CMRR

[ccaudle]

So if we want to use two or four core screened cable unbalanced which is the correct end to connect the screen , instrument or amp end of the cable ?

For balanced connections you want the source end of the screen connected. I think that would apply to unbalanced as well, the principle is that the screen is connected to the reference voltage (aka "ground") in the source, so the common mode voltage of the source and the potential of the screen would move together, effectively cancelling the common mode part of the capacitance.

Electric guitars are getting difficult to record with all the RF mush floating around , for the the extra freedom from noise it might be worth a try , ok we take a hit on cable capacitance compared to typical low noise single core guitar cables .

Star quad is primarily effective at low frequencies, so depending on how loose you are with your use of the term "RF" it may not help at all. For interference in the high MHz range like wireless devices, connecting the shield at both ends with a low inductance connection, and filtering the signal lines are the most effective remedies. Star quad is most effective for things like transformer induced noise, or power line noise from lighting dimmers, motor controllers, etc.
And of course it usually doesn't matter what kind of cable you have if you are using single coil pickups, the noise induced in the pickups is usually going to dwarf anything induced in the cable.

 

[DKatz]

Opamp circuit precision relies on loop gain. What mechanism needs elaboration? Dominant pole compensation? Gain error effect on CMRR? Or you can simulate it, or look up data sheets. This is THS3061 for example:

 

[abbey road d enfer]

Opamp circuit precision relies on loop gain. What mechanism needs elaboration? Dominant pole compensation? Gain error effect on CMRR? Or you can simulate it, or look up data sheets. This is THS3061 for example:

Had you followed the thread, you would have noted that the subject is CMRR in a balanced connection, not for an opamp.

 

[JohnRoberts]

Not to feed the veer... at 7,000 V/uSec and 300MhZ GBW your THS3061 won't easily slew limit on audio.

JR

 

[DKatz]

Had you followed the thread, you would have noted that the subject is CMRR in a balanced connection, not for an opamp.

I did see that. Balanced input to ADC. Having designed analog and mixed-signal ICs, I'm fairly sure those feed (hopefully through buffers) into an opamp-based diff amp that will limit high frequency CMRR.

 

[abbey road d enfer]

I did see that. Balanced input to ADC.

I believe either I have a problem reading you or you don't understand what CMRR is in the context of a balanced connection.
In a balanced connection CMRR is dependant on the balance of impedances on each leg.
In most cases, whatever impedance variation due to roll-off exists will be the same on both legs.
Particularly, in the context of DAC's, their differential inputs have a CMRR in the vicinity of 100dB@1kHz that may roll-off by 26dB at 20kHz.
OTOH an impedance unbalance of the source of only 10 ohms result in the CMRR degrading to 60dB. So clearly the principal subject of attention is not silicon.

I'm fairly sure those feed (hopefully through buffers) into an opamp-based diff amp that will limit high frequency CMRR.

Typically, ADC drivers are 100% symmetrical, so what ever effects one side is mirrored on the other side, resulting in almost perfect cancellation.

 

[JohnRoberts]

Perhaps we have different expectations regarding what the HF is in "HF" CMRR... Mr Katz shared a plot from a 300MHz op amp. Indeed pretty much every performance parameter about NF circuits degrades with increasing frequency if we look high enough. Most A/D differential front ends include LPF to scrape off way above audio band signal.

I know just enough about IC design to know not to pick arguments about it.

I guess for difficult CM situations one could feed + and - signal legs into separate A/D inputs and then digitally subtract them inside the digital domain.

JR

PS; just to keep track of how far this veer has carried us, the OP was asking about microphone designs.

 

[Tubetec]

Thanks for the great replies .
The Cosmos ADC has instrumentation op amps at the front end but presents a low impednce 600ohms-13kohms depending on the attenuation settings . Its designed with measurement in mind without any limitations of the bandwidth you find in the usual audio offerings .
The internal settings of the ADC for balanced /single ended are accessed in a software control panel along with other calibration parameters .

Just for comparative purposes I checked another mic cable ,
5.5M lenght.
544.3pf core to core ,
906.1pf pin3 to shield
956/1pf pin2 to shield
Measurement was made with LCR from cold,

So again around 5% tollerance in the capacitance of the conductors to shield , we could be busting our hump to make our ADC with precission components everywhere and the 1 meter patch cable is degrading our CMRR by what , nearly 30 db at 20 khz Abbey? If were measuring right upto around 200 khz probably even lore losses

Belden 8412 cable I used in the examples I measured the worst performer in the group tested in the cable shoot out, but may be a fair representation of your prized collection of mic cables ,

I'd expect quad core cable of modern contruction is likely to be better balanced across the phases for capacitance, maybe 1-2% if were lucky .A 50pf trimmer cap would allow a good degree of compensation on a standard mic cable over a lifetimes useage , although in shorter test cables smaller values might make trimming more precise . It becomes an easy job to check and maintain the integrity of your cables once in a while , a small hole can be drilled in the XLR shell to permit the use of an adjusment tool . In the context of the testing environment you can easily adjust cable capacitance in the xlr while monitoring on screen in your favourite FFT/Spec or other audio measurement software which is neat.

Of course SMD size components make this possible , before you'd have a great big dirty lump at the end of the cable , but its also an easy retrofit for old cables that are a bit tired from getting trampled on .
The Murata component below looks like it would sit nicey across the pin 1 and 2 of the inside the back end of the XLR plug, a plus is the hole for adjustment would appear at the top when plugged in ,not at some awkward angle.

 

[abbey road d enfer]

The Cosmos ADC has instrumentation op amps at the front end but presents a low impednce 600ohms-13kohms depending on the attenuation settings .

My understanding is that there is nothing between the XLR's and the resistors. It's the buil-in diff amp that is responsible for rejecting CM.

Just for comparative purposes I checked another mic cable ,
5.5M lenght.
544.3pf core to core ,
906.1pf pin3 to shield
956/1pf pin2 to shield
Measurement was made with LCR from cold,

So again around 5% tollerance in the capacitance of the conductors to shield , we could be busting our hump to make our ADC with precission components everywhere and the 1 meter patch cable is degrading our CMRR by what , nearly 30 db at 20 khz Abbey? If were measuring right upto around 200 khz probably even lore losses

With a typical source of 100r impedance and a receiver of 20k, the CMRR would be 72dB at 20kHz, 52dB at 200k, assuming all the rest is perfect.
For precision measurements, adjusting CMRR is as essential as calibrating probes on an oscilloscope.

 

[Tubetec]

Where you want to balance capacitance in a cable but adjustment isnt required a ceramic disk cap with the appropriate value or near enough could be used , super simple adjust on test ,put in the nearest of 12 commonly available values between 10 and 100pf and be done with it , its still a big improvement in any case . The lower the tollerance of the components the better chance of finding something suitable from a pile for a given cable , so no need to waste money on fancy stuff, +/-20% is better in this case.

Thanks Abbey ,
So if I add a 50pf cap to my old cable between pins 1 and 3 now we have approaching balance in the capacitances
how much better is the picture regarding CMRR?

 

[Tubetec]

Maybe even adding an extra trimmer cap between pins 2 & 3 so the total conductor to conductor capacitance of the cable can be brought to a known value along side its stereo counterpart , most certainly what you want for measurement maybe even for recording too , to be sure cable A and cable B are exactly the same in terms of their electrical performance.

For clarity I'll add a drawing although I think it applies to any balanced cable. placing the compensation components at the output end .

 

[abbey road d enfer]

So if I add a 50pf cap to my old cable between pins 1 and 3 now we have approaching balance in the capacitances
how much better is the picture regarding CMRR?

If you manage to balance the two legs within 10pF, the CMRR becomes 76dB@20k, 56dB@200k.
Again, law of diminishing returns.

 

[Tubetec]

The trimmers are a few euros each and a ball park figure for over all capacitance is very adequate ,putting the trimmer on pins 1 and 2 for balance then a ceramic disk to bring the cable to the target value on pins 2 and 3 makes better sense and we take a good bite back from diminishing returns .

By trimming the cable in situ on the test bench I think matching better than 1pf is possible .

Below is the noise floor of the cosmos , theres a steady rise in noise above a couple of khz right upto 180khz,
between balancing the capacities and the quad core cable construction you stand to gain upto 25db reduction in CMRR , that should make an impact on the noise floor results. .

I was thinking of using these great mumetal 30/50 ohm to very hi z(1:50) mic input transformers I have in reverse as passive D.I. to ADC . It wont hit anywhere near full scale digital on the output((1.6volts,600 ohms load on the cosmos), in any case noise from the instrument itself should swamp any noise in the ADC , the bonus is theres huge headroom to play around with .


Did you get the Cosmos windows tweak app going Abbey ?
It seems to allow you to compensate for the harmonics depending on source for best performance .
I wonder is it possible to tune an ADC for music recording purposes for best performance at
say -18dbfs instead of tuning around 0dbfs when testing .

 

[Matt Syson]

While having 'exact' cable characteristics and LENGTHS is important for 'competent video'
work it seems a lot of effort is being expended on what may be CMMR 'aspects when fiddling with cable capacitance differences may well not be the limiting factor for a given 'installation'.
While it is nice to get something right it is also good to consider what 'benefit' if any will be seen even it it were perfect. Like being in the 'rat race', even if you win, you are still a rat.
Matt S

 

[JohnRoberts]

A chain is only as strong as the weakest link, as Bill and others have shared the balance/matching of both ends matter.

Of course nothing wrong with trying to make the middle perfect but perhaps lower expectations.

JR

 

[FIX]

When I did the second revision of the 318a distribution amp, used for ABC and NBC, the spec from both networks was at least -100 at 90KHz. We typically his -105 with a cap and resistor trimmer. After that point the bandwidth of the amp pretty much took care of anything higher.

 

[FIX]

The studio was located in the basement of the lawyer's building who backed the studio, so punting was not an option. After I fixed the console I helped them with several other problems as much as I could (on day two). They had AM radio pickup in their plate reverb and sundry other studio units.

JR

Most new home builders have a line that says "occupation" so they can charge an extra 50K for anyone that is a lawyer, to pay the inevitable legal fees that follow moving in...

Our studio had a plate that used to pick up a cab driver that had a linear CB radio in his trunk with a motor driven power supply. You could hear it spin up and then hear his whole transmission. We finally zeroed in on which cab it was and went down to him and said "either let us fix this problem, or we call the FCC" We actually pulled his power back, as he was clipping the transmitter and spraying shit everywhere...

 

[abbey road d enfer]

Did you get the Cosmos windows tweak app going Abbey ?

I just read about it and concluded I wasn't interested. My understanding is that it can be used to minimize 2nd and 3rd harmonics for a given level. Cosmos ADC comes factory set for -5dBfs, which is perfectly fine by me.

 

[abbey road d enfer]

Most new home builders have a line that says "occupation" so they can charge an extra 50K for anyone that is a lawyer, to pay the inevitable legal fees that follow moving in...

Sorry, what's the connection between lawier and RFI?

 

[JohnRoberts]

Sorry, what's the connection between lawier and RFI?

my post about fixing a console with rfi in a studio that was backed by a lawyer....

JR