ARP 2500 - 1005 ModAMP Module - Ring Mods vs VCAs

The 1005 is a really interesting module.

 At first glance it seems simple... a ring-modulator (Balanced Modulator) connected to a VCA (Unbalanced Modulator). Actually ARP (and Tonus) often combined different components which would usually
be separated, into a single module. A good example is the filtamp 1006 -- it's a filter+VCA.

 The interesting bit about this module is how the two parts interact with one another, how you can control them and how they modulate the modulating VCOs ... but more about that later.

First, a brief description of Ring Modulators & VCAs.

Ring modulators & VCAs are of course related.
Both modules accept two signals and both multiply them together, but in different ways.
VCAs are also called Unbalanced Modulators & Amplitude Modulators.
Ring Modulators are also called Balanced Modulators.

VCAs

A VCA lets a signal pass. How that signal is passed is however controlled by the second signal (The control voltage).

Basically, it's multiplying the input signal by the modulation signal.
If the modulation strength (CV) is at full voltage, all the input signal will get through. In effect, the signal is multiplied by 1. (There is a switch to change the slope of the VCA between exponential & linear).

If CV = 0, out = 0.  That is, there is silence when there is no CV.

An important point to remember with a VCA is that the (audio) input signal can be bipolar (+ve & -ve), but the CV can only be +ve. (Any -ve modulation signals are ignored).
So it won't let any audio through if the modulation signal is zero or -ve.
(Because of this, most VCAs have added a DC offset at the modulation input which inverts any -ve voltages to +ve.)  

VCAs are classified as 2 quadrant multipliers because they handle bipolar signals on the audio input but allow only +ve multiplication. 

 

Ring Modulators (Balanced Modulators)

It's basically an electronic multiplier. 

If there are 2 input signals, A & B, the output is the product.

A x B = output.

Actually, in the case of the ARP 1005 module, the output is really (A x B) / 10 .

This is to protect the synthesizer from overloads, where the product of A and B is greater than +/- 10V

 

In general, Ring Modulators differ from standard VCAs in that they allow both negative & positive multiplication of signals.

You may ask the question "where did the ring modulator get it's name?"

In the early days, the modules used diode rings to change the polarity of the modulation & carrier signals.
Most modern modules have done away with this old technology

So the ring modulator is just a VCA with a special bipolar (+/- ve) input.
Because of the difference , the Ring Modulator is also called a four quadrant multiplier

Convention, states that the audio input ("A" in the ARP 2500) is called the carrier & the modulating / multiplying input ("B" in the ARP 2500) is called the modulator.

The Modulator can be activated or disengaged from the audio signal by pushing the UNMOD/MOD buttons. You can also use gates & triggers to do this. 

The obvious modules to supply gates are the 1027 sequencer, or a pulse from a LFO/Oscillator.  However, the slow random CV outputs of the 1016 Random Voltage Generator  can also trigger these switches. The "internal clock out" of the ARP 2600 (Sample & Hold section) also works.

 

MOD = audio signals A & B modulate one another.

UNMOD = Audio signals are passed through the modulator unaffected.

The ring mod section of the 1005 possibly uses a potted module: the 4014 sub module (I'll check this one day).
This sub-module was used in the 2600.
Here is part of the schematic:

With a RingM you can create a wide range of new frequencies that have no relationship to either of the original inputs A or B. Inharmonic overtones can be produced. This brings us to the lower half of the module.

The 1005, in addition to the ring mod & VCA, contains additional circuits that produce two sets of "Inharmonic Preset Control Voltages". They are named CVa & CVb. 

 

These can be fed back to the two VCOs (which are supplying the inputted audio signals A & B) to detune them a preset amount when the ModAMP  is switched from its "Unmod" to its "Mod" function.

The preset detuning of the oscillators permits control over any apparent key change which could result from the modulation of the two signals, A & B.

 

 

 

The manual suggests using 1004 & 1023 VCOs.

The difference voltage (Va - Vb) controls the frequency ratio of CVa to CVb

The average voltage (Va + Vb)/2 controls the tuning of CVa to CVb

The output of the ring modulator is always passed through the VCA.
I understand that the VCA used in the 1005 is identical to the 1006 Filtamp. 

Electrical Specifications

 

Balanced MoD section:

Output : V out = (INa x INb)/10 +/- 1.0%

Audio Inputs : +/- 10V max (100 Kohms impedance)

VCA section:

(input attenuators fully clockwise)

Vout = (Vin x Vcontrol)/10, Linear Mode.

Vout = Vin¹⁰ (Vcontrol^-10)/2. Exponential Mode.

Control input Impedance = 100 K

Power requirements: 

+/- 15 V @ 35 ma, regulated to +/-0.1 % 

+ 12 V to +15 V @ 20 ma, unregulated.

ARP was planning to make the 1035 Triple Modulator.

I have never seen one of these.
It offered balanced modulation without voltage controlled amplifiers.
The 1035 contained three balanced modulators and six microphone pre-amplifiers.

The inputs and outputs appear at the top and bottom of the module. For each ring modulator there are two audio inputs & one audio output. It looks like you can also plug in low level external instruments like mics and guitars.. you can select the gain (High: x 100 or low : x10).

A switch labelled "AC,DC" selects either AC or DC coupling for each of the 6 inputs. So you should be able to process control voltages as well as audio. I reckon (though of course I haven't tried this) you could mix audio with LFOs.. possibly creating a tremolo.

 The Mod/Mix switches

Mod = Normal Ring mod operation

Mix = simple 2 channel mixer  

These 3 switches allows a simple way to switch between a Ring Modulator or Mixer operation on the two input signals.

In the Mod setting, it can also be used as a VCA when one input is audio, and the other is a CV.

The 3 overload lamps indicate voltages over +/- 10V at the outputs of the modulators. If they are lit, the signals are most likely distorted & clipping... you should thus either reduce the amplitude of the inputs or reduce the gain of the amps.

 

 

 

 

It's interesting to compare the 1035

 and the ring modulator of the ARP 2600.

They both have preamps, DC / AC coupling etc.

 

Links

+ ARP index
+ The ARP 2500 & the 1005 MODAMP module
+ Guitar fool
+ TTSH (ARP 2600 clone) build notes - Ring Modulator

PS: Please let me know if there are any mistakes or errors.

Cheers Jono