POP problems when preamp is turned on

----- Original Message ----- From: "tempus fugit"

Many audio amplifiers use a JFET as a power-on/power-off mute (an RC circuit on the gate to turn the FET on for the first second after power-on. The JFET pulls the input of the last stage to ground until things settle down.) to deal with these turn-on transients.

Also, can anyone account for the DC voltage at the output? I

Unless I misunderstand your problem, it's not really "DC", but a sudden change in voltage at turn-on that causes the problem. The trick is to have everything else settle down first, and then allow the amplifier to amplify. That's where the mute circuit helps you.

Thanks Blake. I knew that they used JFETs for quiet switching, but never really thought about how until now (never needed it before!). A couple of questions: Why a JFET (wouldn't a BJT work too)? Will any old JFET do, or is there one or another that will do the job better (quieter)? Does the audio signal go through the JFET?

As for the DC, I actually measured a jump to about -15v DC which gradually worked its way to 0 when I turned the phantom power off. That issue is fixed now, though. I wired the on/off switch a little differently and there is very little noise when I turn on the phantom power.

Thanks

circuit

JFET

amplify.

Have a look at Q1 in zone A6 of this schematic for an example of how it's done.

FETs make nice voltage-variable-resistors, such that the Rds is relatively constant with small changes in Vds. BJTs don't have this characteristic.

better

fixed

maybe

to

DC

ve

I see it. That's pretty slick - so the signal never goes through the JFET, the output is just grounded by the JFET, hence no signal gets through (is that right?)

I can't see what kind of JFET they're using - will just a standard runofthemill JFET work, or does it need to be a power JFET, or something special?

Thanks

gradually

The

the

sudden

ha

Exactly. The RC time constant gives a delay on power-up, but the diode eliminates the delay on power-off so turn-off transients are muted too.

It's a J111. See the data sheet at:

Ahhhh. I did see that - I thought it was like Q111 - the part # on the schem, not the actual part#.

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