Oh absolutely it will do more harm than good. It acts as a shorted loop to the signal wire, which means a parasitic load on the signal wire, bigger than anything else one could do. It also provides no electrostatic ground. It is a terrible idea .... some "cable" guy came up with it :-)
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Perhaps it works, but I would think that it can do more harm than good in comparison to plain grounded shield. High frequency noise currents induced in the signal wire flows on the surface (skin effect) - shield to ground. I'm not sure replacing it with shorted turn will be better, but who knows. I'm just skeptical, but if it works it works. |
In the experiment presented, and the "concept" for shielding presented, their is an external magnetic field noise source. As the shorted loop is passive, and it's current is only due to the external magnetic noise field, the field it would create cannot be larger than the field already created by the magnetic noise source. It cannot effect the other wire more than the noise field would have already. It would direct the field away from the wire. |
roberttdid, Sorry I read it wrong and I understand argument about shorted turn, but transformer works both directions. Any closed loop will pick-up ambient electrical noise. Induced electric charge will flow in this loop thru/along the shield producing magnetic field around signal wire, that will induce electric noise current in it. |
I was not saying that shorting the shield with a loop will pick up noise kijanki. It will act as a shorted turn and will direct an external magnetic field away from inside conductors to a degree. However, it will also act as a shorted turn to the current carried by the internal conductor, which could seriously impact it's performance. |
roberttdid, Of course. I posted only to state that shield, IMHO, plays important role in defense against electrical noise (when connected properly). I also strongly agree with you that shorting open ends of the shield with wire creates large loop that picks-up electrical noise. Noise currents in such loop produce magnetic field that will "transform" noise into signal wire. |
kijanki, My last post was specific to the link luisma31 posted. I am not disputing anything you said, just not sure you were aware I was making a specific response to that. |
Twisting a pair of wires works great against capacitive or electromagnetic pickup, exposing both wires evenly to electric or magnetic field. That way induced noise currents are exactly even and cancel. It works fine as long as twist is even and its pitch is much shorter than the wavelength of offending signal. Shield in addition to twisting adds more protection against electric or magnetic field by being a Faraday cage, but even some induced currents flow on the surface (shield) only if frequency is high enough (skin effect) while some are lost as eddy currents. Both Faraday cage and skin effect becomes less effective for electromagnetic radiation below 100kHz. Combining both provides good protection against low frequencies (twisting) and high frequencies (shielding). Of course keeping cables short is very important, since cable as an antena for electromagnetic field becomes very ineffective when shorter than 1/10 of the wavelength. If shield creates ground loops, then I would at least ground it at the source end only. It is much better, than not having shield at all, IMHO. |
luisma, This shielding "concept" is flawed and I am going to assume comes from a lack of understanding of what is happening. The person who came up with it is giving himself a bit too much credit. https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwio... When you run a signal through one "loop", and measure the induced voltage in another magnetically coupled loop, what you have done is created a transformer.
So what is the problem with this?
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Dear almarg: Your posts and advice deserves historically (at least from my perspective) a complete and proper answer. Correct While doing the same thing with an unshielded cable resulted in buzz but no hum.Not exactly, the unshielded cable poses no hum and no buzz (on the true balanced connection) as tested And replacing the "looped," shielded, and balanced DAC-to-preamp cable with an unshielded one essentially resolved the remaining buzz.Correct Also, I believe that when Luis refers to a shield being "looped" he simply means that it is not connected at the corresponding end.Also correct, looped means the cable shield is not connected to ground at one end as usually should be, but connected to itself at the ends by means of an external wire or outer added shield over the cable jacket which is isolated from everything else. JSSG for wire loop or JSSG360 for external braid loop. If those interpretations are correct I would not attribute the differing results to differences in capacitanceWell I'm not an EE but somehow I thought the cable jacket will resemble and act as a dielectric and the shield will get "charge" as a capacitor hence increasing the Mogami speced capacitance for such cable, but I could be wrong on my assumptions. I can certainly envision that a cable of any type hanging off of the input of a component, while not being connected to a signal source, could result in hum as a result of EMI effectsMy actual testing with an unshielded cable showed no hum or other effect. Of course to have rigorous testing I should have tested a shielded cable not looped over itself but shield connected to ground at the ends "mea culpa" As I and others have said in past threads it is often ***very*** easy in audio to attribute an observed difference to the wrong variable. And that is especially likely to be the case when the observations involve a very limited number of components, cables, and circumstances. I believe that in this case differences in cable capacitance were not the cause of the observed differences.You are correct I did not "used" all the available variables, now the shield loop in the cable it is my understanding it will affect capacitive coupling per the article below https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwio... Note that I am not a John Swenson detractor, it is completely feasible that the looped shield on certain specific applications (DC cables, signal cables, digital cables etc) and with certain signals/currents (DC, AC, digital streams etc) could provide some "enhancement" (I would even dare to say tone control) on certain cables and possibly be of detriment on others. I think we have deviated enough from this thread :) My conclusion for balanced connections on my equipment (per manufacturer's recommendations) standard Mogami console cable with no tricks will do the trick :) And this is one of the reasons I wanted to go "seriously" into analog as well, digital is "so convenient" but "so hard" to get it right and there is just no real reference on what to look for that I decided to listen to both (without spending a fortune) OMG I hijacked this thread, my apologies to the OP, I will shut up now |
Luisma31 7-9-2020 @luisma31 @kijanki If I am interpreting correctly (and I’m not sure that I am), I believe Luis is saying that completely disconnecting the "looped," shielded, and balanced preamp-to-amp cable from the preamp (not just disconnecting the shield) resulted in hum and buzz. While doing the same thing with an unshielded cable resulted in buzz but no hum. And replacing the "looped," shielded, and balanced DAC-to-preamp cable with an unshielded one essentially resolved the remaining buzz. Also, I believe that when Luis refers to a shield being "looped" he simply means that it is not connected at the corresponding end. If those interpretations are correct I would not attribute the differing results to differences in capacitance. Consistent with one of Kijanki’s comments I cannot envision a means by which high interconnect cable capacitance (within reason of course, and the capacitance of Mogami, even in stock form with the shield not looped, is certainly well within reason) could result in hum or buzz. But on the other hand I can certainly envision that a cable of any type hanging off of the input of a component, while not being connected to a signal source, could result in hum as a result of EMI effects, depending on the design of the specific component and the specific cable, as well as on the surrounding EMI environment. And likewise when the cable is connected at both ends but the source of the signal is turned off (assuming the component providing the signal is not one of the relatively few designs in which the output is grounded via a relay when the component is turned off). Also, while Luis describes the shields as being "looped" I’ll mention FWIW that in a conventionally designed balanced cable (i.e., one in which the shield or some other kind of conductor connects XLR pin 1 at one end to XLR pin 1 at the other end), the resistance of that shield or other kind of conductor can significantly affect the degree of hum and buzz that may result from ground loops. That would also depend on whether the components connect pin 1 to chassis (as they should) or to circuit ground (as is often incorrectly done), as well as on the relation (i.e., the impedance between) circuit ground and chassis ground within the components. The bottom line: As I and others have said in past threads it is often ***very*** easy in audio to attribute an observed difference to the wrong variable. And that is especially likely to be the case when the observations involve a very limited number of components, cables, and circumstances. I believe that in this case differences in cable capacitance were not the cause of the observed differences. Regards, -- Al |
oldears ...the specs for my Ayre P-5xe: the gain for the balanced output is 6 dB higher than single ended out for each of the 3 gain selections. However the gain does NOT change with the choice of RCA or XLR inputs ...Thank you! Exactly. That’s been one of my points from the beginning of this thread; the choice of connector has nothing inherently to do with whether the circuit is balanced (or not) or differential (or not). And one of the best places to use differential circuits is in the phono section, for both higher gain and better immunity to noise. |
To clarify, I can only add the specs for my Ayre P-5xe: the gain for the balanced output is 6 dB higher than single ended out for each of the 3 gain selections. However the gain does NOT change with the choice of RCA or XLR inputs. You should be able to look up the specs and recommended phono cable wirings on the Ayre website in the manual download. The specs are minimal and no S:N specs are given. |
roberttdid Except with a phono input, you don’t gain 6db, as it is an application specific input. It amplifies the difference between the two leads. There is no "differential" output that has 2x the signal level (6 db).I do not know what you mean by "application specific input" or why this matter isn’t more clear to you. My phono preamp gain is adjustable and is spec’d up to a maximum of 68 dB single ended and 74 dB balanced. That seems consistent with my experience. |
roberttdid4 As a counter to that cleeds, and not taking anything away, it is also true that because an RCA connection is used, does not mean that the input is not true differential ...Quite so, and I made that same point earlier in the thread (fifth post). The Audio Research Ref Phono preamps are examples of RCA connections into a differentially balanced preamp. ... differential gives you the ability to increase gain while decreasing common mode noise, but you are still amplifying differential noise as well as noise internal to the amp. You are not getting 6db for free.That's a fair point. There's no such thing as a free lunch. It's still 6 dB. You could always crank the gain 6db on a single ended connection ...Actually, no you can't "always" - unless you change preamps. At maximum output you do gain 6 dB with a differential circuit. |
As a counter to that cleeds, and not taking anything away, it is also true that because an RCA connection is used, does not mean that the input is not true differential. With great gain, comes great responsibility. But seriously, w.r.t. phono, differential gives you the ability to increase gain while decreasing common mode noise, but you are still amplifying differential noise as well as noise internal to the amp. You are not getting 6db for free. You could always crank the gain 6db on a single ended connection, which again, for a cartridge, is already pseudo-differential due to the floating source. |
danvignau There is little, if any, advantage to XLR on short runs.That’s a pretty broad statement and it isn’t clear exactly what you mean. As I’ve already pointed out, the use of XLR connectors does not necessarily mean that the unit is true differentially balanced. There is great potential advantage in using differentially balanced circuitry and connections in a phono preamp. In addition to the increased immunity to noise, there’s a 6 dB increase in gain. That’s a huge advantage in a phono section! |
luisma31, sorry, I got confused with initial post about TT problems. You keep saying "unplugged from pre side" but you probably mean that shield was disconnected, otherwise what are you listening to if nothing is connected to pre? I suspect that cable might create ground loop. Lifting shield on pre side might still produce buzz if shield touches connector shell. Anyway, there is very little chance that anybody can help you if Ralph couldn't. I use XLR cable grounded at both ends between DAC and power amp and it is dead quiet. |
Please read my initial post again, I have described my digital rig not TT connected (yet) I described hum with a shielded balanced cable when unplugged from the pre side, when an unshielded cable was used there was no hum, my understanding is the shield adds capacitance and IMO is not needed and undesirable. |
luisma31, Unplugging connector? You probably mean - disconnecting shield? If you had no hum with shield connected at both ends, then why to change it? I would guess that disconnecting shield at the Phono side only left TT ungrounded with floating antena (shield), while unplugging on TT side or both sides should have less of an effect. How is your TT grounded? |
@kijanki luisma31, If it is not grounded at either end, then capacitance to what? To electrically floating object?honestly I don't know, I'm not an EE, but the effect of the hum is similar to what high capacitance cables will do, and since capacitors are isolated (just like this cable) I think HIGH CAPACITANCE is right on target |
@oldears If its not humming I would not worry, but I suspect there is no need for the 'drain' wire, as the preamp is likely electrically grounded via the ground connection of the AC power cord. We've been making DIN to XLR phono cables for decades; we just tie pin 3 of the DIN (ground of the arm) to the shield which is pin 1 of both XLRs. |
@atmasphere thanks for your clarifications. I am using an Ayre P-5xe which has balanced phono inputs. I was bothered that if one uses the rca inputs to customize resistor loading ( I do not) that the use of one resistor per channel might indicate it was not truly a balanced input. Your explanation of the differential connection clarifies that worry.I have wired my own balanced DIN to XLR phono cables (refusing to pay $600+ for something so simple). Of course pins 2 & 3 are signal, and pin 1 is shield on the xlr ends only. I have a drain wire to the center DIN pin as a drain connecting to the P-5 ground. Is this correct? I have no hum. |
Almost none. I ran unshielded tonearm cable in my home system for some years. You could crank up the volume and no hum or buzz from the cable, even if grasping it or moving it around. You do still have to ground the tone arm and it works best if that wiring travels with the signal wires. One advantage here is this allows for much lower capacitance in the cable.@atmasphere Wire to shield capacitance should play role only if either output or input circuit is ground referenced. I thought it was the advantage of fully balanced, not ground referenced (floating) configuration. There is still wire to wire capacitance (increased by twisting), but there is no wire to shield capacitance, making even less expensive cables to sound better. I agree that shield does not improve much, since twisting wires makes them very immune to electromagnetic or capacitive pickup, but it should not cause buzzing. What lusima31 described sounds like ground loop. XLR cable shield is grounded at both ends, but shells shouldn't be, since it might create such loop. I've read that in recording studios they often have problems with that and fix it by cutting shield at one end. |
Forgot this, I understand with turntable to pre interconnect a shield is recommended if single ended or single ended to balanced adapter cable, I am in the process of setting up this, I guess I will find out. Ideally I would like to go straight from the cartridge balanced to my preamp but we will see. |
Good you mentioned no advantages with the shield Ralph, I was about to send this in the morning before you even answered in the thread and I think this although been mentioned before it is largely overlooked. I had 1st a Hifi digital system and then I started getting into serious and educated analog. The digital audio community was introduced to a concept to improve SQ on digital rigs, specifically on DC connectors which basically states that a shield on a cable, not connected to any ground but looped at the ends of the DC cable will improve SQ, John Swenson was the engineer mentioning the concept, if it works or doesn't up to you to decide, I don't take sides, with the exception some people started applying this concept to all sort of cables, USB, digital interconnects and SE and XLR interconnects, some reported improvements, I tried these and notice nothing but since there is shielding involved I thought (at the time) it was great. Fast forward 1 year, got a new preamp from Ralph (Atmasphere) and there is an accentuated buzzing sound on my tweeters, could be heard from 8 ft away, very annoying, my entire rig is balanced XLR. Started analyzing and graphs showed peaks on 120 Hz, 180, 240 and 300 Hz, AC harmonics of course, some of these 15 db peaks some others 30 - 40 db, very annoying. Turned off the entire house, got into a fight with my wife for it, moved things around, added some power conditioners, buzz was there, called Ralph, he suggested an isolation transformer, got one, some improvement, minor, buzz was still there. Ralph asked me to do a few things and he very nicely sent me some plugs and attenuators (at no cost), attenuators worked taming the buzz but at the cost of power as you could imagine, because of the gain on his pre and amps is high still got plenty of power on the preamp for my speakers so all good there. But me I can't rest until I figure what was wrong, got my spectrum analyzer and scanned the entire house and even the neighbors, nothing out of the ordinary. Got a long extension cord (daisy chaining 4 extensions) and connected my entire system to a neighbor on the other side of the street (with his consentment), same results. Talked to Ralph on the phone about something else but me being me (annoying asking things) and Ralph being Ralph (willing to explain and educate you with his great knowledge) we mentioned cables used, I use Mogami which he recommends but the topic stayed lingering on my mind for several days. Sorry for the long rant, getting finally to my point, playing with the Mogami interconnects I noticed no hum, not even the faintest hum with the XLR in place BUT when unplugged one of the XLR's from the preamp and amp on, disconnecting from the pre there was a nasty hum, this is not supposed to happen, then I remembered when ordering these Mogami's interconnects everyone said better to be shielded and at no cost I got them JSSG (looped shield at the ends), there was one guy don't remember where online which told me before all this, for balanced interconnects never get these shielded but since it was only ONE guy I did not.Replaced the balanced interconnects (between pre and amp) with unshielded ones and when unplugging one end the hum was gone, but the buzz was still there.My source DAC was connected to the pre via XLR shielded, looped interconnects and even with the DAC OFF there was buzz, replaced these and the buzz was gone almost completely, turns out not all the balanced XLR equipment out there is "fully differential" or following the balanced standard, Atmasphere's are but the DAC wasn't. Summarizing and my conclusions.Shielded cables with tricks like loops etc. have A LOT of capacitance, truly balanced equipment won't care but better to have no shields as there are not needed because of the design of these circuits, single ended equipment will suffer from high capacitance with hum, buzz etc.Of course this is what Ralph's have been saying for decades so not so much new here except that now I don't want any capacitance near any of my cables.I'm sorry Ralph I don't think I told you these findings directly but I just discovered this over the last month or so and since it came up publicly in this thread I thought it was best to share.Luis |
How much advantage did you find by adding shielding of the XLR over just using a differential input w.r.t. noise?Almost none. I ran unshielded tonearm cable in my home system for some years. You could crank up the volume and no hum or buzz from the cable, even if grasping it or moving it around. You do still have to ground the tone arm and it works best if that wiring travels with the signal wires. One advantage here is this allows for much lower capacitance in the cable. As roberttdid said about differential filter "it seem obvious", but it escaped me completely, being stuck on "matching".@kijanki I think a lot of people think of matching 'equal but opposite' circuits when they hear the word 'balanced'. EQ circuits might be the best example of that. I once went to see a demo of the new (at the time) Mark Levinson preamp (IIRC the ML-29) which was their first balanced line preamp. But the phono was single-ended. When I asked why, I was told that matching the EQ was the problem the 'engineers' were worried about weird effects if the parts weren't matched really carefully! That was when I knew they hadn't tried it at all. At any rate if you want to do balanced right with active balanced circuitry, the way to do it involves differential circuitry. That's how you develop power supply noise immunity (Cross-Mode Rejection) and Common Mode Rejection. (As a side note, the CCS is critical and frankly, most CCS circuits I see are pretty terrible.) But what is less obvious but also important is the simple fact that you should do wiring in differential mode too- common grounds and common power supply points being the most obvious examples, and of course the EQ can be done differentially as well. Now we use passive EQ for our phono (based on the formula of Stanley Lipschitz) so imagine dual EQ networks, one for each phase; this made it go easy. Those networks should use a common ground of course, but if you think about it, you don't need the ground at all. And if you got that far, then you can see that resistor values are in series as are capacitors- meaning that only one resistor need be used, only one cap (of half the value; you can see where this is going) and now there's one network instead of two. Much, much easier and all you're doing at that point is trimming to the values you need, rather than a Sisyphean task of matching to some extreme; pointless when you have tubes or semiconductors that won't come anywhere close to a similar match. A nice result is you don't have to do crazy amounts of tube matching yet the EQ will be spot on from both phases even as the tubes age. As far as I know, we were the first to do this (1990) and I think it was only about 8 years ago before I spotting any circuits that did the same thing- apparently its not common knowledge even today. |
We are all told that cables make a difference. Absent knowing what cable or that there was a change, my experience in controlled environments says that is the cable is competent, there may not be a difference. Diminishing returns much quicker for interconnects as well versus speaker cables. Noise rejection is important yes. |
Atmasphere, How much advantage did you find by adding shielding of the XLR over just using a differential input w.r.t. noise? Interesting on the differential mode filter network. When you say it it seems "obvious", but certainly wasn't when the question was first posited. Very interesting post, thank you. |
Gains of both amplifiers have to be exactly the same, otherwise common mode noise will get converted to normal mode signal. It requires some form of cross-feedback to keep gains exactly the same. Phono preamp in addition has RIAA equalization that changes frequency response within audio band by 40dB (100 times). I suspect, that it would be very difficult to match it in both "legs" of true balanced phono stage.This statement isn't quite correct. To prevent issues with RIAA equalization, you simply do the EQ network in differential mode. This eliminates issues with matching and gain differences. A bit of audio history since this is relevant: we built the first fully differential phono sections for home use in our MP-1 preamp, introduced in 1989. As such is was also the first balanced line preamp, tube *or* solid state (again, for home use). At the time the XLR connector was the only game in town so that's what we used for the phono input. We had no idea at the time that a product like this should do anything other than support the balanced line standards (AES48) so that is what it did and does. The correct phono connection is: + and - outputs of the cartridge are pins 2 and 3 of the XLR. The tone arm and turntable ground is the shield of the interconnect cable and ties to pin 1 of the XLR of each channel, so no ground wire is needed. You'd be surprised how many cable manufacturers there are that think the ground wire is still needed :) This connection is so quiet that we've used unshielded twisted pair with a ground wire (what we've come to call a 'twisted triplet'; but we've also used Kimber braided wire) and its noise-free. But one thing that has been pointed out here that is really important is the CMRR of the input stage. Of course transformers do this really well, but since we're sort of known for transformerless operation, the other way to do it is with an input gain stage that has as high CMRR as possible, combined with the lowest noise. Obviously opamps work great for this but we wanted to use tubes, so the input circuit is a differential cascode which employs a 2-state CCS (single-stage CCS circuits leave performance on the table). If you want low noise and high CMRR, this type of input circuit (which is common in a lot of solid state circuits as well) is really the only way to go. Because tubes drift and because matching components is tricky, we sorted out the differential mode EQ and as far as I know we're the first to do that by decades prior to anyone else. Differential mode EQ insures that both outputs of the phono section (inverted and non-inverted) will have the same EQ regardless of how much gain imbalance there might be. To prevent common mode noise from becoming differential mode the simple bit is making sure that any ground in the gain circuit is common to both phases- common sense :) In this regard layout plays a role as well, as the designer will find that certain parts in the circuit are best placed as close to each other as possible to maintain noise immunity. The main downside we encountered was the reluctance on the part of dealers to deal with the balanced input since it meant changing out the tone arm cable; some thought the arm had to be rewired which of course it doesn't :) The advantage of operating balanced at this point in the signal chain is that as long as the tone arm cable is low capacitance, the cost of the cable isn't important- how its wired is. So you can have a $200 cable sound just like a $2000 cable. Cable immunity was a big incentive for why we developed a balanced preamp. |
Maybe a point that needs to be made, you don't need an XLR to have a balanced input connection. You can use an RCA and still have a balanced input connection. BAT incorporates its “Flying RIAA Network” differential circuit here, which the company claims has fewer parts yet because of ease of adjustment at the factory and dual differential common mode noise rejection, produces greater RIAA accuracy, with typical measured error of less than 0.1dB. |
kijanki, When you short a oscilloscope and probe to ground, you are creating a loop antenna and creating a differential signal which is not to say that common mode noise and/or common mode voltages are not a problem in measurement. That is why Picoscope makes a differential input scope, and Tek and Lecroy sell high voltage differential probes for about $2,000/each (or more). While you are "floating" you are also a big capacitor to ground which creates a path for common mode signals. Also remember that most oscilloscopes probe grounds are also connected to chassis grounds. Floating oscilloscope inputs (battery powered or otherwise), can offer most of the benefits of differential inputs for many measurements (and tend to be much quieter). And are you assuming the noise was common mode? Are you sure it was not differential mode? I had TT long time ago and always had problems with low frequency hum (single ended phono stage). Current thru this ground wire causes tiny voltage drop, seen by the input as input voltage (input is referenced to BNC GND and not to end of the cable GND). But again, we are talking a cartridge and absent the shielding and other metal structure (and antenna effects isolated by the cartridge itself), it is a loop. Absent the shield/structure capacitance to ground, there is no other current path, whatever current flows in one wire must flow in the other, and hence whatever drop you have on one wire is cancelled by the other. Transformers are already used for MC coils, so we know they work. |
