Hello all,

I've got a 0.5V line level audio out source that I need to connect to a pre-amp mic in (which shows 14.2V).

I found something like this searching around, but I'm not sure if this applies:

                      C1
+Line level in      --||----R1----+--     +Mic level output
                                  |
                                  |
                       +----R2----+
                       |
     Ground (input)----+--------------- Ground 


R2 = ?
R1 = ?
C1 = ?

I'm slightly new to designing circuits, and have no clue if this is what I need. Any help is so much valued and appreciated.

 

hi,
The C1 will block the DC component, the 14.2V and allow the AC audio frequencies to pass through. A value of 100nF to 470nF would be a reasonable test value.

The R1 and R2 resistors will attenuate the AC audio signal, a reasonable test value would be 10K for R1 and R2.

If the audio is attenuated too much then make R2 47K or higher, if not enough make R1 higher.

Not knowing the details of the input and output resistances of the Line and Mic the C1,R1,R2 values are 'typical'.

The 14.2V is higher than I would expect on a preamp input.?

How did you measure it.?

E

 

why do you need to run line level OUT to a line INPUT mic? shouldnt the mic OUTPUT go to the preamp INPUT?

 

Some mic inputs have DC on them to power electret microphones. Rather than have that DC go through the divider, I recommend moving C1 to the other side of R1, so the circuit looks like a T with R1 on the left going to the source, C1 on the right going to the input, and R2 to GND.

Multimedia PC Level 2 (1999) defined the line input (max volume, no clipping) as -10dB, where 0dB is consumer level, 1mW into 700 ohms (yes, 700). This works out to 0.221 Vrms. OTOH, PC99 design guide (1998) defines the full scale inputs as line = 2.0Vrms and mic = 0.1Vrms. That's a 26dB difference. I recommend 20K for R1 and 1K for R2. If that is too much or too little, adjust R2 only.

ak

 

Thank you all for your suggestion and responses. I'll try to answer your questions as best as I can:

What I'm actually trying to accomplish: I have a low level audio source I need to wire directly into a Motorola mobile. The mobile has a built in pre amp for the mic line.

The 14.2V is higher than I would expect on a preamp input.?
How did you measure it.?

E, I took a digital multimeter set on DC, red lead into MIC wire and black lead into GND

Some mic inputs have DC on them to power electret microphones

AK,
I believe that's how this works!

I just would like to be sure so I won't fry anything or damage any components.

 

use a capacitor around 1 mfd to isolate the dc from the audio line. it should couple the audio through.

 

use a capacitor around 1 mfd to isolate the dc from the audio line. it should couple the audio through.

The way you specified above with C1 on the higher voltage side?
Which way would the polarity of C1 be?

 

if it were nonpolerized it wouldnt mater. otherwise, + to the radio side.

 

Ok so I gathered everyone's suggestion to this:

                                    C1
+Line level in      ------R1----+--||--     +Mic level output
                                  |
                                  |
                       +----R2----+
                       |
     Ground (input)----+--------------- Ground 


R1 = 20K
R2 = 1K (adjust this to attenuate)
C1 = 1uf

If that is too much or too little, adjust R2 only.

I don't want to sound stupid by asking this, I'm just trying to make sure I understand.

You mean: too much sound = higher R2 resistance
Too little sound = lower R2 resistance?

 

Nope, the other way around. If it is too loud, reduce the value of R2.

The output voltage is:

R2 / (R1 + R2) * Vin

Bob

 

Thanks for that formula!

I have this: 271-343
http://www.radioshack.com/product/index.jsp?productId=2062308

A.) Could I use this in place of R2?
B.) How do I hook it up? It has 3 pins.

C.) Does the following schematic look correct?

                                       C1
+Line level OUT(.5VDC)  ------R1----+--||--     +Mic level (14.2)VDC
                                    |
                                    |
                         +----R2----+
                         |
      Ground (out)   ----+--------------- Ground 


R1 = 20K
R2 = 1K  OR Potentiometer/Trimmer -(adjust this to attenuate) Lower Value to Decrease sound, Higher value to Increase Sound
C1 = 1uf