So I'm sitting here, tuning my algorithms and parameters to do just what the circuits do in that little black box next to me. I'd match some parameter ranges and curves, such as mapping cutoff frequency to its control voltage down to a few cents and then I'd try some oscillator and envelope on that control voltage. Quite often when it gets to extreme settings I'd have the feeling there's no way this model sounds like its analogue counterpart.
So then I turn to the analogue synth and set the knobs to similar settings. With extreme modulations though just touching a pot ever so slightly may alter the sound quite a bit. So it takes a minute or two until everything lines up and sure enough, the result may be closer than was expected. Which my colleagues next room notice by my giggles, or because I'd call them to demonstrate the effect.
It so happens regularly that on the next day or maybe just a coffee break later, that match is gone. Suddenly the analogue synth doesn't do the same thing anymore and sometimes it isn't even possible to dial the same tone back in. It may be subtle, but it's an audible difference.
So there it is. No two analogue synths of the same make sound the same, and actually not even a single analogue synths sounds like itself at two different points in time.
What it means is, it doesn't really make much sense to do those AB comparisons between analogue hardware and softsynths that are supposed to model said hardware. They're not only controversial because of bias on both the maker's and listener's side, because of sound design skills, choice of settings or genre and numerous other possible reasons - they're most of all controversial because it's inherently unlikely, if not impossible, to create the definitive match. This alone leaves the door wide open for people to hear what they want to hear. I'll get back to that later.
So where do these differences stem from?
First of all the most prominent candidate is temperature. Everybody knows that analogue synths need to warm up until the oscillators keep in tune. But not just the oscillators, also the filters, envelopes, amplifiers, glide circuit, modulation depth and whatever else are subject to change with temperature. Only, we're not as perceptive to these effects as we are for out of tune oscillators. But it's not just the pitch that drifts over time, it's also the sound.
In the same vein, climate also plays a role albeit a tiny one. Humidity and air pressure may have very little effect, but they do have some effect. Hey, a cent off is a cent off. I was told that the developers of Monark had a Winter model and a Summer one. I don't know which one was chosen in the end, I'd be curious to know which one they thought sounded better.
In any case, when you start matching a digital model to an analogue synth in the morning in an office that - no offense - gets more humid over the day, it'll simply sound a tad different in the evening. Likewise if the afternoon sun comes round and you turn the aircon on...
With this in mind, it may make sense to record the synth in the parameter ranges you're trying to match, and verify how the beginning of your work matches up with the end. Keep track of your drift, and if it's bad, maybe model to a snapshot (samples...) entirely.
It's in the Parts
Secondly, there are part tolerances that may create some difference between two specimen of the same synth. Most people have heard of matched transistors. It's often noted in schematics whether or not transistors must be matched. Which in turn means, there's a whole lot of them which are not matched. If they aren't matched on the board, chances are great they aren't matched among machines. Thus they're all a tad different, as are diodes, resistors, capacitors, knobs, faders and all the complex parts glued into chips as well.
Those aren't earthshattering differences, not much at all. But they add another dimension of subtlety to the aforementioned temperature drift. For instance, with our Pro One, when I dial resonance from start of self oscillation to maximum, the pitch of the self oscillating filter goes up by about a quarter tone. When I asked people to send audio samples from their Pro Ones, the pitch would usually go down towards maximum resonance, some as much as a third or fourth - as it happens in pretty much the majority of synths out there.
One can get datasheets for most parts that are used in vintage synthesizers. Not only do they contain formulae that are useful for modelling, but component tolerances are often quoted i.e. how much their properties are allowed to vary. If some resistance or so can be off by 5% it's sometimes quite surprising how some synths actually sound similar. Which brings up the next bit:
Your Service Technician of Choice
What's baked into part tolerances is usually compensated by little trimpots that let your synth repair man dial in correct frequencies and voltages in critical sections. While this is supposed to level out the differences between units it may also bear potential for sonic differences. These trimpots can also be used to change the sound in many ways, and like their counterparts on the front panel, a tad to the left or right may change the sound ever so slightly, yet audibly.
Furthermore when synths get serviced, parts may be exchanged. Some people have collected filter or envelope chips that match each other in order to equip polysynths with a set of chips that sound as similar as possible. Some people also modify some of the resistors and capacitors for smoother or more aggressive operation. That's not necessarily circuit bending, it should merely be seen as improving the hardware surgically.
So, analogue synth afficionados - including myself - will often point out that they made this or that sound on a freshly serviced device. This doesn't mean that all freshly serviced units will sound exactly the same. Not only because of everything else but also because the people who calibrate synths take their own pride in how they make that synth sound better than when it came fresh from the factory!
But even a good service won't always bring back what's lost through wear and tear. It's not only the pots that become scratchy and all that, some parts simply break gradually over the years. Most classically it's electrolyte capacitors that run dry. But of course there's also dust, things getting loose or just old and squishy, who knows. A synth that has been locked in the basement for a decade will need service, there's no question about that.
Talking about age in a synth is a tad ambiguous. The more vintage the higher the price. But how likely is it that the sound of a synthesizer ages as well as a great port in a barrel? I secretly hope that the sound doesn't really change that much, not least if parts can be replaced. However Hans Zimmer told me that his newly built Moog Modular sounds a great deal better than the vintage wall at the back of his studio - same modules, 40+ years apart.
Lo and behold, climate, parts, service and age are the main reasons of sonic differences between two specimen of the same synth, heck, even of a single synth. No-one expects their synth to sound the same before and after service, but why not also before and after lunch?
So, how can we ever do those popular AB tests again when we know all this?