[guf]

I'm asking here because " How stuff works " is in the title.

I'm demoing a new preamp and when I connected there was a hum that came from my speakers, which has happened with other preamps.  No big deal. But then noticed  the woofer was slowly moving in and out. Way more movement than when there was music playing. ( unless it was a very heavy bass song). I'm sure it's a issue with the new preamp because with other preamps or my current one the problem doesn't exist.

Can anyone think of a reason for this?

[srb]

Sounds like some kind of subsonic oscillation that might be caused by a severe impedance mismatch between preamp and amp.  What are the specific components being used?

Steve

[guf]

Thank you for your suggestion Steve. That inspired me to do some testing with 3 differents amps that i have. My Main amp is a first watt SIT2, and it has switchable input impedance either 100k or 10k. When placed on 10k there was much LESS movement in the woofer. I have a DIY  tube amp that i think the impedance is also 100k. Then was a lot of hum and a lot of movement. And i have a Pass labs xa30 with 20k impedance and the movement appeared to be right in the middle of the other two.  I'm not sure if thats what you meant by impedance mismatch but that definitely had an impact on the issue.   Output of the preamp is stated to be 150. These numbers should not be causing all the problems but it appears to affect it somewhat.

thanks again!
guf

[srb]

Just out of curiosity is there anything unusual about the cables between the amp and preamp, perhaps very high capacitance or inductance, that might be having an effect, beyond the preamp and amp's fairly standard output and input impedances?

Perhaps trying a pair of "nothing special" garden variety interconnects might shed some light (or not)?

Steve

[guf]

Yeah threw some monoprice shielded ICs in and hoped for the best but still the same problem. I appreciate the great suggestions. There must be something out of my control.

[Speedskater]

OK, unplug everything but the sub. Make a RCA shorting plug out of an old throw away interconnect. Cut the old cable about 6 inches from the plug, somehow strip the insulation from the center conductor and twist it and the shield conductor together, it doesn't have to be pretty and it only has to hold together for a few minutes. Plug it into the sub and turn the sub on.  Does the woofer move?

On second thought, many subs won't auto turn-on with-out music, soin that case use a battery powered music source to turn the sub on, then turn the volume to zero.

But in most cases it's a bad power supply capacitor.

[Quiet Earth]

Hey guf,
Are you are auditioning a DHT preamp for your SIT2?  Can you say which one it is?   

Also wondering :
Does it have a proper earth ground?
Are all of the tubes seated properly and in good working order?
No shipping damage??

Interesting problem.
 

[shooter]

Most likely the power supply is unstable, it is call motor boating. Since it also has hum adding more capacitance to the power supply may help. If it is a commercial product it is poorly designed.

[avahifi]

Your issue is a low frequency oscillation in your preamplifier.

The reason the amplitude decreases as you switch to a power amplifier with a lower input impedance is that the input impedance of the power amp in combination with the output impedance of the preamp forms a voltage divider that reduces the gain of the low frequency signal from the preamp.  The lower the input impedance of the power amp, the less the low frequency movement will be, as you demonstrated.

10-20 mV of low frequency drift at the output of a tube preamp is not unusual, but it appears your preamp is doing more then that.  Look at the output of your preamp on a scope to see what it is doing.

Changing cables won't help this issue.  It probably is caused by the normal variations in AC line voltage, perhaps intensified by improper coupling capacitor values (too large).  The preamp probably is not "motorboating".  This would usually cause much more violent issues then a very slow in an out woofer movement.

Frank Van Alstine

[guf]

Hey guf,
Are you are auditioning a DHT preamp for your SIT2?  Can you say which one it is?   

I did audition a Coincident Line Stage (dht). It was more detailed than the Shindo, just not as engaging, whatever that means.  It didn't bring out any extra magic from the sit2 like described, or it didn't meet my expectations.   

Thanks everyone for the input. Thanks Frank for your professional insight and description. I'll keep everyone filled in on the progress.

[paul79]

Is your new preamp actually new? Or just new to you?

Bad capacitors can do this.

[guf]

An update:

I removed 2 bypass caps as directed by the manufacture  of the preamp. I also noticed that a couple of leads of the resistors on the were touching on one of the ecc82 input tubes. I easily separated them and suspected maybe that had something to do with the hum problem which was worse in the right speaker. I switched left and right tubes. And powered on and i think blew the fuse. No power. Replaced the fuse, and the ecc82s back in original left and right positions. Powered on. Everything was the same. still hum, and osculating speaker driver problem.  Turned everything off.

went to bed

This morning I inspected the right channel ecc82 tube on the pins where the resistor leads had touched. It was hard to tell but something seemed off with the filament or leads inside the tube. I have a 12au7 in my bottlehead crack so i tried it in my pre. I think at the same time i switched the input impedance on my first watt SIT2 to 10k instead of 100K, remembering yesterday that the movement seemed less in the driver as i went down in value.

Powered everything back up and very little hum and NO movement in the drivers!  It sounds very good. I have been listening all morning with no problems.  There is a slight hum, My Shindo is silent until the 10 or 11 o'clock position, but both of them have the same hum with no music playing when turned up halfway. I'll keep listening.

thanks!