Amplitude Modulation Strange Behaviour

[NormanFreundParticipant]

Pulsar 23 Help Request 16-Jun-2026:

I have found a strange unexpected behaviour of my Pulsar 23 with the voltage controlled amplifier (VCA). I am using it to perform amplitude modulation (AM). If this operation is done all internally to the Pulsar 23, it operates as I expect, the carrier signal is from the Pulsar23’s LFO of about 300 Hz (audio range) and the control voltage (CV) is from the 3BIT S/H output pin of the SHAOS. This simply results in a triangular wave form with step changes in amplitude according to the SHAOS output pulses.

When I record a copy of the Pulsar23’s LFO signal to an external hardware looper (BOSS RC-1 Loop Station), and feed that into the VCA as the carrier signal (IN), that is when the output signal looks wrong, having an appearance similar (but not exactly) to adding the carrier signal to the CV signal. I got the same response if I took the audio signal from my Soma Lyra 8, feeding that in as a carrier wave form.

Please find attached two diagrams that explain, one is the Pulsar23 patching diagram, the other is the unexpected output when the carrier signal is feed externally into the Pulsar23.

Has any one else experienced this?

Have I understood how to use the Pulsar23 VCA correctly?

Any help would be appreciated.

Regards,
Norman Freund

(There is a possibility that the attached diagrams did not come across properly, in which case I will attempt to edit the post to correct).

[subconscious0Participant]

Hi Norman,

I suspect the issue is the BOSS RC-1, or more generally the way the external signal is being reintroduced into the Pulsar-23.

The RC-1 is an audio looper, not a DC-coupled device. It is designed for normal audio signals through its 1/4″ inputs and outputs, so it will likely remove any DC offset and may change the level or voltage reference of the signal. That means the copied LFO coming back from the RC-1 may look similar as a waveform, but electrically it may no longer be equivalent to the original internal Pulsar-23 LFO.

That could explain why the VCA output looks like the carrier and CV are being mixed together, rather than producing the expected amplitude modulation.

A useful test would be to repeat the experiment using a known DC-coupled device. For example, you could record and play back the signal through a DC-coupled audio interface, or use a sampler/module that supports DC-coupled CV/audio recording and playback, such as the Rossum Assimil8or. If the behavior changes with a DC-coupled device, that would strongly suggest the RC-1 is the source of the problem rather than the Pulsar-23 VCA.

I would also try attenuating or offsetting the external signal before feeding it into the VCA input, since the Pulsar may expect a particular voltage range or reference level.

My guess is that the Pulsar VCA is working correctly, but the externally looped signal is no longer electrically the same as the original internal LFO signal.

Regards,
Max

[NormanFreundParticipant]

Thanks subconscious0,

Yes agree, doubt very much my Puslsar23 is faulty rather a case of understanding the limitations. The Pulsar is my first modular synthesiser, so unfortunately I do not have any devices (to the best of my knowledge) that are DC coupled, all AC coupled.

I did try attenuating the incoming signal to the Pulsar23 1/4 jack, made no difference.

As a side note, the whole reason I wanted to do this amplitude modulation was so I could better understand what the CHAOS was doing, creating an AC signal means my AC coupled gear can read it and hence display on my digital oscilloscope in Max (Cycling’74) once the signal is in my computer. I needed two AC signals, one for the 1BIT SHAOS output and another for the 3BIT SHAOS. So I wanted a carrier wave without an Attack/Decay envelope, the Pulsar23’s LFO was perfect for this and taught me a lot, but there is only one LFO, hence why I thought get one from outside of the Pulsar23. Thinking it is a -1 to 1 signal, should have been fine. But yes understand your point, that extra circuitry in the AC coupled devices (a capacitor) might be the culprit upsetting the apple cart once it gets into the Pulsar23’s CV line on the VCA.

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