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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)
