### Author Topic: Audio Voltage biasing  (Read 1268 times)

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#### zathrasb5

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##### Audio Voltage biasing
« on: August 29, 2017, 06:36:32 pm »
Hello.

I have been working on an audio switch, using a 4053, and have breadboarded my design so far.  One of the problems I am having is that none of the low frequency is passing through the IC.

I have attached a simplified version below.  Basically.  Line level comes in, and a high pass filter filters out the existing DC.  I then add in my VBias, and run it through the switch.  On the other side I have another high pass filter on the other side.

What I think my problem is, is that I have a problem with my math.  The formula f=1/(2piCR) should calculate the cutoff frequency, which I know I want to keep much lower than 20hz.

I have two theory's of what is wrong.
a) because I have two resistors on each high pass filter, is 1/R=1/R1+1/R2?, or
b) Is it actually 1/R = 1/R1+1/R2+1/R3+1/R4, or
c) Am I completely wrong.

I know the practical answer is just increase the capacitors and resistor values until it sounds good, but I would like to understand my error.

#### Benta

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##### Re: Audio Voltage biasing
« Reply #1 on: August 29, 2017, 06:59:50 pm »
If the switch is open, you have a)
If the switch is closed you have b)

(disregarding switch resistance)

#### zathrasb5

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##### Re: Audio Voltage biasing
« Reply #2 on: August 29, 2017, 07:11:24 pm »
Thanks, that's what I thought my problem was.  Just based on the sound quality, and where it was cutting off.  Hard to tell for sure, but the correct formula (b) gives the cutoff frequency that I have right now as 63Hz, which is way too high.  Time for a redesign.  Need to order more caps.

-Z

#### Ian.M

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##### Re: Audio Voltage biasing
« Reply #3 on: August 29, 2017, 07:29:02 pm »
Why not just increase the resistors by an order of magnitude?

#### Benta

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##### Re: Audio Voltage biasing
« Reply #4 on: August 29, 2017, 08:03:22 pm »
When doing AC analysis like this, VCC and ground are the same.

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#### zathrasb5

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##### Re: Audio Voltage biasing
« Reply #5 on: August 29, 2017, 08:14:13 pm »
Ian,  The main reason I am changing the caps, and not the resistors, is I actually already have a resistor array that I am going to use, to save space.   I could also change the design to use two resistors to get my Vbias, and then an resistor array feeding off that to each line, and reduce the number of resistors to 8 overall, from 12, but the resistor array should still be smaller, overall.

Benta, Thanks.  That's what i found out.  Not mentioned in any of the audio guides on the internet though.  Definitely a trick for new players.

-Z
« Last Edit: August 29, 2017, 09:38:52 pm by zathrasb5 »

Smf