[sdiy] Becoming better at understanding difficult analog schematics

[rburnett at richieburnett.co.uk]

I simulated many of the TR-808 drum voices with LTspice to get a grasp 
of what some of the more cryptic sub-circuits were doing.  It's 
excellent for that.  Another good example is simulating the Dunlop 
Crybaby pedal.  It's a really simple circuit with just a handful of 
parts, but I've never seen the schematic in any textbook!  There's a 
decent write-up on this site if interested: 
https://www.electrosmash.com/crybaby-gcb-95

Most of the 808 voices are just bandpass filters, as is the heart of a 
Wah Wah pedal.  But the simulations are great for showing you what all 
the little tweaks to the otherwise "textbook" circuits achieve...  Like 
what happens when the signal is injected here instead of there?  What 
happens if some of the output signal is fed back to the input via a 
resistor and capacitor in series?  etc. etc.  Sure you could work out 
the effects on the transfer function by chugging through a load of 
complicated maths if you have those skills under your belt and the time 
spare, but a Spice simulator will do that in seconds and produce a 
multi-coloured graph, and even output a listenable audio file!

I totally get that simulations don't always match real life because of 
stray capacitance, winding resistance, saturation, finite op-amp GBP, 
etc.  But for many audio frequency circuits they generally do pretty 
well.  I'd go as far as to say that any audio circuit that depends 
heavily on specific stray capacitance, ESR, etc, probably isn't that 
good a circuit to start with.  But there are surely plenty of dodgy 
designs from back in the mists of time...  Some of the 808 drum voices 
simulated perfectly with generic diodes, transistors etc, whilst a 
couple of the voices required transistors with a specific hfe, etc.  One 
even required a transistor with a specfic reverse hfe!  But all of the 
simulations were easily close enough to give a newbie a big leg up when 
trying to decipher an otherwise meaningless schematic.

-Richie,



On 2024-05-17 11:38, Ullrich Peter via Synth-diy wrote:
> I am also into simulating with LT-Spice.
> 
> Ritchie wrote:
>> +1 for simulation from me. And LTspice can even write waveforms out to
>> WAV files so you can play and listen to the results!
> 
> An ex-college built a little drum synthesizer with LTSpice that write 
> WAV Files.
> I can load his files onto a SD card for my Alesis Samplepad and 
> Samplepad4. It so cool!
> 
> Ciao
> Peter
> 
> http://ullrich.great-site.net/
> 
> 
> 
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