Some of the world’s most respected, coveted synthesizers went through production revisions that audibly impacted the instruments’ sonic character. Here are four such synths that come to mind:

• Korg MS-20
  (to quote Korg’s MS-20 Kit press release: The original MS-20 used different VCF circuits depending on the date of production. Units produced in the earlier period used a filter noted for its distinctive distortion and self-oscillation, while the filter used in later units was a low-noise design with a more mellow character.) The earlier filter is known as the “Korg 35″, while the later revision is a more conventional OTA-based filter.
• ARP Odyssey
  (The Odyssey is notorious for bearing three different filters in its lifetime. The first Odysseys shipped with a Moog-style ladder filter. ARP was threatened with legal action so they quickly replaced it with a buzzy 2-pole filter, which was quickly replaced by a unique, aggressive sounding 4-pole filter.
• Sequential Prophet-5
  Rev. 1 and Rev. 2 Prophet 5s used SSM ICs for their VCOs and VCFs, while the Rev. 3 Prophet 5 used CEM ICs. Some say the Rev. 3 Prophet 5 sounds weaker as a result.
• Roland MKS-80 Super Jupiter
  The infamous Supe Jupe rack’s filter had an agressive, high-resonance character (IR3109, same filter as the Jupiter-6). But Rev. 5 units (serial number 511800 and up) used Roland’s new IR3R05, the same silky smooth filter that would find its way into the JX-8P, Super JX, and MKS-70.