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    ST-70 buzzing

    ViperZ
    ViperZ

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    Post by ViperZ on Tue May 28, 2019 11:49 am

    Switched from my MK-IIIs to ST-70 this morning to give it some use time. Connected to PAS-3. 1.56V exactly in each channel.
    I have noticeable buzz through both speakers.

    Power supply was rebuilt by me around 10 years ago. I never changed the rectifier tube, that GT 5AR4 must be pretty old. Could this be the culprit?
    peterh
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    Post by peterh on Tue May 28, 2019 11:55 am

    ViperZ wrote:Switched from my MK-IIIs to ST-70 this morning to give it some use time. Connected to PAS-3. 1.56V exactly in each channel.
    I have noticeable buzz through both speakers.

    Power supply was rebuilt by me around 10 years ago. I never changed the rectifier tube, that GT 5AR4 must be pretty old. Could this be the culprit?
    It's very unlikly that the GZ34 causes hum.
    Grounding issue ? Also measure B+ voltages ( DC and AC )DC is in the manual, tell us the AC
    component on B+ and i'm sure someone will tell if it's reasonable.
    ViperZ
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    Post by ViperZ on Tue May 28, 2019 12:06 pm

    Weird. It is a ground loop. Turned off PAS-3, still buzzing. Disconnected PAS-3 from ST-70, buzz is gone. Never had this before in this room.
    Peter W.
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    Post by Peter W. on Tue May 28, 2019 12:52 pm

    ViperZ wrote:Weird. It is a ground loop. Turned off PAS-3, still buzzing. Disconnected PAS-3 from ST-70, buzz is gone. Never had this before in this room.

    What is connected to the PAS and exactly how?
    Does the PAS have a grounded plug?

    Does the 70 have a grounded plug?


    Are there any extraneous grounds?

    Do you have another monolithic (stereo) amp to test the PAS?
    ViperZ
    ViperZ

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    Post by ViperZ on Tue May 28, 2019 12:57 pm

    ST-70 has 3-prong plug. I'll have to double check, but I'm pretty sure PAS-3 has a polarized 2-prong plug.

    Speakers continued to buzz even when PAS-3 was turned off. Only unplugging shielded RCA cables between preamp and amp eliminated the buzzing.

    I do have another SS amp that I could try - Apt Model 1.

    PAS has a Lenco table plugged in, but I was listening to CDs. Also have FM-3 plugged into PAS.
    Peter W.
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    Post by Peter W. on Tue May 28, 2019 2:04 pm

    ViperZ wrote:ST-70 has 3-prong plug. I'll have to double check, but I'm pretty sure PAS-3 has a polarized 2-prong plug.

    If the 70 is OEM, lose the third prong. A polarized plug is OK, with the "hot" being controlled by the switch after going through the fuse.
    As to the PAS, if the "hot" there also goes through the switch, it is OK.


    Speakers continued to buzz even when PAS-3 was turned off. Only unplugging shielded RCA cables between preamp and amp eliminated the buzzing.

    Being "on" or "off" is not relevant to the problem. It is the extraneous ground(s) that is(are) causing it.

    I do have another SS amp that I could try - Apt Model 1.

    PAS has a Lenco table plugged in, but I was listening to CDs. Also have FM-3 plugged into PAS.


    The question *Exactly How* is directed specifically to any turntable. Many TTs have factory 'grounds' manifest as little wires with a spade lug on the end. This wire MUST NOT be connected to the PAS chassis - or any other external ground. It MUST be connected to one-or-another headshell on one-or-another of the RCA plugs. AR, Dual and Revox suggested the left headshell, with AR and some Duals doing it in the factory.

    The FM3 is likely not part of the problem.

    Make sure you use patch-cords of decent quality - they do not have to be 3-figure devices, nor even high-number 2-figure devices. But decent quality from a reliable source.
    Make sure that both the plugs and jacks are clean and tight. That will include removing the top (bottom) of both the pre and the amp to make sure that the inner part of the jack is tight. How often this step is neglected!!

    https://www.dealsanimg.com/d/l400/pict/362587637177_/koh-i-noor-rapidograph-electric-eraser-model-no-2800.jpg Is, perhaps, the best device on earth for cleaning electrical surfaces. Hard to find these days, and the inserts are pricey, but I am blessed with one that has followed me around since college, together with a lifetime supply of inserts.
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    Dale Stevens

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    Post by Dale Stevens on Tue May 28, 2019 3:29 pm

    The question *Exactly How* is directed specifically to any turntable. Many TTs have factory 'grounds' manifest as little wires with a spade lug on the end. This wire MUST NOT be connected to the PAS chassis - or any other external ground. It MUST be connected to one-or-another headshell on one-or-another of the RCA plugs. AR, Dual and Revox suggested the left headshell, with AR and some Duals doing it in the factory

    PeterW, I don't much understand this. My Thorens table has that small grd wire and it is connected to the PAT5 chassis lug. All is well. Dale
    Peter W.
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    Post by Peter W. on Tue May 28, 2019 3:49 pm

    Dale Stevens wrote:

    PeterW, I  don't much understand this. My Thorens table has that small grd wire and it is connected to the PAT5 chassis lug.  All is well.   Dale  

    That is because you have carefully avoided other 'contributing factors' - either deliberately or by pure blind luck. But I betcha dollars to Krispy Kremes that if you were to modify the ground (actually a static-bleed wire) as described, it will be yet better. Back in the day, pretty much every TT manufacturer suggested this arrangement. And AR even put out a 'white paper' as to why.

    NOTE FOR THE RECORD: Turntable "grounds" are not electrical grounds in the classic sense, but designed to bleed off the inevitable static electricity generated by moving a large piece of plastic past a smaller piece of plastic. With that in mind, connecting it as if it were an electrical ground provides a potential source of mains hum directly to the cartridge. And why it is that it should NOT be so-connected.
    ViperZ
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    Post by ViperZ on Tue May 28, 2019 3:53 pm

    So with no sources connected to PAS, the buzz is there, I'd say at an acceptable low level. No buzz at all when PAS is disconnected from ST.
    My California Audio Labs CD player has a 3-prong plug. When I connect the CD player, I get quite loud buzz through speakers.
    I guess, I'll find a 2-prong plug for the CD player... That should eliminate most of the buzz. Still not all of it.
    ViperZ
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    Post by ViperZ on Tue May 28, 2019 4:57 pm

    I plugged PAS into isolation transformer, and everything is dead quiet... I really don't want my 1KVA iso transformer in my system lol...
    ViperZ
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    Post by ViperZ on Tue May 28, 2019 5:54 pm

    I honestly don't know what to say... My Dynaco system never sounded so good after I plugged PAS-3 into the isolation transformer. You don't have to be an audiophile to hear the difference. It's staggering. Should I dig into PAS-3?
    Peter W.
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    Post by Peter W. on Tue May 28, 2019 6:23 pm

    ViperZ wrote:I honestly don't know what to say... My Dynaco system never sounded so good after I plugged PAS-3 into the isolation transformer. You don't have to be an audiophile to hear the difference. It's staggering. Should I dig into PAS-3?

    Much more tomorrow, but the short answer is yes.
    ViperZ
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    Post by ViperZ on Tue May 28, 2019 6:26 pm

    This PAS-3 has a 1-year-old power supply, new electrolytic capacitors, two film caps on the output, and new tubes in the line stage.
    It was originally well built, so I don't really know if I can achieve better shielding/isolation and wire routing to make a difference.
    How much better can it get in terms of getting it silent, short of building a new preamp inside?
    Dave_in_Va
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    Post by Dave_in_Va on Tue May 28, 2019 9:07 pm

    Is there a picture or a diagram available somewhere that shows the turntable ground Peter is talking about?
    I have my turntable ground is hooked to my phono preamp (PH 14). I don't have any buzzes or hums but I sure have alot of static electricity when I finish spinning a disc. The cork mat comes off with the LP.
    I have a Zerostat but frankly that's a royal pain to use.
    I've seen record cleaning brushes with a ground wire connected to it. Do they work??
    I apologize for going slightly off topic.
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    ramon68

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    Post by ramon68 on Tue May 28, 2019 9:55 pm

    Dave, I bought a leather mat from Analogue Seduction in Britain and static cling left forever. 30 bucks plus shipping and you will find leather superior to cork and felt on your platter.
    Lowers the noise floor and no static, period.
    peterh
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    Post by peterh on Wed May 29, 2019 1:23 am

    ViperZ wrote:I honestly don't know what to say... My Dynaco system never sounded so good after I plugged PAS-3 into the isolation transformer. You don't have to be an audiophile to hear the difference. It's staggering. Should I dig into PAS-3?

    Try a "ground wire" between the PAS chassies ( choose a screw at the back) and the st-70 ( choose
    a screw here too). If this helps then you know that the PAS would benefit from a 3-wire powercord
    attatched to the same 3-wire outlet as the st-70. Try it!

    As for the ground-wire from the thorens, try co connect it to the PAS at the same location as the
    above wire. If it is more silent when connected, then connect. Otherwise skip it.

    Same goes for other connected stuff. Note that if the Television uses cable and is in some way
    connected to the PAS it will cause hum unless a ground isolator is mounted between the cable and the
    TV-set.
    Peter W.
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    Post by Peter W. on Wed May 29, 2019 8:33 am

    }GLEEP{

    RANT WARNING


    Look at your PAS schematic. You will find that the Phono & Tape Head grounds are not common to the line-level grounds. Why? Because both are designed for moving parts using plastics that *will* cause static. Keep in mind that a true “head amp” takes the output from the tape head only, entirely unamplified. As the phono stage from the cartridge.
    Here is a link to the AR XA service manual. You will note that the “Shield” is the center wire between the shielded signal pair.

    https://vinylnirvana.com/ar-turntable-resources/ar-turntable-history/1969-ar-xa-service-manual  

    Its purpose is to bleed the static off the cartridge, and eventually, it is connected to the headshell of the left signal RCA plug. Consider that the same arrangement was used by Crown and Ampex for their unamplified decks – and for the same reason.
    Now, consider what happens if this ground is connected to a common (mains) ground, or, worse, to an internal ground to the system that eventually reaches a mains ground. Any potential on that ground will be amplified. Twice.

    Star vs. Daisy grounding:

    • Generally, accepted best-practice is star-grounding. That is, anything that needs grounding is individually managed *TO THE SAME POINT*, resulting in a single ground point with multiple leads to multiple points. Such results in every point being at the same potential.
    • A Daisy-Chain ground allows different points at different potentials. Any time this happens, that difference may be amplified, and, often, results in hum.
    • So, the desired condition is that all components be at the same potential going to a single ground. Where this is difficult to achieve – such as in a multiple-component audio system – this common potential is achieved by keeping everything “above ground” (two-wire cords).
    • As an aside – look at modern components with “factory” three-wire plugs. The electronics are well-isolated from the chassis – and only that is grounded.
    • Vintage components that use the chassis as the common rail DO NOT like to be grounded.
    • Channeling grounds through a mis-wired (3-wire plug on an OEM/Vintage 70) is the worst form of daisy-chain grounding.
    Guys and gals, if your PAS is humming, there are several potential causes:
    • Misplaced wired. Pathway is important. Twisted pairs are twisted for a reason.
    • Component failure – new, old, indifferent, capacitors age and fail. Some fail because they can, some because they are old, some because they are of poor quality. That a component has a ‘new’ power-supply does not eliminate caps from consideration.
    • Stray hum sources. The PAS will put out something like 14V at full bore. So, anything that may induce hum – which includes fluorescent lamp ballasts (CFL), dimmers, motors, ignition transformers (oil burners), using two different outlets on two different feeds and much more may do it.
    • And, the Isolation Transformer *IS* a miracle-cure for much of all that. But it should not be necessary. At the same time, there is no reason not to use one.

    This is not hardly rocket science. But that it may sometimes appear exotic and/or obscure has more to do with a lack of history than a lack of knowledge. A modern UL® inspector would recoil in horror were a vintage 70 be submitted. That does not make it unsafe, but it does mean that we need to understand the intent and the design.

    As to Euro standards – they put the system ground at the primary transformer – which could be from hundreds of meters to several kilometers from the end-user. Which DOES require an entirely different understanding of the process and how to manage hum.  Here in North America, and using our house as an example, we have 3-wire triplex coming into the house – and a ground-rod at the panel, a ground directly to the water-service, and the service-transformer on the pole is grounded by a ground rod at the base of the pole. We can be pretty sure that “ground” is exactly that.

    Bottom line:

    • It is not productive to retrofit any Dynaco – and by implication any other – tube product with a 3-wire cord if not OEM supplied.
    • The PAS/70 combination is inherently dead-silent by design. So, if that is not the actual result, there is something else going on. Look for that something else.
    • Hum may come from sources not actually active – such as the Turntable, tuner, tape deck, DAC, whatever. All that needs to happen is that a hum source is introduced into the system and amplified from there. Back-feeding from grounds at differing potentials is a perfect example.

    In the immortal words of John Muir: Come to kindly terms with your ass, for it bears you. Which applies equally to audio equipment of a similar vintage.


    Last edited by Peter W. on Wed May 29, 2019 8:37 am; edited 1 time in total (Reason for editing : pagination)
    ViperZ
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    Post by ViperZ on Wed May 29, 2019 8:58 am

    Thank you for your detailed response, Peter.

    I agree with pretty much every point - but I have had a 3-wire prong on this ST-70 for 10+ years (mostly in a different house though). Like you said, it could be so many other things in the house that could cause ST-70 or MK-IIIs to hum when connected to PAS-3.

    The cause of the "big" hum was my CD player, which has its own 3-prong cable. When I disconnected its power cord, hum was still there, but at lower level. Still noticeable though. So came the isolation transformer.

    I'm just looking at the course of action now...

    1) I can open ST-70 and MK-IIIs and try lifting the 3rd wire off the chassis (or even try a cheater plug for a quick check). This is probably worth doing.
    2) Try to reroute wiring in PAS-3, not sure how much difference it will make. I will be replacing the volume control with Alps in a couple of months anyway, I hate how uneven the existing volume control is.
    3) I'm in Canada, so same thing as you - we have a ground rod outside, a watermain ground connection, and panel ground plane connected to neutral in the panel.

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