Switching power supplies are not usually noisy, some go bad and are noisy.
The noise from a switching power supply is ultrasonic >30 KHz, and so you wouldn't be able to hear it anyway.
High frequency noise is much easier to filter out than low frequency noise, smaller bypass capacitors/inductors are both small in value and size to do the filtering.
If I needed to get the 640p back working, I would make something similar to the original +/- 18 V supplies, using a two half-wave rectifiers running off a 24 VAC transformer and 7818 and 7819 regulators.
A 24V transformer to power a low-power device such as a per-amp shouldn't need much current, so the size of the AC power transformer needn't be too big or expensive. You probably don't need more than 100mA per supply.
The size of a small 24V transformer would be much smaller than a 18V laptop brick, and the brick only gives you one positive voltage supply. A big disadvantage of a linear supply using a power transformer is you must make sure the transformer is well-away from the magnetic phone cartridge or you will hear a lot of hum pickup from the transformer.
You can download the 34063 datasheet which explains the circuit below, even gives you a printed circuit pattern to build it.
http://www.alldatasheet.com/datasheet-pdf/pdf/392304/ONSEMI/SC34063A.htmlIf the preamp is going to anywhere near a magnetic pickup on your record player, build the power supply in a box that lies on the floor below and run three wires up to the preamp to minimize hum and noise from the power supply.
if all I had was a 18V laptop brick, I would use the +18V output for the positive supply and use a 34063 IC (see below with --18V) output for the negative supply. ll you gotta do is replace R1 with a higher valued pot (2K) and adj. the output voltage to -18V.