Making a switch using JFETs

[apakhira]

Hi
Could u list some common JFETs which can be used as bi-directional analog switches.I want to make a "soft" switch for switching audio signals on or off.Is that possible.Could u please give me the pinouts of the JFETs and specify whether they are n-Junction or p-Junction?I am going to put them after the output of a Flip-Flop.Are any other components necessary to make the JFETs work or should i connect them directly to the output of the Flip-Flop?

Thanx

 

[Exo]

For switching audio signals it's best to use a cmos switch like the 40 series
(4066 - 4053 and others), or a bus switch - **broken link removed**

If you want to use a single fet choose carefully as many of them have a high capacitance wich will allow audio to pass even when turned off.

 

[apakhira]

Thanx
Could u draw or post a link of a Schematic usig the 4066 as a switch, as i don't know what the other components (Capacitors, resistors,etc) are required.

 

[Exo]

The 4066 is all you need. It contains 4 independant switches. Here's a datasheet https://www.fairchildsemi.com/ds/CD/CD4066BC.pdf

You can only use it to switch preamp signals tough, don't think you can switch speaker outputs using this, use relays for that.
What devices do you want to switch?

 

[apakhira]

Yeah, i won't switch speaker outputs.Im making a 2-effect guitar processor and i needed something like a DPDT soft switch, which i guess can be done with a 4066.The signal which i m going to switch isn't preamped.It is the direct output of the guitar.Will it pose a problem?
And btw, can NPN switching transistors be used to switch the audio signals, as they r cheaper, i think.Actually i dont know the price of the 4066, the shops being closed as it is a Sunday.

Thanx

 

[Nigel Goodwin]

Yeah, i won't switch speaker outputs.Im making a 2-effect guitar processor and i needed something like a DPDT soft switch, which i guess can be done with a 4066.The signal which i m going to switch isn't preamped.It is the direct output of the guitar.Will it pose a problem?

No, that's fine, CMOS switches are commonly used in effects units and guitar amps. Bear in mind CMOS switches usually have a negative supply pin, this should be connected for switching audio signals.

And btw, can NPN switching transistors be used to switch the audio signals, as they r cheaper, i think.Actually i dont know the price of the 4066, the shops being closed as it is a Sunday.

They can, but the performance isn't as good as a CMOS switch, I remember an old well respected amplifier which used diodes to switch the inputs, that worked well - but was well before the advent of CMOS switches.

 

[Exo]

Yeah, i won't switch speaker outputs.Im making a 2-effect guitar processor and i needed something like a DPDT soft switch, which i guess can be done with a 4066.The signal which i m going to switch isn't preamped.It is the direct output of the guitar.Will it pose a problem?
And btw, can NPN switching transistors be used to switch the audio signals, as they r cheaper, i think.Actually i dont know the price of the 4066, the shops being closed as it is a Sunday.

Thanx

If you want a DPDT then the 4053 is what you need, it contains 3 separate SPDT switches, so if you use 2 of 'em togheter then you'll have a DPDT.

**broken link removed**

About the price, 40-series are cheap, they cost less then an euro over here. if you have a defective TV or VCR with scart somewhere then there's probably a 4053 in there as it's commonly used to switch the scart signals.

 

[apakhira]

Well Exo, i already said that i am building a 2-effect guitar processor.So i'll need two DPDT switches for true bypassing.

Bear in mind CMOS switches usually have a negative supply pin, this should be connected for switching audio signals.

-Nigel Goodwin, I don't understand what u said.And do u mean to say that i do not need any other external components for this IC to work? And if i supply 9V to the Vcc, what should i supply to the control pins to make the switches "on"?

btw, how can i use diodes to switch audio signals.Do u have a link for a schematic for it.Is it as good or close to the CMOS switches?

 

[Exo]

Well Exo, i already said that i am building a 2-effect guitar processor.So i'll need two DPDT switches for true bypassing.

Well, that's what i said, a 4053 will suit your needs perfectly.

Bear in mind CMOS switches usually have a negative supply pin, this should be connected for switching audio signals.

-Nigel Goodwin, I don't understand what u said.And do u mean to say that i do not need any other external components for this IC to work? And if i supply 9V to the Vcc, what should i supply to the control pins to make the switches "on"?

What he means is that these cmos ic's can be connected to a dual power supply (a negative and a positive voltage, for example +5/0/-5V )
For switching audio it is recommended to do so, even tough they can work with a single supply.

And yes, you shouldn't need any other components, except for filtering on the supply lines (.1µF).

To switch them, supply Vcc to the switcher input and it will switch, supply Gnd and it will open.

btw, how can i use diodes to switch audio signals.Do u have a link for a schematic for it.Is it as good or close to the CMOS switches?

It's far better to use the cmos switches.

 

[john1]

Hi,

Using diodes to electronically switch between inputs
is very effective and quite easy.
Many of the older multi-trace oscilloscopes did it like that.

When a diode is conducting current, both ends can be
considered as the same connection (more or less),
When a diode is reverse biased (insulating), both ends can be
considered not connected (more or less)

Well, as far as signals are concerned anyway.

The circuits are simple and well known.

John

 

[apakhira]

apakhira wrote:
Well Exo, i already said that i am building a 2-effect guitar processor.So i'll need two DPDT switches for true bypassing.


Well, that's what i said, a 4053 will suit your needs perfectly.

But u had said that 4053 had 3 switches.
And if im gonna supply negative voltage, which pin will i supply it to?Would it be any use if i converted afor example a +9V supply to +9V/0/-9V using a circuit with zener diodes, will it be of any use?

Thanx

 

[Exo]

But u had said that 4053 had 3 switches.

it has 3 SPDT switches, Use 2 of them to create a DPDT switch, the 3rd switch is just unused.

And if im gonna supply negative voltage, which pin will i supply it to?Would it be any use if i converted afor example a +9V supply to +9V/0/-9V using a circuit with zener diodes, will it be of any use?

check the datasheet for the exact pins.
You will need a real split power supply, you can't just create -9V with a zener.