Damage Protection
The input stages of most audio equipment are unprotected. This approach appears to save on parts cost, complexity, and sonic degradation; however, in reality, it may indeed cause costs and degraded sonics. The inputs of power amplifiers are certainly among those most likely to sustain input voltages that may be damaging to the active parts inside.
Causes
Typical culprits include first, large signals from line level sources, and from amplifier outputs, experienced through accidental connections (see Section 8.5.2). Here, excessive signal voltages that could be applied could range from a few volts, up to 230 V rms, and from below 10 Hz to above 30 kHz.
Second, the outputs of crossovers or consoles, or misconnected amps, which are kaput and have DC faults, so the output voltage might range from +/–10 V to up to +/–30 V for line sources, and up to +/–160 V DC for power amplifiers, but more typically +/–30 to +/–90 V DC.
Scope
The parts most at risk from excess input voltages are the solid-state active devices, particularly discrete BJTs, and most monolithic IC op-amp input stages.
Valves are relatively immune to input voltage abuse. They are most likely to be harmed by gross overdrive conditions that bias the grid positive so a damagingly high current flows.
J-FETs and MOSFETs are next most rugged. MOSFETs are most susceptible to gate- source overvoltage, but gate-source protection is straightforward and effective.
IC input stages are the most fragile. Due to IC structure, even FETs, when monolithic, may have parasitic weak points. For long-term reliability, currents flowing into or out of IC op-amp pins12 must always be kept below 5 mA.
Harmful Conditions
There are two kinds of potentially damaging input voltages: (1) common mode and
(2) differential mode. Either may occur when a power amplifier is in (i) the on state or
(ii) the off state, giving four possibilities.
On-State Risks
When an amplifier employing BJTs at the front of its input stage is on, powered up, and settled down, it can sustain relatively high differential (signal) voltages without damage. Generally, in high NFB op-amp and other dual-rail based designs, the max differential voltage is a volt below the supply rails, hence a maximum differential voltage ranges from +/–14 V for input stages working from +/–15-V supplies, up to +/–30 V or even over +/–100 V, where the input stage transistors operate from the same or else similarly high supplies, as the output stage.
Long before differential overload, the input stage will be driven strongly into clip. Provided the amplifier has clean recovery, an overvoltaging may pass unnoticed if the high differential voltage only lasts I mS. Yet this is plenty long enough to damage a semiconductor junction. In BJTs, the most vulnerable junction is the base emitter, when reverse biased.
Under the same powered-up conditions, common-mode voltages above +/–10 V can damage unprotected BJT input stages. In large systems, the common-mode voltage can be this high, commonly comprising 50/60-Hz AC and harmonics, and arising from differences in grounding or AC power potentials.
The input stage’s supply rail voltage usually has a large bearing on the maximum safe DM and CM input voltages. Here, low supply voltages may do no favors.
Off-State Vulnerability
When an amplifier using BJTs is switched off, both differential and common-mode voltages as low as +/–0.5 V may be damaging. Users are advised to always power-up preceding equipment before the power amps. This is universal practice among informed users, both domestic and professional. However, if the prepowering of the source involves the passage of signals above 0.Sv peak to amplifier inputs, then unless the transistors behind the sockets are protected before the amp is powered-up, they may well be damaged. This mode of subtle, progressive damage and sonic degradation to analogue electronics has yet to be widely recognized. It can be overcome without changing otherwise sensible practices, by suitably designed input protection.
Occurrence Modes
Damage to input devices may be catastrophic if the overvoltage causes high currents to flow. This is rare.
Otherwise, with BJT inputs, damage may be subtle. Transistor parameters are degraded but NFB action initially hides the worst. Telltale signs would be changed or, reducing sonic quality, raised, increasing and/or intermittent noise, higher %THD, and possibly increased DC offset at the amp’s output.
With ICs, damage may be cumulative, caused by peculiar metal migration effects occurring in ICs’ microscopically thin conductors. This means an input stage can appear to handle abuse repeatedly until eventually the catastrophic failure occurs when all the conductor has migrated away!
Protection Circuitry
Power amps have been designed to survive likely levels of both CM and DM overvoltages by the use of some combination of the following.
1. Series input resistors, which may already be part of the input stage’s RF filtering, will limit the current flowing into inputs. If the resistance between the input
socket and the active device is 5 kΩ, then above 25 V DC or peak signal would be needed to get more than 51xA to flow.
2. Back-to-back zeners to 0 V, working in concert with series current-limiting resistors (which may already be part of the input stage’s RF filtering). Both CM and DM voltages can be clamped to any available zener voltage. Designers must allow for quite wide variations with tolerance and temperature, and possible sonic degradation. Programmable zeners may also be used or zeners may be combined with BJTs.
3. Ordinary, fast diodes across the active differential inputs, in concert with series input resistors in both legs. Protects against DM overdrive only. Internal to some IC op-amps, for example, NE5534. External diodes with larger junctions may be used to enhance protection.
4. Clamping relays. Placed after the series input current limiting resistors, inputs are shorted to 0 V until power is up on all rails. With suitably rapid action and power sensing, relays in this configuration can provide complete protection against both DM and CM input signals.
5. Bin13 describes a method developed at the BBC, using VDRs, zeners, and current sources, providing input protection to audio balanced line inputs (including power amps) up to 240 V ac. Alas, sonic quality may be detracted from.