Behringer 1004 VCO

Behringer 1004

Introduction One of the projects I worked with Behringer on during 2019/20 was the range of ARP 2500 clone modules, and the toughest modules to get right were the 1004 VCO and 1050 Mix Sequencer. I bought a Behringer 1004 module in September 2021 to see how well the VCO had turned out and to consider using it in a 5U replica of the ARP 1045 Voltage Controlled Voice.

The Behringer 1004 The Behringer VCO is a close replica of the original ARP schematics and the early prototypes proved impressive and worked first time! SMD components are a big advantage when it comes to VCO accuracy and stability, the B2600 VCO’s also proved precise (possibly too precise). High quality SMD Tempco resistors are used in the VCO cores.

The Behringer Manchester team completed the production version and added LDO regulators to the VCO to provide well regulated +/-11.34V voltage rails, which helps VCO stability. This is the same approach I use at AMSynths to create precision voltage rails for VCO circuits. There are five 4580 dual Op Amps, which are low noise and use BJT’s not JFETS at the input. They are a good replacement for the original 1339 and 301’s.

Front of the PCB

Timing Capacitors The original ARP 1004 used a set of seven timing capacitors in the VCO cores which are way too big to fit in a Eurorack module, and required manual selection. Behringer have chosen 4x polyester capacitors for the larger values (yellow and grey boxes in the photo) and SMD ceramics for the lower values. The low range capacitors are larger than the original; 1,150nF, compared with 710nF.

What is surprising is that the 3.45:1 ratio of capacitor value between the triangle and sawtooth cores in the original has not been retained, it is more like 10:1. This probably explains the extra frequency scale trimmer for the Triangle core (VR5).

VR9 HI END ADJUST

Trimmers The B1004 has a total of six trimmers, five of which are blue multiturn versions and not accessible, and one single turn which is accessible by the customer from the rear of the PCB (VR9). The Quick Start manual provides no details about the trimmers, so here is what they do:

• VR10 OFFEST – adjust the overall frequency of the VCO
• VR11 SCALE -adjusts the overall volts per octave scale of the VCO
• VR12 SAW ADJUST -adjusts the shape of the sawtooth waveform
• VR5 TRI SCALE – adjust the volts per octave of the Triangle core
• VR6 TRI OFFSET – adjust the frequency of the Triangle core
• VR9 HI END ADJUST – adjust the scaling at higher frequencies (4-8KHz)

The additional triangle frequency trimmer is probably used to compensate for using standard capacitor values in the two cores and avoids capacitor selection.

Incorrect Sawtooth

Waveform Accuracy All the waveforms were accurate and calibrated correctly by the factory, except for the sawtooth. The SAW ADJUST trimmer adjusts the shape of this waveform, which is generated by the sawtooth VCO core. The triangle VCO core is separate and is used for generating sine and square/pulse waveforms. The sawtooth waveform that has not been calibrated will show a step in the waveform, but you are unlikely to hear the difference in the audio range. You will notice it when using the B1004 as a LFO.

My B1004 VCO showed exactly this issue and I removed the front panel and adjusted VR12 to create a linear sawtooth with a 10v p-t-p amplitude. I used an expensive oscilloscope to adjust the VCO but a free software plugin can be used, in fact I detected the issue using MScope running in Cubase. The pictures show the before and after, note also the corrected waveform amplitude.

Corrected Sawtooth

The pulse waveform has a nice wide range from 5% to 95% (in fact 1.2ms of 25ms at 40Hz, which is spot on), and is exactly the same as the original ARP specification. The waveforms are all 10V peak-to-peak, again matching the original specification. The Triangle and Sine are bi-polar +/-5V whilst the Sawtooth , Square and Pulse are unipolar 0 to +10V.

The 1004 uses 100R Tempco resistors which are at R1, R2 and R19. These are also used in the Behringer 1047 filter but in series to give 200R.

Frequency Accuracy Once warmed up for 20-30 minutes the B1004 proved to be calibrated accurately at the factory, with a good set of results from C1 to C4;

• C1 is -4 cents
• C2 is 0 cents
• C3 is +1 cent
• C4 is +3 cents
• C5 is +7 cents
• C6 is +11 cents

The VCO was factory tuned to C1, with the Coarse control at minimum and the Fine control at midway. The VCO goes sharp above 1kHz and the calibration notes of the original ARP 1004  suggest only 1 cent inaccuracy at 1, 2 and 4kHz – this is a pretty demanding specification. There is a High Frequency trimmer (HI END ADJUST) that may help achieve better scaling above 1 kHz, in my own tests this does not do a lot.

Conclusion The Behringer 1004 VCO has worked out very well, good waveform shape accuracy and pitch stability over 5 octaves. I am building one into a 5U 1045 replica, more details in a future post.