Regardless of the title, this DI doesn’t describe a gadget to let you know when to don your shades, however as a substitute incorporates a helpful add-on to (analog) audio equipment. Constructed right into a mixer, for instance, it should present when the output of any stage is approaching clipping, maybe on account of extreme bass or treble increase. Constructed right into a venture field as a stand-alone unit, it’s helpful throughout circuit improvement. It could not present you the place the issue is however will present that some stage is in peril of changing into overloaded. It’s proven in Determine 1.
Determine 1 The diodes mix the alerts to be monitored, and the comparators examine if any of them is near your chosen restrict, both unfavorable or constructive. If that’s the case, the LED flashes.
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The outputs of the circuits to be monitored—as few or many as you select—are every related to a pair of the enter diodes. Essentially the most constructive and unfavorable peaks of the inputs, lower than a diode drop, then seem throughout R1. The inputs are usually not measurably loaded, neither is there any important interplay between them.
Comparators A1 and A2 examine these peak voltages towards references decided by R2/3/4, their commoned outputs pulling low when the related limits are exceeded. That quickly discharges C1, turning on Q1 and thus LED1. C1 slowly fees again up by way of R5 and R6, holding Q1 on whereas it does so. Q2, an n-channel JFET, is used as a constant-current diode, limiting the LED present to its IDSS or saturation drain present worth, which is round 7–8 mA for the 2N5485 proven and largely unbiased of rail voltages from <9 V to >30 V. Be certain the system can face up to the height provide voltage, although information sheet values are normally conservative. When constructed into gear the place the provision is fastened, an appropriate resistor can be utilized as a substitute, however a JFET is greatest for the stand-alone model, the place provides will differ.
With the values proven, peaks of >~10 µs shall be detected, comparable to a half-cycle at 20 kHz, giving LED flashes of ~20–50 ms length relying on the provision voltage. If that voltage is nice sufficient to trigger breakdown of Q1’s gate-source safety diodes, the flash time shall be decreased considerably as R5 will successfully be partially shorted, however no harm will happen owing to the excessive resistor values. For an extended flash time, improve R5/R6; rising C1 will slug the response time. DC ranges above or beneath the related limits will flip the LED on constantly.
Solely +V and -V energy rails are wanted, a central floor being pointless, so it may well freely be used with both single or cut up provides as much as a complete of 30 V or so. Connecting C2 throughout the provision proper by the LED is nice apply, although the latter’s present pulses are small. An additional decoupling cap throughout U1 shouldn’t be wanted.
To permit for various power-supply voltages, enter swings, and headroom, it’s solely essential to alter R3, which can be discovered by utilizing the next equation:
R3 = (R2 + R4) / (VSS / (VCLIP × 10^(-h / 20)) - 2 VF) - 1)
the place:
- R2 = R4 = 10k
- VSS is the whole rail-to-rail provide voltage
- VCLIP is the pk-pk voltage, at clipping, of the phases being monitored
- h is the chosen headroom in dB
- VF is a p-n diode’s typical ahead voltage, say 600 mV
A few examples: With ±15 V rails, a ±14 V most enter swing, a selection of three dB headroom, and R2 = R4 = 10k, R3 comes out as 32,736 Ohms; or 33k. A single 12 V rail, a ±4.5 V enter swing, and a couple of dB headroom offers 19,663 Ohms, or 20k, for R3. (For the stand-alone model, I used a 50k pot plus a 10k resistor to cowl all eventualities.)
Observe that the voltage throughout R2 should be higher than 2 V, or the LM393 will misbehave. Whereas it may well sense at or beneath floor, although that doesn’t concern us right here, a minimum of one enter of a comparator should be greater than 2 V beneath the constructive rail.
Whereas not proven on the schematic, the enter traces ought to use screened leads (the screens being earthed, naturally) ideally with a number of hundred ohms at their enter ends to isolate the phases being monitored from the leads’ capacitance.
Easy as this circuit is, I’ve discovered it helps to provide warnings of mismatches between the positive factors of cascaded audio phases. In use, it should usually be simply flickering on musical peaks; if not, you’re most likely not utilizing your full dynamic vary or S/N ratio. If it’s flashing a lot of the time, which will simply be all the way down to Mahler, Wagner, or loss of life metallic, but when it’s stable, examine for a blown op-amp someplace! In fact, you might have heard the results anyway.
—Nick Cornford constructed his first crystal set at 10, and since then has designed skilled audio gear, many datacomm merchandise, and technical safety equipment. He has finally retired. Largely. Kind of.
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