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    Pas 2 Subsonic pulsing?

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    F.Sheng.WuChi

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    Join date : 2016-12-20

    Pas 2 Subsonic pulsing?

    Post by F.Sheng.WuChi on Wed Dec 21, 2016 3:11 pm

    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!
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    Peter W.

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    Re: Pas 2 Subsonic pulsing?

    Post by Peter W. on Wed Dec 21, 2016 4:00 pm

    Please note the interpolations:

    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!
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    peterh

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    Re: Pas 2 Subsonic pulsing?

    Post by peterh on Wed Dec 21, 2016 4:59 pm

    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!

    Sounds like altered components values ( or some moved/bad grounding).
    Line stage is of feedback type, thus it may be unstable if "things are outside spec".

    What exactly is done with this unit ? Try to backtrace any mods until it's stable again.

    GP49

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    Re: Pas 2 Subsonic pulsing?

    Post by GP49 on Wed Dec 21, 2016 8:12 pm

    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."

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    PeterCapo

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    Re: Pas 2 Subsonic pulsing?

    Post by PeterCapo on Thu Dec 22, 2016 1:29 am

    I was in touch with someone who had a problem that sounded similar.  I provided him with instructions for bypassing the tone controls, which included adding the 1.0uF capacitors at the outputs (per original Dynaco X-mod).  When he made the changes, his woofers pulsing stopped.

    I assume your PAS is running into the Stereo 70?  If the Stereo 70 is original, and if the PAS still has its tone controls in their original configuration (non-X-mod), it might be the DC offset at the PAS outputs(?)  If this is the case, try this: http://www.audioregenesis.com/documents/PAS%20Tone%20Control%20Bypass.pdf

    If not, then I don't know what else to say at the moment.
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    ArlanB

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    Re: Pas 2 Subsonic pulsing?

    Post by ArlanB on Fri Dec 23, 2016 12:18 am

    Something else you might consider is acoustic feedback between your turntable and speakers.  This is an old problem that occurs when your turntable is to close to one or both of your speakers and they set up a low frequency feedback loop. It is easily remedied by isolating the turntable from the surface that it sits on and or moving it away from your speakers.  I have two Dual turntables, a 1219 and a 1249.  The 1219 can be as close as two feet from the speakers, the 1249 needs to be at least 3 with a dampening material between it the surface it sits on to make the feedback go away. So there is nothing wrong with you equipment.  Microphones do the same thing however it is a high pitched feedback.   Using the loudness control does enhance the chances of acoustic feedback.  I use mine until I reach about the 11 o'clock position on the volume control then I have to turn it off to keep the feedback from beginning.   Good luck.
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    F.Sheng.WuChi

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    Re: Pas 2 Subsonic pulsing?

    Post by F.Sheng.WuChi on Thu Dec 29, 2016 6:11 pm

    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."


    The power supply caps were replaced; the selenium rectifier deleted and replaced with diode. Effort was made to keep close to stock specs.

    Is it possible that the capacitors on the phono board are responsible for the resonance? They are the original components. As of two months ago, all caps tested "ok"; but I have been using the preamp regularly during that time.
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    F.Sheng.WuChi

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    Re: Pas 2 Subsonic pulsing?

    Post by F.Sheng.WuChi on Thu Dec 29, 2016 6:21 pm

    Thanks for your considerate replies. I've found a lot of food for thought here.

    A couple of you suggested changing the placement of the TT in relation to the pre and speakers. I will try that first, since it is the least intrusive. If that does not solve it, then I'll try the same placement, with new 12ax7 tubes, in case the current (original) ones are microphonic.

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    F.Sheng.WuChi

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    Photo of my setup

    Post by F.Sheng.WuChi on Thu Jan 05, 2017 12:09 am

    The problem is slightly remedied by pulling the TT out of that niche.
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    PeterCapo

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    Re: Pas 2 Subsonic pulsing?

    Post by PeterCapo on Thu Jan 05, 2017 12:15 am

    Can you restate whether or not it happens with a high level source like a CD player?
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    F.Sheng.WuChi

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    Re: Pas 2 Subsonic pulsing?

    Post by F.Sheng.WuChi on Thu Jan 05, 2017 5:24 pm

    Other than the TT connected to phono input, I stream apple lossless via an Airport Express that is connected through the "spare" jack. I don't currently own a cd player, but I think that the level of this wireless doodad would be nearly equivalent to what a CD player would deliver.

    At the moment, the wireless streaming is my main source; and it has been pretty flawless, in other words, no pulsation, just normal woofer action.

    I have only seen the pulsing while playing the turntable...and come to think of it, I'm not sure if all records are producing this effect. I guess that is another test that I should perform...to try to play a range of records to see if others are equally prone to cause the pulsation.

    GP49

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    Re: Pas 2 Subsonic pulsing?

    Post by GP49 on Thu Jan 05, 2017 7:12 pm

    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.
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    Peter W.

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    Re: Pas 2 Subsonic pulsing?

    Post by Peter W. on Fri Jan 06, 2017 9:01 am

    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.


    Following up on this - see if you can beg/borrow/steal a professional grade stethoscope. Listen to the TT on the top or side. Then also the surface on which it sits. This requires the TT to be in operation with a record, and the system volume down to zero - to start. Then, very, very gradually turn up the volume and see if any artifacts that you hear through the scope are translated to the speaker cone. This could be subtle as this will be at an extremely low frequency - more of a pulse than an audible sensation. But your ear is sensitive to these low frequencies more via bone-conduction than actual hearing, and why the stethoscope may transmit that 'pulse' so that you can make the connection.

    Putting it another way - you might get a migraine from eyestrain, or ear-strain... pale
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    F.Sheng.WuChi

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    Re: Pas 2 Subsonic pulsing?

    Post by F.Sheng.WuChi on Sat Jan 14, 2017 2:53 pm

    I'm still investigating this issue. On the tip that this effect may be caused by resonance between speakers and TT, I listened with a stethoscope but could not detect anything. I do not have another cartridge, so, I cannot switch in another with different specs. Moving the TT did not eliminate the pulsing. This test was not conclusive imo.

    I performed another test by switching the TT to Line-level output and connecting it to the PAS line input (FM-MPX). This path utilizes the TT's integrated preamp. The woofer action was completely normal and not at all exaggerated.

    Since the problem is not present while utilizing the PC-5 line stage, Does this test indicate that the problem originates on the PC-6 Phono board itself? Could it be that a board component(s) are problematic? Bad/ caps? They are the original "Black Cat".

    Edit: They are General Instrument caps. No visually apparent defects. I replaced all these caps on the phono board with new orange drops. Still getting the abnormal pulsing.

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