I have an NAD 7150 integrated amp (circa 1983) feeding Boston Acoustics A150 speakers (same age). The amp has a phono output, but there is no phono connected. When the "listen" knob on the amp is turned to phono, I get a loud hum from the left speaker. Any idea what might be causing it?

Also, I get a bit of static-like noise when turning the volume knob (from both speakers). Again, any idea as to cause?

Thanks for your help.