There is a lot of theory put into amplifier design and when one looks at these complicated circuits often the protection circuits are more sophisticated than the gain stages. Purists believe the only true amplifier is a piece of wire with gain. Engineers often criticise the protect circuits of creating their own problems, along with causing inferior quality reproduction.
These are the major problems come across in the design of any amplifier:
Heat – is there sufficient cooling for heavy duty work?
Can the output transistors go out of their Safe Operating Area? (known as SOA).
Can the amplifier drive a 2 Ohm load when minimum load spec is 4 Ohm?
The dangers of mains voltage boosting and sagging?
Oscillations and instability.
Over driving and distortion.
Loudspeaker protection – expensive lessons to be learnt.
Modifying the STR-DG500/600 and Protecting the speakers
The one issue I have picked up with the Sony amplifiers in this range is their ability not to switch on when there is high humidity. Many owners are not aware it’s a humidity problem and with all the functions these amps have one would have thought an LED indicating thus would have been relevant.
High humidity causing no sound
Actually there are quite a few amplifiers on the market which exhibit loudspeaker protection mode so it can be fairly common problem, I don’t like the idea of switched mode supplies and humidity though. Use a hair dryer to get the engine running again.
The reason why one has protection to the speakers is usually due to one or both power stages going faulty and allowing DC through to the load. Plus or minus 50V across a voice coil is going to cause a serious disruption to service – in some cases it’s after the damage to the speaker that the fuse blows.
Current limiting, fusing your speaker rails
Some ways to limit the probability of this happening is to put a current limiter in series with the speaker, a pos temp coefficient resistor or poly-switch. A light bulb in series with the speaker is also a solution. In both cases purists don’t like any resistance in the signal path, especially one which is not pure and can be non-linear.
Active speaker protection
The audio amplifier when switched on usually holds the speaker protection relay open, tests for DC, stabilises and then connects output signal to speaker load.
Assume the circuit Figure 3 is in active mode i.e. protected or has just been powered up.
How does it work: Looking at Fig 2, R1, C1 and C2 makes a potential divider chain, the audio signal is passed through the rectifier D1~D4. When the DC voltage starts climbing towards the switch on voltage of T1, the transistor conducts pulling down the base of T2 in figure 3, switching the Darlington T2 and T3 off, switching off the relay (or opening the circuit to the loudspeaker).
C3 and R3 have a time constant and acts as a switch on mute circuit. When the circuit is powered up the capacitor keeps the base of T2 low, charging up through R2 until the devices T2 and T3 conduct, switching on the relay.
One problem which can rear it’s ugly head is that the relay contacts burn closed which may not then protect the speaker. Figure three shows a circuit with a SPST relay which for most smaller amplifiers should be adequate but the proper way is to use a single pole dual throw relay and earth the NC contacts. See the article on halfgaar.net.
Proper wiring for speaker protector – SPDT with NC to ground
I purchased the Velleman K4700 (pdf) from the US a few years back and shipping to SA took about three weeks, customs and excise clearance done prior in the USA. The kit is well made and simple to follow. Highly recommended.
Due to the higher powers that we encounter in audio amplifiers one needs to also be cautious as to whether the relay contacts are adequate.
A general rule of thumb is to allow for 50 to 100mS switch-over in the event of a fault. 1/10 of a second is pretty fast. 1/20 of a second is better but here we are nearing the extreme where erratic switching may take place at peaks.
In a new build it may be best to test with a coupling capacitor. Check rating and polarity. Coupling capacitors to loudspeakers pass oodles of current, check current capacity as well. (ripple).