Popping sound on power down

[ashok]

As the subject says, I hear a popping sound from the
speakers when turning the amp off. When turning the system
on, I power up the source (solid-state), preamp (solid state)
and then the ST-35. Everything is fine.

On power down, I turn off the amp first, then the preamp and
then the source. I hear a pop from the speakers. Not very
loud but distinctively audible. The pop is much louder if the
preamp is powered off before the amp.

My speakers are of unknown sensitivity with an impedance of
6 ohms (connected to the 8 ohm tap on the ST-35). This is only temporary
while I wait for my real speakers. Could this be the cause of the problem?

Also, I did not put in the standby switch in the amp, only the ON/OFF, but
I do not think that is the problem. Any suggestions on where I might start
looking? Over at the bottlehead forum, there are suggestions for putting
a capacitor across the on/off switch, not sure of the exact value, but
will check there again.

Thanks

Ashok

[Shannon Parks]

Ashok-
Something like a .05uF, 1kV cap should be fine.

Shannon

[ashok]

Thanks Shannon. I will try it out. Another possibility I suppose is
to use a variac for the amplifier, something I was planning to do
anyway.

Ashok

[EWBrown]

I put in an ICL CL-90 in Dyna ST'35s and SCA-35s that I have repaired or modified, this also seems to eliminate any pops at turn off, (I do have the cap across the switch). The original idea with the ST-35 was to have the preamp (PAS-2/3X) turn it on off through a switched auxillary outlet, and teh preamp has the cap across the power switch. The ICL does "soften" the initial power on surge, when using silicon rectifiers, which don't turn on slowly as does a tube rectifier.

/ed

[tubes4hifi]

Hi Ashok,
I'd bet the popping sound is your preamp.
The amp has enough capacitance to keep putting out sound for 15-20 seconds after shutoff. I have this problem alot with my preamp.
You can add a mute switch or circuit to the preamp, or just wait longer to shut off the power amp.
Roy
www.tubes4hifi.com