AM1035 Triple Balanced Modulator


2024 Update I revisited the ARP 2500 in the Spring of 2024 and designed a set of ten “lost modules” that ARP mentioned in 1970 but never manufactured. These Eurorack modules are commercially available from the AMSynths webstore here.  The AM1035 was relaunched as a Eurorack module, initially as a 16HP wide version in April 2024.

ARP 1035 in situ

Overview This module is a replica of the 1035 Triple Modulator module from the ARP 2500 synthesizer. The module was in the original ARP Tonus announcement and it can be seen in the first sample system and in the first ARP 2500 catalog. However it was never put into full production and the rack space was typically used by an extra dual envelope generator.

It is a very interesting module which each circuit having three modes;

  • In modulation mode (MOD) it can be used to produce complex ring modulator sounds when all signals are audio/AC.
  • In modulation mode (MOD) it will act as a VCA when the X input is an audio signal and the Y input is a control voltage
  • In mixer mode (MIX) it mixes the two signals together with no modulation.

I have recreated the module using ARP designs from the 2500 and 2600, but if anyone has the original schematics or blueprint do let me know.

ARP 1035 Module

Description The 1035 module has three identical and independent  balanced modulators, with over voltage indicators and three low level microphone pre amplifiers. For each modulator there are two audio inputs and one audio output. In addition, external signals from low level sources such as microphones, guitars or other electronic instruments may be plugged directly into the front panel jacks on the 1035 module. A front panel switch associated with each low level input jack selects either high gain (x100) or low gain (x10) amplification.

Each balanced modulator will accept two input signals in the frequency range from DC to 20 kHz, which means both control voltages and audio signals can be used. A switch labelled “AC/DC” selects either AC or DC coupling for each of the six inputs.

DC Mode In the DC position the input bypasses the gain amplifier (and front panel control) and goes straight into the balanced modulator via the unity gain buffer op amp.

AC Mode In the AC position the input goes via the AC coupled amplifier with the front panel control knob adjusting the gain from unity to 10x. The frequency response is flat to 6 Hz.

External signals patched into the low level input amplifier are mixed into the adjustable gain amplifier and with the main inputs, but only when the switch is in the AC position. The two audio signals which are being modulated by one another can also be simply mixed in equal proportions by sliding the MOD/MIX switch to the MIX position. This enables an easy comparison of the pre-modulated (MIX) and post modulated signals (MOD).

There are three overload lamps that indicate the presence of voltages in excess of +-10V ate the output of the modulators. If the lamp is illuminated, the signal is probably distorted and clipped. If this happens the amplitude of the input signals or the gain of the amplifier should be reduced.

AM1035 Module

Eurorack AM1035 In 2019 I envisaged the 1035 as a 5U module, but moved to a Eurorack format after the release of the Behringer 2500 modules. I have used the ARP 4014 circuit as the basis of the balanced modulators, it was developed as part of the 2500 system, and used in the 1005 Mod Amp. The high gain amplifier looks like the microphone input of the ARP 2600 but without the 1000x gain setting. I have left this feature out.

I have added a buffering Op Amp to the X and Y inputs of the 4014, and upgraded the Op Amps from the original LM301’s. The signal path into the 4014 has been modified to replicate the 1035 separation of AC and DC signals, and to create the MIX mode.

The AM1035 uses SMD components, including matched transistor pairs for the current mirrors. The MIX mode is implemented by switching input B into the buffering Op Amp on input A and by connecting the B input to a fixed control voltage to provide unity gain. This is similar to how the ARP 1005 module works. Orange 3mm LED’s have been used for the overload indicators rather than the original current hungry and slow incandescent bulbs.

Layout The Eurorack panel design closes follows the original layout and graphics, with the omission of the pre amplifiers. Each channel has two level potentiometers on the input side of the balanced modulators, slide switches for selecting AC/DC and MOD/MIX and an overload 5mm red LED.

The 1035 has the following connections:

  • 6x Signal Inputs
  • 3x Signal Outputs
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