Pas 2 Subsonic pulsing?

F.Sheng.WuChi wrote:       >>Snippage<<

Recently bought a PAS 2 off the "bay" and had it gone-over. It's been functioning well - except that the phono stage is demonstrating a strange phenomenon: While listening, even at low levels, the woofers pulsate rapidly and strongly. It is only visually noticeable, but it is quite pronounced - so much so that I refrained from turning up the volume beyond 8 o'clock for fear of blowing out the woofs.

Keep in mind that the loudness switch is to be used only at very low volumes, and its purpose is to make up for the fact that the human ear becomes less sensitive to highs and lows at such low volumes. Above very low volumes, the loudness switch can cause what you described.  Further, the visceral effect of very low notes is minimal at low volume, so exaggerating the high and low ends brings some of that back. Additionally, you may have some rumble artifacts from your TT that manifest as cone movement. If you have a stethoscope, see if you can detect it mechanically. And, lastly, your phono input stage should be shorted when not in use. See if the pulsation goes away when the inputs are shorted.  

Turning off the loudness gets rid of the behavior. Line stage does not behave this way, or is hardly noticeable.

Sure. And the line stage has far less amplification and no mechanical inputs.

The preamp has refurbd power supply. Stock equivalents. Some newer caps for the pots. For specifics, I will consult my bill, and post some pics shortly.

Would anyone please offer some insight into my issue? If it is a simple replacement of a cap or other component, I may make an attempt to perform the fix. If it requires diagnostic equipment beyond a dvm, then I will likely be dropping it off with my go-to fixer.

Sure: Some suggestions:

a) Don't use the loudness switch. Or, if using it, below 8 o'clock only on the volume control. For above that, use the tone controls as-needed.
b) Bypass the tone & loudness controls altogether - this is a very common fix for the pre-X versions as the tone controls are always 'in-circuit'.
c) Try isolating the pre-amp from the TT and any other source of vibration, and also the TT. The two 12AX7s in the phono stage are often microphonic, picking up vibrations from the local environment and 'translating' them to the speakers.

Good luck with it!

Thanks for your help!

F.Sheng.WuChi wrote:Hello, I've been a long time reader of the forum since acquiring my ST-70 many years ago. Tube audio has brought a lot of listening pleasure to me since then. I have been learning a lot from this forums and the plethora of online info. Currently, I rely on local techs who do it for a living for necessary repairs and refurbishment - and I have been very happy with the results. I've have very few issues all this time.

Recently bought a PAS 2 off the "bay" and had it gone-over. It's been functioning well - except that the phono stage is demonstrating a strange phenomenon: While listening, even at low levels, the woofers pulsate rapidly and strongly. It is only visually noticeable, but it is quite pronounced - so much so that I refrained from turning up the volume beyond 8 o'clock for fear of blowing out the woofs.

Turning off the loudness gets rid of the behavior. Line stage does not behave this way, or is hardly noticeable.

The preamp has refurbd power supply. Stock equivalents. Some newer caps for the pots. For specifics, I will consult my bill, and post some pics shortly.

Would anyone please offer some insight into my issue? If it is a simple replacement of a cap or other component, I may make an attempt to perform the fix. If it requires diagnostic equipment beyond a dvm, then I will likely be dropping it off with my go-to fixer.

Thanks for your help!

GP49 wrote:Something like this has arisen before, in the 1980s when Dynaco modifications were all the rage, and some (truth: MOST) were not well-designed.

One had an "enhanced" power supply that just happened to have a resonance in the low frequency range, which interacted with a low frequency pole in the PAS phono stage.  The result was ultra-low frequency motorboating, which you are hearing.  Anything that causes a bass boost...turning up the bass, or using the loudness contour (though this is OUT OF CIRCUIT at high volume levels) would exacerbate the problem.

The solution was to remove the power supply "enhancements."

GP49 wrote:Mismatch in phono cartridge/tonearm always causes a mass/compliance resonance that can vary in frequency (too high a mass for the compliance = lower resonant frequency) and in "Q" (damping...dependent upon physical damping in the stylus assembly and pivot damping in the arm, the latter probably being none in your turntable).  It is possible that the resonance fall right where there is a resonant pole in the PAS power supplly or phono stage, and s the PAS and is amplified and perhaps resonates.  You can vary this resonance with a different cartridge with a higher or lower compliance and more or less damping (Grado cartridges with very low damping are known for their "Grado Dance"); with a higher or lower mass headshell; and by adding tonearm pivot damping if practical (it usually isn't).  

You MIGHT be able to see this while a record is playing by carefully watching the stylus and cartridge and seeing if there is a rapid "jiggling" happening.  Or all you might get is aching eyes and a migraine; you'd be looking at something VERY small.