A75 problem

[ckwong99]

Dear Friends,

Can anyone help me solve the problem about A75 .

This is my final changing applied on this AMP (PCB and schematic design follow to Norman and Pass original that posted at passdiy) :

1) Only use single end signal input design.
2) increase input series resistor R4 to 3K
3) decrease input GND resistor R30 to 10K
4) increase input GND cap C1 to 200p
5) increase cap C2 to 200p.
6) Optional C9 and C10 cap were installed with 100p.
7) 2 x100u cap installed on the broad +/- DC source
8) R81 feed-back resistor equal to 500 ohm.
9) Back-end uses Hitachi J56 & K176 , so the bias network Q11 I changed it to Hitachi version match to the characteristic of back-end FET.

Test result 1 :
The front-end when input signal didn't short to GND and completely isolated from back-end. I can adjust a DC offset tends to zero (around 1mVDC) and without any oscillation but only got 2~3mVAC at 100Hz. When feed a signal to input , I can measure a clear signal output maximum peak-to-peak is 46VAC and the bias applied about 1.8VDC.

Test result 2 :
When the whole circuit was connected , the input was connected to GND . I have a slight adjustment on P1 & P2 then the speaker output DC offset was controlled in 4mVDC and 5mVAC at 1KHz. The system can stable lasting for an hour long.

Problem find :
Keep the setting of above test result 2 , I try to open the input and GND wire connection. The meter showed VAC was fluctuated and very fast the resistor R31=5.1 ohm was over-heated on front-end broad. Even the input connected by pre-amp the problem is still exist.

My questions :
1) What happen to my system when I open the input and GND wire connection ?
2) Why the system can stable lasting for an hour long when the input short to GND ?
3) How to verify and address this problem ?
4) The front-end connect to back-end with 2x13cm signal cables for +/-DRV, is it will too long ?
5) R31 and C8 they are the filter network, should it be take out from the front-end broad ?

CK