Attenuation resistors for inline mic to line level

Use the balanced version at the end. Use high quality matched
resistors, match them by hand with a good meter:
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Line level signal to microphone input adapter

Sometimes there is need to convert line level signals to such signal
that it can be connected to microphone input. Because the line level
signals are typically in range of 0.5..2V and the microphone signals
are in millivolt range, quite much attenuation is needed to match the
signal levels. This means that typically you will need 40-50 dB of
attenuation.


40 dB PAD

Consumer audio line (-10dBu) level to microphone matching can be done
using a "40 dB L-pad attenuator". This circuit is indeed quite simple:


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

R1 = 10 kohm
R2 = 100 ohm


Circuit technical data:

* Attenuation: 40 dB
* Input impedance: 10 kohm
* Output impedance: 100 ohm
* Input signal: Unbalanced line level input
* Output: Unbalanced microphone output


50 dB PAD

Professional audio audio line level (+4dBu) to microphone matching can
be done using a "50 dB L-pad attenuator". This circuit is indeed quite
simple:


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

R1 = 33 kohm
R2 = 100 ohm


Circuit technical data:

* Attenuation: 50 dB
* Input impedance: 33 kohm
* Output impedance: 100 ohm
* Input signal: Unbalanced line level input
* Output: Unbalanced microphone output


Constructing the circuit

The circuit itself can be built to a small metal box or even can be
fitted inside an audio connector. The resistors in this circuit do not
need more than 1/4W power rating (even lower ratings will do). I would
recommend using metal film resistors in this circuit, because they are
less noisy than cheap carbon film resistors.

I don't recommend building the circuit to a plastic box because we are
playing with low level microphone signals, so the interference from
nearby equipments or cellular phones can be a signnificant if you
build this circuit to a plastic box or without any box. If you plan to
use the circuit in any serious audio system do yourself a favor and
build a circuit to a good metal box.


What to do if I get humming noise when I connect this to my system ?

Unbalanced microphone inputs of audio equipments are very sentitive to
all noise in the system, especially when you connect something else
than a flowating microphone to them. Usually at short distances there
are no problems in usign the adapter. If you get any 50/60 Hz noise
problems, put an audio isolation transformer to teihter input or
output side of my circuit.

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Balanced version

Balanced Taper Pad: 50dB (for 600 ohm impedance, XLR connector)


XLR in XLR out

1 ------------------ 1

2 ---R1---+---R2--- 2
|
R3
|
3 ---R1---+---R2--- 3

Component values:


R1 = 300 ohms
R2 = 300 ohms
R3 = 4 ohms


Equations to calculate different value attenuations



---R1---+---R2---
|
-> Z1 R3 Z2 ->
|
---R1---+---R2---




R1 = ((Z1 + Z2)* 0.994 + (Z1 - Z2))/4

R2 = ((Z1 + Z2)* 0.994 - (Z1 - Z2))/4

R3 = (Z1 + Z2) / attenuation


Example: Z1=600 ohms Z2=600 ohms attenuation=317.3 (=50 dB)
Then R1=298 ohms, R2=298 ohms, R3=3.78 ohms

Tomi Engdahl
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Fernando


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