Question about HA1457 IC chip discrepancy in my Yamaha PM2K console

[JW]

Attached schematic.

I noticed today that IC #2 in this circuit has a 1K resistor between pin 6 and ground on two of my buss channels. The other channels follow the schematic, and have a direct link to ground from pin 6 (no 1K)

Can somebody perhaps educate me as to what is going on? (Resistor does look stock from the factory in these channels, as it is exactly the same as the 1K to ground from pin 5 after the 33pF cap.) I'm inclined to follow schematic, but if it doesn't really matter, maybe I'll just leave the 1K resistors in those channels. Not sure. . . . . .

 

[Khron]

http://nice.kaze.com/av/HA1457.pdf

Judging by the schematic on the first page of the datasheet, pin 6 is the non-inverting input of the opamp, and pin 7 is the inverting input. While usually, in the inverting configuration, the non-inverting input is connected straight to ground, adding a (trimmed / trimmable) resistor there helps with balancing the impedances, and as such, minimizing the output DC offset.

 

[JohnRoberts]

Attached schematic.

I noticed today that IC #2 in this circuit has a 1K resistor between pin 6 and ground on two of my buss channels. The other channels follow the schematic, and have a direct link to ground from pin 6 (no 1K)

Can somebody perhaps educate me as to what is going on? (Resistor does look stock from the factory in these channels, as it is exactly the same as the 1K to ground from pin 5 after the 33pF cap.) I'm inclined to follow schematic, but if it doesn't really matter, maybe I'll just leave the 1K resistors in those channels. Not sure. . . . . .

The 1k resistor in combination with the 100pF cap from - to + input shunt some of the negative feedback signal harmlessly out of the input.  This will improve stability while scrubbing off some open loop gain. The inductor in series with the program bus (not buss) is to decouple bus capacitance to ground, also a source of instability.

The other 1k to ground in the compensation network is coincidental, and not related.

JR