POP problems when preamp is turned on

[tempus fugit]

Hey all;

I've built a mic preamp that has one xformer for the opamp (+/-15v), and
another for phantom power (+48v). When I switch the power on, I get a good
sized POP (about 1v or so according to my scope). I get another POP when I
turn it off. When I switch the phantom power on, it pops again, and when I
switch the phantom power off, it pops yet again, and I measure about -15v DC
on my meter at the audio output.
Can anyone clue me in as to how to get rid of these annoying (and maybe
damaging) pops? Also, can anyone account for the DC voltage at the output? I
have a DC blocking cap at the opamp's output, so I'm not sure how the DC
voltage is getting there.

Thanks

 

[Blake]

----- Original Message -----

Can anyone clue me in as to how to get rid of these annoying (and maybe
damaging) pops?

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

have a DC blocking cap at the opamp's output, so I'm not sure how the DC
voltage is getting there.

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.

 

[tempus fugit]

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

 

[Blake]

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

http://www.mrgearhead.com/faq/schematics/fender/Frontman 15G Schematic%2
068F.pdf

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

 

[tempus fugit]

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

 

[Blake]

. . . so the signal never goes through the JFET,
the output is just grounded by the JFET, hence no signal gets through (is
that right?)

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.

I can't see what kind of JFET they're using

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

http://www.fairchildsemi.com/pf/J1/J111.html

 

[tempus fugit]

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

Thanks