Loudspeaker relay with DC protection

Project presentation

Abstract

A good power amplifier should disconnect the speaker during power-up and power-down. The signal chain is likely in a chaotic state during this period and the chaos produced better does not reach the speaker in order to protect it from potential damage. Furthermore, the loudspeaker should be protected from potential catastrophic failure of a DC coupled power amplifier driving the loudspeaker. The typical failure mode is that one of the amplifier output stage power transistors forms a short circuit with the power supply, leaving the loudspeaker connected to the rail, resulting in destruction in most cases. I will discuss electromechanical relays and solid state relays and present my project based on a solid state relay.

Electromechanical relays

Usually the loudspeaker relay is an electromechanical one. While this is a good solution for muting the loudspeaker, an electromechanical relay may fail to interrupt the current that flows during catastrophic output stage failure, especially for high power amplifiers. The advantage of electromechanical relays is that they may have low distortion in theory. This benefit may vary from relay to relay and also dependent on age and wear of the relay. AC operated relays may induce noise into the signal. The disadvantage of electromechanical relays is limited DC current breaking capability. Wiebe Cazemier conducted a very illuminating study how to properly connect electromechanical relays so that they are more likely to disconnect the speaker during a DC event. He has also demonstrated that relays are prone to failure interrupting large DC currents. Conventional relays have very limited capability to switch high DC currents at high voltages. Special relays become very expensive or even unobtainable. Power relays that are used in electric installations may be able to break high current at high voltages, but fail at low signal level since they are not designed for this application. The result is crackling sound and even signal interruptions at lower levels due to oxides on the contacts. Actually those relays are built for switching mains voltage only and high current operation. Summary is that electromechanical relays may work fine for low power amplifiers, but are not suitable for loudspeaker protection in high power amplifiers.

Solid state relays

Solid state relays discussed here are made of two N-channel MOSFETs in series with their sources connected together. There are other types as well, which are unsuitable for audio applications. The advantages of such solid state relays are that they have low and predictable on resistance with generous current conducting capability, near zero "contact" wear, constant properties over lifetime and they can switch very fast, which allows to actually switch high currents. Disadvantage is sensitivity to excess voltage and non-linearity, which contributes to distortion. The main non-linearity of concern is that the RDSon is a function of the VDS across the MOSFET. Also, the CDS has impact on the ability to mute the speaker and is frequency dependent. Someone has performed a very interesting study how the non-linearity of solid state relays may contribute to distortion. Unfortunately I can't find the website anymore. Conclusion is that the contribution to distortion is negligible given that the ratio between relay resistance and loudspeaker impedance is reasonable.

Project presentation

Design considerations

There are some considerations regarding the DC detection and control circuitry. The DC detection shall not falsely trigger during normal operation. Music may contain some DC due to asymmetric wave forms and low frequency content that may also trigger the detection. When using low pass filters to distinguish between signal and DC, less sensitivity towards false triggering means slower response time. Control circuitry needs to be reliable and robust. It can be kept very basic or more advanced by adding just a few low cost parts. For this project, versatility was given priority and therefore, higher part count and larger PCB real estate were accepted.

Design features

  • Delayed connect of loudspeaker after power on
  • Instant disconnect of loudspeaker on power down (AC loss) triggered by power supply via SHUTDOWN interface
  • Automatic restart after AC loss
  • Fast disconnect of loudspeaker in case of DC on amplifier output
  • Does not false trigger with 10Hz signal at amplifier output
  • Latch off on DC fault (optional)
  • Manual or automatic reset after DC fault possible if fault is cleared (configurable)
  • Galvanic isolation between channels to fit dual mono amplifiers with separate power supplies
  • Wide supply voltage operation from 12VDC to 90VDC
  • No extra power supply required
  • Extra SHUTDOWN input for manual shutdown (used for shutdown on mains AC loss by transformer control module)
  • Extra AUX_CONTROL output that switches together with relay to control auxiliary circuitry like input muting relays
  • Status indicator LEDs: Relay status, DC status, latch status
  • Functional as module inside amplifier and also as external unit

Dependent on transistors used

Theory of operation

The image above shows the operation of the loudspeaker protection in simulation. For the simulation, I enabled the latch so that a manual reset is required. With the latch disabled, the loudspeaker would be reconnected automatically following a DC event after the fault has cleared and the delay time has passed.

At t=0s, the modules power supply is powered up. The delay is starting to run and once the delay time is over at t=4s, the loudspeaker relay engages connecting the loudspeaker to the amplifier output. The delay time is adjustable of course.

At t=15s, a high DC voltage is present at the amplifier output. The loudspeaker protection trigers quickly and disconnects the loudspeaker. Also, during the DC event, the red LED indicates DC volatge being present and the yellow LED indicates that there has been a DC event. The loudspeaker remains disconnected.

At t=17s the circuit is reset manually using the reset button. The start-up delay begins to run again.

After the delay time has passed at t=21s, the loudspeaker is conected again.

At t=30s, a DC event with an amplitude of -2V occurs. The reaction time is much longer now and it takes almost one second to disconnect the loudspeaker. The DC event persists for longer time and during this time, the red LED indicates DC being present. Also, the yellow LED remains on and the loudspeaker disconnected until a manual reset occurs at t=36s.

After the delay has passed at t=40s, the loudspeaker is connected again. At t=50s, there is a low voltage DC event with an amplitude of +2V this time. The sequence is basically the same like before. Reset occurs at t=56s.

At t=67s, the AC loss detection triggers, which would happen in case the amplifier is being turned off. Now the loudspeaker is disconnected immediately in order to avoid any grunge, likely being produced somewhere in the collapsing signal chain, from reaching the loudspeaker.

Missing from above test case is a DC event during power-up of the amplifier. I simply didn't have this in mind when designing the test case, although this is very likely to happen in reality. With the latch being enabled, this would result in the loudspeaker never being connected since the DC event is stored. Both the latch and the LED indicator are very useful for debugging this novel circuitry and may be left away without any relevant loss of functionality once the circuit is debugged and running well.

Prototype

Block diagram

Interface

Left channel
Connector Description Usage details
AMP_L_SIG Power amplifier output left Connect to the left amplifier speaker output. Reference to AMP_L_GND
AMP_L_GND Power amplifier ground left Speaker return left
AMP_L_PWN Amplifier negative supply rail Optional connection. Reference to AMP_L_GND
AMP_L_PWP Amplifier positive supply rail Optional connection. Reference to AMP_L_GND
CTL_GN1_L
CTL_GN2_L
Control circuitry ground Has no galvanic connection to amplifier ground or right channel ground
CTL_SUP_L Control circuitry power supply Wide voltage range input from roughly 12V to 90V. Reference to CTL_GN1_L / CTL_GN2_L
CTL_SDN_L Control circuitry shutdown input Leave floating for normal operation, pull to CTL_GN1_L / CTL_GN2_L for instant speaker disconnect
CTL_AUX_L Control circuitry auxiliary output Open collector output which switches together with speaker relays. Reference to CTL_GN1_L / CTL_GN2_L
Right channel
Connector Description Usage details
AMP_R_SIG Power amplifier output right Connect to the right amplifier speaker output. Reference to AMP_R_GND
AMP_R_GND Power amplifier ground right Speaker return right
AMP_R_PWN Amplifier negative supply rail Optional connection. Reference to AMP_R_GND
AMP_R_PWP Amplifier positive supply rail Optional connection. Reference to AMP_R_GND
CTL_GN1_R
CTL_GN2_R
Control circuitry ground Has no galvanic connection to amplifier ground or left channel ground
CTL_SUP_R Control circuitry power supply Wide voltage range input from roughly 12V to 90V. Reference to CTL_GN1_R / CTL_GN2_R
CTL_SDN_R Control circuitry shutdown input Leave floating for normal operation, pull to CTL_GN1_R / CTL_GN2_R for instant speaker disconnect
CTL_AUX_R Control circuitry auxiliary output Open collector output which switches together with speaker relays. Reference to CTL_GN1_R / CTL_GN2_R

Status LEDs

Green Yellow Red Status
off off off Module is offline, Loudspeaker disconnected
on off off Loudspeaker connected
off on on DC present, Loudspeaker disconnected
off on off DC transient stored, Loudspeaker disconnected

PCB assembly drawing

PCB assembly 3D preview

PCB assembly

Schematic left channel

Schematic right channel

PCB and assembly options

In the schematic I used two transistor pairs: High voltage small signal transistors and high voltage medium power transistors. They have the same pin-out, which allows to populate the PCBs with either type. Both transistor types can be replaced by any other pin compatible transistors dependent on availability and supply voltage used. The circuit will work with any transistors with the only limiting factor being VCE and SOA.

There are two schematic revisions: One that is designed for small signal transistors only and one that has the larger power transistors designed in. The second revision illustrates which transistors neeed to have higher power rating for higher power supply voltage operation. The schematic presented here is for the high power variant, while I built the low power variant using only small signal transistors because I have a 12V supply available for powering the loudspeaker protection modules and there is no benefit for me wasting a lot of power using a higher voltage supply. Usually one would run the control circuitry from any low voltage supply like 12V or 15V or whatever is present in the amplifier. In this case, literally any transistors can be used for the control circuitry. Only the DC detection needs higher power transistors since they are subject to the audio output signal.

Simplification

The attentive reader might have spotted that the design is actually two completely separate circuits with one channels circuit being an exact copy of the other one and so is the PCB design. This has several reasons:
First, the circuit is a proof of concept prototype and copy and paste in the EDA environment seemed more appealing than spending more time at this stage to optimize the design.
Second, the modules form factor is determined by other modules that have the same form factor so PCB size reduction was not an obtainable improvement anyway and built for low supply voltage operation, the extra component cost is negligible.

There is a lot of redundant control circuitry that can be shared between channels while maintaining almost all features listed previously. Only the DC detection and solid state relay circuitry needs to be separate for each channel. Meanwhile I have drafted a variant with optimized topology and added some additional circuitry that makes the module even more versatile.

Status and outlook

One PCB for the low power variant using TO-92 transistors is assembled and being tested. I also improvised the high power variant using TO-126 transistors for the CCS and found performance at high supply voltage being as good as the low power variant. I will use the protection curcuit as low power variant since I have +12V for such control circuitry available in my amplifier.

I found an issue with using the DC protection in my amplifier: The amplifier generates some DC voltage at the output during power on and this event is long enough to trigger the protection circuit. As explained previously, this would mean that the DC protection circuit needs to be reset after every power up. I would need to redesign the circuit so that DC events at the loudspeaker output only trigger the protection after the turn-on delay is over and the loudspeaker is connected. Prior to that, DC events should be ignored.

There are two possible solutions to deal with DC events during power on:
Disable the latch by removing R14 and R42. This will let the circuit reset itself a few seconds after end of a DC event. This is within the intended mode of operation. Manal user reset is mostly a debug option and automatic reset is a much more user friendly option.
Better would be to change the circuitry in a way that DC events during startup are ignored. U1/U2 and U4/U5 could be connected to the auxiliary control output instead of ground. It culd be feasible to improvise this on the PCBs without creating a mess. However, there is little point enabling the latch with manual reset being required.

When I was testing my audio amplifier to the limits, I noticed that the protection circuitry trigers at a sine wave output level of roughly 10V with no DC being present at all. I figured out that the DC detection circuitry does some rectification of high frequency signals and small capacitors added to the DC detection circuitry suppress this effect. The capacitors C100 to C103 are added to the schematic. The additional capacitors do not have a footprint on the PCB and therefore, assembly and connections needs to be improvised. I found this easy to do.

Another useful feature to add would be a bleeder resistor to the DC detection low pass filter capacitor. This allows to set sensitivity without affecting reaction time. With the current component values shown, the circuit is too sensitive. I installed resistors parallel to diodes D1/D2 and D13/D14 to add this feature. Increasing the DC detection filter capacitors C1 and C4 to 33µF would also reduce sensitivity and avoid false reaction to high amplitude low frequency signals.

Next generation: Simplified and improved

Based on previous experience with the circuit and how it performs, I simplified and improved the design. I removed the redundant control circuitry and added a simple rectifier so that the module can generate its own supply voltage from any AC power supply. The added power supply also allows to sense AC loss directly. Another option for instant shut down on power loss would be the shutdown input in case any other module provides the AC loss detection.

Schematic

PCB assembly drawing

loudspeaker protection module PCB assembly
Due to the simplified circuitry, I was able to reduce the size of the PCB. In order to maintain compatibility with the other PCBs, I added a small section that can be cut away to reduce the PCB size.
The interface for debugging, namely the LEDs and button, were relocated slightly though. I did not drill any holes for the debug interface into my rear panels anyway.
In the schematic, the possible assembly options are indicated. Most assembly options are leaving away optional components. By skipping the LED indicators, a lot of components can be removed.
Additional to the Speakon connectors, the loudspeaker output is available on Wago 236 series connectors or Faston blades, that can be put on the PCB instead of the Speakon connectors.

PCB assembly 3D preview