to provide to the user the proper signals so that he can readily detect distortion a final stage amplifier is employed including frequency compensation in the form of high frequency boost circuit 105 a bass boost circuit 106 the circuitry of this invention is readily portable and useable with minimum distraction for the user by reason of the effective design with the only operational controls and indicators located on the small top surface 10 setup controls are on the side away from the user it must be brought to mind however that operation once the appropriate controls are set involves merely the operator listening for distortion and when he encounters distortion reduce the gain on one of the appropriate one of the four controls 21 through 24 if he is only using channel 1 then his attention by his hand only need be directed to the first knob when distortion no longer appears at his earphones then he can always be assured that a good clean signal is reaching the recorder or whatever sound device he may be using the versatility of this device is obtained in part due to the fact that it is totally self contained including batteries and is equipped.