summing large amounts of channels.

Help Support GroupDIY:

ruffrecords

Well-known member
Joined
Nov 10, 2006
Messages
13,931
Location
Norfolk - UK
Winston O'Boogie said:
I think slugged might be a Brit term.    I've had a few sceptic tanks look at me funny when I said it.  Dunno?

Yours,
Slug.

Funny how you take a lot of colloquial English for granted. The alternative term might be to 'pad' it with so much resistance.

Back at Neve in the 70s my boss was a very nice but very vocal man. One day I bought him a packet of sticky toffees. I gave them to him and said "! Hey Tony, here's a packet of 20dB pads"

Cheers

Ian
 

emrr

Well-known member
Joined
Apr 12, 2006
Messages
7,915
Location
NC, USA
ruffrecords said:
Funny how you take a lot of colloquial English for granted. The alternative term might be to 'pad' it with so much resistance.

Back at Neve in the 70s my boss was a very nice but very vocal man. One day I bought him a packet of sticky toffees. I gave them to him and said "! Hey Tony, here's a packet of 20dB pads"

Cheers

Ian


Haha ouch!
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
21,408
Location
Hickory, MS
Winston O'Boogie said:
I think slugged might be a Brit term.    I've had a few sceptic tanks look at me funny when I said it.  Dunno?

Yours,
Slug.
Slugged is common vernacular for bending pot tapers with resistive loading.

JR 
 

pucho812

Well-known member
Joined
Oct 4, 2004
Messages
12,588
Location
third stone from the sun
Winston O'Boogie said:
As far as I  could tell, Pucho (and the folks at the studio)  are happy with the desk as is with 990 op-amps.

Seems counter productive to be recommending a change over to some "better" scheme,  I don't think he's interested in re-engineering the whole desk, but I could be wrong?
Maybe he already lost the will to pick up a soldering iron after reading that digital summing is the way to go. 
     

I have not lost any will... I am offended at such a remark.

believe you me my boy,  soldering is second nature and has been for decades. As is doing any of the maintenance work at the studio.  Our desk does require it at times, especially since no one has looked after it for years prior to me. I proposed this question like all the others to further understand what would be best practices for doing "insert something here". As always there are trade offs. If there is an end goal here outside of learning more stuff, it would be to do a desk that 48 channels and 24 monitors. so in total  possibly summing 72 channels down to stereo,  72 channels down to an aux output and so on. As always there are things to consider when doing such as a task so might as well ask people way smarter than me on this who have ventured down this road before.
 

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
13,456
Location
Marcelland
Going back to the root of the subject, what mix scheme is used in this mixer? Voltage-summing or VE? Unbalanced or balanced?
What is the noise spectrum, hum, buzz, hiss, whatever? A schemo would help.
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
21,408
Location
Hickory, MS
Winston O'Boogie said:
As is probably obvious,, my dig wasn't really aimed at you Pucho, I've never doubted you'll go down fighting, soldering iron in hand :)
?
As for the distributed schemes I posted tables for - the Focusrite desks you referenced used it, the big  Neve VR's did, as did some higher end Soundcraft desks. 
I addressed this in my 1980 console design article.
I'd personally like to hear from any of the folks here that say this scene won't  help noise from the summing amps to show why 'Small Signal Audio Design' got it wrong. 
As Abbey's signature says, it isn't about who's right, but what's right. 
Not wrong, but perhaps not significant...  As has been shared by more than just me, other noise sources will dominate the noise floor in use, so don't get fixated on optimizing that one variable (bus amp ein) while ignoring the rest.

Distributing across multiple bus amps would also improve the phase shift/distortion issue caused by inadequate loop gain margin, that I am actually concerned about.

Extensive modification could hurt the resale value of the console (but perhaps that ship has already sailed). Limiting tweaks to just the master bus sum amp would be easier to unwind, and keep the strips relatively stock.  I have already given my best suggestions and even I am tired of hearing them, so I won't bore ya'll .

JR

 

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
13,456
Location
Marcelland
pucho812 said:
If I had schematics, I would gladly share  ;)
OK, but what brand is it? Someone may have more info. I think you can determine without a schemo the value of the feed resistors and confirm the bus are balanced or not. And you didn't answer my question about the noise character (hum, buzz, hiss...).
You have been quite vague, you're not helping us helping you.
 

pucho812

Well-known member
Joined
Oct 4, 2004
Messages
12,588
Location
third stone from the sun
abbey road d enfer said:
OK, but what brand is it? Someone may have more info. I think you can determine without a schemo the value of the feed resistors and confirm the bus are balanced or not. And you didn't answer my question about the noise character (hum, buzz, hiss...).
You have been quite vague, you're not helping us helping you.

You have helped me plenty, Knowledge is good. The studio desk is fine, no hums buzzes, hiss, our any other unwanted noises, yes it does 990 amp blocks for auxes, busses and stereo bus. it's nice.  literally I asked to learn more stuff.  ;)
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
21,408
Location
Hickory, MS
Winston O'Boogie said:
I wasn’t suggesting at all that lowering the noise from the summing amplifiers is also a panacea for the noise from channel amplifiers and  noise from mix feed resistors plus other areas.  These, and other improvements, are separate issues.

Distributed summing will give a very definite noise advantage though  (7dB  here),
perhaps in theory with bus feeds shorted, not using real world sources.
  as well as relax the requirements on the  loop gain which is what you acknowledged as being a concern.    Having locally summed blocks also alleviates the issue of the VE buses extending over the full length of the desk.
is bus length really an issue...? Bus capacitance "can" be an issue in extreme cases (like marginal stability).
I don’t know the whole details of Pucho’s desk but he indicated the summing amps were switched to Jensen 990’ s after the mod was done to extend it with extra channels.  He also wants to keep using those 990’s
Good practice when adding a side car, or adding a number of extra channels to an existing design is to sum them separately with their own bus sum amp. This will not degrade the performance of the original design.


If we knew the details of the bus feed resistors etc., suggestions could be made there as to  whether the values might  be reduced to lower noise from there.  There could be another 6dB or so improvement to be had, depending on what’s in the desk already. 

Etc., etc, rinse and repeat.

It is easy to make what-if suggestions for other people to try modifications....  Another 6 dB improvement?? I am not convinced you will even get the first 7 dB (in practice) let alone 13 dB total.  Console design is not rocket science, but it is science involving multiple variables.

JR
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
21,408
Location
Hickory, MS
Winston O'Boogie said:
An old desk may have relatively high mix feed resistors.  Some I've seen are 20K.  If it's possible to reduce these by 3 times, (let's say that the pan-pot is buffered so the law on it wouldn't be changed).  then Johnson noise is lowered by square root of 3 so, a little over 5dB.

It's not out of the question that on a man stereo bus, you lower the resistors even more to 4K7. 

Q.E.D.
The Johnson noise of the bus feed resistors are not the controlling parameter so reducing that to zero will still not make any difference to the actual noise floor. 

Go ahead and modify your (or any) console's sum bus resistor's values (I used 2.4k in my last big un, but that was to manage crosstalk, not Johnson noise).....

provide the before and after noise measurements of your modified console (as actually operated). I will be glad to admit I am wrong , if I am wrong.

JR
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
21,408
Location
Hickory, MS
Winston O'Boogie said:
This wasn't done on the desk Pucho asked about though was it, hence why he asked. 
And there is also a preferred arrangement of breaking down the sub-mixing that results in lower noise.
I posted an excerpt from a book on it. 
and there are quickly diminishing returns from the cost complexity.  IMO A separate bus amp for the extra added channels will preserve the design integrity of the original design
You may have not bothered to read it though since you yourself  wrote the definitive article on mixers in the 1980's and nothing anyone else has said on the matter since is of consequence.
And did you read the article I linked to?  I shared that concept and a few more (even better ones.)
I'm out.  Good luck Pucho.
Promises promises...

JR
 

zamproject

Well-known member
Joined
May 11, 2010
Messages
1,098
abbey road d enfer said:
I don't know why this is turning sour. Pity. Several good contributions. Could have almost been made a sticky.

I nether get it  :eek: I was following this topic with interest

With all respect due, I don't understand why JR ( I'm generally pleased to read your contributions !) with moderator responsibility,  publicly challenge a member ?

(I probably miss something as English is not my main language)

Best
Zam
 

madswitcher

Well-known member
Joined
Feb 6, 2010
Messages
287
Location
Buckinghamshire, UK
Hi Pucho,

If you can, get hold of a circuit diagram of an EMI TG console, or even the Users Manual.  The designer) used a transistor as a voltage to current converter for the channel mix output and a simple virtual earth amp which simply summed the currents.

Simple, no mix resistors and did the job effectively.

Regards

Mike
 
Top