The invention relates to hearing aids, and more particularly relates to ITE and canal hearing aids.  In its most immediate sense, the invention relates to programmable ITE and canal hearing aids. 
It has long been desired to manufacture a hearing aid in which many circuit variables may be adjusted by the dispenser.  In practice, this has proved difficult in ITE and canal hearing aids. 
The difficulties have resided in the severe constraints which space considerations impose on the hearing aid circuitry.  Where adjustment of the hearing aid has been accomplished by adjustment of potentiometers, the number of potentiometers has been limited by the small volume in which they must be installed.  Where adjustment of the hearing aid has been accomplished using hand-held programmers which activate digital circuitry in the hearing aid, the space requirements for serial ports, microprocessors, filters etc.  have likewise limited available design options. 
Circuit adjustability has also long been recognized as desirable for reasons of manufacturing economy.  Where an ITE hearing aid has been manufactured and proves, on final test, to be out of electrical specifications, it is necessary to cut the device open, to replace one or more hard-wired parts, and to reseal the housing.  This is not only labor-intensive and therefore expensive, but the fine wires inside the hearing aid may become overstressed and break.  Major rework may consequently be required.  While it is possible to avoid this sort of quality-control-related rework by providing additional potentiometers to be adjusted at the factory, this limits the number of potentiometers which can be adjusted by the dispenser and can even be disadvantageous because dispensers might meddle with adjustments which are intended exclusively for factory technicians. 
It would therefore be advantageous to provide a hearing aid circuit which would permit many electrical variables to be adjusted, both at the factory and at the dispenser's office, without being so large as to require a BTE construction.