Question about the MAX333CCP

[bluepunk]

Hey everyone!

I've been playing a little bit with switching audio (guitar) signals with a MAX333. Using a 5V to 0V supply with a voltage devider for the ground reference.

My audio is being sent trough the MAX333 from the COM1 pin to the NO1 pin. (NC1 is shorted to ground).
All of this seems to work as expected the only issue I'm currently seeing is that when I apply a high signal to IN1 I hear it switch.

Everything is powered with the 5V from an Arduino and the IN1 is connected to digital 13 on the Arduino. (using the simple led blinker sketch with delays set to 1000ms to test it)

Is there any way to get rid of this switching noise?

Ps. I'm fairly new to this kind of electronics so if I talk crap somewhere let me know 

[Stefan]

Have you tried connecting NC1 to the 2.5V of the voltage divider?

NO1 will be at those 2.5V, I assume. So you are switching the output between 2.5V and 0V. This step has a wide frequency band and will be hearable.

edit: I've just realized that you feed the signal into COM1 and out of N01, sorry, I was thinking you get it in NO1 and out of COM1. Where does the signal go after NO1? Do you maybe let an input float there?
-Stefan

[bluepunk]

I've made a simple schematic of what I've done.

https://dl.dropboxusercontent.com/u/865542/capture.png

U6 and U7 are connected to a guitar and amplifier respectivly.
It works as I'd think it would. Its just that the moment you apply a signal to IN1 you hear a click of sorts.

Edit: sorry about the weird layout of the wires. Stupid 123d.circuits.io schematic auto routing/layout which you can't edit. 

[Stefan]

From the datasheet, I'm fairly sure that GND of MAX333 should be connected to GND of the arduino, not the 2.5V. That GND pin is the reference for the logic level applied to the IN-pins, not the audio signal.

Current though the IN pin will go out through GND and through the voltage divider, affecting the voltage there. That voltage is the reference of the audio signal, which is why you might hear it.

You might want to disconnect NC1 again, now that I see the schema, I don't think that is it.

What values do you have for R1 & R2? Also, how is the arduino powered?

p.s. I assume we have to thank this auto-layouting tool for shortening GND1 and VIN on the arduino

[bluepunk]

Darn I missed that connection to VIN (its not connected). The resistors are 20k each. The caps are 1000uF (rated 16v and 25v because that's what I had in my bin)

I'm powering the Arduino from USB.

From what I understand the GND connection is set to 0V whilst the V+ and V- are 2.5V and -2.5V respectively. I did this after a suggestion from my Dad.

I think I'll have to lend a scope somewhere to actually see what it's doing.

[Stefan]

If you define MAX333's GND as 0V then indeed V+ and V- are +2.5V and -2.5V respectively. But the output D13 of the arduino is 5V or 0V relative to Arduino's GND, thous +2.5V and -2.5V relative to MAX333's GND. The MAX333 is quite robust in this respect, that's why it is working at all.
I suggest you try it.  Easier than organizing a scope

-Stefan

[bluepunk]

In this particular case if I disconnect NC1 I get a lot of noise on the amplifier. Not sure what kind of noise but its loud and nasty if it's coming out of a small Marshall amplifier

Edit: also looking at the chip layout I got the idea that the input pins were not affecting the signal. Could be very very wrong tho.

[bluepunk]

Okay after some further testing (read removing NC1). I've come to the conclusion that it still works but there is sound coming trough the chip if IN1 is disabled. It kind of sounds like a pot that doesn't ground the guitar signal. Only audible if you strum really loud.

[Stefan]

The disconnected NC1 probably picks some of the signal up due to capacitive coupling. Just leave it connected to the 2.5V.

Have you tried connecting the MAX333's GND pin the correct way?

-Stefan

[bluepunk]

I had GND and V- connected to the Arduino's GND before. That made a ton of noise where my dad gave me the idea of the voltage divider to see if that solved it.