I do not see anyone above has referred to the 6dB noise floor reduction (theoretical at least) offered by running true balanced. This is particularly beneficial on phono as cartridge noise floor is not inherently low. I have run fully balanced for nearly 30 years with quite a few different pick-up and amp components. I would never go back to the dark side. |
Roberttdid, I would compare single ended phono preamp input to, for instance, oscilloscope. When you short oscilloscope probe (tip to probe GND) there will be still some noise on the screen - bigger if you touch shorted inputs (in-spite of you "floating"). It is because for electrical noise one of the wires (GND) provides lower resistance path to GND. 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). Of course scope can be used in two channel differential mode but it suggests that second input is required. Amp could be truly balanced, but it is overkill, IMHO - costs money and introduces more components in the signal path. Remaining choice is likely between differential amp and transformer. Transformer, in spite of some distortions at low frequencies (most likely inaudible) is a clean simple solution. What do you think? I had TT long time ago and always had problems with low frequency hum (single ended phono stage). Stingreen - you stated that your Ayre phono is dead quiet. It's likely because of balanced input. XLR cable wires are usually twisted - a very strong defense against electrical noise. Do you know by any chance what is inside (transformer, differential amp etc.)? |
Sorry kijanki, I thought we were talking about discrete level or discrete within an IC circuit, and that the 3 op-amp instrumentation amplifier you showed was a different part of the discussion. The circuit shown is missing the output buffer that would be on a practical circuit and those two output resistors have to be matched as they dominate CMRR in the calculation. I now get completely where you are coming from :-) If the noise current induced are not the same in both wires, then they are not common mode, they are differential mode, and a balanced connection only removes common mode noise. As for the balanced connection - it is not balanced anymore when you connect it to unbalanced input. Noise currents induced in both wires will be different and noise will go thru. In a typical connection of two components, you have multiple paths for current flow which is how CM noise generates a signal. With a catridge/phono stage, you have a single loop so induced common mode noise does not generate a signal. Where it gets dicey, is when you start adding new current paths like shields. Fix your DM noise, and now create CM noise. I think this is perhaps was jcarr was referring too? |
That's not how it works. I think you are confusing something. For a 40db CMRR, you must have gain matching of gain matching between the two channels to +/- 0.5% (1% total), and you must have perfect transistor matching (which they never are).Look at the picture I provided. Each of two amps operates in non-inverting mode and is referenced to inverting "-" input of another amp. Since voltage at inverting input is the same as voltage at non-inverting input then voltage across R2 is V2-V1. For common mode signal V2=V1 and voltage across R2 is zero. It means you could remove R2 (no current flow) and without it you just have two amplifiers with perfect gain of one each (followers) even if R1s are a little different. R2 plays role only for normal mode signals. That way you can set any gain for this stage and gain for common mode will always be one. If you set gain of 100 then CMRR will be 40dB. As for the balanced connection - it is not balanced anymore when you connect it to unbalanced input. Noise currents induced in both wires will be different and noise will go thru. The fact that cartridge is floating won't help, unless input is balanced. The differential input of an op-amp does not have a gain of 1Nobody said that. Instrumentation amp is not an op-amp. |
Nice discussion. Ralph needs to chime in. JCarr, You wrote, "Phono cartridges are floating sources rather than balanced, and unless there is a low-impedance connection between phono amplifier ground and both sides of the cartridge signal coil, and/or the common mode rejection of the phono stage input stage is extremely high across a wide bandwidth, there most likely will be substantial pickup of electrical noise from the environment (i.e., connecting the phono cartridge in balanced mode will probably yield worse noise performance than an unbalanced connection)." In this paragraph, you used the terms "most likely" and "probably" at very key points in your statement. Have you actually done the experiment and made measurements? Also, what would you say happens when the balanced phono circuit is floating with respect to ground, just like the cartridge itself? |
I perceive this would only be of benefit at frequencies outside the audio range, as within the audio range, effectively the cartridge completes the loop and common mode noise would be rejected. Do you have examples of commercial products that use this technique and are they measureably quieter?
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It's common mode. By definition the currents must be matched. If they are not matched, they are not common mode, they are differential and then you cannot tell them apart from the signal. The coil in the cartridge also completes the loop. It is by analogy the isolated end of the transformer. If it is a powered microphone that has an active ground connection, power and signal, then yes, the benefit is there. The benefit is mainly in better shielding for a non powered microphone.
The differential input of an op-amp does not have a gain of 1. It's a current source into a differential pair, that will share current based on the transistors being perfectly matched, and it will convert that current into a voltage by the load resistors ... who's matching also impacts the common mode rejection of this stage. As for Instrumentation amps, yes they have laser trimmed resistors, but front (two amp) differential section has always gain of 1 for common mode signal independently of resistor tolerance. That's not how it works. I think you are confusing something. For a 40db CMRR, you must have gain matching of gain matching between the two channels to +/- 0.5% (1% total), and you must have perfect transistor matching (which they never are). If you set gain of 100 for this section you get automatically 40dB CMRR independent of resistors tolerance. It happens because each of two amps is referenced to input of another (instead of the GND). Typically you use op-amps with laser trimmed resistors and/or you calibrate. You of course need tight temperature tolerance matching which is easier at the IC level, since with low power draw, temp will be very consistent between the temperature co-efficient. That is the only way I can see, for truly balanced amps, to function without converting common mode noise to normal mode signal. Otherwise matching resistors and keeping them matched to some sensible number (like 60dB=0.1%) is not practical, while adding RIAA frequency correction and matching capacitors to 0.1% is next to impossible. Which would probably be pointless as you pointed out, you are not going to get capacitors to that tolerance. |
Phono cartridges are floating sources rather than balanced, and unless there is a low-impedance connection between phono amplifier ground and both sides of the cartridge signal coil, and/or the common mode rejection of the phono stage input stage is extremely high across a wide bandwidth, there most likely will be substantial pickup of electrical noise from the environment (i.e., connecting the phono cartridge in balanced mode will probably yield worse noise performance than an unbalanced connection). An input transformer magnetically isolates common mode noise, and is a good solution to building a quiet balanced phono stage. |
one would have to introduce a true balanced phono circuit internally. While the idea of duplicated, bridged, or fully balanced (whichever term you like) is sexy, the idea that this is the only advantage is really not correct. Professional gear has been using balanced signals long before audiophiles got it into their heads to duplicate circuits. Having a differential input which is not referenced to ground has a lot of benefits for noise. And while I don't think we need full-sized XLR plugs, there are mini-XLR plugs which would work a treat for this. I've often wondered this myself. |
Thanks for the mention, Kijanki. To add further technical elaboration to this erudite discussion of esoteric matters, IMO the main reason "most phono preamps lack XLR input even though cartridges are naturally balanced" (quoting from the subject line of this thread) is simply what Lew said earlier: ... it’s a longstanding tradition to offer only RCA inputs. :-) And of course traditions often tend to be self-perpetuating. It’s perhaps also relevant that not too many decades ago I believe a considerable majority of high quality turntables had pendant (non-detachable) phono cables terminated with RCA plugs. Finally, regarding RIAA equalization Ralph’s (Atmasphere’s) MP-1 and MP-3 preamps are of course fully balanced, provide transformerless balanced XLR inputs for their built-in phono stages (RCAs can be added as an option), and are spec’d as having RIAA accuracies of 0.07 db and 0.1 db respectively! And given especially that it is Ralph who has provided those specs, I have no reason to doubt them. I’m not in a position to elaborate on how he accomplishes those numbers, of course, other than pointing out that he uses triode-based differential stages rather than separate signal paths for the two legs. But as far as purely technical considerations are concerned his designs certainly speak to the practicability of accomplishing what the OP has asked about. Best regards, -- Al |
With a cartridge, you only have a ground connection on one end. Equal currents induced in the same direction on each wire (common mode) "induce" the same voltage and cancel each other out (it is a loop), at least at practical audio frequencies. Differential noise, will of course look just like a signal. The cartridge is already "isolated".That is true, as long as currents flow in the loop only. If one of the wires go to input while the other goes to GND (unbalanced input), currents in both wires are not even anymore. Perhaps, that's why floating (for audio frequencies) balanced output, like transformer is not enough and has to be connected to balanced input (not single ended). Microphones also benefit from balanced input. As for Instrumentation amps, yes they have laser trimmed resistors, but front (two amp) differential section has always gain of 1 for common mode signal independently of resistor tolerance. If you set gain of 100 for this section you get automatically 40dB CMRR independent of resistors tolerance. It happens because each of two amps is referenced to input of another (instead of the GND). https://www.researchgate.net/figure/Fig-Two-Op-amp-Instrumentation-Amplifier_fig1_299514235 That is the only way I can see, for truly balanced amps, to function without converting common mode noise to normal mode signal. Otherwise matching resistors and keeping them matched to some sensible number (like 60dB=0.1%) is not practical, while adding RIAA frequency correction and matching capacitors to 0.1% is next to impossible. |
Most phono stages, preamp and amplifiers aren’t true balanced. A balanced input doesn’t necessarily mean it’s balanced. My Atma-sphere mp-1 is fully balanced with XLR phono input. I had Ralph add rca phono inputs so I could use any tonearm without adapter. Not many arms have xlr connectors . All the arms I’ve ended up with over the years except one has been rca. |
@kijanki instrumentation amplifiers use laser trimmed resistors internally and/or external precision resistors as well as significant open loop bandwidth to achieve high CMRR and fixed gain levels. If you mean the differential pair / long tailed pair on the input, they also need somewhat matched resistors and in op-amps they are trimmed to provide higher CM range. <<$1.00 op-amps have 100db+ common mode rejection, but not in practical circuit which would be about 70db with 0.01% resistors. Low cost practical instrumentation op-amps with high CM range readily achieve 90db CMRR. Your amps may have a few precision resistors, and/or they are calibrated. The main point of differential connections in audio is to eliminate the noise from having ground references which are different at the receiver and the transmitter whatever they may be and how they are caused. In a home audio system, that is going to be predominantly through noise induced through the AC, with the capacitance of the power supply, even EMI caps completing the circuit. With a cartridge, you only have a ground connection on one end. Equal currents induced in the same direction on each wire (common mode) "induce" the same voltage and cancel each other out (it is a loop), at least at practical audio frequencies. Differential noise, will of course look just like a signal. The cartridge is already "isolated". |
@lewm One advantage of true balanced is to remove reference to GND in order to get rid of wire to shield capacitance, but the same can be achieved with input transformer. The other is that even harmonics produced by both "legs" cancel, but some people looking for "warm" sound might not see it as particular advantage. Matching resistors is unnecessary if cross-referencing of both "legs" is used (like in the input stage of instrumentation amp), otherwise it is almost impossible. As for matching RIAA curve - it is very difficult. Typical good capacitors are 1%. Matching them by hand is not practical for production, not to mention changes over time (aging). Phono stage amplifies 60Hz by about 6 times vs 1kHz signal. As a result of both you will get very poor CMRR @60Hz in order of 30dB only. Good Instrumentation amp, like one in my Rowland amp, have 90dB CMRR @60Hz (and it is just plain line-in). My current power amp (Benchmark AHB2) has CMRR=80dB (equivalent to 0.01% gain matching) If they really don't make balanced inputs for such low level signals, they should. True balanced is out of the question, IMHO, because of capacitors tolerances, but simple solution would be to use input transformer. Why not use instrumentation amp and drive shield with common mode signal? It is very common in low level amplifiers. Interesting subject. Perhaps Almarg or Atmasphere can chime in? |
A cartridge has no ground reference so common mode noise will not induce a signal at the pre-amp except at frequencies well outside the audio band. Matching gains of gain elements is only critical if sufficient feedback at bandwidth is not used to ensure gain is set by the resistive/capacitive elements and not the gain elements themselves. |
kijanki, To the degree that signal handling by the two phases of a balanced circuit is not exactly symmetrical, you are correct in saying that will reduce Common Mode Rejection, and it does for sure in tube-based balanced circuits, because exact matching of vacuum tubes is not possible or stable even if it's momentarily possible. (Your argument about RIAA equalization does not hold as much water as does the argument about tube or transistor gain matching, because it is much easier to exactly match capacitor and resistor values, or to tweak the values for good balance, than to match the gain components.) To all of that I say, so what? CMR is reduced from some perfect value to some lesser value, but you still do get CMR. Whereas, in an SE circuit, you don't. I have two fully balanced phono stages; neither of these ever drove me crazy with hum and noise problems that I read about all the time in relation to SE phono stages. But the OP asked "why" we don't have (more) balanced phono circuits, and I think it's mainly the profit motive. |
I can see one possible problem with true balanced operation. 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. |
First, it's a longstanding tradition to offer only RCA inputs. Second, it is not enough to offer XLR inputs alone, in order to take advantage of the balanced nature of phono cartridges; one would have to introduce a true balanced phono circuit internally. That requires nearly double the parts count, which increases the cost of building the device, which reduces profits. Some inherently SE circuits do use a transformer interface between an XLR input and an SE pathway, which does glean some of the benefits of balanced operation. So, there are some phono stages that do offer XLR inputs and do offer balanced circuitry. There is no "con" view of balanced designs, unless cost is an object. I have read the rationale in favor of RCA inputs and SE circuitry, and it usually involves a story about colorations due to added parts count. |
