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NEA-4019 Discrete VCA User Manual & Build Notes
Thank you for purchasing an NEA-4019 VCA module. This module is an adaptation of the ARP 4019 VCA
found in the ARP 2600 synthesizer using . This document will provide information about the basic
functions of the module, build notes, as well as the trimming procedure.
Power
The red stripe on the power cable is always -12V. Be aware of the orientation of this cable. The cable
and the PCB header are keyed to avoid accidentally plugging the power cable in backwards. The VCA has
a red LED indicator on the rear PCB to indicate that the module has successfully powered up. If you do
not see this LED lighting up, there is a problem and you should disconnect power immediately.
Front Panel Jacks
Audio Input 1 & 2 - These are the signal inputs of the VCA. Although they are labeled as audio inputs, DC
signals can be amplified as well. There may be slight DC offsets when using the VCA to amplify DC
signals. Audio input 1 in non-inverting, and audio input 2 is inverting.
Linear CV Input - This jack is a VCA CV input that has a linear response. Linear CV gain is typically
slightly less than that of the exponential CV input. Ideally a 0-8V signal should be fed into this input.
Exponential CV Input - This jack is a VCA CV input converts incoming CVs into an exponential response
curve. This input is useful for percussive, snappy envelopes. Ideally a 0-8V signal should be fed into this
input. The exponential CV input is capable of higher gains than the linear CV input. They can be made
equal by reducing the EXP GAIN trimmer, but it can also sometimes be useful to have more volume
output.
Audio Output - This jack is the output of the VCA.
Front Panel Controls
Audio Input 1 & 2 - These two knobs attenuate the incoming signals plugged into the Audio Input 1 & 2
jacks.
Linear CV Amount - This knob attenuates incoming CV signals fed into the Linear CV Input jack.
Exp CV Amount - This knob attenuates incoming CV signals fed into the Exp CV Input jack.
VCA Gain - This knob controls the master gain of the VCA.
Trimmers
The NEA-4019 has 4 trimmers on the PCB. The trimmer functions are as follows:
Exp Gain Trimmer - This trimmer sets the maximum gain of the Exp CV Input. Note that the OFFSET ADJ
trimmer also affects the Exp gain. This trimmer should be set so that there is maximum volume and
minimum thump. This trimmer is normally set between 6:00 and 11:00
Offset Adjust Trimmer - This trimmer sets the maximum gain of the whole VCA. The 4019 does have
some CV offset. This DC offset becomes greater as this trimmer is increased. Note that the OFFSET
ADJ trimmer also affects the Linear gain and Exp gain. If you hear any CV bleed, try reducing this slightly.
This trimmer is normally set between 7:00 and 12:00.
Lin Gain Trimmer - This trimmer sets the maximum gain of the Linear CV Input. Note that the OFFSET
ADJ trimmer also affects the Linear gain and Exp gain. This trimmer can be set fully counter-clockwise
unless this trimmer is causing signal bleed, in which case this trim should be reduced until bleed
inaudible.
Hi Freq Trimmer - This trimmer nullifies some of the high frequency signal bleed. It should normally be
fully counter-clockwise.
Trimming Procedure
To adjust the trimmers, it is best to feed an envelope generator into both CV inputs simultaneously while
running a square wave at 440Hz into the audio input. We find this to be easiest if done by ear.
Connect VCA Output to mixing board where you can monitor the audio signal.
1- Set Exp CV knob at 0
2- Set Lin CV knob at 0
3- Set VCA Gain at 0
4- Set the Lin Gain trim at 50% (6:00)
5- Set the Offset Adj trim at 50% (12:00)
6- Set the Hi Freq Trimmer at 50%(12:00)
7- Set the Exp Gain Trimmer at 0% (2:00)
Ideally, an envelope generator with 0-8V output should be used to plug into both the Linear CV and Exp CV
inputs.
8- Set Audio Input 1 to 100%
9- Slowly turn up the VCA Gain knob. You should hear the audio source plugged into Audio Input 1.
10- Set Audio Input 1 to 0%, then confirm that Audio Input 2 is working as well. Set VCA Gain to 0% when
finished.
11- Set Lin CV knob to 100%, then turn up Audio Input 1 & 2 to 100%.
12- With envelope generator, set attack to 0%, decay to 0%, sustain to 100%, release to 0%, now send this
envelope to the Lin CV and Exp CV input jacks. A 0-10V pulse will do if you don't have an envelope
generator.
13- You should now hear the audio signal being modulated with the Lin CV signal. Adjust Offset Trim for
maximum output volume, but stop when you hear the volume of the "thump" increase. You want to set
this trimmer for maximum volume and minimum thump. You'll need to compromise.
14- If you hear signal bleed when you turn the Lin CV knob up, try reducing the LIN GAIN trim. If you do
not hear signal bleed, you can increase the LIN GAIN trim to get more gain.
15- Set Lin CV knob to 0%
16- Set Exp CV knob to 100% and adjust the EXP GAIN trim with the 0-10V signal opening and closing the
VCA. As you increase the gain of this trimmer, you'll hear the output volume increase, as well as the
volume of the thump. Adjust this trimmer so that the volume of the VCA roughly matches that of the
Linear CV response. If there's too much thump, try reducing the Offset Adj trim. Exp Gain and Offset
trimmers interact with each other, so some experimentation will be required to achieve optimal results.
Steps 11 - 16 may need to be repeated to find the VCA's "sweet spot".
17- This VCA has a little high frequency bleedthrough in most cases. Sometimes this can be minimized
by adjusting the Hi Freq Trimmer.
Normalized Connections
The NEA-4019 PCB has a DIP switch on the rear. This switch toggles the normalized connections to the
NEA-4019 switching jacks. When nothing is plugged into a jack, the jack can be either grounded, or it can
be connected to the Normalized Connection Panel pads on the PCB. This allows for a normalized signal
path when nothing is plugged into the module, like the ARP 2600 has. The DIP switches should be set so
they are closest to the power header if you want the jacks grounded when no plug is inserted, and should
be set so they are closest to the Normalized Connection Panel if you are planning on using the
normalized connections.
Build Notes
R2 & R4 - These are the audio input resistors. Making these smaller will increase the gain of the audio
inputs, but may cause signal bleed if they are too small. They are indicated as 220K in the schematic,
but we've been using 75K in the REV E boards. These can be lowered to 56k for more signal input, or
raised to around 120k if you're getting any signal bleed from this.
R14 & D2 - These parts can be omitted if you do not want a power indicator LED.
R7 - This part is optional. R7 is a 2.2k-15K resistor that's in parallel with the Exp CV knob. Adding this
resistor will give the Exp CV knob a slightly more logarithmic taper.
R32 - Reducing R32 will increase VCA gain. Increasing will reduce overall gain. 4.7k is the value we like
best.
C7 -For stock 2600 configuration, there is a 0 Ohm resistor in this space, but if you want to add a DC
blocking capacitor (.47 film capacitor) you can place one in this pad. Be sure to use a film cap since this
is in the audio path. trr5.
Troubleshooting
No audio when using Linear or Exp CV inputs - Check Q4 & Q8
Only Linear CV works - Check Q3
Only Exp CV works - Check Q5, Q6, Q9
Excessive signal bleed - Replace Q4, repeat trimming procedure.
If you have any questions or problems, please contact us. For the fastest response, please use email at
[email protected]. Thanks!
NEA-4019 VCA Revision E Bill of Materials
RefDes Name Value Quantity Type Distributor Manufacturer Part Number
C1 ,C2 CAP_1206 4.7uF 2 Tantalum Mouser F931E475MAA
C3, C5 CAP_1206 100pF 2 Ceramic Mouser 12065A101JAT2A
C4 CAP_1206 30pF 1 Ceramic Mouser VJ1206A300JXAAT
C6 CAP_1206 220pF 1 Ceramic Mouser 08051C221KAT2A
0 Ohm 0805
Resistor or Film ERJ-6GEYJ000V (0 Ohm) or
C7 Cap 1 Mouser LDECD3470KA5N00 (Cap)
D1 1SS319 1SS319 1 Mouser 1SS319
D2 LED (optional) SML-211UTT86 1 Mouser SML-211UTT86
D3, D4, D5, D6,
D7, D8, D9, D10,
D11, D12 DIODE_1206 1N914 10 Mouser 1N914BWS
3.5mm Inline
Switching Jack
J1, J3, J4, J5, J6 plus nut 5 Erthenvar 3.5mm Inline Switching Jack
IDC2X5 Shrouded
J2 Header 1 Tayda A-2939
1x5_2.54mm
Breakaway
J7 Connector 1 Tayda A-197
Q1, Q4 DMMT3906 2 Mouser DMMT3906
Q2, Q6, Q9 MMBT3906 3 Mouser MMBT3906
Q3 MMDT3946 1 Mouser MMDT3946
BC850BW or
Q5, Q7 MMBT3904 2 Mouser BC850BW or MMBT3904
Q8 DMMT3904 1 Mouser DMMT3904
R1, R3 POT_RV09 100k Aud 2 A-1855
R2, R4 RES_1206 *75k (see notes) 2 Mouser ERJ-6GEYJ224V
R5, R8, R12 POT_RV09 100k Lin 3 Tayda A-1848
R6, R31, R10,R34 RES_1206 100k 4 Mouser ERJ-6GEYJ104V
R7 RES_1206 2.2k 1 Mouser ERJ-6GEYJ222V
R9, R13, R17,
R18, R23, R33 RES_1206 1k 6 Mouser RK73H2ATTD1823F
R34 RES_1206 180k 1 Mouser RK73H2ATTD5492F
R11 RES_1206 150k 1 Mouser ERJ-6GEYJ154V
R14, R22, R32 RES_1206 4.7k 3 Mouser ERJ-6GEYJ472V
R15 RES_1206 1.5M 1 Mouser ERJ-6GEYJ155V
R16 RES_1206 220k 1 Mouser ERJ-6GEYJ474V
R19, R26 RES_1206 220 2 Mouser ERJ-6GEYJ221V
R20 RES_1206 3.3k 1 Mouser ERJ-6GEYJ332V
R21 RES_1206 182k 1% 1 Mouser RK73H2ATTD1823F
R24, R29 RES_1206 56k 2 Mouser ERJ-6GEYJ563V
R25 RES_1206 18k 1 Mouser ERJ-6GEYJ183V
R27 RES_1206 33k 1 Mouser ERJ-6GEYJ333V
R28 RES_1206 470 1 Mouser ERJ-6GEYJ471V
R30 RES_1206 47k 1 Mouser ERJ-6GEYJ473V
DIP_4LPST
S1 (Optional) 1 Mouser DIP_4LPST
TRIM1, TRIM2 POT_TC33X 10k 2 Mouser TC33X-2-103E
TRIM3 POT_TC33X 100k 1 Mouser TC33X-2-104E
TRIM4 POT_TC33X 50k 1 Mouser TC33X-2-503E
U1 TL071BCD 1 Mouser TL071BCDR