using a transistor as switch

[thebadtall]

hello,

i need to use a bc557 or 547 to switch on or off two audio signals from one spst switch

any help ?
thank you

 

[alexan_e]

So you want to use the transistor as a switch to an audio signal (AC)?

 

[thebadtall]

well yes, i actually intended to use relays but currently i have none,
i guess that since the transistor has polarity cannot switch an audio signal, but in combination pnp&npn could be?

 

[alexan_e]

The only way I see this working is if you make some kind of preamp or buffer but them this is not a switch as you want.

If you intend to follow this path then I would suggest a couple of opamps

Check this page http://www.decdun.me.uk/gainclone2.html you will find schematics for a transistor, a jfer, and an opamp buffer

 

[FvM]

Selectively enabling BJT buffers isn't bad. Under certain conditions BJT can be also used as analog switches, but with restricted signal level. Since 30 or 40 years, people prefer FETs or CMOS switches for this purpose.

 

[thebadtall]

thank you for your help, but i will use relays. (as soon as i receive them)
about bjt analog buffer switches, it will be quite complicated for me
thank you again

 

[zsolt1]

hi,
you could use 1 spst switch to turn on & off your preamp's power suply , supposing that both preamp's are supplied from the same source . I think that audio signal should not pass when spst switch is off .

 

[alexan_e]

The problem with cutting power to turn off and providing power to turn on a preamp/buffer stage is the booming you may get on the speakers since there is a power amplifier still on.

 

[thebadtall]

there will be a 3" delay circuit to prevent the boom, and a slow turn on power supply.
i have a couple of ne5532 and another couple of tl072, wich i could use, but this all about is a tube preamplifier, and i dont know if it is correct to use opamp before, even if the power of the signal stays low..
however the input signal will come from digital source , from pc or mp3 player... it will not be pure analog (ie phono)
so i dont know ...... wait for relays or build it with opamp?

 

[alexan_e]

I would prefer the relay solution

 

[D.A.(Tony)Stewart]

thebadtall<< When you need to design something, it is very important to define the interface parameters. e.g. source and destination impedance and signal level. Keep this in mind next time.
You can also use Reed Relays which are very reliable and miniature.

 

[Fe(II)man]

MC14066B Quad Analog Switch

- - - Updated - - -

MC14066B Quad Analog Switch

 

[D.A.(Tony)Stewart]

When I used the 4066 a few decades ago the on resistance was a few hundred Ω, so perfect for line impedance of 10KΩ , but not so good for speakers or headset. Maybe they have improved or you can drive it with +/-9.0V split supply or center biased on 18V

 

[tpetar]

4066 all the same even today.

 

[BradtheRad]

I used the 4066 IC to switch rapidly between two signals going to my oscilloscope.

It can work at audio frequencies.

Internally it is constructed from a set of mosfets, which are turned on and off by a hi or low signal.

 

[Jestin_cubetech]

dealing with audio signal ?

analog switching ic /analog multiplexer
1, 74HC4053
2, ADG 884

digital controlling switching ic
1, pt2258
2, digital pots [up-to 255 position resolution, 6 channels ...]

 

[D.A.(Tony)Stewart]

Just came across a great OPTO-ISO MOSFET 16 Ω switch

**broken link removed**

It was used as the front-end DSO probe shunt for the AC coupling capacitor. with 1Meg dc load.