One of my phono stages is the same ARC PH-3SE as you have. I have used this with at least a dozen different tables and arms and have never had a hum problem.
You are absolutely right that a "shielded" cable/wire should have a much better resistance to RFI/EMF/hum.
I think something is amiss with your setup. Who did the rewire and what kind of wire did they use?
Did you have the hum before the rewire? If not, it has to be in the rewire somewhere. |
You also may have wired your cartridge incorrectly. If your wire joins the shield and the "negative" lead then you could get hum (I don't know the specifics of your wiring harness).
But make sure you've plugged in the right color wire to the right cartridge pin. One way to be sure is to use an ohm meter with the phono cable unplugged. Then check ohms between the blue wire and the shield for the RCA jack of the left channel. Ohms should be very low 0.3 or something like this. Then do the same for green and the right shield for the RCA jack. Now do the same for the right channel RCA jack tip and the red wire (0.3 ohm or less) and the white wire and the tip of the left RCA jack.
Now check a metal part of your tonearm (between the headshell and the bearing in case your bearing has a plastic cup) and see if it has a low ohm reading to your ground wire. |
Noisy tube in the AR unit? |
thank you, will try all these things. I did try different tubes, not the problem. |
when I do not connect the ground wire from the tt, just leave it dangling unconnected to anything, the hum is greatly reduced. What does this imply? problem with the way the entire system is grounded? I live in an apartment, and have no control over the non-audiophile wiring setup. |
There are probably a dozen reasons why one phono stage might be more likely to hum with a particular tonearm and cartridge where some other phono stage will not. First of all, lets get the terminology straight; when you say "hum" it implies a 60Hz or 120Hz tone (in countries where the mains frequency is 60Hz). Hum is almost always due to inadequate grounding. By that definition, tube phono stages are no more likely to hum than SS ones. However, true balanced phono stages of either type are much more immune to hum than are single-ended circuits, because even if the ground scheme is not perfect, hum in a balanced circuit is often cancelled by the mechanism of Common Mode Rejection (CMR). The better the CMR (i.e., the more the two halves of the balanced circuit are truly in balance), the more the phono stage is impervious to hum. One kind of noise that is akin to hum is the rare instance where the cartridge will interact with the tt motor, some types of which can induce hum in the cartridge. "Bad" tubes can sure be noisy, but the noise does not usually have the quality or pitch of hum. |
Lloydc:
When you disconnect you TT ground and the hum is reduced, it implies you have a ground loop. You cannot have multple points of ground in different locations, as these will create hum as ground current flows from one location to another.
For example, if your TT base is metal and grounded to the third wire AC ground, and the analog input ground in the pre-amp is also grounded to the third wire ground, you have a ground loop, with the RCA cable shields making the connection.
The reason why some pre-amps have different hum performance may be due to how they reference the analog signal grounds at the inputs. Some are hard connected to a chassis or analog ground, and some are referenced with a capacitor or resistor. The resistor or capacitor provides a high enough impedance to interrupt or reduce ground loop currents.
Differential inputs are useful for CMR, but phono carts are almost alway connected single ended, which means the (-) and shield are common, and you cannot get a true balanced connection to the pre-amp input. So the CMR of the input is of limited utility. |
Something that worked for me with a P75 phono stage was running a wire from the ground post of my phono preto the chassis of my preamp |
Dear Dhl, If the phono circuit has a balanced topology, then the cartridge may be connected in balanced mode, and believe me there is CMR for the phono signal which does cancel hum along with other noise common to both halves of the signal. You can get a true balanced connection no matter how the cable is built, because the cartridge per se has no true ground, but it is indeed better if the cable provides identical conductors for the pos and negative halves of the signal, and a separate ground wire that generally grounds the tonearm body only. I don't know why you seem to think that is a problem or not available. |
Lloydc:
I am wrestling with a similar condition after replacing my ss pre with a tube unit. Are you sure that your problem is a 'hum' and not a 'buzz'? Mine is definitely a buzz (heard only in the tweeter), and my current thinking is that it relates to external circuit noise which is induced into the system and picked up by the tubes and sent through the rest of the system. I must admit that I am early in my process of diagnosis, but nearby lighting dimmers cause similar a similar buzz when activated. Anybody else have comments on that line of thinking? |
Rtilden, Check to see whether any of your ICs, particularly the one from tonearm to preamp, are running parallel to or anywhere near an AC cord or a PS umbilical. I had an identical buzz that drove me a bit nuts until I noticed that my phono cable was running near to BOTH the AC cord from the turntable AND the PS umbilical from the phono stage PS. Re-routing it made a huge difference. Sometimes we lose track of what's going on with all the spaghetti hanging down at the rear of our system shelves. |
Thanks, Lewm. I will check out that possibility in my next step. |
Rtilden, just FWIW, tubes are no more prone to picking up noise from lighting than transistors. That is a power supply problem, not a tube/transistor problem. |
Lewm:
The cartrigde may be connected in "balanced mode", but when you connect the shield to the minus input, you convert it to single ended. You have to reference the input voltage to somewhere, and for phono inputs its the shield (-). If you could get a cartridge with a true balance output (like a center tapped transformer), with the split (or center tap) connected to the analog ground/shield, then you can have a true balanced input. Some microphones are wired this way. But phono cartriges are not.
For example, I have a PS Audio GCPH that uses a "differential" input pre-amp. The input RCAs are ground referenced via a 100 ohm resistor to analog chassis ground. The two phono inputs (V+) and (V-) are twisted pair from the tonearm/cart with a separate isolated shield. Shield is connected to chassis ground. Is this balanced? NO!!! Because the potential between the shield/chassis ground and the (V-) input is essentially zero, because no current flows in that 100 ohm reference resistor. In a true balanced system, V- and V+ would be the same, only opposite in polarity. In this phono setup, V- is 0.
So while this system may give you some CMR, it will not be anywhere nearly effective as a true balanced system. |
Dhl,
To take your first paragraph, ALL cartridges (except for a very few, among which are some early Deccas and probably some others of vintage origin) are inherently balanced. The typical modern cartridge does not know or care which side of its coil is referenced to ground. Since historically 99% of phono stages were SE, most cartridge makers label one of the two pins per channel as "ground", just so that the two channels will be in phase, I guess. When we use an SE phono stage, we then treat the signal as SE.
But I did think some about your other point, whether connecting the shield to the negative phase would convert the signal to SE. This only comes up if one is using a cable that was built for handling the signal as SE, but now we are connecting that cable to a balanced phono stage. If the plug is RCA, then the negative half of the signal would have to be carried on the ground side, as far as the phono stage is concerned. As long as the "ground" side of that RCA is connected to the negative phase of the balanced circuit (and NOT to chassis ground, as is the case for your PS Audio), I don't think it would make any difference that the shield was also connected to the negative phase (it would not actually be grounded and so would operate as an unshielded cable). The shield is typically only connected at one end. But this is really a hypothetical situation; if one has a balanced phono section, one should use a balanced cable (two wires for signal, one pos one neg, plus one wire for ground) terminated with an XLR plug.
I know nothing about your PS Audio GCPH, but I take your word for it that you are listening in SE mode. Sounds like it is not a true balanced circuit, if the RCA jack is connected to chassis ground. Or if the circuit is balanced, they don't want you to operate it as such. |
thanks to everyone for the responses. It is definitely hum, and not tube or other noise. I suspect that Dhl93449 correctly diagnosed a ground loop. How to solve it, in an apartment with exceptionally noisy wiring (even a flourescent light on the same circuit) and few outlets, is an open question.
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Noise issues with phonostages are often quite unpredictable and hard to figure out.
While it might be the case that certain gear is generally more susceptible to noise problems, it can also be the case that it is only more sensitive in certain applications; e.g., sometimes the problem can be cured by moving the location or orientation of the phonostage or interconnect cables.
I have a preamp with a built in phonostage that, in my setup (preamp on a shelf two levels below my turntable) I have hum issues, whereas that same preamp in another system is dead quiet with a phono cartridge that has a .05 mv output (my problem is evident with a .30 mv cartridge).
Hum generated from the power supply of the phonostage itself might be a bigger issue with tube linestages, but, again, when things are right, tube stages can be essentially dead quiet (like my stand-alone phonostage).
I have heard both solid state and tube units that had noise issues. I don't think "type" tells you that much about proclivity to be noisy--one has to actually try the unit. In any given application, a unit that is otherwise known to be quiet may behave badly. For example, a friend's very expensive Boulder phonostage buzzes badly from over the air interferene (from lighting) where a tube unit we tried is quiet (Boulder is NOT known to be prone to ANY kind of noise problems).
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You have to reference the input voltage to somewhere, and for phono inputs its the shield (-). If you could get a cartridge with a true balance output (like a center tapped transformer), with the split (or center tap) connected to the analog ground/shield, then you can have a true balanced input. Some microphones are wired this way. But phono cartriges are not. All phono cartridges made today are balanced sources. None are single-ended. You don't need a center-tap to be balanced- that is a common myth. I don't know of any microphones that have such a thing- my Neumann U-67s sure don't. Neither do Shure SM-57s- these are mics at far extremes of cost and performance. Any tome arm that has 5 connections (stereo signal + tone arm ground) can be run balanced by replacing the interconnect cable between the arm and preamp. |
To clarify my comments, phono carts are ISOLATED devices, but are not inherently balanced. A true balanced voltage source would produce a symmetric V+ and V- signal referenced to common.
However, I agree they can be run in a balanced configuration with the correct wiring configuration and true differential input phono stages.
Sorry Lewm if I misread your original post. Its just that true differential phono stages are very rare, and neither of the OP preamps have true differential inputs, so using balanced wiring with these single ended input stages converts the system to single ended with little to no CMR advantages. |
The OP asked whether one or another different types of phono stage might be more or less susceptible to hum. In responding, I brought up the fact that true balanced phono stages are much more immune to hum. Then we got off on this tangent. I don't think the discussion was out of line with the intended subject of the thread, but it was prolonged by your insistence that a cartridge is not a balanced source. Anyway, it's all good. |
Higher level phono sections like mm carts rather than low output mc carts are less sensitive to EM induced hum from nearby transformers, etc. Shielded cables and/or supplemental shielding around the low level amplification device(s) like a step up transformer helps. See my system listing here on agon for an example. Adding shielding around my phone step up amp allowed me to use my preferred unshielded IC in my case and eliminate significant audible hum. |
Lloyd, have you tried temporarily using a cheater plug (a 3-prong to 2-prong adapter) to isolate the safety ground pin on the power plug of the phono stage and/or the turntable? That would break a ground loop between the two components, and enable you to determine if a ground loop is the cause of the problem. For that matter, is there still a hum if the turntable is unplugged from the AC? Also, is the hum level affected by whether or not the turntable is on or off, and also by the position of the tonearm relative to the rotating platter and/or the motor? Lloydc 05-07-12:
Do phono stages have different grounding schemes, making them more compatible with certain tt/tonearm/wiring combos in unpredicable ways? That strikes me as a very well put question, to which I would answer "yes." Schematics for your phono stage can be seen by clicking on the two corresponding thumbnails near the bottom of this page. It appears that circuit ground is connected to chassis (and hence AC safety ground) via a 10 ohm resistor (shown on page 2), and a 560 pf capacitor (shown on page 1). Other designs can be expected to handle the interconnections between those grounds differently. Also, differences in the amount of stray capacitance that exists within the power transformer of different phono stages, between the AC line and chassis, can affect ground loop susceptibility. On the question of cartridges being balanced or unbalanced, I would put it that it could be reasonably argued that a cartridge is inherently balanced, and it could also be reasonably argued that it is inherently neither balanced nor unbalanced, since when unconnected its outputs are floating ("isolated" as DHL said). But it's a moot question, because the design of what they are connected to causes the two signal lines from each coil to be treated in a balanced or unbalanced manner. For balanced operation, the phono stage has to closely match the impedances between each signal line and its internal circuit ground, as well as processing the inputs differentially. Best regards, -- Al |
To clarify my comments, phono carts are ISOLATED devices, but are not inherently balanced. A true balanced voltage source would produce a symmetric V+ and V- signal referenced to common. A common method I use to identify a balanced source is what happens if you swap the + and - outputs of the device? Do you get hum or is phase merely reversed? If the latter then its a balanced source which can be treated single-ended or balanced depending on the wiring it drives. We first started building balanced phono sections (all tube) about 1989. They have XLR connections, and the way a cartridge hooks up is the + output is pin 2, the minus input is pin 3 and the tone arm ground (which has no connection to the cartridge) is pin 1 (apparently we were the first to do this...). In this hookup, you can see that that ground is essentially ignored as the output of the cartridge occurs between pin 2 and pin 3 of the XLR. So there is no need for a separate ground wire like you see in single-ended connections. Certainly there is no need for a center tap! A center tap can never be perfectly centered in the windings, so the Common Mode Rejection Ratio is reduced if you use a center tap. IOW, you don't want or need a center tap. Such a connection is immune to hum. The 'shield' connection can be a simple wire and it will work the same. |
