Hafler 9130/Pro1200 circuit differences?

[reiver]

A question for fellow Hafler owners.

I'm modifying my Hafler 9130 amplifier, and have been comparing the schematics between it and the Pro1200. They both appear to be identical except (except for one resistor, R21) and the separate circuit that looks like the power supply to drive the front end.

I'm no expert on circuit design, can anyone suggest the difference between these two circuits, I assume the 9130 version is simply a "better" design and the Pro1200 version simplified to lower costs?

-Bryan

see attached

 

[reiver]

re:

I figured it out.

The "left" side of the circuit is the thermal circuit which shuts down the amp when overheating. The new "right" side is the soft on/off circuit which came out on the 9130, but not on the Pro1200.

 

[DavidT]

The other change is the VAS bias resistor R16(R44). It was raised from 27 ohm to 33 ohm, perhaps to lower heat dissipation in the associated transistor string, to perhaps lower service repairs...have not cranked any numbers on that change...

O.K....just cruising by, looking into another matter and noticed this thread, 14 years later...HTH.

David

 

[DavidT]

While out in the garden building a squirrel screen, I remembered a better reason R16 was changed: to move the operating point of the VAS. Cannot remember the details, as my VAS exploration effort was 10 years ago...and the files are in a box...somewhere.