# Urgent help needed for effect of a capacitor in an audio amp circuit

Discussion in 'Homework Help' started by matthew798, Feb 9, 2015.

1. ### matthew798 Thread Starter Member

Jan 16, 2013
38
2
Hello everyone. I have a report due for tomorrow and I am pulling my hair out over this one.

We have a radio that we have to test. Once the initial test is done and the radio has passed, we are asked to change out a capacitor and test the radio again. The capacitor completely kills the radio`s performance so we are then asked to replace the old capacitor. Now we need to explain and prove why the new capacitor didn't work.

Here is the schematic of the part of the radio we are working on:

The capacitor we are replacing is C215 with a 0.01uF. Now, I was able to get some hints from my teacher as to why the new cap ruins the radio and he said it has to do with the reactance. The bandwidth is 350Hz to 5kHz.

My hypothesis so far is that with a smaller capacitor, the reactance goes up according to Xc = 1/(2*pi*f*c). But what I cant figure out is how I can justify the massive difference in the output without knowing the reactance of the coil in T202. Furthermore, what the heck do R215 and C214 accomplish? Do they have something to do with it?

Thanks in advance.

2. ### #12 Expert

Nov 30, 2010
16,676
7,320
Work backwards. If the frequency range is known, and the capacitance is known, what must be the reactance of the coil?

3. ### matthew798 Thread Starter Member

Jan 16, 2013
38
2
Good idea, but I don't know what the voltage on the secondary is supposed to be nor the xformer winding ratio, so I don't know what voltage i'm supposed to have over the coil to begin with...

My teacher insists that it's simple to solve without that info...

4. ### #12 Expert

Nov 30, 2010
16,676
7,320
It is.
What capacitive reactance formula do you have that requires a voltage to work the equation?

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
I would at least ask myself what the relative reactances are for the 4.7uF and the .01uF.
Then perhaps what is the actual reactance of each at say 1kHz.
Why is the capacitor there in the first place? Knowing that (plus the working bandwidth) should give some inkling as to it's desirable effect the AC load voltage.
This might give some insights - keeping in mind one would expect the 4.7uF to have either some or very little significance (your choice) in relation to the actual load voltage....

Last edited: Feb 9, 2015
6. ### matthew798 Thread Starter Member

Jan 16, 2013
38
2
Alright it would seem there is something obvious that i'm just not seeing.

The reactance of the 0.01uF at 1kHz is rouhly 16k and with the 4.7uF it's around 34.

I think part of my problem is that I wasn't seeing the capacitor as a coupling cap. I had forgotten about the DC component. But to be honest, I have no idea what you guys are getting at. I could solve for the coil, yeah, but in an ideal world, the entirety of the AC signal would be across the coil, so as to have the highest signal on the secondary. So it would make sense for the coil to be extremely high impedance to avoid losing amplitude over the cap. Anything over 1MOhm would enure that no matter what the cap, the signal isn't to badly effected. Am I wrong?