OG
& MOP HOME > CHEAP YAGIS FOR VHF AND UHF
PO Box 544 Aitkenvale QLD 4814 Australia - +61-7-47791161 - 0427182469
Technical Information
Line level signal to microphone input adapter
Tomi Engdahl
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
Other attenuation values
+Line level input -----R1----+-- +Mic level output
|
+----R2----+
|
Ground (input)----+--------------- Ground (output)
The attenuation of the circuit is determined by
equation:
attenuation (in dB) = 20 * log10 ( (R1 + R2) / R2 )
This equation is quite accurate when the impedance
of the mic input where this circuit is connected
has much higher impedance than the resistance of R2.
Typical microphone inputs in equipments have input impedance
of 1.5 kohm or higher (which is much higher than 100 ohms).
Component selection tips:
- R2 should prefeerably be something between 10 ohms and one kilo-ohms
- R1 should be from few kilo-ohms to few hindred kilo-ohms..
Those value ranges generally works bets for audio applications.
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 the input or output side of my circuit.
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
A BLAST FROM THE PAST !
theTARCinc
website and this page was orginally hosted courtesy of http://vk4tub.org
(Thanks Tubby !)
Send comments to theTARCinc WebMaster at qrmvk4wit@wia.org.au
(make sure to remove the anti-spam qrm first)
Page rendering Gavin Reibelt VK4ZZ ogSOFT Software Support.
hand crafted using XT-Gold 3.0 1word Text Editor
Free DHTML scripts provided by Dynamic
Drive
Last updated 8th January 2005
This page is now hosted courtesy of VK4ZZ.NO-IP.ORG on a computer in Vincent,
a suburb of Townsville Australia.
Contents of this page can be used freely for non-pecuniary purposes - making sure you acknowledge the great work done by Tomi Engdahl !