Best Console Summing amp/Master Buss comp Ideas....

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JohnRoberts said:
leigh said:
Would a load being driven to a virtual earth input count as being "driven to ground", or as "driven differentially"?
It would count as driven differentially since the AC input current is matched by current supplied by the feedback resistor, which comes from the op amp output and ultimately the PS rails, so no current flows into ground for that example. 

JR
That's not my analysis. If the PS rails were infinitely stiff, the current would be flowing into the PS 0volt, but they are not, and the decoupling caps are there to provide a stiffer path  to whatever "ground" they are connected to, thus diverting the current(s) to this local "ground".
In fact, to my knowledge, just about every mixer has either protectyion resistors or local regulators that definitely prevent local currents to be reflected to the PS rails.
 
abbey road d enfer said:
JohnRoberts said:
leigh said:
Would a load being driven to a virtual earth input count as being "driven to ground", or as "driven differentially"?
It would count as driven differentially since the AC input current is matched by current supplied by the feedback resistor, which comes from the op amp output and ultimately the PS rails, so no current flows into ground for that example. 

JR
That's not my analysis. If the PS rails were infinitely stiff, the current would be flowing into the PS 0volt, but they are not, and the decoupling caps are there to provide a stiffer path  to whatever "ground" they are connected to, thus diverting the current(s) to this local "ground".
Yes but.....  You could make the same exact argument for differential loads, while in theory that PS ground current would net out to 0. .

I see a difference here between a grounded load and  VE summer since the current is not flowing directly into an actual ground trace, but directly into the PS rails. Where it goes after that is a slightly different consideration. 

As I posted earlier with my AM radio station anecdote signal ground and power supply ground can be too separate for system stability. 
In fact, to my knowledge, just about every mixer has either protectyion resistors or local regulators that definitely prevent local currents to be reflected to the PS rails.

I have used per channel regulators and flameproof resistors to provide system robustness. A single channel failure should never take down the whole console.

I advocate always following the current. It is shame that many sensitive PCB layouts are made by mechanical PCB artists and not the circuit designers who have more appreciation for subtle current routing issues, but major layout problems generally reveal themselves during prototype development and testing. In critical circuitry it is not unusual to create multiple schematic grounds (or 0V nodes) that ultimately get connected together.  This can annoy PCB artists... ;D

Driving faders. pan pots, and send pots, will generate signal related local  0V (or local ground) currents that need to be accounted for so they do not corrupt other channels causing crosstalk.

I even killed some brain cells back in the day trying to envision a topology where all local channel signal currents netted out to zero. I gave up because it seemed impossible (or at least very impractical), so a little bit of local brute force is indicated to maintain signal integrity. When sending signals across any distance they will often benefit from differential treatment (IMO).

I try to avoid sweeping pronouncement about grounding (even while making them)  8).  I don't even like to think of ground as part of signal flow while is clearly is impossible to ignore.

JR
 
JohnRoberts said:
I see a difference here between a grounded load and  VE summer since the current is not flowing directly into an actual ground trace, but directly into the PS rails.
That's where I beg to differ. Since there is a not unsignificant resistance and inductance between the opamp supply pins and the actual hard-regulated PSU, most of the AC currents are diverted via the decoupling caps to the closest "ground".
Indeed if there are on-board regulators, they may offer a stiffer path than the decoupling caps and reflect the currents back to the PSU.
 
abbey road d enfer said:
JohnRoberts said:
I see a difference here between a grounded load and  VE summer since the current is not flowing directly into an actual ground trace, but directly into the PS rails.
That's where I beg to differ. Since there is a not unsignificant resistance and inductance between the opamp supply pins and the actual hard-regulated PSU, most of the AC currents are diverted via the decoupling caps to the closest "ground".
My apologies for this veer...

The AC current sunk by the VE input does not go to the "closest" ground but to the dedicated PS common for that op amp's power supply source(s).  In fact if multiple caps are on those those PS rails, current could be coupled in to different parts of that ground trace in different amounts.

The clean signal ground or 0V for that sum amp will ultimately connect to the PS common, but IMO  it is not necessarily the same exact circuit node. 
Indeed if there are on-board regulators, they may offer a stiffer path than the decoupling caps and reflect the currents back to the PSU.
As I mentioned I have though about this (albeit, years ago) and whenever we send audio signals single legged from a strip to elsewhere inside the same console each send creates a small ground current error between the local PS ground and other local PS grounds, There can be a dozen different single legged sends coming from an input strip going to other different channels strips each with their own local PS.

One thing I liked about my current source summing was that it literally sent currents that were independent of the receiving end's voltage.

Note: If I were designing a broadcast console (I never have) that has much higher crosstalk requirements, I might be tempted to distribute signals around as dual-polarity pairs so the ground currents always netted out to zero.

JR
 
JohnRoberts said:
As I posted earlier with my AM radio station anecdote signal ground and power supply ground can be too separate for system stability.
This is an important point and it also affects RFI.  I explain how to deal with one simple case in my MicBuilders LNprimer.doc

[quote author=JohnRoberts]I even killed some brain cells back in the day trying to envision a topology where all local channel signal currents netted out to zero. I gave up because it seemed impossible (or at least very impractical), so a little bit of local brute force is indicated to maintain signal integrity. When sending signals across any distance they will often benefit from differential treatment (IMO). [/quote]leigh, it should be pretty obvious that some of us have spent much time hunting this unicorn.  While you can do it for eg a simple preamp, a perfect grounding scheme for a large mixer is probably Unobtainium.

However, those of us who agonise over such matters, usually design & build better stuff than those who are ignorant of such mythical creatures.

I'll throw in another 'philosophy' from my LNprimer to confuse you further  ;)

Certain EQ circuits will unavoidably inject stuff into our Clean earth.  For these, each stage is connected to the Clean earth at a point in the order in which they occur.  This is Heirarchical Earthing like the busbar approach used in old valve gear.  The idea is that later stages do not contaminate earlier stages.

Like all the principles here, they do not exist in isolation but must all be part of an integrated approach.

http://edcorusa.com/boards/topic/183/tn42-hum-and-noise-in-a-circuit

Best description of Heirarchical Earthing I’ve come across.  He calls it “ground connection order.”

Some of his thinking on earths vague but this is typical of the valve era with their bus bars.


Various people have pointed out you can only do so much with an existing console.  Kingston who achieved some very worthwhile gains, tells us the final PCB was a mess of mods.

I think your approach, checking at each stage/mod to see if it improves things, is the correct one.  Keep at it.
 
JohnRoberts said:
The AC current sunk by the VE input does not go to the "closest" ground but to the dedicated PS common for that op amp's power supply source(s).
That's exactly what I meant. Thanks for re-phrasing it more precisely.
In fact if multiple caps are on those those PS rails, current could be coupled in to different parts of that ground trace in different amounts.
Indeed the currents will be shared according to the relative "stifness" of the various decoupling points.
current mixing.
In the early 80's I had made exactly the same suggestion to the two console manufacturers I have been working with, Midas and Soundcraft. While the former were interested but never pursued because they thought the improvements would not be worth the added complexity, the latter simply dismissed the idea because it was not "made in house". When I submitted the same idea to THAT, when they were looking for ideas for expanding their product line from VCA's and RMS detectors, they just considered that the large console market was dead. I guess they can't be blamed.
 
abbey road d enfer said:
In the early 80's I had made exactly the same suggestion to the two console manufacturers I have been working with, Midas and Soundcraft. While the former were interested but never pursued because they thought the improvements would not be worth the added complexity, the latter simply dismissed the idea because it was not "made in house". When I submitted the same idea to THAT, when they were looking for ideas for expanding their product line from VCA's and RMS detectors, they just considered that the large console market was dead. I guess they can't be blamed.
I mentioned the current source summing in my console performance article published in RE/P magazine back in 1980. Some in the industry didn't believe it would work. I won't name drop but one well respected console designer shared with me at a trade show years later that he used it in a broadcast console he designed for a small british console company and it worked fine.

I may have mentioned this before, several years ago I talked to an engineer at THAT corp about repurposing the resistors already in their balanced line drivers to make a precision current source and they would have done it for me if I paid for the cost of a new metallization layer (a few tens $k). They were apprehensive about making a current source chip for commercial release because they did not have enough protection circuits already in the silicon to protect every pin from the outside world, so a proper release with full protection would require a complete scratch silicon design (millions$).  Not to mention there would be little market for them. THAT knows from their VCA sales how the analog console business is going. 

As i have shared before this is all academic at this point. Bus noise with modern ICs is already well below  mic/room noise (actual benefit is more phase shift and distortion reduction).  Analog consoles are archaic technology. I have some friends still trying to sell analog consoles and that market segment is really difficult these days and never getting better (IMO). Finally the last nail in the coffin, digital summing can be even better than my very good analog current source summing. I wish it wasn't. Then I'd have something to sell.  8)

JR
 
JohnRoberts said:
THAT knows from their VCA sales how the analog console business is going. 
Coincidently SSL has just announced their latest "large" analog console, which has about 20 channels! Improving summing amp performance in a 100 channel mixer is a valuable task, but if the future of analog is 20 stems, it's not really worth the hair-pulling.
As i have shared before this is all academic at this point. Bus noise with modern ICs is already well below  mic/room noise (actual benefit is more phase shift and distortion reduction).
I know it's a pet peeve of yours, and I agree that on a purely metrologic point of view, you are absolutely right. But consoles are not only measured, they are used by people who have only their ears as measurement tools, and noise is something that anybody can detect and instinctively know is not good. I have personally never met anyone who complained namely about phase-shift and/or HF distortion, but certainly a large number who could express preference or dislike for certain mixers based on the bus/summing amps noise performance.
Finally the last nail in the coffin, digital summing can be even better than my very good analog current source summing. I wish it wasn't. Then I'd have something to sell. 
Maybe you could start a campaign, with the help of a few recognised names in the industry, explaining that digital mixing cannot be as good as analog, because of the stairstep effect and the loss of micro-details due to truncation and blah-blah... oh, but, I think I've heard it somewhere... 8)
 
ricardo said:
leigh, it should be pretty obvious that some of us have spent much time hunting this unicorn...

It is indeed. And glad to see that healthy debate continues, even between such experienced folks as yourselves!
 
Thought I would post this here, since I repeatedly sang the praises of the Texas Instruments LME49710 opamp for a summing amp. I just got notice from Mouser that the LME49710 (as well as the quad version, the LME49740) has been "Scheduled for obsolescence and will be discontinued by the manufacturer."  :'( :'( :'(

http://www.mouser.com/Search/ProductDetail.aspx?R=0virtualkey0virtualkey926-LME49710NA/NOPB
 
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