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

[Moby]

About DC...I'm sure that you will destroy mic input TX if you put dc on it... that's not question, that is fact... from my knowledge hd192 has no output caps, so there is real for some dc to come on primary.... I planned to make some custom input TX for that purpose. That will be gapped step up TX , not so small , but who cares, I have all the space in the world :grin: so I'm thinking of about 150-600 ohm gapped TX for input... maybe more step up, depends on agronomy of windings. In that case I'm gapped and able to handle DC :wink:

 

[Bjorn Zetterlund]

The output of your PT unit will probably want pin 3 grounded; most modern designs prefer that. Sometimes you get electronically balanced outputs where connecting pin 3 to ground will basically short an op-amp output, which is not a good thing. As was already pointed out, you may get into **** with DC offsets unless your unit has decoupled outputs.

Bjorn

 

[Moby]

all modern chip based sym drivers like bb DRV has small DC only in balanced mode. In unbalanced mode they act very bad from DC side. Say about 20-30mv . Simple calculation will show that "hair diameter wire" like is ins mic TX cant handle that current. Not to mention HI MU cores. So that's why I'm talking about coupling and maybe gapped version. But like to hear other opinion. :grin:

 

[Ian MacGregor]

Well... I guess I could use a nice op amp to unbalance and then couple the ouput of the amp with a capacitor to the input of the 1272? What do you guys think would be best?

Ian

 

[Tekay]

You can read more about Neve's summing here!
http://lists.topica.com/lists/neve/read?sort=d&start=50

If you're using a 31267 in reverse with all windings connected you will have a peak around 20kHz and then a fast roll off! ( depends on the cap value you're using over the sec, winding )

When your using a 31267 as a line input it will have it's -3dB point 5- 10kHz above the 10468!

 

[Technetium]

ok this DC thing has got me nervous...
i'm in the process of making a VERY simple 8ch summing 2 bus w/ 5k resistors on hot & cold, & .1uf cap on ground. i was planning on using my UA 2-610 as makeup gain for it...will i have to worry about ruining the mic input transformers in my UA pre? or will i be ok since i am going balanced?

btw i'm coming out of a MOTU 828MKII, if thats any help.

thanks,
-dave

 

[Tekay]

Hi!
Normaly Neve calculated their busses for a 1200 ohm input impedance for the mix amp! A thumb rule is that the buss impendance is 1/2 of the mix amp impedance!

With a Neve styled makeup you don't need the first ampstage in the 1272 for this! outputstage incl output transformer is +15dB and 6 more dB from the input transformer, 6k8 ohm to boost the gain 1dB and 3k3 to boost 2 dB.

Your Motu has less than 100ohms output impedance and with 8 channels balanced build it balanced and use 2k2 for each +/- you will then end up with 377 ohm and the gain loss is -22dB
An unbalanced 8 channel buss with 4k7, total imp = 394ohm, gain loss -22,2dB

Goodluck!

 

[Technetium]

hey tekay,
thanks a lot for the info!
i'll let u know how it goes.
-dave

 

[Steffen]

ops...deleted....didn´t read the full thread....
steff

 

[Moby]

2k2 for each +/- you will then end up with 377 ohm

Tekay, i think that A/B class op amp cant drive that impedance. Im afraid that less then 10k is real pain for that small chips I believe that NE5534 can drive 600ohm , but how that sounds.... Again about DC... I dont understand why not go with balanced summing? And if input TX is centar tapped its possible to connect CT thry capacitor to ground... value depends from impedance of primary, and its not small cap, but thats only one, not 8 or 32 caps for every channel.

 

[Ian MacGregor]

Ok, here's what I think you guys are saying. I have no values for R's or C's, just trying to get the general idea so far. Please feel free to critique, modify, flame...

This is just for example, 3 channels going into one Neve 1272 I/P Xformer:

Thanks,
Ian

 

[Tekay]

Yes! This is the circuit I'm thinking about! But you won't need the caps!
In a 8 channel unit he sours will see 2 x 2k2 + the total impedance = 4k8 and I don't think that this should be any problem for the output drivers in a Motu or similar unit!

 

[Ian MacGregor]

You wouldnt need the caps becuase each side of the transformer is seeing the same DC right? So no DC current flows in the Transformer primary?

Now I'm starting to get it. What gain would the 1272 want to be and does this gain want to be variable, depending on how many channels are being used??

Ian

 

[Moby]

Looks ok, I suggest to place resistors near TX so, make some PCB with resistors and caps on it... that's also NEVE :grin: Tekay, yes from electronics point of view I agree, but that's the matter of taste, I simply don't like chip op-amps running under 10k impedance (crossover). For "lower" impedance I like class A and current capable drivers. Anyway, this will wok cool :thumb: