Quote:
What I don't understand is what are the analog components needed to build one
I hope i'm not wasting your time with this. I don't think you don't need to know the intricacies to make a "lazy" emulation that basically yields the same result to your ear.
Exactly what components, and in what configuration, really depends on what topology we're looking at, and i'm not too well read on all of them. So the following goes for what i do know. I'll just list some building blocks that are common.
A few concepts first that informs the rest.
-An amplifier consists at least of one or two transistors.
-A so-called operational amplifier is really a whole block of more specific amplifiers with different tasks.
-A diode and a transistor is based on the same technology (semi conduction), except diodes consist only of a passive channel and your sole mode of definition is in what direction you put it and exactly what semi-conductive material you're using, while a transistor will have an extra lead which can modify the operation of said channel.
To build a gain reducer:
At the very core, to reduce gain you need an "inverting amplifier". It will subtract one signal from the other and output the difference. This is where the (negative) feedback comes in (which you will sum with the input, while the other input is tied to ground). If both input and feedback are 1:1, gain is *0. If no feedback is provided, gain is *1. The balance is done with two resistors, one on the direct input and one on the feedback. They are joined post-resistors. The formula is -feedbackresistor/inputresistor. Again the amp will, as the name implies, invert the polarity of the signal (hence the -), so is common to place two amplifiers in a daisychain to revert it if phase is important.
Typically the job of the second amp is just to pass the input to the output with unmodified gain and just revert the phase, but since you already have two amps in place... maybe the first is used for setting the volume and the second is used for some compression, since it is there anyway.
Now, i've only built this kind of circuit with whole OpAmp IC:s (operational amplifiers). Doing a differential amplification with these looks like this:

...but iirc you can build one discretely with just two "common emitter" transistors and three resistors, not counting any output buffer measures taken or such. You can look up "differential amplifier" if you want to look into it. Depending on case, you might not need extra buffer amplifiers.
A super simple/hacky way to create a threshold for gain reduction/compression (and i don't know if it was used here) is to pass the feedback through a diode. The diode must reach over a certain voltage to "break down" and start conducting. It also introduces a voltage drop, which means gain can't really be reduced to 0, but that's not what we are doing here anyway. IF this method was used, note that there'd be *some* variance on exactly what voltage the diode will conduct, from die to die. It probably also needs to be a diode with a low breakdown voltage since, i assume, audio voltage levels are quite small even within the cartridge synth? (it is generally good to keep them high. studio audio has a pvp of 1.7, consumer audio outputs 0.45, modular synthesizers anything between 5 and 12 haha. the typical silicon diode breaks at 0.7 and the typical germanium diode breaks at 0.3 - these aren't as common in consumer electronics and are more or less extinct today. You can make your audio level match the breakdown voltage by adding a "voltage divider" circuit).
About building the simplest possible compressor in the world:
For a fully passive diode bridge compressor, all you need is really 7 resistors, 4 diodes and 2 capacitors, just to point out how simple it can get. That is a little academical - you still need an active buffer stage for the signal to keep the intended volume and impedance. I'm not sure of its applicability here since i don't know how you'd add a threshold to this passive scheme, but those resistors would at least let you define ratio, attack and decay. There probably is a way to introduce a threshold, and i just don't know about it.
About building a full VCA compressor and why it probably isn't what we want:
A full-sized VCA circuit will first use two inverting amps (= 4 transistors and some resistors) to produce a control voltage (which you can easily bias or gain control in itself via the specifics of the VCA block), which can then be used to in turn to gain-control another OpAmp pair that is solely responsible for the end volume (how much goes to audio out, and how much goes down the drain). This volume is fed back into the VCA block for partial self correction in a compressing circuit.
Note that the purpose of a full-blown VCA is to be able to provide easy means to control the amount of gain reduction. You might not need all its bells and whistles. A lot of the hobbyist schematics you'll find are unnecessarily feature-heavy for our purposes even when they're labeled simple, because they're designed by someone who wanted some specific interface in the first place. Like being able to voltage control the ratio, threshold, knee or whatever. All we want to do is voltage control the volume and that's it.
Rectifiers:
All a rectifier does is nullify the negative swing. And all it takes is a diode. This is more elaborately called half wave rectification.
But if you add together two rectified signals (one for each pole), you get both polar peaks on the same 0-maxV pole. This is called full wave rectification.
It looks like
this.
Partly because of the choppyness introduced by the diodes' breakdown voltage, a smoothing capacitor is used. and/or you can use germanium diodes, which have a lower breakdown voltage.
A VCA block will need this for producing a control voltage out of the rectified, smoothed signal.
I've mentioned diode bridges twice in this post. Note that their applications are different. One is to be a compressor in itself, the other is to precondition a source material for the VCA.
I'm just wondering why they might've intentionally wanted a compressor. Maybe they were worried about overdrive or just general loudness? On the other hand, the APU squares can get too loud compared to the rest as it already is. Maybe it's just a fluke effect or a hacky "well, we might aswell".