Help with line input switch

[RomeyRome]

I write software. This is all new to me.

So I cooked up this project to keep myself busy...

My car has a unit that takes an iPod/iPhone input, and intergrades it with the car unit & controls for skipping tacks, etc.

I wanted to be able to switch between 2 different iPhones. Line out -> line in, plus controls just get passed straight through - Sounded simple enough.

I have things working. The problem I'm having is with the sound quality. It didn't sound right. Sounded like clipping. So I added 10uf caps to the left & right channel. It cleaned the bass up, but the mids & highs still sound like there's some crackling going on. Not a lot, but enough to annoy you. Like the speakers are blown. Like I should turn down the gain - that I don't have.

I'm obviously playing compressed audio & internet streaming, so high-fidelity isn't the goal.

Here's what I have:
The switch. It's enabled when pulled low.
Powered by a prefab 12v ->5v step down. It also constantly powers both iPhones independently outside of this diagram.

What am I missing?

Maybe I need to "pull down" RX,TX, & Vid Out since there's probably no constant signal, so it's floating? Should probably just ground the video line. It's useless. I dunno....

Thanks!

 

[Mooly]

I'm not convinced these are optimal devices to use tbh. The spec says 0 to Vsupply as the range of input signals permitted.

An idea. AC couple the inputs to chip. Also arrange for a "half supply bias" on the input side of the chips. That can be just two 100k resistors to generate that. The output will have to be AC coupled to. Also good practice to have something like 470k on the inout and output side of the caps going to ground to define ground so that he cap doesn't assume a reverse bias condition (depending what its connected too).

Beyond that and I suspect you have to look at different devices that can operate on a higher voltage. As it is you are limited to -/+2.5v or so signal level capability.

No guarantees... you will have to try it.

 

[RomeyRome]

Beyond that and I suspect you have to look at different devices that can operate on a higher voltage. As it is you are limited to -/+2.5v or so signal level capability.

No guarantees... you will have to try it.

Thanks for the feedback.

How high should the signal level be? I didn't think it would need a whole lot.

I have some of these on hand. Datasheet says -1.2v/7v. I kind of need it in an SOIC-20 package if possible.
http://www.nxp.com/documents/data_sheet/CBT3244A.pdf

 

[gofar99]

Hi I ran into a similar problem about a year ago with a digital volume control. The problem was the input signals that are symmetrical around the ground level would drive the control into a negative area that it could not handle. If high enough (positive) they could drive it over the top of the level it could handle as well. It was only able to accept a +/- 1.5 volt signal in either case. I had to develop a symmetrical power supply for it and AC couple the inputs to stop the problem. It sounds a lot like what you are experiencing.

 

[Mooly]

Thanks for the feedback.

How high should the signal level be? I didn't think it would need a whole lot.

I have some of these on hand. Datasheet says -1.2v/7v. I kind of need it in an SOIC-20 package if possible.
http://www.nxp.com/documents/data_sheet/CBT3244A.pdf

I would assume the levels need to be within the supply used for the chip. So that 5 volts peak to peak as a maximum.

The real test to do is to feed it with a test signal (sine wave) and look on a scope at what is happening. That will take all the guess work out of this.

Have you tried what I suggested earlier with the biasing and AC coupling ?

 

[RomeyRome]

Have you tried what I suggested earlier with the biasing and AC coupling ?

Not yet. I have to go pick up some components. Just trying to make a hit list of things to try.

 

[Mooly]

Not yet. I have to go pick up some components. Just trying to make a hit list of things to try.

OK. A "to do" list