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

[Richie Burnett]

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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