Plum Audio ARBA User manual

PLUM AUDIO – ARBA
Thanks for purchasing ARBA from Plum Audio!
ARBA provides four Voltage Controlled Amplifiers (VCA) with linear response. It may look like a
simple VCA module but under the hood it is packed with features that unleash a variety of uses.
Three features that distinguish ARBA:
• Normalization: Everything is normalized to everything – all of the inputs, CV inputs and
outputs are normalized between the four channels so you can use a variety of
configurations depending on how you patch it.
As a result of this feature, you can get a significant gain boost (with CV control!) when you
chain several channels together.
• Variable sensitivity: Unlike most VCA's in Eurorack that clip easily due to the 5V unity gain
– the cause of this annoying clipping is that many envelope generators output a standard
voltage of 8V – with ARBA you can set the sensitivity of each of the channels (or - how
much voltage needs to reach the CV input to get unity gain at the output) using the four
trimmer resistors located on the back of the module.
• Limiters: soft clipping diodes will limit the outputs to 10.2 volts peak-to-peak and warm
up the sound (the limiters can be disabled using switches on the back of the module).
The combination of these three features allows a variety of uses, for example:
Quad simple linear VCAs
Stereo VCAs
Summing VCAs
Soft clipper
Distortion with CV control
Level booster (instrument or line level to Eurorack level)
And much more…

PLUM AUDIO – ARBA
Installation Instructions:
● ARBA is made according to the Intellijel 1U standard and should only be installed where the
Intellijel 1U standard is supported.
● Before connecting ARBA, make sure to turn off your case’s power supply.
● Connect one side of the supplied ribbon cable to the power header of ARBA and the other side
to your case's power bus board.
The red stripe of the ribbon cable (-12V side) must be oriented to the same side as the “red
stripe” marking on the header.
● Use the four screws supplied to mount ARBA to a free 18HP space in the 1U row of your
Eurorack case.
Power Supply Requirements:
● ARBA requires a -12V/+12V power supply (2x5 pin connectors).
● ARBA draws 50mA from the +12V rail and 50mA from the -12V rail.
INTERFACE:
INPUTS AND OUTPUTS:
1. VCA Inputs – DC-coupled signal inputs
2. Signal outputs
3. Gain control voltage (CV) inputs
4. CV indicator LED – its brightness represents the CV level

PLUM AUDIO – ARBA
OPERATING PRINCIPLES:
In this part of the manual you will find a few examples of patches that can be done with ARBA and
demonstrations of various aspects of its operating principles, please take your time and try out these
patches for a better understanding of the ARBA.
EXAMPLE 1 - QUAD MONO VCA:
The most basic usage of ARBA is as a quad mono VCA.
Use one channel to control the level of one mono source.
Connect the output of the source (oscillator, filter or any other audio source) to the input of this
channel, connect the CV source (an envelope generator, LFO or any other CV source) to the CV
input and connect the output of this channel to your listening path (a mixer or any other audio
interface).
When the CV level rises, the level of this channel will increase.
ARBA provides four identical channels that can process four mono audio sources.

PLUM AUDIO – ARBA
NORMALIZATION:
All the jacks in ARBA are normalized to the next channel, meaning that when a cable is unconnected to
the jack, the signal is coming from the previous channel (input, CV, and output signals).
To break the normalization, all you need to do is to connect cable into the jack.
EXAMPLE 2 - DUAL STEREO VCA:
To use ARBA as a stereo VCA you can use the normalization of the CV inputs and share one CV
for two (or more) channels.
In the following example, ARBA is divided into two sections, each of which functions as a stereo
VCA.
The first stereo VCA including channels 1 and 2:
CV input of channel 2 is unconnected so it’s receiving the same CV signal from the CV input of
channel 1.
The second Stereo VCA including channels 3 and 4:
The second CV signal is connected to the CV input of channel 3. This will break the normalization
from the previous CV inputs and will be used as the CV for channels 3 and 4.
With the same principles, you can blend examples 1 and 2 to create one stereo VCA and two mono
VCAs.

PLUM AUDIO – ARBA
EXAMPLE 3 - SUMMING:
ARBA can sum together a few channels.
When you leave an output unconnected the signal is summed at the next connected output.
In this example, only the output of channel 4 is used. This output sums together the outputs
signals of channels 1, 2, 3, and 4.
In this configuration, you can also use ARBA as a four-channel mixer, and use offset modules like
Plum Audio VASAT as CV source to control the gain of each channel manually.

PLUM AUDIO – ARBA
EXAMPLE 4 - SHARED INPUT:
ARBA can share one input to a few channels.
When you leave an input unconnected, the signal is shared from the previous connected input.
This is great to sequence different processing for one audio source like several effect units or
different filters.
In this example, only the input of channel 1 is connected. This input is shared with the inputs of
channels 2, 3, and 4.
You can connect outputs 1-4 to different paths and use different CV generators to create cool
rhythm sequences.
BOOSTING:
When using the normalization of the input and output together (chaining channels) you will get a boost
of about 6dB* in each channel after the channel from which the signal enters.
*Depending on the settings of the trimmers on the back of the module. It can be up to 9dB per channel
when setting the trimmers fully clockwise.

PLUM AUDIO – ARBA
LIMITERS:
ARBA is limiting the outputs to 10.2 volts peak-to-peak using soft clipping diodes.
By default, your module is shipped from the factory with the limiters enabled. There are significant
advantages to using the limiters for audio processing, such as warming up the sound, musical saturation
at high gains, and protecting the modules after ARBA from hard clipping and distortion.
But, if you want to process DC signals that are outside the -/+5.1V range using ARBA, you can only do so
with the limiters turned off.
On the back of the module, you will find four small dip switches in groups of two. Each switch is a
controlling limiter of one of the channels.
Use a pencil, small screwdriver, or any other sharp tool to move those switches to the off position.
When the limiters are turned off, the natural clipping point (hard clipping) of ARBA is about 23.2V peak-
to-peak (-/+ 11.6V), Be careful! - ARBA can produce high gains.
EXAMPLE 5 – BOOSTING:
Connect the audio signal to the input of channel 1 and your envelope signal to the CV input of
channel 1.
Now connect the patch cable to your listening path from one of the following outputs:
• At the output of channel 1, you will get 0dB*.
• At the output of channel 2, you will get a boost of about 6dB*.
• At the output of channel 3, you will get a boost of about 12dB*.
• At the output of channel 2, you will get a boost of about 18dB*.
* Depending on the trimmer’s settings.
Change between outputs 1, 2, 3, and 4 and listen to the impact on the signal.
You can make this patch even more interesting by using CV inputs of channels 2-4. Those CV
inputs will control only the relative boost, for example, if you connect another envelope
generator with a different rhythm to CV input 2, it will control only the boost between 0-18 dB,
a CV input 3 will control the boost between +6dB to +18dB and so on… (in case the active output
is channel 4).

PLUM AUDIO – ARBA
EXAMPLE 6 – VOLTAGE CONTROLLED DISTORTION:
For this example, we will assume that the limiters are left active (default settings).
Insert the audio signal to the input of channel 1 and the envelope generator to the CV input of
channel 1.
Use the output of channel 4 and connect it to your listening path.
Use the CV input of channel 2 to control the amount of distortion. When the CV signal on this
input is 0V the output is clean. Any CV above 0V will increase the distortion.
EXAMPLE 7 – VOLTAGE CONTROLLED SOFT CLIPPER:
As in the previous example, you can use half of ARBA to control the amount of soft clipping of
one signal by using the output of channel 2 and the second half of ARBA is left free for different
use.

PLUM AUDIO – ARBA
CHANNEL SENSITIVITY CALIBRATION:
On the back of the module, there are four small trimmers that control the overall gain of each channel.
Use a small screwdriver if you wish to change the default setting. They are single-turn trimmers. Be
gentle so you can feel when the trimmer is reaching its end – rotate clockwise to get higher gain or
counterclockwise for lower gain.
You don’t must to use our suggested “calibration procedure”. The best way is to use your ears and set
those trimmers according to your taste.
You even don’t need to set all channels equally, if you want 2 channels with higher gain and 2 channels
with lower gain you are welcome to do so.
If you wish to calibrate all channels equally to get unity between channels, please follow the calibration
procedure.
CALIBRATION PROCEDURE:
First, you need to choose what amount of CV will cause unity gain (unity gain is the point when the input
gain and output gain are equal).
- For super clean and natural low output gain you can use 8V or above.
- For low saturation at high gain you can use 6V-7V.
- For a higher saturation, you can use 5V or less.
1. Turn off all limiters by moving the four dip switches from the ON position to OFF.
2. Use a multiplier or stackable cable and insert the same DC source to the audio input and the CV
input of the channel you're calibrating, DC voltage should be at your desired settings (8V for
clean, 6V for low saturated and so on…).
3. Connect the output to a multimeter or oscilloscope and measure the voltage at the output.
4. Set the trimmer until the output is approximately equal to the voltage at the input.
5. Repeat this procedure on the next channel.
6. When you're done – don’t forget to turn the limiters back to ON.