[Sugimoto, Enomoto]

We made the following changes to summing node (spare one) and replaced it with the old one, to utilize it as a differential receiver (see also 9920).

= Change summary =
- Comm. comp. gain was increased by 20dB (same mod. as the old one). 
- The fast path was hacked so that the point just after VGA of IN2 is connected to the point just before the Fast Pol, and two 5Hz HPFs in the fast path are changed to voltage followers.
Signals from the common path is no longer sent to the fast path.

= Change detail =
- R68 120k --> 12k  
- C279, C280 1nF --> 0Ohm  
- C275, C276, R178 and R179 are removed  
- Pin #7, #9 of U47 are cut  TP4 and TP27 are connected by a jumper wire  
- U73 was replaced by a new one since one channel of it had been broken