weird problem

[parabellum]

I made a passive preamp with a 100k alpha log pot. I got a bit of a hum out of my ST70 which is a different issue.

So now when i hook up a a cd player (ps1 or a dvd player) and mess with the volume pot the cone on one driver (fostex fe108 sigma) starts pulsating at a certain point starts pulsating really visibly even without music playing. When I increase volume it stops pulsating like that. The other speaker doesnt pulsate like that. Both drivers had less then 10 hours of play.


what the hell?

[parabellum]

when i play music using my imac as a source the cone doesnt pulsate like that

[kheper]

When I increase volume it stops pulsating like
that. The other speaker doesnt pulsate like
that.

when i play music using my imac as a source
the cone doesnt pulsate like that

It is probably an impedance mis-match.

Measure the voltage of the imac and the other
sources, then compare them. The imac should
be higher. Passive pre-amps do this, due to a
lack of signal getting thru.

[parabellum]

When I increase volume it stops pulsating like
that. The other speaker doesnt pulsate like
that.

when i play music using my imac as a source
the cone doesnt pulsate like that

It is probably an impedance mis-match.

Measure the voltage of the imac and the other
sources, then compare them. The imac should
be higher. Passive pre-amps do this, due to a
lack of signal getting thru.

how do i rectify the problem, and why only one driver?

[Shannon Parks]

Is everything DC coupled in your linestage and ST70? Maybe you need a large AC coupling cap on the output of your linestage to prevent (the small) DC grid currents traveling all the way through your linestage, since at most volume settings your log pot would be 20k or less in parallel with the 470k grid resistor at the input of your ST70.

[parabellum]

I just wired some caps before the preamps out's. No change. I'm giving up on a passive preamp. Gonna try to score an adcom gtp 450

[Shannon Parks]

I just wired some caps before the preamps out's. No change. I'm giving up on a passive preamp. Gonna try to score an adcom gtp 450

Just a FYI that the GTP450 has no phono stage. If phono was important and the tuner wasn't, checkout the GFP555.

[parabellum]

I just wired some caps before the preamps out's. No change. I'm giving up on a passive preamp. Gonna try to score an adcom gtp 450

Just a FYI that the GTP450 has no phono stage. If phono was important and the tuner wasn't, checkout the GFP555.

I've been scouring the craiglist for a gfp555 II or gfp565 with no luck. Finally decided to build a phono stage myself and get a 450 if I can get it cheap. It does have a remote though which is nice :)

[kheper]

Check ebay. I have seen GTP 400s go for
$100.

These GTPs do have phono stages.

I once preferred the GFPs with no tuner, but
now I go straight into the amp with a quiet
equalizer.