Mod a Pa amplifier

To move a sealed sub whith LT i have buy an used QSC RMX 1450.
Yesterday i have open the case amp and dowloaded the schematic from qsc site:
http://www.qscstore.com/rmmodaschan21.html

Now i want to do some mod.
First one i want to add more capacitant, 6600uF for a rail of a channel is not to much; the second mod is to add best decoupling cap for the 2 opamp.
The third mod is to add 2 pair of transistor fo channel (1943/5200) to have more current capability at heavy load.

The questions are:
Which is the acceptable ammount of capacitant for this amp? (for a rail of a channel)

Can i add 2 pair of 1943/5200 for channel whithout any other modifications?

After that modification must i do reset bias current?

Is this amp in class AB?

Thanks for any reply
regards
D.N.
 

Villaw

Member
2005-02-09 1:49 pm
Ni
You can carry out all the mods you ahve suggested.
If I was to modify that amp I would,
1. change the caps to better quolity metalised polyprops
2. add more capacitance in the PSU should space allow and by pass them with polyprops
3. change (if possible) the opamps to burr browns.
4. change the fets to bipolars (difficult)


Have fun.
 
Hi,
the output bias controller will deliver the same voltage to the extra output pairs. The doubled up output pairs will now dissipate twice the heat. Have a think about running temps and heatsink capacity.

The doubled up outputs will run at reduced current into the SAME loading and REDUCE the load on the drivers due to improved current gain in the outputs.

BUT you want to increase the low impedance capacity. This WILL increase the loading on the drivers. You need to check carefully the SOAR of the drivers into your proposed worst loading conditions. I recommend a substantial increase in driver heatsinking to extend the driver SOAR when under severe load.
Another alternative would be to double up the drivers as well.

I recommend PSU caps @ +-2mF for each amp of peak output current into your proposed new load. e.g. 40Vpk into 8R needs 10mF, 56Vpk into 4R needs 28mF. Smaller // caps have less inductance and higher ripple capacity so 4 times 6.6mF is better than 26.4mF, but not everyone agrees with this view.

All your mods make sense. I would add one more, increase the onboard decoupling and bypass with PP(MKP) and ceramic.