The problem is caused by the switching that turns the air conditioner system on and off. Assuming its a relay of some sort, its contacts are supposed to be bypassed by a small capacitance to prevent arcing (which creates bursts of RFI) of the contacts as they open and close.
I am sure this capacitor is present in the air conditioning system but it has failed. The relay that operates it will not be far behind as the contacts are now subject to arcing as they operate. It would be a good idea to get this serviced.
Proper grounding of the preamp can assist with making the preamp less susceptible to this sort of noise. If properly grounded, the chassis of the preamp will be grounded by the ground prong of the power cord. The actual circuit ground of the preamp will not be the chassis, but will float at chassis potential and will be the same thing as the shield connection of the RCA inputs. Meanwhile the tone arm ground will connect to the chassis of the preamp. In this way you will have the maximum immunity to noise of this type.
Unfortunately many preamps are improperly grounded, which means that IMMV; resulting in things like isolation transformers and such to fix this problem. But the most elegant way to do it is to have the phono circuit grounded properly to begin with.
I am sure this capacitor is present in the air conditioning system but it has failed. The relay that operates it will not be far behind as the contacts are now subject to arcing as they operate. It would be a good idea to get this serviced.
Proper grounding of the preamp can assist with making the preamp less susceptible to this sort of noise. If properly grounded, the chassis of the preamp will be grounded by the ground prong of the power cord. The actual circuit ground of the preamp will not be the chassis, but will float at chassis potential and will be the same thing as the shield connection of the RCA inputs. Meanwhile the tone arm ground will connect to the chassis of the preamp. In this way you will have the maximum immunity to noise of this type.
Unfortunately many preamps are improperly grounded, which means that IMMV; resulting in things like isolation transformers and such to fix this problem. But the most elegant way to do it is to have the phono circuit grounded properly to begin with.