MPA681, MPA683 and MPA685 "Overload" LED Indicator

Features and Design Notes


The MPA681, MPA683 and MPA685 "Overload" Indicators are powered by a wide bandwidth positive and negative peak detector that is buffered from the audio chain. The overload LED is set to illuminate at the threshold of 1% distortion when the input stepped switch is set to "0" and the output level is adjusted to its maximum setting. When Overload conditions are encountered, the input stepped switch may then be decreased by 1 step position. This will realize a minimum of 2dB headroom for any pre-amp setting.


When the design of the overload indicator circuit was first undertaken, it was found that many of the typical overload indicator circuits have a bandwidth of only several kHz. Therefore, high frequency transients and overdrive conditions that cause the active circuits to clip are not indicated. With these circuits, the only evidence that the distorted peaks occurred may be found in the poor audio quality that ends up on tape due to high audio frequency or even ultrasonic distortion. The overload indicator circuit was therefore designed to have a bandwidth of >30 kHz, and was tuned to track the distortion vs. frequency curve of the active devices.


Due to the slightly asymmetric clipping of the pre-amp JET circuits, the onset of distortion due to a negative going peak that degrades the measured distortion to 1% occurs at a peak level slightly different than the onset of distortion due to a positive going peak that degrades the measured distortion to 1%. Therefore, the circuit is designed to respond to positive and negative peaks independently and indicate the onset of distortion 1% and higher no matter whether the peak occurred on a positive or negative peak.

The overload indicator circuit is buffered from the audio chain to prevent the indicator circuit from adding its own distortion to the audio signal. Many inferior circuits use un-buffered diode type peak detectors that actually inject non-linear currents into the audio chain during overload conditions. With the Ingram Engineering pre-amps, the only distortion that is experienced is due to the analog circuits going into soft compression as the audio circuits transition from the linear to non-linear region.


Granted, this is perhaps more design effort applied to a secondary function of the pre-amp than might seem necessary. However, after studying possible implementations of the overload function and understanding the shortcomings of common circuits, it was felt that the improved usability of the overload indicator function was well worth the extra attention paid to these details.