I'm giving it four stars only because it's been exactly 6 years since it was updated, and it desperately needs quality of life and bug fixes. Otherwise I'd give it a 5.
At first glance, Charlatan presents as a very simple and limited virtual analog synth, but what sets it apart is its quality analog oscillators and filter. It's oscillators and filters sound excellent and lively, made better by the subtle (almost unnoticeable) analog 'drift' effect employed under the hood, which is uncommon in free plugins. Also, it's kind to your CPU meter, so don't be afraid to use more than one instance.
In essence, Charlatan is somewhat comparable to Roland's Juno-60 or Korg Polysix in terms of capabilities and philosophy. But it actually has more features, such as a proper second oscillator, oscillator sync and ringmod. It could also be considered a stripped down Jupiter-8 or Prophet-5, but in general, it's its own take on the 80s poly synth.
Because of its limitations and wanting to keep things simple, you likely will not stray too far from typical 80s sounds like basses, pads and poly sounds. It's just not as versatile as your typical semi-modular hybrid soft synth. Though I'm sure it can be useful for House, Techno, and other surprising areas. Charlatan is particularly great for basses; some of the best 80s style basses I've heard in a free plugin! It holds up even for synth brass and stabs. It can do string machines, transistor organs, and all sorts of other keyboard sounds as well. With its noise oscillator, it makes surprisingly useful filler FX. Some have even managed to squeeze out dissonant drones and usable drum sounds.
So in essence, it's dead simple and limited, but that's not a bad thing. Limitations breed creativity as they say. It sounds good enough to easily fill the 80s polysynth role, but is versatile enough that you can still find new and interesting timbres by experimenting. Also, don't neglect on the Unison and Stereo modes. Older versions of Charlatan did not have this, but they can really add a lot to the sound.
That said, it's not perfect. Osc 2 can only be detuned up to 6 semitones, which means... no 7th chords. Does the creator have a vendetta against 7th chords, to stop at exactly 6?! There are also bugs. Sometimes I found patches don't load properly, glitches out, or don't sound right, particularly if in POLY mode. And it doesn't seem to honor initial pitch bend messages for fine tuning, though this may not be exclusive to Charlatan. An additional way to alter the drift amount would also have been huge in a free plugin. But these are not deal breakers.
Using it with a chorus is a necessity: The free TAL Chorus LX is a great load-once-and-forget chorus, but any decent BBD or multi-voice chorus will do, but that's a whole other topic. Also you'll want a good reverb. TAL's free reverb plugins are well-suited for the job (I personally like the older TAL-Reverb-2 for its tone) Then there's also Supermassive (free), which can be warm and spacey.
3.5/5. Vivaldi is a straight-forward 2-op FM synth, where you can layer up to 4 parts. Each part is made of very basic components: Three oscillators (carrier, modulator, LFO) and two ADSR envelopes. The envelopes control the level of the oscillators, but the LFO can be sent configured to multiple destinations extensively.
Unfortunately, if you want to emulate classic Yamaha-style FM, this is not the place. I tried to recreate the classic Brass sound, but the envelope or feedback just wasn't providing enough 'umph' on the attack. Even the extremely limited 2-op FM synthesizers Yamaha released are capable of this, as found in various OPL emulation plugins JuceOPLVSTi and vst2413, so it is down to differences to the architecture itself.
Though while not a strict emulation, it does its own thing rather well, and has a pleasant soft FM sound, with still being capable of its own flavor of harsher tones. Definitely more pleasant than other free FM plugins such as Uno FM or mda DX10.
I would recommend Vivaldi MX over the original Vivaldi, as it's essentially a free upgraded version (more polyphony, multitimbral etc.). Also, it's big brother Ganymed is a 3-op FM synth with 2 parts instead of 4.
I love going back to old synth plugins just to see what can be done with them. Neon is apparently the oldest VST instrument there is (came with Cubase 3.7 in July 1999), so that's right up my alley. I've given it a go, made a handful of tracks and sounds with it, and here are my thoughts.
Pros: The 3 waveforms (tri, saw, sq) ... they could have just made it a plain square. Being the first. Woodgrain panel.
Cons: Only 5-note polyphony (even though it's supposed to be 16). Oscillators waveforms are low quality and sound like they have a permanent low-pass filter. Filter lacks an "Envelope Amount" control. No individual waveforms for oscillators, or Osc volume mixing control. Osc2 detune only goes 7 semitones back and forth, so no octave higher/lower possible. ADSR envelopes are a bit clicky even when turned all the way off LFO depth (mod wheel) is weak, as well as the pitch bend range is weak.
In summary: It's not good at all. And being old isn't an excuse. Both Model-E (Steinberg) and Pro-Five (Native Instruments) were created before Feb 2000. That's less than a year later. Then Native Instruments came out with FM7 in Feb 2001, which to this day is one of the best FM synth plugins ever made. And Neon could have been better.
It's oscillators are pretty bad. Neon is advertised as having two oscillators, but the only thing you can do is detune Osc2 up or down 7 semitones, not even a full octave... There is no oscillator mixer or individual waveform selection. And that's fine, the real crime here is how horrible the oscillators sound. Look at the output through any analyzer and you'll wonder what happened to the high end. The high frequencies are all filtered out, so instead of rich harmonic square or saw waves, you get these bland waveforms. And they get worse the lower you go. It's almost like they're using a single-cycle .wav file for each oscillator!
The filter is also pretty bad. It's weak and you can barely hear the effect of turning the cutoff down or resonance up. And the filter envelope is hard-coded to react to the "sustain" level as the amount, no "amount" knob to speak of.
Even heavy EQing can hardly save this synth. Give it a shot, and see for yourself. Steinberg still offers it as a free download. It can make borderline acceptable basic sounds thanks to the ADSR envelopes, but even the waveforms of the NES sound better than these do.
mda's DX10 is a simple 2-operator FM synth (3-op if you use the special FL Studio version). It has no GUI, instead you are presented with 16 parameters in your host's default interface.
A lot of people seem to compare it to Yamaha's classic FM synths, but let me tell you: it's not comparable. It cannot even be compared to a Yamaha 2-op, such as the OPL sound chip. Where Yamaha-style FM is more warm and punchy, DX10 is more metallic and acid-like. Everything seems to sound like that UK "Donk" bass sound as you crank up the modulation amount. Also, its envelopes are rather limited. Instead of ADSR, it uses a sort of strange ADR/DSR envelope. In comparison, the OPL2/3 used ADSR, the DX21 used a ADDSR, where a secondary decay rate is applied to the sustain, and in the DX7, a freeform 5-point envelope is used.
Suffice to say, you cannot model 2-op Yamaha sounds accurately using DX10 (without losing your mind that is), you can however use it for its own merits. One of those merits include a waveform modulator, where you can modulate between a sine and "saw" approximation that contains additional harmonies added to the fundamental sine.
It can definitely sound interesting in its own right if you play to its strengths, but don't be fooled, it's not a Yamaha clone, and you will be hard pressed to make any decent brass sounds.