Using Neve 1272 Line Amps Like they were meant!! (Summing...

Ian MacGregor

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I want to build a 8-stereo channel (maybe more, but let's start with 8) summing box using Neve 1272's (real or DIY'ed) for makeup gain. I am kinda familiar with how passive summing box's work (like the Folcrom); is this the same concept? Using the 1272's kinda how the Folcrom uses mic-pres? Except that 1272s were designed for the lower gain range?

Ok, so here's what I'm thinking: Have each pin 2 and pin 3 on the input XLRs connected via a summing resistor (5k??? have no idea...) to the primary of the 1272 input xformer. All the pin 2s will go to one leg of the primary and the 3s go to the other leg. My main question is how do I know how much gain I need to have in the 1272? It would be cool to have some sort of trim control or something as well. I'm very open to doing this the way Neve would have as well, even if the noise/distortion isn't steller.

Anyway, lots of questions...

Ian
 

Ian MacGregor

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Here's a... ummm.. rough idea! Any thoughts?
1272_SUMMING_BOX_ROUGH.gif


Ian
 

fallout

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Wouldnt that be more or less the same as building New York Dave's LCR summing circuit and patching it into some Neve 1272's? I would think being able to patch the LCR summing circuit into any preamp you want would be more desireable because more you'd have more options... Hmmm... Today I feel like a 1272, nah maybe an API 312..... Or am i missing something?

-Jay
 

Ian MacGregor

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Ehh.. I guess you are totally right. I was just thinking it might be cool to have it all in one chassis, sorta like the dangerous 2-buss, but Neve-ish. I guess if I wanted to get really wicked I could put a switch that could select which line-amp to use :twisted:

I need to dig up NYDave's awesome rundown of the summing stuff...

Ian
 

fallout

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Yea, I hope New York Dave's posts are intact somewhere, as I was planning on doing this myself.. I do remember though, if you are going to patch this summing circuit into a mic pre you need to pad the output down to mic level. I'd definately rather have a summing bus that I can patch into whatever amps I'm feeling that day. Some mixes might call for different amps. I'll probably just use a patch panel to change amps...

-Jay
 

Plexibreath

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I'm interested in this too as I'm building an 8 pre rack of API 312 preamps. I'd like to take the outputs of that rack and feed them into a minimal signal path mixer I could build just to mix levels only to 2 bus. Is there a basic schematic of how this is done? I guess it's done passively then the amps are to boost the level back up? I had thought that summing could be done through the use of transformers so you don't have to step on the tone as much, I obviously have no idea. Enquiring Swines want to know :twisted: .
Kelley
 

fallout

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Your right!

http://www.technicalaudio.com/reading/neve_xfmrs/neve_edo7113.html

Not sure what i'm thinking of... Or if i'm thinking.... :idea:

-Jay
 

rascalseven

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Yes, the 10468 was used for mic inputs and the 31267 for line. However, the turns ratios are practically identical. You can use a 31267 'backwards' for a great Neve mic preamp.

Check out http://www.jlmaudio.com/Neve%20transformer%20info.htm for more specific info.

BTW, I have done this with the 31267 (Marinairs), and they sound awesome. Not only that, but the 31267's are much easier to find and cheaper.

Peace,

Joel
 

Ian MacGregor

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Ok, pretty cool. Does anyone know how Neve did this? Did they just have the channel outputs running via a resistor and into the 1272's input xformer? If so, anyone know what value resistor they used? And here's a complicated one: What happens when all 8 inputs are not being used?

Ian
 

rascalseven

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Actually, all the 80-series boards and BCM10's and all the class-AB-output broadcast consoles, etc. that followed were passive, voltage summing devices that used 1272's or 3405/3415's etc. for the 40dB or so needed for gain makeup.

And they were even unbalanced busses :shock:

:green:

JC
 

Ian MacGregor

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[quote author="Crusty"]Yes, the 80 series was the passive type mixing into one leg of the transformer, the other leg going to ground. It is a single ended system with an aluminum extrusion used as a shield running the length of the console. This was done on all busses with an additional copper buss bar running along the LF/RF-LR/RR and aux buss extrusions. This elaborate buss/shielding system was a BBC spec I'm sure, as these consoles were mainly sold to the BBC for broadcast purposes in radio frequency saturated environments.

I believe the summing resistors for the most part are 7k5, if I remember right. One sheet of my console docs has all the info, including buss impedance and levels. I can scan it if anyone's interested.

The bus impedance is critical when doing this kind of summing. I remember experimenting with running the buss psuedo-balanced: running resistors from the ground of the module to the other end of the transformer. Results were not good, as I remember. I have found that making sure there are no gound loops in the actual buss shield made the console extremely quiet.
This passive suming system has a definite sound to it and behaves differently from the virtual earth system. I think its an underated feature of the old Neves and contributes to their sound immensely.

Ian, the resistors are located on the buss bar, so when a channel is "unselected", the signal leg of the resistor is grounded, preserving the buss impedance.

Paul[/quote]

Wow! Cool info here. I would be very interested in any info you could scrounge up on the 80 series summing. Sounds like it might be a good idea to go unbalanced?

Ian
 

Moby

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Thats nice, but i think that you must take care that no dc comes on primary of tx...that is possible with coupling caps in front of summing, or connecting low side thru large cap to the ground... im talking about if you wish to connect some of "modern" DA w like motu 192 or lynx....there is no caps on the out and its possible to ruine ungapped tx like all mic input is...just thinking about ....maybe im wrong??
 

Moby

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And if you go with 7k5 summing res. what will be best primary impedance? I think that depends on number of channels :?:
 

Ian MacGregor

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Wow Paul! Waay cool. I appreciate you checking this out for me. I think the studio manager where I work just *might* have some sort of console documents. I'm gonna see if I can dig 'em up and scan em. Some pictures would be RAD!

I have another general question tho: I would really like to set up a box that does the summing just like the Neve 10 series console, unbalanced and everything. I am planning on summing the balanced outputs of a PT HD 192. Can I just use the + and ground from the 192 output and disregard the - output?? Or do I need to have a balanced -> unbalanced converter (op amp) on every summing buss input channel?

Thanks again,
Ian
:sam:
 

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