe.

The poor audio performance of early op-amps was due to the combination of a horribly non-linear amplifier, low gain-bandwidth product, and low slew rate. Feedback could not hide that the amplifier was basically crummy.
Ed
 
  • Like
Reactions: OnAudio
#22
Mark Johnson said:
Just because the uA 741 designer chose to perfectly match his R3 and his R12 {uA 741 schematic in post 12}, and then to make both of them 50K, doesn't automatically mean that 50K is the best value for your input stage with your bias currents.

So what IS the best resistor value for your discrete opamp? Assuming you'll build it using thru hole components, on a standard PCBoard made of standard FR4 expoy fiberglass.

_
Click to expand...
30k or infinity.This resistor will be optional. Sometimes there are diminishing returns on how low a current you can run in the LTP. The 50k they used was for a current half what we are using.
 
  • Like
Reactions: lineup
#23
EdGr said:
The NPN/PNP cascode was used because the PNP transistors had very low Hfe.

The poor audio performance of early op-amps was due to the combination of a horribly non-linear amplifier, low gain-bandwidth product, and low slew rate. Feedback could not hide that the amplifier was basically crummy.
Ed
Click to expand...
Yes technology has changed much, however these techniques are still musical
 
#25
Both editions of Bob Cordell's textbook praise (Q6 + emitter resistor), and he includes it in his most recent amp, called the BC-1 . Bob also used it in his error correcting power amp, published in 1984 in the Journal of the Audio Engineering Society (download link) . His choice of emitter resistor is much, much lower though.

(Q6 + emitter resistor) improves the current mirror but adds another 2 components.

John Curl sometimes prefers to improve the current mirror by adding two different components, this time resulting in a full 4T Wilson mirror. 4T Wilson has great performance but can sometimes be awkward to interface to the subsequent (second) stage.
 
Last edited:
  • Like
Reactions: OnAudio
#26
Thanks Mark.

Its been a while since I read all those books 🙂, am not sure how much I retained. The last I remember is that one should not leave an unused OPAMP hanging but should connect its output to its inverting input 😀. The limit on LTP current is based on the cutoff current which can be as high as 50uA and as low as 50nA. I still use Cordell's models. I had looked at Self, Pass, Sloan and many more university texts , how much I remember or makes sense is another matter.

Lets discard Q6 for now.😉
 
#35
I humbly request that each of the following gentlemen take the time to put together and hear these particular OPAMPs. Self, Paul, Cordell, and Pass. Sorry for being so confident, and best wishes if you already knew their secret. Hope you enjoy the treat.
 
You must log in or register to reply here.
Top Bottom