Post by thread:[PIC]: Making a Audio Switcher

On April 16, 2005 11:28 am, L030010 wrote: > Hi all > > Need some advise in now to design an audio switcher > > This switch programmable and can route the sound signal to the > correct speaker . If you don't want to hear clicking sounds as you switch from one speaker to the next, you will need to have a variable resistance between the speaker and the output going from infinite (open) resistance to zero resistance (closed circuit). One clever design did it by making a light box using photosensors and a light moving on a track, which is a very mechanical way of doing it, but it works and is easy to fix.

For switching ahead of the amplifier, I use FET switches for the Audio. It takes a junction FET to do the job. If the right resistances and capacitor sizes are used on the Gate, the switching is very smoth without clicks and pops. If you are switching on the speaker side, then HIGH Current AC devices must be used, easiest are relays, but are not smooth switching devices. I havent used a power FET, but it could possibly work. I have drawings for the FET switches if anyone is interested. Vern ----- Original Message ----- From: L030010 <spam_OUTl030010TakeThisOuTsingnet.com.sg> To: Microcontroller discussion list - Public. <.....piclistKILLspam@spam@mit.edu> Sent: Saturday, April 16, 2005 11:28 AM Subject: [PIC]: Making a Audio Switcher > Hi all > > Need some advise in now to design an audio switcher > > This switch programmable and can route the sound signal to the correct > speaker . > > regards > > -

On April 17, 2005 07:14 am, L030010 wrote: > Please comment on this design i have > the design my not make sense as i am not electronic trained The transistors are biased to 0v to turn them off (open circuit), otherwise they are biased to 5v to turn them on (closed circuit). The speaker should use a reverse biased diode across it (similar to relays), otherwise your circuit needs to guarantee that you will quiet the speaker before/after use.

L030010 wrote: > Please comment on this design i have > > the design my not make sense as i am not electronic trained AFAIK pretty much all speaker drivers drive the speakers with AC, with (ideally) 0 V DC component. How exactly that is generated depends on the final amp stage. Your switch circuit seems to assume a non-0 V DC component. You weren't clear about whether you want a pre-amp switcher or a post-amp switcher. This looks like a post-amp switcher, for amplifiers with little power. As it is, it wouldn't work with the bipolar currents that a speaker driver will generate. You would probably have to make a circuit like what is inside a 4066 that can stand the currents you need. It is /much/ easier to switch on the input side of the amp (unless you use relays to switch the outputs). Maybe you tell more about your objective, to get some ideas. Gerhard

"L030010" writes: >Thanks for the input > >actually i am looking at tapping the signal from the PC sound card, switch >it to one of 10 speakers I have and then to the speaker. if the signal >after the switch is too weak i got no problem to ampify the signal Someone else on this list gave you the advice of using a circuit like the FET's in a CD4066 quad bilateral switch. This is excellent advice and will definitely switch the output of your sound card as long as it is the kind that doesn't have a power amp output to a small set of speakers. The SB16 card, for example, has a thumb-wheel volume control on the back plane and it will actually feed audio to small speakers that do not have their own amplifiers. The amp on the sound card is probably a Watt or two. Don't use a CD4066 on that because they do have internal resistance that would probably make the sound output too low. Most of the modern sound cards send a line-level output to the audio jacks and that should be in the 2 or 3-volt range like the output of a piece of hifi gear. If you build your switcher so that the 4066's have a VDD of 15 volts or even 12 volts, audio peaks will not leak through the switch unless they exceed plus or minus half the VDD, only referenced to ground. In other words, audio peaks would have to be at +-6 volts in order to break through if you use 12 volts for VDD on the 4066's. Of course, if you use a PIC for your controller, this means you'll have to have a level converter between the output of the PIC and the control lines on the 4066 gates. You can use 5 volts VDD on the 4066, but audio peaks of +-2.5 volts are sufficient to start coming through the switch even when it is off. You will definitely need amplified speakers at each location. They can be any kind of speaker that has a line-level audio amplifier, from a really good sound system down to the cheapest amplified P.C. speakers you can find. If there are any distances involved, you might want to use shielded audio cables since you can pick up mains hum. Martin McCormick WB5AGZ Stillwater, OK OSU Information Technology Division Network Operations Group

Martin McCormick wrote: >>actually i am looking at tapping the signal from the PC sound card, switch >>it to one of 10 speakers I have and then to the speaker. if the signal >>after the switch is too weak i got no problem to ampify the signal > > Someone else on this list gave you the advice of using a circuit like > the FET's in a CD4066 quad bilateral switch. That would have been me :) > This is excellent advice and will definitely switch the output of your > sound card as long as it is the kind that doesn't have a power amp > output to a small set of speakers. The SB16 card, for example, has a > thumb-wheel volume control on the back plane and it will actually feed > audio to small speakers that do not have their own amplifiers. The amp > on the sound card is probably a Watt or two. > > Don't use a CD4066 on that because they do have internal resistance that > would probably make the sound output too low. Given that you said you weren't opposed to using an amp after the switch, I think using the 4066, possibly with a resistor divider in the input, and an amp afterwards is probably the easiest solution. Look into app notes for the 4066; you'll probably find circuits like this (without the input attenuator, though). Gerhard

Gerhard Fiedler writes: >That would have been me :) >Given that you said you weren't opposed to using an amp after the switch, I >think using the 4066, possibly with a resistor divider in the input, and an >amp afterwards is probably the easiest solution. Look into app notes for >the 4066; you'll probably find circuits like this (without the input >attenuator, though). I started playing around with the CD4016 and the CD4066 in the late seventies and those are the first chips I go to if I want to switch audio since the input and output terminals pass AC as long as the peaks are below VDD/2. I probably shouldn't admit this in public, but I have been known to connect a pot as that divider you mentioned and then turn the gate off while cranking up the pot until I hear the clicks and pops of the audio peaks breaking through the switch. I don't know how close I am to blowing the FET's, but I then back it down a bit until there is silence. I haven't blown one that way yet. Be patient, though. There isn't a McCormick-proof chip on the planet. If you need to switch more signals, there are some similar chips with one-of-N type functionality, I think. They are multiplex encoder/decoders, just the thing for the outputs of a PIC to control. Martin McCormick WB5AGZ Stillwater, OK OSU Information Technology Division Network Operations Group

L030010 wrote: > can you kindly enlighten me a bit more on the following > > "possibly with a resistor divider in the input, and an amp afterwards is > probably the easiest solution." what is ment by a resistor divider in > the input? > > sorry I am not a electronic trained guy Take for example the 4052; this is a dual 4-channel analog (de)multiplexer. Meaning it serves to connect one input to four outputs, or one output to four inputs (your case) -- and this two times (like with stereo signals). Of course, whether this is exactly the configuration that serves your need, I don't know; you'll have to think about how you want to control those switches and research the available circuits that have elements similar to this one. Now you need to see what voltage your sound card puts out. This is probably in the range of +- 12 V max; you can measure this. Then you have to decide how you want to supply the chip; a dual supply would be good (because then you won't have to deal with shifting the AC input). It stands max. +-7.5V, maybe you can use the +-5V from the computer supply. This means that you have to divide the input voltage so that it doesn't exceed the chip's supply voltage (or stays a bit below it). Say you want to divide it by 3. Then you use two resistors, one double the size of the other: in ------ R1 ----+----- out (to chip input) | R2 | gnd R1 would be double the size of R2. Maybe R1=220R and R2=100R. You probably should read up on some electronics introduction (sorry, but I don't have any pointers to online material) to get a rough understanding of this all while experimenting with it. Gerhard