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Hard to find Parts
Overview The AM Modular clones a variety of old circuits, many of which include obsolete parts. This means substituting a modern equivalent. Here is our guide to getting the right
replacement to work !
556 Op Amp The 556 Op Amp was manufactured in the mid 1970s. It was widely used in the audio path of Eľ modules.
It was slightly better quality than the 307 and 741 Op Amps, and thus used in
pre-JFET/Bi-FET days, where a good frequency response was critical The RC1456 and RC1556 Low Noise, Fast Op-Amps were manufactured by Raytheon.
They are pretty much impossible to locate now, but it is safe to substitute with a
Bi-FET (TL08x or TL07x) or the inimitable NE5532 & NE5534.
My approach to circuit replication is to use original parts first and then try close substitutions. So I have replaced the 556's initially with 5532's. Dave Rossum did
switch to Bi-FET's in later module revisions as well using the LF353 in the Audity.
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 Upgrade Tips
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Here are some analog synth upgrade tips from Paul Schreiber and Tony Allgood, they also apply to
cloning old analog circuits.
1) Upgrade the CV Op Amps.
To provide improvements in drift, tacking and stability. Use the Linear Tech or TI LT1013 (dual op amp). Its a pin for pin replacement for the 1458/4558/TL072, etc.
You don't have to do *anything* to the circuit, just swap it. There is also a quad version the LT1014 and a good single version is the OP-07 or OP177.
2) Replace PSU capacitors.
Over time, they age, the ESR (internal
resistance) goes up, they heat up, and you get output ripple. Use high temperature versions (105C)
3) Replace the S&H capacitors in the CV multiplexors of poly synths. Use 1% polystyrene capacitors.
4) Add power
supply bypass capacitors. Use 0.1uf, 50V ceramic axial capacitors and solder these on the solder side of the PCB, on the ICs from power pin to ground. Use 1 cap for every 3 ICs. Op amps will need 2.
5) Upgrade the Audio Op Amps. Start by using the TL072 to replace 1458, LFxxx and 4558s, but beware inducing Op Amp instability and fatal oscillation with the replacement. Its not as easy as it looks!
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