Skema ini adalah rangkaian unbalanced audio lines to balanced audio
lines converter. rangkaian ini sangat sederhana karena hanya menggunakan
1 IC opamp NE5532, beberapa resistor dan kapasitor. Walaupun cukup sederhana akan tetapi rangkaian ini bekerja dengan baik.
Audio Unbalanced to balanced converter sederhana
IC opamp NE5532
Fungsi rangkaian unbalanced audio lines to balanced audio lines converter ini adalah untuk kebutuhan pengkabelan audio
profesional
antara alat audio satu dengan alat lainnya agar tidak terpengaruh oleh
noise yang di karenakan panjang kabel. jika peralatan input tujuan
pengkabelan tidak mendukung sistem
input balanced,
agar dapat bekerja tentunya rangkaian ini harus di padukan dengan
rangakaian kebalikannya yaitu balanced audio lines to unbalanced audio
lines converter. Tapi alat-alat audio tingkat menengah atas sekarang ini
sudah mendukung sistem pengkabelan balanced jadi anda tidak perlu
kawatir akan hal ini.
Rangkaian ini saya
posting
tujuannya adalah untuk sharing saja, mungkin saja di antara pembaca ada
yang punya peralatan audio lama yang belum mendukung sistem pengkabelan
ini anda dapat memanfaatkan rangkaian ini agar alat anda dapat berfungsi
dengan baik tanpa noise.
Alternatif
Introduction
A Direct Injection (or DI) box is a very handy piece of equipment for
any public address rig or recording studio, whether for band or general
use. It will allow you to connect the output from guitar amps, keyboard
mixers, tape machines and just about anything else directly to the
mixer, without using a microphone, and with no hum loops.
The unit described will convert unbalanced inputs (such as from a
guitar or bass amp) to balanced, allows the level to be set to
something reasonable, and comes in two flavours. There is a completely
passive version that uses a transformer to create the balanced send, or
an active unit which can be operated from a 48V phantom feed or a
couple of 9V batteries.
Description
Firstly, for those who may not know about phantom feed, Figure 1
shows how it is done. The 48V supply in the mixer is connected to both
signal lines, so causes no current flow in transformers since both ends
of the winding are at the same DC potential. At the remote end, the
current is tapped off the lines using a resistance value suitable for
the electronics. Again, this is done with both signal lines to ensure
that there is no DC imbalance in the circuit.
Figure 1 - Phantom Powering
After filtering (and in some cases regulation as well), the DC is
then available to power the circuit that drives the AC signal down the
very same pair that provides the power. In all cases the shield must be
connected at both ends, since this provides the DC return path (hence no
earth lift switch). In this example a microphone has been used, but
the same concept applies to virtually anything that can function on the
limited power available.
Figure 2 is the passive version of the DI Box, which is very easy to
build. The only problem is that to get good sound quality, you will need
a good transformer, and these are expensive. As can be seen, the input
is simply two 6.5mm phone jacks to allow a speaker lead to pass through
the unit. The output is a male XLR connector, and is balanced. Have a
look at the Jensen Transformers (or any other audio transformer
manufacturer) web site to track down a suitable unit. There are many
other manufacturers, but I don't know them all. See if you can find one
in your country. The transformer is 1:1 ratio, and needs to be rated
for 600 ohm operation (or higher).
Figure 2 - Passive DI Box
The switch selects either line or speaker level from the phone jacks,
and the 1k pot allows you to set the level when using a speaker source.
When using speaker input, the attenuator is variable to allow for the
widely differing output levels available from amps. No "earth (ground)
lift" switch is provided - these are often used to completely isolate
the signal source, for those occasions where there is a hum loop created
between the mixer and the stage equipment. Instead, there is an earth
isolation circuit (the 10Ω resistor and the 100nF cap), which will be
more than enough except in the most extreme cases. The earth lift is
only fully effective when the transformer circuit is used, and will
prove worse than useless in an active unit.
The active unit uses the 48V phantom feed available in many mixers,
but can be run from batteries if this is not available. To ensure that
there is no unnecessary battery loading a LED has not been included.
The connections to the XLR have been shown on all the drawings, and
the pin numbers are clearly marked on the connector, designations are
...
Pin 1 | Earth (Ground) |
| Pin 2 | Hot (+ve signal) |
| Pin 3 | Cold (-ve signal) |
Note that in some cases (especially with older equipment of US
origin), pin 2 is 'cold' and pin 3 is 'hot'. This connection scheme is
not recommended, and should not be used. The above is as close to an official standard as you will find, and should be used in all cases.
Figure 3 - Active Phantom/ Battery Powered DI Box
An earth lift switch cannot easily be used with phantom powering
without excessive complexity, and has not been included. The 10 Ohm
resistor and 100nF cap will be quite sufficient in all but the most
stubborn of cases.
The opamps require some degree of protection from the applied 48V
when the unit is connected, and this is provided by the diodes from the
opamp outputs back to the power supply. Without these it is possible to
damage the opamps as the output capacitors charge. Because some degree
of mucking about would be normally be needed for the output capacitors
to make the unit truly universal, these are specified as bipolar
(non-polarised) types - standard electrolytics
must not be used.
All resistors should ideally be 0.5W 1% metal film for lowest noise
and best matching. Capacitors must be rated at 25V or more, and diodes
are 1N4148 or similar. If you need as much level as you can get and
don't care about a bit of distortion, then a low power opamp (such as
the LM358) can be used. These draw a lot less current, so the supply
voltage will be higher. This allows more signal before the opamp clips.
Bear in mind that many low power opamps can supply less output current
than the TL072, so you may not get any real benefit. This does not apply
to the LM358 - it can supply more than enough current (and more than
can be provided by the phantom power scheme).
Two versions of the active unit used to be shown here, but by using
bipolar output caps the unit can be dual-purpose. When plugged into a
phantom supply, make sure that the switch is in the phantom position to
eliminate unnecessary battery drain. Likewise, always leave the switch
in the 'Phantom' position when not in use.
If you want to make the unit phantom or battery only, simply leave
out the parts that you don't need. For battery only, you don't need R8
and R9, and D3 (24V zener) can also be omitted. If the unit will only be
used with phantom, then you can omit the Phantom/Battery switch and the
batteries.