changing DC voltage

A power MOSFET circuit would do the job nicely. You may need to drive the gate via an amplifying transistor (like a simple 2N2222) to get the voltage range you need.

BRW

I built something to control one of these things a while back. The manufacturer said that it is okay to use pulse width modulation at frequencies above 1kHz, so that's what I did and it worked fine! If you don't get any joy with an analogue solution, you could think along the lines of 555 - H-Bridge.

Good luck!

Andy

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Thanks- One of my dilemmas is that I have run out of digital output lines but have these DACs sitting idle. So I'll try that first.