Well this is frustrating ...

[Paul Joppa]

It sounds to me like the problem is capacitive coupling from a power transformer winding to the signal ground. The Quickie has no power transformer.

This is described in one of the Jensen Transformer white papers. These papers of course reflect the reasons you should use transformers, but it is perfectly true that isolation transformers will break such a current better than pretty much anything else.

It's too late (and I had too nice a wine with Friday dinner) to follow up in much detail, but a ground lift in one device and/or the other should make a difference, if my analysis is correct.

Usually we look at a preamp / amp, and because the power amp transformer is larger, we expect it to be the "source," but I imagine both contribute in the real world. That may be why reversing the phase with a cheater sometimes works and sometimes doesn't.

[Deke609]

Many thanks PJ. I will track down the Jensen white paper and see if I can make sense of it tonight.

cheers and thanks, Derek

[Deke609]

Here is Bill Whitlock's (Jensen) White Paper that PJ referenced. Since hum is a common problem this may be of help to others now and in the future.  It is pretty accessible - no calculus, just a bit of simple algebra based on Ohm's Law and the formulas for voltage dividers, parallel resistors and capacitive reactance. And it's a really cool example of how you can use these simple formulas/concepts to work through a pretty complex problem.

Many thanks PJ for flagging it.  I am still digesting it (as fully as I can) and will probably have a couple of follow-up questions (a) to make sure I am at least getting the basic idea, and (b) about the practical solution: which looks to be a diy interconnect box with a wideband, low distortion 1:1 isolation transformer.

cheers, Derek

[Paul Joppa]

For what it's worth, you don't have to DIY; Jensen makes isolator boxes of many kinds; here's an example:

https://www.jensen-transformers.com/product/ci-2rr/

That's a little pricey, and some listeners don't like the resulting sound quality. I see it as a convenient diagnostic tool. If you like the sound, it's also a solution, but as I said a ground lift (back to back diodes shunted by a 10-ohm resistor and a small capacitor to bypass it at radio frequencies)  in the preamp and/or in the power amp will often do the job. That's because the source impedance of the capacitive coupling is very high, so the 10 ohm resistor almost completely shorts that current. Merlin Blencowe's article on grounding has a treatment of the ground lift:

http://www.valvewizard.co.uk/Grounding.pdf

[Deke609]

Excellent! Thanks PJ. Yes, the Jensen "IsoMax" stuff is pricey - which is why I thought I'd diy, plus it's fun to make one's own stuff.

I will try the diodes/resistor/cap approach. If it completely kills all audible hum I will just go with that. Otherwise, I will look into getting a good 1:1 transformer.

cheers and thanks, Derek

[Jamier]

So, this is a slightly OT, but I’m a little confused about PJ’s comment that earth grounding is not  important when it comes to noise reduction. I completely believe him, he has more experience than, well, maybe anyone, but a recent build that PB helped me on, led me to believe that earthing is important to noise reduction so I’m trying to understand. PB advised me to ground the signal input ground at the point where the input entered the audio circuit ( in this case it is a PCB). I ran a wire from the signal input ground connection to a star ground that I located next to the IEC. The star ground connects directly to the ground pin on the IEC. When I first powered up the amp(s), they are monos, this wire was not connected and I had some audible hum. When I connected this wire to the star ground that hum ceased. The amps are push/pull K-501s. Also, connected to the star ground are the signal input cable shield ( from the load end) and two wires from aluminum plates that the binding posts and input RCAs (these are on opposite sides of the amps from each other) are mounted in, they have insulated shoulder washers. If the earthing isn’t the reason for the hum reduction, what is? Can you help me understand? I haven’t used the amps in a few weeks so I will experiment with this connection and see if the hum repeats when disconnected but I remember that this connection reduced the hum considerably, I’m pretty sure.

Jamie

[Paul Joppa]

Jamie - the noise reduction was because the signal ground connected to the chassis, not because the chassis happened to be connected to earth ground for safety reasons.

[Jamier]

PJ,

     Thank you for correcting my misunderstanding. If I had made the chassis from something metallic that might have occurred to me, but I used wood and created the star ground from a screw terminal block, which became, I guess, the conductive part of the chassis. At least I had the good sense to attach everything metal to it. Well, at any rate, I’m learning, albeit slowly. Apparently, I ‘m not afraid to ask stupid questions, so for those of you who are, stay tuned and I will probably ask every one of them and probably some twice.

Thanks again, Jamie

[Deke609]

I scoped the voltage of mains neutral to mains ground a few minutes ago.  It looks a lot like what I saw coming out of the SII-45 when it was fed by the Nickel Wonder.

Top attachment is mains neutral:ground. Bottom is the SII-45 output I posted before.

Any thoughts?

I tried adding one diodes/resistor/cap combo to the SII-45 and it did nothing. I will try adding one to the nickel wonder tomorrow (ran out of time today)

cheers and many thanks, Derek

[Edit: COuple of additional points. (1) If you look toward the bottom of the images, there are some basic stats about the waveform. (2) The hum has gotten worse over the last month. It started as a barely audible hum coming out of the Kaiju a month ago. It is much louder now.]

[Deke609]

No change with with the diodes/resistor/cap ground elevators installed in both SII-45 and the Nickel Wonder. 

Don't know how to make sense of this.  No hum with DAC directly connected to the SII-45. And no hum with DAC -> Quickie -> SII-45.  But connect the Nickel Wonder (which when originally built caused no hum) to the SII-45, or a retail purchased and unmodified SS headamp, and I get hum.  Had the same issue (but not as loud) when the Beepre was still put together (but didn't test it with other amps - just the Kaiju -- b/c disconnecting the BP made the hum go away and so I assumed that it was the source of the problem).  So something has changed in the environment - maybe corrosion in a receptacle or at the mains sub-panel caused by the August upstairs water leak; maybe the City's new line on the street; or a host of any other things -- Arghh!)

In Bill Whitlock's White Paper, he suggests that the power transformer in each connected amp produces a parasitic capacitive coupling with the amps's chassis ground, and that the interconnects then connect the two chassis, and that this opens the door to a ground loop that becomes part of the signal going to the second amp (b/c somewhere in the amps the signal wire is referenced to ground). 

If the above interpretation of the white paper is correct, then doesn't it follow that elevating the chassis ground from earth ground in both amps leaves what I'll call the "Whitlock problem" untouched - can't a loop still circulate between the two chassis by way of the interconnects?  If so, then maybe I need to try isolation transformers.  The specs of the Jensen CI-2RR look good on paper: frequency range of <10Hz to >40kHz and low distortion - but is $200 USD. [Edit: and I'd really like to diy them]. Does anyone know of equal or better isolation transformers (wideband, low distortion) that would be suited to the task? I came across a couple pcb-mount transformers made a company called Audac that look even better to me on paper (models TR106 and TR266: http://audac.eu/products/c/transformers/line-transformers, but I can't find any reviews of them and haven't figured out where/how I could buy them.

cheers and many thanks, Derek

[Jamier]

Derek, you mentioned in some of your previous posts, in other topics, that you experimented with shielding, it seemed like a fair amount of shielding. Do you have more than the “stock” shielding in the SII, and do you have it in the NW pre? If so, is it the refrigeration tubing type that you experimented with in other builds?

Jamie

[Deke609]

Thanks Jamie - I thought of that too. There is one piece of copper tubing in the NW that shields the incoming signal wiring (from DAC) to the vol. pot. But it is wired to star ground at only one end and otherwise doesn't make contact with anything conductive (chassis is acrylic).  I suppose it could act as an antenna, though, but I didn't see anything when I scoped a 1 ft piece of wire (used as an antenna) - a bunch of stuff in the really high kHz and MHz that is likely coming from my wireless internet, but nothing in the 0-200 Hz range.  Plus I get the same hum 1 floor down - so if it's RFI, it's a pretty powerful and consistent signal - so I doubt that's it.

But yeah, I have a ton of extra shielding in all my BH amps - but none of them has hummed before.

I guess a good test would be to take a cheap headamp and the NW to my buddy's place down the street and try them there. Although I'm not sure if he's on the same City power line as me.  Would be a better test if he was on a separate line (in case I were to hear hum at his place too).

This is driving me nuts!

cheers, Derek

[Deke609]

Follow-up: same hum at my buddy's place. Took the NW preamp and retail-bought SS amp there. So I thought for sure this pointed to the preamps and something about the way I built them.  But ... I mentioned that prior to testing at his place I thought it might have been the new power line the City put in. Turns out they put a new line along his street too (perhaps the same line, since we live a 15 min walk from each other) and finished about a month ago (same time my hum problem popped up).

So, since I've already inspected both the main panel, sub-panel and a bunch of receptacles for water damage and found none, my next step will be to rebuild the BeePre completely stock.  If I still get hum, it's the mains power. If I don't, it's my build mods.

cheers, Derek

[Deke609]

Some additional observations in case they mean something to someone.

As I mentioned before, I get no hum with the Quickie feeding the SII-45. Same goes for feeding the Kaiju. EXCEPT, today I noticed that if I touch either of the tube shields on the Q, exactly the same hum shows up in the touched channel. BUT if, while touching a Q tube with one hand, I then touch the Kaiju chassis with the other, the hum disappears.

Does this point to or confirm anything?

many thanks, Derek

[Jamier]

Derek, you have tube shields on your 3S4s? Are they connected to the ground bus?

Jamie