antto wrote:what's different about the DAC and glide? it's a resistor network with a buffer and a switched lowpass filter what interaction is there?
The glide time is slightly dependent upon the R2R network and buffer because it is unbuffered. It connects directly to the FET and capacitor and the "glide" pulse can introduce spikes into the CV signal. It is obviously arguable that this is only a very minor difference but just as I said: ignoring it does make a difference even if small on its own.
The effect of poor matching in the R2R network or current from the buffer is more likely to slightly throw off the tuning than influence the glide time as is commonly believed. That effect (feeding an R2R into a capacitor) is not actually present at all if the network is matched (1% or better = near zero effect.)
The small pulses from switching the glide as well as poor tuning though may have a far more significant effect on the sound than people assume. It is easy to hear C2 being slightly out of tune with C3 especially when you mix instruments. In poly synthesizers this often creates that "fat" detuned/drifting sound.
antto wrote:other than the accent being derived from the main envelope, what other interaction is there?
There is a small effect on decay time by the accent pulse changing the output current. The circuit uses a darlington but it is far worse than a FET buffer and the effect on timing gets significant especially at the maximum decay time setting.
- Gate + glide + accent pulses from sequencer (notable effect on other modules)
Small pulses are introduced due to EMI as well as the varied current from the PSU. This can be very significant due to the 5.33v reference generator.
It's very common to ignore the PSU and simply assume the voltages are static and have infinite current supply potential at all frequencies of interest as well as that all necessary supply bypassing is accomplished in the circuits but this is not 100% true.
- Custom exponential current source, oscillator and waveshaper
The exact structure of the current source (including input clipping) and its temperature coefficient may contribute to how the oscillator drifts as well as how it responds to different changes in CVs and parasitic signals from the supply rails.
- Specific filters (high-pass, low-pass) in signal path
It is easy to assume these passive filters do not significantly influence the timbre. Again, little things can add up to make a more significant difference.
- Additional components of the filter itself including effect of res accent filter, buffer and custom expo current source
This should be obvious. The accent circuit generates an additional envelope based upon the res setting. This is probably one of if not the
most essential part of the "303 sound".
- Complicated accent handling and high level input to VCA
Matching various factors such as the simple amplitude compensation for the res control can be critical due to their influence upon non-linearity later in the circuit. The way the gate + accent envelopes are mixed into the VCA current source is easy to over-look but has a non-trivial effect on the result. Without actually building your own (lm13700, etc) and testing it you might never realize the exact scaling of the envelope signals and the non-linearity of the current source. As we all know: you can definitely hear the difference between different amplitude envelope shapes. Again a very small influence that can add up to others.
- Input mixer in signal path introduces significant non-linearity
This one is simply obvious: if you put an amplifier in the signal path you should probably account for any additional non-linear effect it has as well as its effect on low/high frequency content.
antto wrote:yes, the power supply, this is the most famous difference, but i still don't know exactly what it affects in the sound/behavior of the synth
It allows for signals to be transmitted along the supply rails. Just because the rail is "v+" or "+5.333" doesn't mean it isn't like any other wire: it carries a voltage and current which are both variable and influenced by every circuit they are connected to.
Changes in the amount of current drawn due to the pulse buffer circuits switching (slide, accent, gate) can introduce small impulses into the other circuits.
antto wrote:yes, and i may add the actual PCB layout being significantly different, which is also a famous difference
but i also ask the question - which of these differences result in a "significant" audible difference, and where for example, i know (i've read it somewhere) that in general, the x0xb0x audio is less noisy than the 303 - (assuming this is true) is that a bad thing?
Yes because the PCB layout influences EMI and how effective the supply bypassing is which has a significant and very audible effect on which signals are transmitted across the rails between circuits.
For example with two oscillators connected to a poor quality supply with the main rails correctly bypassed but something like a reference rail "5v / 5.333v" not bypassed as much, the oscillators may use the reference as: surprise! A reference voltage. If small spikes are introduced on this rail by the oscillator sync (reset) pulse it can cause oscillators tuned near fractions of each other (1/2, 3/4, etc) to sync together on their own!
This sort of effect is absolutely essential to accurately reproduce a synthesizer which suffers from it. Again, all small "insignificant" effects that add up to produce notable differences in how the entire unit functions.
antto wrote:there is (since a year or so) a PCB replica project (re-303), it is said that it's a copy of both PCBs (on modern FR4 material) and has formed a comunity of folks.. basically, they are trying to build complete replicas.. does that not count as 100%?
It may or may not, depending upon how perfect the reproduction is. I doubt a duplicate PCB is needed but in the very least all these "minor" effects need to be taken into account and reproduced by any modern circuit.
Reproducing the PCB 1-to-1 is just an easy way to have zero understanding of how/why and get a near perfect replica, assuming the components used are matched correctly.
antto wrote:when you have so many extra knobs, you tend to tweak them away from their "neutral" positions too often, so keeping this model into "303 territory" is a mind battle for the user.. i mostly use this model to make it produce completely non-303 sounds
Absolutely: The tb-303 is an interesting sound but really not applicable to the majority of places you'd want a general-purpose synthesizer instead. That's because the sound is so ultra-specific and unique to the 303 that people almost instantly know "that's a tb-303 or similar".