DIP Analog Switch source

[Mike]

I'm trying to build a rather tough device for me to build - a PIC 16F84 controlled audio amplifier.

The hard part of my goal is to take 4 amps (4 x LM3886) and have them run in stereo, parallel mode. In other words, the amp would be able to drive 2 x 2-ohm speakers.

I also need the option to bridge both sides of the amp, for higher power into 2 8-ohm speakers.

Finally, I want to be able to take those two parallel channels and have them run in bridge/parallel mode to allow the highest power into a single 4-ohm speaker.

My plan is to use DRV134 chips for the bridging, so I don't have to worry about having to setup one amp to invert or not. My original plan was to use a few relays to either use or bypass the DRV134 chip to set stereo bridge or parallel mode. I haven't put much thought into it yet, but I'd somehow figure a way to set mono bridge/parallel mode using the relays.

I plan on adding an LCD and three buttons on the front of the amplifier box to switch between the modes. The LCD would display amplifier mode and temperature (if I hooked up a temp. sensor). Programming the PIC shouldn't be too big of a deal because I have PicBasic Pro.

But, rather than using relays, are there some sort of audio switched that could be controlled by a logic signal from the PIC that are in DIP style cases? I've looked all over and only can find surface mounted audio switches.

Does any company make these, or would I be better off just using relays?

Thanks,
Mike

[Enzo]

Not sure if it fits the bill, but plain old vanilla CMOS 4051, 4052, and 4053 are popular analog switches.

You can also set up all kinds of signal routing with a 4016 or 4066. These are independent quad analog switch ICs. You can make a SPDT switch with two sections. Wire one end together, then add an invert between the control pins for each section. That way when one is on, the other is off. Voila - SPDT. And of course paralleling more for DPDT or 3PDT or whatever is simple.

These are all available in DIP and are very cheap. I am sure there are more sophisticated and modern analog signal routing ICs out there, but these came to my mind.

[Robert Reed]

Mike

If you are talking any kind of power here, I doubt if you will find anything in a DIP package to work. If I understand correctly you want to configure one pair of amps to drive a load in bridge mode or drive the load by parrelling the two chips in non bridge mode. Most of the conversion will have to be done with the outputs of the amps.This entails handling the highest power and lowest impedance points in your circuit. As I mentioned the DIPs can't handle this kind of power or even approach thru-put impedances this low. Relays are awfully simple and there are some cute little printed cicuit relays available that will do the job for you, and relatively cheap. I think your only alternative would be hi-power low Z fets that you could swittch outputs configurations as needed. Save the DIPs for the inter circuit switching as needed for phase splitting,etc. as needed. Remember you will be switching 2 Ohm loads at times. As an after thought, if you decide to go with relays, Google up Teledyne and check out their DIP relays that are of the coil type. I've used some of these and they handle a surprising amount of current and take up very little board real estate.

<small>[ October 26, 2005, 09:10 PM: Message edited by: ROBERT REED ]</small>

[paulsantangelo]

Enzo:

Can you give me a wire diagram of your SPDT switch using 4016 IC. I came up with a couple of PNP transistors to do the job, but I do like the idea of using IC's. I figure your using 4000 series which meet my power requirements.

Thanks
-paul

[Mike]

Robert,

The switching will all be done at line-level signals. The highest power will be audio ran through the pre-amp. No high power signals.

All the output switching is done by hand, by re-connnecting the speaker wires.

Because bridging requires inverting of one amp, I will use DRV134 chips so, rather than switching modes of the opamp, It will just feed an inverted signal to one amp. The relays or ICs will only toggle the DRV134 on or off. The DRV134 will run at all times, but the relay will either send the input signal through it or bypass it. When it's bypassed, the amp can drive 4 4-ohm loads at 68W/ch (or two bi-amped speakers), or two 2-ohm loads at 136W/ch. Or, if all channels are fed the same input signal, it could drive a single 1-ohm load at 272W.

In bridged mode (through the use of the DRV134's) I could get that same 136W into 8-ohm.

Finally, in bridge/parallel mode (using one DRV134 and sending the signal to all 4 amps), at least 272W RMS would be provided to a 4-ohm speaker. It would probably be closer to 300-350W.

I did forget to mention, the chip I use must be able to pass the signal through it with none or as close to no quality degration as possible. This is going to be a Hi-Fi amp. I want to see just what I can do compared to the high-end manufacturers. I've got very close with sound quality (with my LM3875 amp which will soon be retired to use the nice toroid transformer in it for this amp), but not with power. It's as clear as many middle to high-end amps, but is just lacking in power. In this amp I need to add the power to that sound quality.

[ecerfoglio]

All of the "plain old vanilla" CMOS (4051, 4052, and 4053, 4016 or 4066) analog switches have a very low "on" resistance and a high "off" resistance.

Look up the actual values in the data sheets, but remember that they depend on the (+V) voltage used. "Ron" will be higher if you use +V = 5 V than if you use 12 or 15 volts (but then you will have to level-shift the logical inputs )

Just choose your cicuit's impedance so that the "on" resistance can be considered cero and the "off" resistance can be considered infinite:

For instance, if Ron = 10 ohms and Roff = 10 meghom then you can switch a circuit with 10 K impedance without loosing sound quality.

[Mike]

Thanks for the reply.

Looking around and thinking about my options, I think I'm leaning towards the relays - because they are easy to controll and very simple to setup and use.

But, will I have problems with electromagnetic noise in the signal?

[Robert Reed]

Mike
Wow1 you are talking power!
My experience with the 4066 series chips yeilded an 'on' resistance of 70 to 200 Ohms from a 15V supply. Howevever the 'off' resistances were quite high, on the order of 100 megohms or better. As Ecerf pointed out, keep insertion point impedances in mind--good advice. I dont think these will work for you in a straight audio path block due to some distotion that they will introduce. This wont be a lot, but at the extremely low levels you are shooting for it would degrade that performance. As to electro-mech. relays--no problem with EMI (other than possibly during a very breif switching period) from DC relays. If you were using AC relays (and I dont know why you would at this point in the circuit) it would depend on how much amplification the system has after this point and the audio level being switched. As to the output configurations--will you be changing back and forth frequently or just at setup and breakdown?. If this is a stationary setup, you may want to consider relays to do this task for you. They probably will need some beef to them as it sounds like you will be handling upwards of 10 amperes at times.

[Mike]

Robert.

No, the only power those relays or chips will handle is line level.

I was just saying my plans for the system and how it will work. All of the high-power switching will be done by hand by connecting the outputs differently.

Only line level will be switched

[Enzo]

That on resistance of a few ohms or even a couple hundred is pretty much negligible in the high impedance world of the line signals.

There are many examples of pro audio gear using 4016 as a routing element. It works. Of course a couple JFETs will also work.

Paul, I have no way to draw here. But it is simple. If you have the pinout diagram for the 4016/4066, use these numbers. The chip has four identical sections. Each section has a pair of in/out pins and a contro pin. Grounding the control pin turns the switch off. Putting a positive voltage on the control pin turns it on - meaning the connection goes to low resistance between in and out. There is no direction to the in and out, use the chip in either direction.

So in the first section, pins 1 and 2 are the ends of the first switch. Pin 13 controls them. Ground pin 13 and there is extremely high resistance between 1 and 2. The switch is OFF.

So pin 13 makes pins 1 and 2 into a SPST switch. Well, more like a SPST relay I suppose.

Pins 3 and 4 are the in/out pins of the second section, with pin 5 the controller. Pins 3 and 4 are then another SPST switch.

To make a SPDT switch, connect pins 2 and 3 together. Now if we turn on the first section, then pins 2,3 are connected to pin 1. But if we turn on the second section, then pins 2,3 are connected to pin 4. SO we have now a switch with pins 2,3 as the common, and pins 1 and 4 as the two switch positions - pins 2,3 are the pole and pins 1 and 4 are the throws.

Of course if we turn both on or both off, we get connections not normally found on a SPDT switch. Since the control pin is controlled with simple logic voltage levels for the CMOS chip, if we wire a simple inverter stage from pin 13 to pin 5, then whatever we put on 13, the opposite will appear on 5. Turn 13 off, and 5 will be on. Turn 13 on, then 5 will be off. So in this example the control voltage will always be applied to pin 13.

Inverters abound. You can use one section of a 4049, or one section of a 4011 wired as an inverter. (That just means the two input pins wired together.)

These are CMOS, so they don't use much current. and the power supply can be anything up to 18VDC. Since your line signals are not more than a couple volts, you have headroom there.

This just one option of many. If my description is not understandable, I can make a simple drawing, but you would have to get it from me directly. My addy is tmenzo "at" msn "dot" com. I admit the description is a whole lot larger than the circuit diagram.

If you write, make the subject line "nutsvolts analog switch" to get past the spam filters.

[dr_when]

Mike,

Can't you use any of the audio/video crosspoint switch IC's that Maxim and numerous other companies make. They are typically used for what you seem to be doing.

Google on "audio crosspoint switch"

[jwax]

Thanks guys! I was just listening in, but think I now know how to build my 4 stereo channel switcher- the 4016! I want to select one of four stereo inputs (TV, DVD, VCR, or Tuner) to go to the computer, and out to the main speakers.
Sweet!

[jimandy]

Jwax, use the 4016 if you want, but the CMOS cookbook says "The 4066 is an improved version with lower on resistance".

If anybody's interested, I think I have an RCA 16 switch xpoint (4x4) chip languishing in the basement, bought years ago to build my audio center switch box. Now it's just another "Round Tuit" and the chip, I believe, is obselete.

[jwax]

Thank you more, jimandy! 4066 it is!

[ecerfoglio]

Just remember that those CMOS analog switches don´t like signals that swing outside their power suply´s levels.

If you have (or may have) coupling capacitors, be shure to bias the signal to 1/2 your supply´s "+V".

If you can´t use capacitors (for instance, if you have to go down to DC) you have to use a bipolar suplly (up to +/- 9 V, or better +/- 7 V) and power the analog switch between +V and -V. Of course, that implies level shifting the logical input values.