The 150 is quite potent and convenient. You can use that module “as is”, yet, if you don’t mind tinkering, some aspects can be taken up a notch. Basically, I’d suggest three tweaks: 1. Increasing the accuracy of the sample and hold, 2. Tweaking the sample and hold so you can use its lag section for proper portamento (and, as a side-effect, have “gated modulations”), and 3. putting an insert to the LFO section, so you can process control voltage or audio with the Attack-hold-VCA of the LFO “DELAY” section.
The info here is open source, and if you like these mods, support any any of the organizations in the following list: https://drlizdobson.wpcomstaging.com/2018/02/18/feministsoundcollectives/.
For more technical details, see Robert Keeble‘s analysis here (thanks for the mention, btw, Robert!): https://amsynths.co.uk/home/products/modulators/am8152-ring-mod-sh-noise/
0. (no mod): mystery jumper on the back of the PCB
The little jumper next to the LFO TRIGGER IN at the back of the PCB couples/decouples LFO reset. Jumper in = gate/trigger resets LFO phase *and* engages “delay” (i.e. fade-in), and jumper out = only LFO “delay”.
1. Improve S&H hold accuracy
In terms of component specs, the Sample and Hold section of the Behringer is faithful to the schematic of the original, yet I found the voltage held between clock pulses dropped relatively quickly. This makes driving the 1V/OCT input of a VCO or a self-resonating lowpass filter at very slow S&H times less pleasant. Increasing the value of the hold capacitor (22nf, incidentally C8 on both the Roland and the Behringer) fixes this.
First I went for a much too large value (thanks to KSS@Mod Wiggler for the pointer) and then just settled on an extra 68nf film box across C8. At very slow S&H times, there is less drift, and on very fast S&H clock pulses, the difference is minimal.
I find the FET terminal in the yellow circle above the easiest solder spot. Best solder some wire to it, to which you can then connect your additional capacitor. If you are a confident solder-type, you can also wire the cap terminal directly to either the FET terminal or IC1, PIN5.
2. Switch for portamento and “track and hold” in the S&H
This modification is very simple and gives you two extras in the Sample and Hold section: portamento functionality and, as a side-effect, track and hold. All you need to do is to change a resistor and wire up an on/off switch, for which, and here is the catch, you need a new hole in the panel if you don’t use an extension panel.
Some background on the S&H and LAG circuit: A trigger pulse (from the internal clock or an external trigger/gate) makes the S&H take a short “sample” of incoming voltage, which is then held until the next trigger. Since the sample taken is very short, you can do fun stuff like turning white noise into some stepped modulation (“computer room” sound, here we go!). The LAG section (slew limiter) after that introduces smooth glides between each sampled voltage piece.
Now, if we bypass the sampling bit, we can do things like portamento or even turn a gate into an Attack-Release Envelope.
For setting this up, we first need to establish unity gain in the entire section, because otherwise your pitch voltage would not track properly, even at zero portamento time. Luckily, we just need to reduce Roland R39/Behringer R18 to 100ohm for that.
Secondly, we need to bridge Behringer C4 (C10 on the Roland), so that the S&H part of the circuit passes on incoming CV “unprocessed” to the LAG circuit as long as gate voltage is held at EXT CLK IN. What we get by doing this is “track and hold,” where an incoming modulation signal is passed through “as is” while a gate is open (much like an on/off switch), and the value of the modulation signal present when the gate closes is held until you send another gate. Since S&H and T&H are cool to have, we just bridge C4 with switch and wire.
Setting up portamento: plug 1V/Oct signal to S&H input and the S&H output to the 1V/OCT input of your VCO. Also plug gate to EXT CLK IN and flick your switch that bridges C4. Play notes and use the LAG slider to set portamento time.
Setting up track and hold/ “gated modulation”: Flick the C4-bridging switch to on and send gates to EXT CLK IN. Try this: play two VCOs with the same 1V/OCT source but feed the pitch voltage for VCO2 through this gated setup: both VCOs play the melody in unison when gate is on (you use a dedicated gate for that) and as soon as gate is off, VCO2 keeps the last pitch while VCO1 continues playing your melody. Instant interval shenanigans…
Processing audio with S&H: an old trick to do some sort of sample rate reduction effect on audio is to send it through and S&H and clock the S&H at audio rates. The sonic options this gives you are further increased when keeping the gated modulation/C4 switch on since this increases your sample window. Try, for instance, a pulsewave oscillator at your clock input and change its pulse width while modulating signals.
Turn a gate signal into an A-R Envelope: split a gate signal with a multiple and send it both to ext clock in and to signal in, then pull up the LAG slider. Doesn’t give you independent control of attack and release but hey, still not bad as a freebie… 🙂
3. Turn part of the LFO circuit into a utility VCA with Attack-Hold-Envelope
the LFO of this module has a DELAY section, which when you pull up the DELAY fader and send a trigger or gate into TRIGGER input. When your gate hits, the LFO level is pulled to zero and then fades up to maximum amplitude. (I.e. this should rather be called “fade in” than delay.) The circuit responsible for this fade in is a simple VCA with a variable attack-time envelope, which we can repurpose for a couple of fun things.
Quick description of the circuit: When powering the module up, C16 (4.7uf; Behringer C47 is 10uf) is charged, keeping Q19 (=Behringer T19) open, which in turn makes sure that voltage as set per VR10 passes through Q20 (=Behringer T15) and keeps the VCA (IC9, Behringer IC6-B) open. A trigger/gate opens Q18 (=Behringer T21), which instantly discharges C16, closing the VCA. C16 then charges up again at a rate set by VR7, so the VCA opens again and stay at maximum level until another gate or trigger starts your rising envelope again.
Since the VCA does not care which modulation or audio signal it processes, we can easily hijack that circuit by inserting a switched jack socket at R102 (= Behringer R116, 330k). If nothing is plugged into this socket, you have your “delayed” LFO as usual, and when you plug external CV in, the LFO signal is replaced by this voltage. Some applications for this would be ducking an audio signal or using other modulation sources you want to fade in at the push of a trigger. NB that LFO is replaced by your external CV at the LFO outputs as well as at the normalized switches and sockets for Ring Mod and S&H as long as you have that external source plugged in.
If you plan on using this with audio signals more often, I’d recomment to declick the attack envelope in this circuit a bit, so you don’t get plipps when the level of the VCA is pulled down. This mod does not interfere with standard LFO fade-in applications. Just wire a 22nf capacitor between the control input of the VCA and ground – you can to this on the PCB back like so: