The MAM RS3 Resonator


The Resonator is a box which accepts mono-in and delivers stereo out. In the midst of the audio path are three resonant bandpass filters, each of which can be set to a separate pass frequency.

For modulating the cutoff of the filters, you'll find an onboard LFO (actually two LFOs) running from 0.01hz - 20hz, an Envelope follower, and a jack for accepting a external modulation source.


"Vactrols" is a word that everyone should get to know... If you don't know it offhand, you've more-than-likely heard the effect of this semiconductor device in an audio circuit.

Some information on Vactrols can be found by searching through the Analogue Heaven mailing list archives via this link. Use "Vactrols" (without the quotes) for the keyword.

The original prototype of the RS3 (which was mono and without the 2nd LFO or envelope follower) was built by Juergen Haible. Juergen discussed a bit of the design and the background of the RS3 on the AH mailing list in Mid 1997. You can find specific info on the RS3 by again looking in the AH Archives and using "mam resonator" as your keywords.

In short, a vactrol is a light sensitive SCR which is encased in a housing along with a light source, basically an optoisolator. When you use one in an audio circuit like a compressor, you get a very nice "mushy" feel to their workings. This comes from the inherent latency of the SCR reacting to the light source.

Digging In

The bandpass filters are wired in parallel with respect to the input signal. The base frequency of each filter can be set separately (no specs in the manual as to range). Once you set the relationship between the filters, then the frequency of all three can be changed at once via a separate "Manual" control.

The resonance of all three filters is controlled in unison by a single control. The width of the band of the filters is fixed at an unspecified amount

The RS3 comes alive in wonderful ways when modulation is applied to the filters. The modulation path is more interesting than it may seem at first glance.


LFO - There are actually two LFOs in the RS3, but only one speed control. LFO2 runs at a rate approximately 30% faster than LFO 1.

There are three modes to modulating the filters from the LFO(s):

Mode 1 LFO 1+ to BPF 1   Mode 2 LFO 1+ to BPF 1   Mode 3 LFO 1+ to BPF 1
LFO 1+ to BPF 2     LFO 2   to BPF 2     LFO 1- to BPF 2
  LFO 1+ to BPF 3     LFO 1+ to BPF 3     LFO 2   to BPF 3

Note the availability of a negative polarity of LFO1.

In Mode 1, a single LFO is driving all filters. This gives a classic "normal" swept filter effect.

In Mode 2, LFO 2 - which is running faster than LFO 1 - is driving filter 2. This gives a very nice non-linear animation to the sound.

Mode 3, however, is where the RS3 will make it's money. The opposing polarities of LFO 1 are routed to filters 1 and 2, then LFO 2 is routed to filter 3. This mode, in combination with the Stereo Pan control (explained below) is where you can find Jean Michael Jarre string pads.

Envelope Follower - This is a classic envelope follower which allows for sweeping the pass frequency of the filters. But, there's a twist. The envelope follower does some frequency separation at three ranges, "Low", "Mid", and "High", with a separate follower circuit at the output of each of these "channels".

The "Low" channel routes to Filter 1, the "Mid" channel routes to Filter 2, and the "High" channel routes to Filter 3. This is great for providing more stereo depth to a signal. See the "Stereo Pan" item below for more information.

The polarity of the signal from the envelope follower is switchable from Postive to Negative allowing for classic Down sweeps, as well as Up sweeps, of the filters.

The individual channels of the follower are not exposed on the front panel, and no information is given in the manual about the frequencies or bandwidth for each.

Manual - Spin the knob to shift the pass frequencies of all filters at once. The relationship that you have set for each filter via the individual frequency knobs is not disturbed.

External - When something is plugged into the External jack, the Manual control becomes an polarized attenuator for the signal. In the Middle position, the external signal will have no effect. Moving clockwise from center will cause the external signal to sweep the pass frequency up, while moving counterclockwise from center will cause a sweep down.

There are no specs for what or how hot of a signal that you can feed into here, but I have used the output of an ARP 2600 ADSR for this with no problems.

One thing to point out... There is no provision on the RS3 to sync the LFO to an outside source, or to otherwise control it's frequency. To do synced filter sweeps on the RS3, you will need to route a synced CV source into the External jack. There are various MIDI->CV converters on the market today which provide this feature, including those from JKJ Electronics, Encore Electronics, and Kenton Electronics.

All of the modulation sources can be used at once, in varying degrees. The exception is the Manual control being overridden by the introduction of an external source.

Other Controls

Stereo Pan - The RS3 gets it's "mono in, stereo out" designation from Filter 1 and Filter 3 being panable from hard left to hard right. With the control fully counterclockwise, Filter 1 is panned hard left, and Filter 3 is panned hard right. Moving the control to the center places all filters "right up the middle" basically making the unit mono, and then continuing to full clockwise reverse the position of Filter 1 and 3/

Refering back to Mode 3 of the LFO modulation, note that the modulation of Filter 1 and 3 comes from different sources. This is where you will get the greatest stereo effect from the RS3, and this is where the "Jarre Strings" sound lives.

This control also has an effect over how the RS3 can create a stereo spread when using the Envelope Follower. Again, Filter 1 and 3 are panable across the stereo field, and with the three follower channels each driving a separate filter, there is magic to be had.

Volume - This is the overall output volume of the box. The signal IS amplified through the RS3, and a full clockwise setting of the Volume control shows this. The output signal can get VERY hot.

Resonance - Covered above, this controls the resonance of all the filters at once.

Deep Thoughts

  • The filters in the RS3 seem to distort easily. Be careful with your input levels, and keep in mind that one resonance setting across all filters means that you may have one filter which will be more susceptible to distoring that others because of sonic energy at a specific pass frequency.

  • The filters are VERY resonant. Watch your tweeters! Especially when doing wide sweeps.

  • I was disappointed to not find a facility for LFO sync or external control of the LFO speed. In my case, I can route enough gear to the external jack to make this a non-issue. But, I can also easily find times where I'd like to have control over the internal LFO.

  • I was also disappointed to find a low range of LFO speeds. I am one who appreciates a VERY slow LFO, and seeing this one run as slow as 0.01hz is great. But... The upper limit of 2hz is too slow in my opinion. Again, the external jack is just a plug away for me, but not for everyone.

  • I would love to be able to externally modulate the pan position of Filters 1 and 3. Once the warranty runs out on mine, I think this is a mod that I'll look into having done.

  • Given the ability to overload the filter stages, it would of been a nice feature to of included a general OVERLOAD monitor LED which might show the highest gain of the three filter stages.

  • I've put the quasi-negative points up first, because NONE of them will keep me from jumping up and down about this box.

    The sound is AWESOME. I have NEVER heard something like this in person. I've referred to the "Jarre String Sound" a couple of times above, and THIS is the only way to get it that I know of.

    Folks have also said that the magic behind the resonator in the Korg PS3xxx line are the Vactrol devices in this box. More correctly, the RS3 is modeled after the resonator design in the Korg PS3xxx synths. If you're familiar with the sound of the resonator in the PS3xxx, then you know what the RS3 can do for you.

    For Best Results

    Use the RS3 in an effects loop so that you can bring as much forward of the effect as you need. There is no wet/dry control on the RS3, though there is a hard bypass.

    Feed the RS3 a sound fairly rich in harmonics. A bright string patch is what I had the most fun with, along with crash cymbals. The source sound is best *not* to have a lot of filter modulation going on within it, as this will throw off the detectors in the Envelope Follower and cause erratic results.

    I also had a lot of fun feeding a "Classic" MiniMoog bass patch into it. Using Mode 3 of the LFO modulation provides some WONDERFUL animations to the sound.

    Bottom Line

    This box is here to stay in my effects arsenal. The RS3 provides a VERY unique effect that I have yet to hear from any other box. Strings and bass lines will come alive with this box in your rack. Simply AWESOME.

    The RS3 is available from various sources, most notably:

    Music Central
    Rogue Music

    Block Diagram, Vactrol Information and Audio Samples

    A block diagram of the RS3 is available here (150k GIF file)

    MAM's Website is located at:

    For more information on Vactrols in general, point and click at:

    EG&G Vactec Corporate Home Page

    Vactrol home page which has links to several pages of application notes including circuits and design formulae.

    Thanks to David Halliday for the Vactrol links!

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    Copyright 1998-2001, Mark Pulver -