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Old 27th September 2004, 11:20 AM   #1
Zombie is offline Zombie  Sweden
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Default Two zeners to ground in audio input?

What is that good for?
Two zeners in series to ground on each channel, one pointing to ground, the other to the signal...there's a resistor to ground, too and the usual input cap...
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Old 27th September 2004, 11:27 AM   #2
Mr Evil is offline Mr Evil  United Kingdom
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They limit the magnitude of the input signal, most likely to prevent the amplifier from clipping, maybe because it behaves badly when it does, or to soften the clipping characteristics.
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Old 27th September 2004, 12:08 PM   #3
djQUAN is offline djQUAN  Philippines
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it can also serve as ESD protection. static discharge will be routed to ground.
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Old 27th September 2004, 12:26 PM   #4
Zombie is offline Zombie  Sweden
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I might be the ESD, yes. The input goes to a 4066 solid state CMOS switch.
How does it affect the sound? Not at all?

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Old 27th September 2004, 12:39 PM   #5
Eva is offline Eva  Spain
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Zeners show non-linear capacitances high enough to affect high frequencies if source impedance is not very low

In order to clamp low level signals, zeners should be used only as continuously polarized voltage references, not directly as clamp elements, since the capacitance of the zeners by itself may be enough to unexpectedly clamp the signal while it gets charged

Thus, a much better aproach is generating a positive and a negative references through a zener to ground in series with a biasing resistor to the corresponding supply rail, and using 1N4148 diodes to clamp the signal. 1N4148 has much lower capacitance and faster 'switch' than any zener
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