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I was studying this discrete voltage regulator used in a guitar amp power supply.

The filtered +-36V supply voltages go straight to the power amp, and if I got it right Q212-Q213 act as variable resistors in order to keep the voltage set by the two zeners. But I don't really get what the rightmost part of the circuit does (the connection from Q214 collector resistor goes to a JFET gate used as a switch, I think, at the power amp input).

enter image description here

How does it determine the reference ground for the circuit?

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    \$\begingroup\$ Ahem... where's the jfet (most probably an antipop device on power-up)? \$\endgroup\$ – Andy aka Jan 14 '16 at 22:49
  • \$\begingroup\$ The reference ground is determined arbitrarily by where you draw the ground symbol. \$\endgroup\$ – jpcgt Jan 14 '16 at 22:55
  • \$\begingroup\$ @Andyaka I've added the power amp input section with the jfet \$\endgroup\$ – 83dB Jan 14 '16 at 22:58
  • \$\begingroup\$ I merged the pictures. Red line connects the two sections. Red box is Transistor in question. \$\endgroup\$ – Passerby Jan 14 '16 at 23:08
  • \$\begingroup\$ Are the two different ground symbols connected to each other somewhere on the schematic? \$\endgroup\$ – Spehro Pefhany Jan 14 '16 at 23:24
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The power supply clearly has symmetrical positive and negative output rails, which only works if the two grounds are connected together. I suspect the grounds are actually connected together at the chassis, and the reason for keeping them separate elsewhere is to avoid a ground loop between the power amp and sensitive input circuitry.

Q214 drives the JFET which switches the input signal into the amp. The JFET turns off when negative voltage is applied its Gate, so the input is enabled when Q214 is off and its Collector voltage rises.

D201 rectifies AC from the transformer and produces a negative voltage on Q214's Base (more negative than the Emitter which is at B-), so the input is only enabled when mains power is present. This ensures that the amp will go silent as soon as it is switched off. R214 and C215 provide a delay between power on and input enabled, which may stop the amp from producing a 'thump' when it is turned on.

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  • \$\begingroup\$ This configuration where the transformer 'sucks down' the base of Q214 when it is ON is really strange, but totally agree! \$\endgroup\$ – Daniel Jan 15 '16 at 7:53

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