Front line hardware for the war on distortion
A low distortion signal generator was needed for various forays into the world of DIY HiFi. after some research, an established design was found over at Elliott Sound Products.
The design is discussed in detail there, so i won’t dwell on it here, rather I will cover practical aspects of construction.
Die-cast aluminium boxes are a winner for all manor of home made test equipment, the alloy machines very well leaving minimal burr and the shielding is excellent. The knobbs and switches were salvaged from some old military surplus equipment. The BNC connector is isolated from the chassis to prevent groundloops, always good practice in test equipment. One day I will get round to machining calibrated graduations onto the dial.
Construction is fairly basic, with fibreglass matrix board and point to point wiring used throughout. Though not pretty this construction method is cheap relitivly low effort compared to design of a custom PCB and can still give very good results at low frequencies.
Voltage regulators and the bulk of the oscillator are on the top board, the lower board houses the Schmitt inverter square wave generator, relay and output attenuator.
Analog oscillators of this sort suffer from amplitude bounce when the frequencey is changed which is caused by slight mismatches in the tuneing resistors or capasitors. This can be quite an issue and great lengths were gone to here to reduce this effect. The brass block is the result of dual ganging 2 single gang potentiometers selected for there match. bounce is still very noticible a markII may be attempted at some point.
Despite the shortcomings, once settled this generator is capable of sine wave output with distortion below the residual in my HP 334A distortion analyser making it below about 0.02%