ARP 2500 - 1042 module

 This is one of ARPs 2500 "lost modules"

I've never seen one of these ... even as a picture.

I don't know if any were even produced.

Please let me know if you have any info .

I have a AM Synths AM1042.

It's probably the closest I'll ever get to one of these.

There are 3 circuits using the ARP 4019 VCA circuit

in each case.

The ARP 4019 is a classic discrete transistor voltage-controlled amplifier (VCA) sub-module originally used in the ARP 2600 .

+ ARP 2600 - VCA

VCA  (4010 & 4019)

The circuits have AC & DC inputs

They are classic VCAs

Plug the audio into the AC input & the CV into the DC input.

The DC inputs are better suited for CV signals.

If you just use the 3 AC inputs (for audio) it's a good amplifier

Each circuit has a black slide switch for VCA mode (linear or expo).

So which do I choose??

Linear VCA modes provide a 1:1, natural response ideal for mixing and modulation, while exponential modes produce a "snappier," faster response suited for percussive audio, aligning with how human ears perceive volume. 

Linear is generally best for CV modulation, whereas exponential is preferred for sculpting amplitude envelopes to sound more natural.

Human ears hear logarithmically, so exponential VCA responses often sound more "linear" in volume to our ears, whereas linear VCAs can sound abrupt.

Mixing Methods: If using a linear envelope, an exponential VCA creates a natural-sounding curve. Conversely, if using an exponential envelope, a linear VCA is sufficient.

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Exponential VCAs (The "Audio/Percussion" Standard)Response: 

Follows an exponential curve; slow initial response, faster later on.Best For: Audio amplitude (VCA as a volume envelope).Characteristics: "Snappier," faster, and more "natural" sounding for percussive or fast amplitude envelopes.Use Case: Percussion sounds, or creating smoother decays if the envelope itself isn't already exponential.

Linear VCAs (The "Modulation" Standard)Response: 

CV input directly maps to audio gain (e.g., 5V = 50% amplitude).Best For: Modulation, controlling other CV signals (FM, PWM), and precision mixing.Characteristics: Easier to predict and manage; sounds more gradual.Use Case: If you need subtle, precise control over modulation depth