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I'm looking at the schematics of the Minimoog model D trying to figure out how much can be cut off for the sake of simplicity and modern convenience, and I'm curious about the purpose of a particular subcircuit. The whole device basically operates out of a dual 10V supply, but also has a dirty +15V rail that serves to 1) drive a lamp and 2) take part in a "decoupling circuit" (at the bottom left).

Since power supply design is just hard, my goal right now is to see if I can just eliminate this +15V rail from the circuit and rely on a suitable dual 10V wall adapter, which would cut down on cost, size, and just be overall more reliable than the 70s discrete regulator circuit the original uses. The lamp bit is simple, and can be made low voltage with an LED, but the other application of this rail confuses me. I have no idea why it's there. Is it just to get a local 9.3V rail within the circuit? If so, why is it called a "decoupling circuit"? Are there any other reasons for that to be there, or just tweaking component values and using the +10V directly would be effectively the same?

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Here's the specific part of the circuit: -

enter image description here

It produces a reasonably regulated 9.3 volts from the 15 volt rail using 10 volts at the base. Because it's an emitter follower, the emitter will "follow" the input (the base voltage) but be about 0.7 volts lower. It's not a bad regulator but it's not perfect.

my goal right now is to see if I can just eliminate this +15V rail from the circuit and rely on a suitable dual 10V wall adapter

Well, you could probably cobble-together a 9.3 volt regulated output voltage for the other parts on the schematic that need 9.3 volts. It could be derived from the 10 volts with a suitable low-drop-out regulator.

Or, you could "manufacture" a 15 volt rail using a boost converter.

Is it just to get a local 9.3V rail within the circuit?

Yes it is. Example: -

enter image description here

why is it called a "decoupling circuit"?

I have no idea.

Are there any other reasons for that to be there, or just tweaking component values and using the +10V directly would be effectively the same?

It probably needs to be 9.3 volts so I wouldn't mess with it but, there's nothing going to detonate if you did connect it to 10 volts from what I can see so, maybe leave that option open (use a solder link to make it easy to swap over rails).

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  • \$\begingroup\$ Yeah, I figured it was probably just bad naming, just needed a sanity check. Thanks. I'll do some transient analysis later to see if it reacts substantially different with 10V, as this doesn't need to be a particularly precise bit of the device. If it does, I might go the route of adding some sort of regulator in place of that subcircuit. \$\endgroup\$ Jan 29, 2023 at 17:45
  • \$\begingroup\$ @GabrielGolfetti that's the right way = simulation and then some more. \$\endgroup\$
    – Andy aka
    Jan 29, 2023 at 18:53
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I have no idea why it's there. Is it just to get a local 9.3V rail within the circuit? If so, why is it called a "decoupling circuit"? Are there any other reasons for that to be there[?]

Multi-stage audio amps are sometimes susceptible to feedback through the power rail. Due to impedance in the power supply and connecting wires, when the output transistors draw a lot of current, a voltage drop on the input stage transistors may result.

The decoupling circuit isolates (decouples) the input stage(s) power rail from the power stage power rail.

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  • \$\begingroup\$ So by this reasoning, is that regulator a fundamental part of the circuit? Or will that be a non-issue with a better quality power supply? \$\endgroup\$ Jan 30, 2023 at 12:45
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    \$\begingroup\$ @GabrielGolfetti If you have a single supply for both the power stage and the input stages of your amplifier, the overall amplifier may be unstable. I'm not saying that it will be, but it might be. If you build the amp without the decoupling circuit, I would definitely check the output with an oscilloscope to see if there are high frequency oscillations. \$\endgroup\$ Jan 30, 2023 at 13:28
  • \$\begingroup\$ as a side note, this bit of the circuit is the only one to be grounded directly to the transformer's center tap instead of the regulated ground. Since the purpose of this bit of circuitry is to generate an envelope (potentially with very sharp transients), I'm starting to think the decoupling is potentially due to very high current requirements of this particular module, which would severely affect the power rails for other bits for the particular regulator being used. Is this a reasonable way of thinking about it? If so, would overshooting the PSU requirements be enough? \$\endgroup\$ Jan 30, 2023 at 13:49
  • \$\begingroup\$ relevant schematics are here, figures 9-8, 9-13 and 9-18: fantasyjackpalance.com/fjp/sound/synth/synthdata/… \$\endgroup\$ Jan 30, 2023 at 13:50

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