Friday, January 20, 2023

High Precision Composite Op-Amps, Part 5 - Cart Before Horse


This post is a part of the series on audio amplifier feedback. The contents of the series can be found here.

In my previous post on this topic, I gave a couple of practical examples of composite amplifiers with the topology described by John D. Yewen's article in Electronics & Wireless World, February 1987 and promised we can do even better.

One of the issues with Yewen's topology:

is that R1 R2 (in the schematic above) attenuate the signal amplified by U1, which causes U1 to work extra hard, or, more precisely, work with a higher input signal. As was discussed previously on this blog, this affects the linearity of U1's input stage and adds distortion that cannot be corrected by feedback.

One way to address this issue is to use a better opamp as U1, but it is much easier to move the divider to U1's input:

R1 R2 should be large compared to Ri to avoid an unwanted noise gain increase and a loop gain reduction (see the discussion in my previous post), but otherwise this works exactly the same as Yewen's composite, only with lower distortion.

Naturally, this approach also works with frequency dependent dividers:

A practical example is a little composite headphone amplifier:

Here the lower leg of the frequency dependent divider is connected between the first opamp's inputs (suggested at RCL-electro, a Russian-speaking DIY audio forum) - the opamp cares only about its differential input voltage. Also, a lead compensation capacitor C1 is connected across Rf to improve phase margin. This composite beats hands down the Objective2 headamp (which is built with the same opamps).