graham-r
pfm Member
With the parts I will be using it will cost a lot more than the kit or the figures you give above. Chassis mounting 12VA transformer alone about £10 plus VAT and I'll probably use heavier duty relays. I may well add protection against oscillation.
I have designed the trickiest parts of an "ultimate" protection unit which uses phased mosfet and relay switching so as to provide the lowest possible resistance and distortion when connected and be able to safely break even the output from 300W++ amplifiers without relay contacts welding together etc. Other possible features include over driving protection for all drive units by averaging the power within the bandwidth handled by each driver. It was mainly a design exercise and "to prove a point" and I'm not likely to do anything with it...
It certainly would be more expensive if you were to build a heavy duty protection device, oscillation protection is definitely a worthwhile addition.