Guardian-86

For some DIY audio enthusiasts, the thought of having an amplifier fail and blow their speakers is a persistent worry. Although the Modulus-series of amplifiers and the LM3886 Done Right are very rugged and reliable amplifiers, some of this anxiety is justified. After all, speakers can be rather expensive and mistakes do happen. However, any circuit added to the output of the amplifier should preserve the high quality of the output signal, which is no small feat. I designed the Guardian-86 to accomplish this goal.

Outline:

Features:

The features of the Guardian-86 are listed below.

  • Mono construction.
  • Compatible with the Modulus-86, Modulus-286, LM3886 Done Right, and other amplifiers with single-ended outputs. For a bridge/BTL-compatible version, please see the Guardian-686.
  • No relays! All switching is done with MOSFETs for the lowest distortion.
  • No relay contacts to wear, thus, no change in performance due to aging.
  • Adds no measurable distortion.
  • Floating photovoltaic MOSFET driver ensures consistent good performance throughout the full swing of the output signal.
  • 5-second delay on power-up. Output disabled when supply voltage falls below 22 V. This dropout voltage can be further optimized for your specific application by changing one zener diode.
  • Input: Molex Mega-Fit connector for easy connection to the Modulus-286 and LM3886 Done Right, and terminal blocks accepting up to AWG 10 (5.2 mm2) wire for use with the Modulus-86 and other amplifiers.
  • Output: Terminal blocks accepting up to AWG 10 (5.2 mm2) wire and two 4.2 mm diameter holes at ¾” (19 mm) pitch for direct mounting onto the speaker output terminals.
  • 2.50 × 1.75 inch footprint. The assembled board is approximately 1.05 inch (27 mm) in height.
  • Gold plated circuit board. Designed and manufactured in Canada.

Two fully assembled Guardian-86 boards are shown below. The cost of the parts needed for this assembly is $31 (per board). Binding posts can be added if desired.

Circuit Description

The Guardian-86 is a high-end protection circuit designed to protect speakers against two common ways that power amplifiers can misbehave:

  • Sharp voltage transients on power-up and power-down, resulting in annoying clicks and pops in the speakers.
  • Excessive DC voltage on the output of the amplifier, for example resulting from amplifier failure or from unruly signal sources.

The main design challenge with this type of speaker protection circuits is the design of the switch itself. The switch must be able to handle low voltages with high fidelity and be able to switch the large output current. These requirements are in conflict as the low-voltage switching tends to be best accomplished by gold switch points, but those points will vaporize if they are used at the high current levels commonly found in audio amplifiers. While it is possible to design a relay with two contact sets in parallel – one optimized for low voltage the other for high current – a much more elegant solution is to use a solid-state switch, such as a MOSFET.

Using a MOSFET switch solves another issue with relays: Arcing. When the relay switch contacts open at high currents, a plasma arc forms. Such an arc can destroy the switch completely. Even in nondestructive cases, the arc will cause the contact surfaces of the relay to wear resulting in a slow degradation of the switch itself. Thus, when used to protect a speaker in a hifi system, the relay wear will cause the sound quality to slowly degrade over time. Using a solid-state switch avoids the arcing, hence the contact wear associated with the switch commutation. This ensures good performance of the protection circuit throughout the full service life of the circuit.

Yet another issue with relays is that some of them cause distortion and in some cases this distortion can be significant. Thus, a better solution is to use a solid-state switch, such as a MOSFET. However, for a MOSFET to provide good performance as an audio switch, it must be biased in a way that minimizes the variation in switch resistance throughout the signal swing. In the Guardian-86, this is accomplished by using a floating photovoltaic gate driver, thereby minimizing any distortion contributed by the switch.

The Guardian-86 provides a 5-second delay on power-up and disconnects the speaker when the supply voltage falls below 22 V. This ensures that power-up and power-down is accomplished without audible clicks and pops. The drop-out voltage can be further optimized by changing two zener diodes in the circuit. The Guardian-86 will also disconnect the speaker if the amplifier outputs more than ±700 mV of DC. The Guardian-86 accomplishes this without using relays and without adding any measurable distortion of its own.

The plot below shows the distortion before (blue) and after (purple) the Guardian-86 when used with a ultra-low distortion amplifier. The sharp jumps in the curve are caused by the range switching of the APx525 audio analyzer.

References

I suggest readers who are interested in the many tradeoffs of amplifier output switching to consult the relevant chapters in the books below.

Douglas Self, Audio Power Amplifier Design, ISBN: 978-0240526133.

Bob Cordell, Designing Audio Power Amplifiers, ISBN: 978-0071640244