A fixed frequency oscillator built into the tape recorder to supply the bias current.
"I think you did an outstanding job. It's hard to believe you were able to take forty year old, paper based records, and get such good sound out of them. I'm sure those records were not designed to last 40 years, so you obviously had to do a lot of work to get them to sound that good. If you were ever looking for an example of what your ..."