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Best electronic switch for audio signals-logic drive
Best electronic switch for audio signals-logic drive
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Old 1st August 2018, 05:59 AM   #1
dreamth is offline dreamth  Romania
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Default Best electronic switch for audio signals-logic drive

I am looking for the best and cheap possibility of switching or muting audio signals with the lowest distortion effect with electronic parts driven by logic levels..Any hint?It's a cassette deck i'm working on and i need to switch very low signals(tens -hundreds of milivolts).
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Old 1st August 2018, 06:38 AM   #2
steveu is offline steveu  United States
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Do a search for CMOS transmission gates. CD4052 is a useful configuration. The analog signal has to be in the middle of their supply rails and they need to be in a high Z circuit so that their resistance non-linearity is less significant, and you may want to both series open and shunt for best bleed. (3x 4016/4066) If you use JFETs, add 1/2 the audio to the control voltage for best linearity. There is an old diode bridge circuit but it probable thumps badly when switched. Some circuits swap bias to multiple diff-amps, sort of bipolar linear multipliers.
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Old 1st August 2018, 12:12 PM   #3
dreamth is offline dreamth  Romania
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Thank you!I was just reading on Doug Self Small signal amplifier book about cd4066 use.I saw the fact that cd4066 needs to be in a high z circuit for best results.
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Old 1st August 2018, 05:12 PM   #4
mlloyd1 is offline mlloyd1  United States
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Best electronic switch for audio signals-logic drive
look for the manual of the hafler 915 jfet preamp. the schematic will be helpful for you.
best regards,
mlloyd1
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Old 1st August 2018, 05:48 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by dreamth View Post
I am looking for the best and cheap possibility of switching or muting audio signals
with the lowest distortion effect with electronic parts driven by logic levels.
For muting, a series resistor and a shunt relay contact to ground will work fine,
and is foolproof. You may need a transistor driver for the relay coil.
https://www.electronics-tutorials.ws...h-circuit.html

Last edited by rayma; 1st August 2018 at 06:06 PM.
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Old 1st August 2018, 08:00 PM   #6
Monte McGuire is offline Monte McGuire
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Best electronic switch for audio signals-logic drive
Quote:
Originally Posted by dreamth View Post
Thank you!I was just reading on Doug Self Small signal amplifier book about cd4066 use.I saw the fact that cd4066 needs to be in a high z circuit for best results.
Loading a CMOS switch into a high impedance will minimize the current through the switch, and thus the resistance modulation. You can also load the switch into a virtual earth, and this minimizes the voltage variation of the switch FETs relative to the substrate, and this also minimizes resistance modulation.

The only issue with using a virtual earth load is that the switch capacitance will de-stabilize the virtual earth amplifier. You can counteract this with a feedback capacitor, but if the switch capacitance and the compensation capacitor get too large, which the amplifier's output sees as a series combination of two capacitors to ground, this capacitive load can also destabilize the amplifier. So, there is a limit to the amount of switch capacitance that can be compensated.

So, loading into a high impedance is the simplest approach, but you still have to be careful to control the switched voltage relative to the substrate voltage. Some CMOS parts can be used with wider supply voltages, well beyond 5 or 7V, and this is helpful. It should also be obvious that the CMOS switch will need to use bipolar supplies to switch a bipolar signal, since you cannot drive the switch with a signal beyond either power supply rail. This makes the logic drive signal more complex, but again, some parts let you use three voltages, two for the signal switching FETs, and one as a logic ground reference, simplifying the issue.
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Old 9th August 2018, 04:18 PM   #7
dreamth is offline dreamth  Romania
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Click the image to open in full size.

I bypassed the switch here, but i know how it works as it is used in my first cd player.

Last edited by dreamth; 9th August 2018 at 04:22 PM.
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Old 9th August 2018, 07:22 PM   #8
mchambin is offline mchambin  France
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Analogue switches is the answer.
Analogue switch - Wikipedia

There are a lot of those from AD, TI, Maxim, etc.

An application note: https://www.diodes.com/assets/App-No...es/AB011-P.pdf
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Last edited by mchambin; 9th August 2018 at 07:28 PM.
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Old 9th August 2018, 08:28 PM   #9
mchambin is offline mchambin  France
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Quote:
Originally Posted by dreamth View Post
i need to switch very low signals(tens -hundreds of milivolts).
Be aware that analogue switches have some feed through. The logic control signal has some effect on the analogue signal.
This mean you might have some DC offset in the audio when the switch is ON versus OFF. You will have some spike in the audio when switching because of capacitive coupling, you better use digital control signal with slow rising and falling edges ( just clubber the digital signal with a capacitor, i guess you do not need fast switching ).
For lowest distortion, as said you need to choose switches with low Ron relative to the series impedance of the audio.
Because you deal with very low audio signals, this is good for low distortion.

In case these issues are troublesome you can better use reed relays rather than full electronic switches.
In extreme cases, one can choose mercury wetted contacts.
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Old 10th August 2018, 05:56 AM   #10
steveu is offline steveu  United States
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Remembered that if you slow/filter the switch voltage, it prevents clicks in the audio. After all, you don't really care if it takes 100mS to switch sources, but a 100mS transition contains no "audio" frequencies, noise.
(C)MOS transistors have no DC path between the gate and channel so there should be no DC in the audio, likewise J-FETS as long as the gate is always reverse biased.
Consider that there is no distortion in the "off" state so shunt switches are distortion free when the source is not muted. You probably don't care about a bit of distortion when muted.
My fav was P channel JFET shunt, which operates off ground (positive gate voltage) and the peak audio voltage levels are not likely an issue. But N channel may be a better shunt (with a negative gate voltage) if you have a negative supply voltage.

Last edited by steveu; 10th August 2018 at 06:12 AM.
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