[sdiy] Guy researches and talks about how aging components change the sound of a synth

Tue Sep 1 14:28:09 CEST 2026

Richie,
amazing take down, thanks a lot. Great points!

Hey, can you just keep going? I could use like 2x more of that.

Cheers,
D.

On Tue, Sep 1, 2026 at 10:38 AM Richie Burnett
<rburnett at richieburnett.co.uk> wrote:
>
> Firstly, thanks for sharing this Cheater.
>
> AI writing style concerns aside, here are my thoughts as a guy who is a well
> aged engineer/scientist by profession, and has spent too much of their spare
> time analysing old analogue drum machines and digitally modelling.
>
> I am very sceptical of his results and the conclusions drawn from them for
> several reasons which I shall try to summarise below:
>
> 1. All resistor and capacitor values are subject to manufacturing spread
> (initial tolerance) on the day that they are made, as well as aging effects
> over their lifetime.  You simply cannot make any deductions about aging
> effects on component values from things like filter cutoff-frequency or
> envelope time-constant measured *today* unless you knew what they were on
> that same machine the day it rolled off the production line!  It may be that
> a whole batch of carbon film resistors came in slightly high against their
> nominal resistance value on the day that they were made.  Electrolytic
> capacitors often have an even wider initial tolerance range.  It was common
> for electrolytics in the 80's to have an initial manufacturing tolerance
> of -20% to +80%!!!  That's very wide, and also highly asymmetric. Right
> there on day one.
>
> 2. You can't draw conclusions about individual component values just by
> measuring time constants.  It is not that simple for several reasons.
> Firstly, even a basic RC circuit has two components, both of which influence
> the resulting time constant.  If one drifted high and the other drifted low
> by the same percentage you would never know because the overall RC
> time-constant would stay the same!
>
> 3. In addition to this the TR-909 single-transistor VCAs are highly
> non-linear, in the way in which they respond to the envelope level and also
> in the way in which they influence how the capacitor discharges into them
> over time.  This means that you can measure radically different time
> constants on the same machine, on the same day, depending on whether the
> drum sound is accented or not, or the exact way in which you choose to infer
> the time-constant from the decaying amplitude of the waveform.  The first
> part of the decay for voices like the Toms decays slower than an exponential
> decay because the single-transistor VCAs tend to compress the envelope at
> high amplitudes.  The same single-transistor VCAs also tend to pinch off
> abruptly towards the end of the decay.  So in the strict sense decay time is
> meaningless for such a non-linear system that doesn't follow the textbook
> exponential decay closely by design.
>
> 4. Similar effects happen in things like the bridged-T (MFB) filters too.
> It is tempting to look at the values of the two Rs and two Cs, do some
> calculations and make deductions about the cutoff frequency and damping
> factor of the resulting filter.  This gets you close.  However, when you
> test the actual hardware (or even simulate with good models) you find that
> the cutoff frequency and the damping (decay time) are quite different to
> what the R and C values alone would suggest.  This in due to two properties
> of real world op-amps.  Finite input resistance and finite gain-bandwidth
> product both act to load the circuit and shift the response away from what a
> basic mathematical analysis of the component values alone would suggest.
> This isn't due to aging, it's just due to the detailed physics of the
> circuit.
>
> 5. Quoted values for things like time-constants and resonant frequencies in
> the Roland Service Manuals for these old drum machines are very approximate
> figures in my experience.  Certainly not accurate to a few percent.
>
> 6. Capacitor and resistor values are also subject to thermal drift
> (temperature coefficient.)  So unless the same machine was tested at
> identical temperatures now and 40 years ago, you can't assume that all of
> that measured drift is due to aging alone.  We keep getting told that the
> world is getting hotter!  :-)
>
> I'll leave it at that for fear of boring readers who aren't interested in
> this particular field, and just summarise it as "Very sceptical as someone
> who has done what he claims to have done."
>
> Interesting article though, thanks for sharing,
>
> -Richie,
>
>
> -----Original Message-----
> From: cheater cheater via Synth-diy
> Sent: Tuesday, September 1, 2026 2:59 AM
> To: synth-diy
> Subject: [sdiy] Guy researches and talks about how aging components change
> the sound of a synth
>
> Really interesting stuff:
>
> https://www.reddit.com/r/DSP/comments/1w2z13c/every_surviving_tr909_is_40_years_old_what_does/
>
> quoted below.
>
> PS Some people say he might have used AI to write this. Best as I can
> tell it's not AI written, but what do I know. It seems like the
> research is legit. Maybe he used it for rephrasing his notes into a
> post. Or maybe we're seeing a phenomenon where people who use AI a lot
> just start talking like it. Historical linguists will have a field day
> on this 100 years from now, in case we survive that long...
>
> -----------
>
> by BeatForge_Dev
>
> Every surviving TR-909 is 40 years old. What does "authentic" even
> mean    when the schematic and every living unit disagree in the same
> direction?
>
> I'm modelling the TR-909 the way I modelled the 808 and 606 before it:
> component by component from the service notes, with one hard rule - no
> fitted or matching EQ anywhere. Every filter, every time constant,
> every gain in the model has to trace back to a component that is drawn
> on the schematic, or it doesn't ship. Validation is against recordings
> of real units (multiple machines where I can get them), but the
> recordings are the gate, never the source: when the drawing and a
> recording disagree, I don't get to turn a knob until they agree. I
> have to find out why.
>
> That rule is what made the following visible, because without it I
> would have EQ'd the discrepancies away in an afternoon and learned
> nothing. As the model got close - close enough that the remaining
> deviations were a dB here, a few percent of a time constant there -
> the residuals stopped looking like noise. They grouped. Specifically,
> they grouped by what the component that owns each parameter is made
> of.
>
> Time constants owned by tantalum capacitors read consistently long, by
> 14 to 19 percent, on every unit measured. The 909's tom body decay is
> set by a tantalum; every reference unit decays 14-19% slower than the
> drawn RC says it should, and they all miss in the same direction.
>
> A recovery time owned by an aluminium electrolytic reads consistently
> short. The hand clap has a roughly 228 ms recovery between hits set by
> an electrolytic; on both units I could measure, the effective
> capacitance comes out around 0.83 times the drawn value. Electrolytics
> dry out and lose capacitance - this is the single best-documented
> aging mechanism in the business, and it points exactly this way.
>
> A clock frequency owned by a carbon-composition resistor reads
> consistently high, by about 16 percent - on four different machines.
> Carbon comps drift upward with age, heat and humidity; four units
> agreeing on the direction and roughly the magnitude is not a lottery.
>
> And the control that makes this believable: not everything drifts. The
> kick's poles sit dead on the drawn values across units. Film caps and
> metal film resistors holding their values while electrolytics sag,
> tantalums stretch and carbon comps climb is exactly what a 40-year-old
> PCB should look like. If every parameter had missed, I'd have
> suspected my own readings of the schematic. The residuals sorting
> cleanly by dielectric and resistor construction is what flipped my
> interpretation: these aren't model errors. The model was measuring the
> age of the reference units.
>
> Which lands you somewhere genuinely strange for validation. The
> schematic describes the machine Roland built in 1984. Every recording
> anyone can make today describes a machine that has been drifting away
> from that schematic for four decades - and drifting coherently,
> because the same chemistry is running in every unit. So a model that
> is exactly right by the drawing will sit at a small, systematic,
> same-signed offset from every reference you can buy or record. If you
> validate purely by fitting to samples - which is the industry default
> - the fit will happily absorb one particular elderly unit's drift,
> your metrics will reward you for it, and you will ship "authentic
> 1984" with today's electrolyte chemistry baked in.
>
> Edit:
>
> After listening more and more, i prefer the old values, Toms and Kick
> have more sustain, it just sounds more alive and less stiff.
>
> Question for the room: has anyone treated component aging as a
> first-class model parameter rather than a nuisance? Tape emulations
> have had "worn" controls for years, but those are usually designed
> sounds, not chemistry-driven parameter drift. And if anyone has good
> literature on solid tantalum capacitance drift over multi-decade
> timescales, I'd genuinely like to read it - the electrolytic and
> carbon-comp stories are textbook, but the tantalum shelf is thinner,
> and right now my 14-19% is a measurement in search of its chemistry
> paper.
>
> Disclosure: I make a commercial drum plugin, this work is in it.
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