I think that's generally true (e.g., preamp to power amp connections, DAC to preamp...), but I make an exception for the one place where we're dealing with microvolts and nanovolts. I used single ended phono inputs for years, but once I went balanced, I never looked back. Like John says, it's one design choice among many possible ones. This is why phono stages are one of the very few pieces of electronics where the design is truly critical to actual audible performance.
Enough RF got by the transformer to overdrive the opamp inputs. Adding a case to ground capacitor reduced it, changing opamps seems to have mostly fixed it.
If you squeeze your walnut hard with two hands, you may come up with more. Nowadays, a lot more.
Mind if I ask just how many of "those" you are close with ?
If it's very little to none, the main part of your target-practice line right after "a" at the least applies to you.
(means the only one you are trying to impress is yourself)
It must be understood that the input level of a MC input might be as low as 100uV, in fact this is almost typical. There are even lower output MC's but they are not as popular, today.
IF I could hear or measure hum pickup of an unreasonable amount from a single ended phono stage of my own design, THEN I would adopt balanced throughout.
However, the Vendetta Research was single ended input, and the JC-3 Phonostage is single ended input as well. So far, no complaints.
However, SOME consideration is necessary for acceptable performance. DON'T have a power transformer near the input cable or even the chassis of the preamp.
IF you run the input wiring next to a power transformer, you are sure to pick up hum, so don't do it! Don't put the power transformer that powers the phono stage INSIDE the phono chassis IF YOU CAN HELP IT! It is much better to have a separate chassis for the power transformer and even the filter caps. However, IF you put the Vendetta power supply chassis on top of the Vendetta phono stage itself, IT WILL HUM! Aluminum is just not a very good magnetic shield UNLESS IT IS VERY THICK, and that is expensive. So just 'obey the rules' and don't put your power transformers' next to the single ended phono inputs or cables.
In this age of legal responsibility, if a slight added hum, due to 'not following the rules' was dangerous, I'm sure that I would have been sued by somebody over the decades.
Another IMPORTANT consideration is the TYPE of input wiring that you have from your phono tone arm. You have to change your input cable from the tone arm TO a balanced configuration (usually with balanced connectors) in order to get significant rejection, and MY tone arm came single ended. However, very wealthy people will most probably have balanced cables coming from their tone arms, so a balanced input MIGHT be useful. End of story. (I think) '-)
IF I could hear or measure hum pickup of an unreasonable amount from a single ended phono stage of my own design, THEN I would adopt balanced throughout.
However, the Vendetta Research was single ended input, and the JC-3 Phonostage is single ended input as well. So far, no complaints.
However, SOME consideration is necessary for acceptable performance. DON'T have a power transformer near the input cable or even the chassis of the preamp.
IF you run the input wiring next to a power transformer, you are sure to pick up hum, so don't do it! Don't put the power transformer that powers the phono stage INSIDE the phono chassis IF YOU CAN HELP IT! It is much better to have a separate chassis for the power transformer and even the filter caps. However, IF you put the Vendetta power supply chassis on top of the Vendetta phono stage itself, IT WILL HUM! Aluminum is just not a very good magnetic shield UNLESS IT IS VERY THICK, and that is expensive. So just 'obey the rules' and don't put your power transformers' next to the single ended phono inputs or cables.
In this age of legal responsibility, if a slight added hum, due to 'not following the rules' was dangerous, I'm sure that I would have been sued by somebody over the decades.
Another IMPORTANT consideration is the TYPE of input wiring that you have from your phono tone arm. You have to change your input cable from the tone arm TO a balanced configuration (usually with balanced connectors) in order to get significant rejection, and MY tone arm came single ended. However, very wealthy people will most probably have balanced cables coming from their tone arms, so a balanced input MIGHT be useful. End of story. (I think) '-)
Non-rich diyers can make their own balanced cables pretty easily.
Let's dig in a bit deeper. 100uV is indeed a reasonable number, but that's at (usually) 5cm/s, which we can think of as 0dB (yes, the realities of analog can mean that much higher values are sometimes observed, but this is a reasonable reference level). Now in a top-quality phono stage, we'd like to have the noise limitation be on the cartridge end, so shooting for an 80dB s/n ratio isn't unreasonable. That's 10nV, input referred!
John, would you disagree that this is a reasonable target?
Let's dig in a bit deeper. 100uV is indeed a reasonable number, but that's at (usually) 5cm/s, which we can think of as 0dB (yes, the realities of analog can mean that much higher values are sometimes observed, but this is a reasonable reference level). Now in a top-quality phono stage, we'd like to have the noise limitation be on the cartridge end, so shooting for an 80dB s/n ratio isn't unreasonable. That's 10nV, input referred!
John, would you disagree that this is a reasonable target?
All you need is a 1% imbalance in the cable geometry to reduce CM to only 40dB. And 1% symetry is hard to keep even with machine made cable. Then you cut it and terminate it and then its something else inside to the circuitry. Sounds easy but not in practice if you want high CM rejection. But sometimes even 20dB is a noticeable reduction if you have a hum issue.
The extra circuit complexity of balanced for greater cmr often causes counter active effects in other areas.
PS -- dont let ac power cords and thier radiated fields (50/60hz and HF/RFI) get anywhere near signal cables. How often is that the case?
Thx-RNMarsh
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The scale of noise pickup that will degrade the noise performance of the preamp.
I don't think that's correct. You have to assume that the noise wavelength is small compared to the twist. See the Whitlock paper that Steve referenced above- even with an unbalanced source, a well-balanced input will still reject the CM noise. Considering that I'm using cheap Guitar Center mike cable to connect my cartridge to my preamp, and I can lay it across power transformers or touch the cartridge pins without any hum, I think Whitlock's point is proved.
Before I am told that an input transformer is a 'must have'. Let me point out what is WRONG with input transformers.
A casual glance at what a transformer has to do, does not seem too difficult, but there is still a serious underlying problem. This is the warped record induced output between 0.5Hz and 2Hz. from slightly warped records. This is a fundamental weakness in transformer inputs for phono cartridges. Mikes, OK, but phono is different.
If you doubt this, just put a DC scope across the input terminals of a phono stage playing a record. Now, can you take a picture, like a square wave, for example? Check it out.
A casual glance at what a transformer has to do, does not seem too difficult, but there is still a serious underlying problem. This is the warped record induced output between 0.5Hz and 2Hz. from slightly warped records. This is a fundamental weakness in transformer inputs for phono cartridges. Mikes, OK, but phono is different.
If you doubt this, just put a DC scope across the input terminals of a phono stage playing a record. Now, can you take a picture, like a square wave, for example? Check it out.
In other words, you have no idea, you were just throwing it out there.
OK, let's estimate it. Worst case, 2 Hz from your numbers. Stylus displacement will be on the order of 1mm if it's a really bad warp. So velocity is 1mm/0.5s = 0.2cm/s, absolute worst case. For our typical cartridge with 0.1mV for 5cm/s, this translates to 4 microvolts. I don't think that's even close to the point where it would bother a transformer. The real distortion source there will be the cartridge if the stylus is being flailed around that much. I think FM as well.
I have a pretty good scope, but I don't think I can see 4 microvolts on it.
edit: For a more typical warp frequency, 0.3 Hz (once per revolution), we're now talking under a microvolt.
yes, it is correct for cables alone... the subject I addressed.... not for high cmr amp circuits.
The key in your words is "a well balanced input". With a well balanced input circuit... high cmr circuit... higher than you get with cable alone, you will reject more cmr signal. If one has a single-end input with low cmr then a balanced cable configuration is better than none. It leads right into what you say. you need a well balanced circuitry to get benfits of cmr rejection.
[Bill Whitlock is not an authority to me... go back to the original sources for that subject which i have - somewhere- given references to]. I'll post something from them later... got work to do.
Thx for allowing me to fine tune comments. -RNMarsh
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