I guess I don't know about filter amps.
I just wanted the old HD repaired, I've never considered ebay. I guess that may be as cheap as getting one repaired.
The problem is 1, sometimes there is no sound what-so-ever from the test set period. I have replaced the 9 volt battery with no results. 2. Sometimes I get a loud squeel when I can hear anything unless I hold my finger on one of the leads,,,,, works real good untill I hit ringing current! It's just broke.