Interfacing split-supply preamp to single-ended amp

I have a stereo preamp built around the LM338 dual amp, which requires
+ and - 15 volts split supply. I want to interface it to a transistor
amp with about 25 volts single supply. The kit manual merely asures
me that it can be done.

I was thinking of putting a voltage divider across the 25 V supply and
returning the tap to the ground on the preamp board. To avoid
shorting out the negative half of the supply, I can have no continuous
earth return to the main board, i.e. cable shields must be earthed at
only one end. Will this work?

Doug.

 

The cable shields are not the problem. The problem is that the 25
volt supply must not be connected to any other circuit that grounds it
differently. I think I would connect the amp outputs directly to the
speakers, but return the speakers back to the center node of a pair of
capacitors that are in series across the supply. You can share one
pair with both speaker outputs, or you can make two series pairs, one
for each channel. This will produce a lot less pop at power up than
connecting the speakers to one side of the supply, with a single
series capacitor between output and speaker.

 

Basically you need to modify the biasing arrangements of the LM338 to work
from a single supply. Assuming the input goes to the non inverting input
then I would suggest you connect a pair of 10K resistors across the 25V
supply each decoupled with 25uF and use this to bias the opamp something
like this (the junction of the two 10Ks is labelled 0V(op) ).

10uF |\
in -- C ----------|+\
| | \____
| --|- / |
in-- R | | / |
| | | |/ |
| | | |
| 0V(op)|--- R --|
| |
0V(25) |
R
|
|
C
|
|
0V (25)

HTH

Ian