I think you have two options. Firstly, just get a phono preamp and use your A500. There are lots of options, from the Behringer PP400 available from £17 upwards, plus a set of phono leads which you can get for around £5. The Project Phono Box is well regarded at £49, or the Cambridge Audio 551P at around £100.
All these are new prices, and you can do better on eBay buying used. There's nothing to wear out on these, so buying used is a low-risk approach.
The second suggestion is to sell your A500, and you should get back what you paid for it, and get an amplifier with a built-in phono stage from any of the mainstream suppliers like Marantz, Denon, etc etc. That will probably end up costing more than the phono stage.I would go for the phono stage as the amplifier you have is pretty good. The Project Phono box is a good bet.
Hope this helps.
S.