BUS Amplifier - function of Inductor in parallel with resistor at the output

[ln76d]

I'm confused what are the functions of some parts at the output of the circuit.
What should be the role of L7/L8 10uH inductors?
What are for  C70/C71 (550pF sum) capacitors in transformer feedback ?
What gives the zobel (can i call it zobel) with R85/C67? 

Schematic of one channel in attachment, parts marked red.

Any help appreciated

 

[JohnRoberts]

Without knowing more context I would speculate the inductance there is to decouple capacitance from a several foot long bus wire.  Too much capacitance to ground at that node can make the amplifier unstable.

JR

 

[PRR]

> What should be the role of L7/L8 10uH inductors?

Look in any transistor power amp.

Read Doug Self's and Codell's power amplifier books.

There are several papers about the Jensen 990 opamp which discuss this topic.

Understand, or just copy.

 

[ln76d]

Multum thanks!

Here's the good explanation from Jensen JE-990, if someone is interested!

"To isolate capacitive loads (in order to prevent them from reducing the phase margin by delaying the feedback) with-out significant losses in the operating frequency range, the parallel network of R15 and L3 is connected in series with the load only.
The feedback is still derived from the output node before the load-isolation network. The 40uH inductance with the 39Ω resistor exhibits an impedance pole at 155kHz. Therefore the series impedance is 39Ω above 155kHz. But below 155 kHz the impedance decreases to about 0.2Ω at dc. At 20 kHz the impedance is 5Ω  at 80°, resulting in only 0.11 dB loss with a 75Ω load."

Source:
http://www.technicalaudio.com/pdf/Jensen_Transformers/Jensen_OpAmps_990_and_related/Jensen_JE-990_opamp_JAES_reprint_1980.pdf