Hafler FET Muting Circuit

GeorgeP

GeorgeP

Journeyman
Joined
Mar 22, 2023
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129
Those two FETs at Q15 have to go. They do degrade the audio a tad and can go "leaky" reducing the output level. Sometimes they short completely and the channel is dead!
I removed the FETs and put a relay there instead. Much better.
I used a 5V DC low current relay with the coil tied to ground through a 17V Zener diode. The other side of the coil goes to the output pin of the comparator.
After 5 seconds or so, the output pin of the comparator goes to -23V, AND THE RELAY CLOSES. At 5/6 volts across the coil the relay current is only 11 milliamperes, easy for the comparator to handle, and low dissipation for the Zener diode (0.190 W. with a 0.500 W rating).. The contacts short the output to audio ground, the same way the original FETs did, with MORE attenuation, and less distortion and loss. Use a diode across the coil in reverse bias, for the coil current release transient.
 
The Hafler FET muting circuit helps reduce noise when turning an amplifier on or off. It uses special electronic parts called FETs (Field-Effect Transistors) to mute the sound for a moment, stopping any loud pops or clicks for smoother sound.
 
fransa said:
The Hafler FET muting circuit helps reduce noise when turning an amplifier on or off. It uses special electronic parts called FETs (Field-Effect Transistors) to mute the sound for a moment, stopping any loud pops or clicks for smoother sound.
Click to expand...
It's an ok circuit, but can be non-linear when the devices are compromised. They go very leaky, even short during AC line spikes from local lightning strikes. Been there...
I removed those two FETs from my two Haflers and put in a sealed DPST Relay. Much better. My new GPulseLinear Preamp design uses these sealed relays on sockets. Using them for source switching also. Sound quality is better.
 
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