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.
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 ohmCircuit technical data:
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 ohmCircuit technical data:
+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:
Those value ranges generally works bets for audio applications.
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.
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 Taper Pad: 50dB (for 600 ohm impedance, XLR connector)
XLR in XLR out 1 ------------------ 1 2 ---R1---+---R2--- 2 | R3 | 3 ---R1---+---R2--- 3Component values:
R1 = 300 ohms R2 = 300 ohms R3 = 4 ohms
---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 firstname.lastname@example.org
(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 !