For most systems (less than +/- 70V) BC639 - NPN (Q2) and BC640 - PNP (Q1) will work happily. The transistors must be rated for the supply voltage. Because of the rapid response and deliberate sensitivity to supply voltage variations, this is possibly the most reliable and accurate clipping published to date. The "pulse stretch" circuit will detect a clipping period as short as 120us, but reliable detection will take place within 1ms with any program material. This is connected in such a way as to detect either of the transistors turning off. Now if the output of either channel rises close enough to the supply voltage to equal the reference voltage, Q1 is turned off by forcing the base voltage to be greater than the emitter voltage - no base current, so the transistor will not conduct. A reference voltage (typically 3 Volts) is created across the 1k emitter resistor by the resistor shown as "See Table 2". Q1 and Q2 are the detectors, and we can examine the operation of Q1 (the positive peak detector) - Q2 is identical, and detects the negative peak.ĭuring normal operation (not clipping), Q1 is turned on continuously. A dual supply is not needed in this application. The +ve supply in this circuit is from the 12V zener, and the -ve supply goes to earth.
Amp clipping detector download#
As always, I suggest that you download the data sheet for the device you intend to use to double check. This is pretty much an industry standard, and nearly all dual opamps use this pin configuration. The pinout for a typical dual opamp is shown for reference. The terminal marked 'External" is to allow additional channels to use the same pulse stretch circuit, making it possible to have multiple detectors (even using different amp supply voltages), all sharing a common clipping LED. There was one detector published many years ago that was similar in some respects (this was pointed out by a reader after this circuit was published), but it was dramatically more complex and included extra functionality that (IMO) is best kept separate. Indeed, it is new to me as well, since this is a method of detection I have never seen published in this form.
Although a simple circuit, it uses a principle of operation that will be new to many readers. (Seeįigure 1 shows the circuit of the detector. If (when) the supply voltage varies, the detector varies along with it, and will detect even a very short peak that crosses the detection threshold. The clipping detector shown here relies on one factor - how close to the supply voltage is the amplifier's output signal at any instant in time. The fact is that neither of these is true, and the amplifier's DC supply voltage can vary quite considerably from hour to hour, and even minute by minute. This would be fine if the mains voltage stayed exactly the same at all times, and if the power supply had perfect regulation. Many clipping detector circuits have been published over the years, but most of them rely solely on an attenuated (reduced) version of the output signal, supplied to a suitable comparator circuit. Well, search no more, because here it is. What is needed is a simple and reliable way of detecting that the amplifier is clipping (or so close that we have no margin for error). One of the problems is that short duration clipping is very hard to detect by listening alone, but is still capable of causing damage - especially to tweeters, and it does nothing for the sound quality.
The same may well happen to one's ears, but the effect is much more subtle (and cannot be fixed !). If this happens too often or is too severe, tweeters are the first to go - they are damaged by the excessive power generated by a combination of "power compression" and the harmonics created when an amplifier clips. At some stage, we will all find ourselves pushing hi-fi equipment just a little too hard, and if lucky, will just find that the sound has become "dirty".
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