Attenuator / mixer

[mick]

I don't know if this has been discussed before, if it has maybe someone would kindly point me to the thread. I'm finishing up a load of 312's and then I will be starting some Neve's, that will give me about 16 micpre's, none of these will have output level controls on them. One of the reasons other than sound for making these was to bypass my desk (soundcraft studio) but if the output of these pre's (as I would like to drive them a bit) is to loud I might be in trouble when it comes to my DAW interface. So I was thinking about some kind of attenuator, but I don't have a clue where to start, what would be the best type/way to go about it, then I thought well I'll need about 20 of them and realised I was now thinking about some kind of mixer, didn't take long for things to get out of hand, imagination is a dangerous thing. Basically my idea is to have a balanced line in from the micpre's going to the attenuator, the signal is then split, with one balanced line out to go direct to my DAW, the other going to a pan pot and then on to a mix bus. What would be the best way to attenuate the pre's ? I mean I would have thought clean with plenty of headroom no ? and then I suppose opamps would be best for splitting the signal to get it to a mix bus ? Oh the questions are endless, so if anyone could help me out here on where to start maybe I can get my head around it, here's a block diagram of what I think I'm trying to achieve.

http://www.twin-x.com/groupdiy/albums/userpics/normal_Trackmixer.GIF

Any help would be appreciated

Mick

 

[peter purpose]

That's quite an undertaking there Michael.
Give me a couple of years and I'll come up with something.

 

[mick]

Mr Purpose, shouldn't you be printing addresses and licking stamps ? :wink: I've been trying to find a schematic from a mixer to see where they would take a direct out from, but I don't have any ... oh well dommage.
I just don't know how to do the first part of this, I'm sure for the summing side I can use one of the many ways which have been discussed here before.

 

[capnspoony]

NYD has a mixer in the drawing board that I built and sounds great. I think you could modify it for your needs. Its a line level simple op amp mixer and he has aux sends that you could use to send to your daw and the rest of your desired chain matches his layout. He splits the signal using opamps as a buffer. I think.

--Richie

 

[mick]

Hi Richie, yes that's what got me thinking about a mixer (NYD is a very bad man putting all these ideas in our heads :green: ), I was planning on making a mixer like NYD idea at a later stage, good to know that you like the way it sounds.
As I said originally I want to control the outputs of my micpre's firstly, so I was thinking of a unit for that purpose alone really, it's just that I'd like to bring the (Balanced) signal in, attenuate it and come back out balanced. The split (bus) is not so important, in fact it could be mono bus I guess, the mixer part would just make life easier for track laying, allowing me to get a rough mix of levels at source before (in my case) my computer, and a way of having zero latency monitoring and a headphone mix. I think I'd need transformers for all the input's with NYD idea, I was hoping this could be done without too much cost.

Mick

 

[SSLtech]

Hi Mick,

You asked me about the SSl 9k Line input... -Too much to scan & post, but there's really nothing to it, that's why it sounds good. No 'character', just clean signal. It's an 3-opamp instrumentation amplifier topology, with variable gain on the 3rd op-amp.

Thassit. -Nothing else to it...

Keith

 

[mick]

Thanks Keith, now where's that emotion icon for 'flew straight over my head' ?

 

[pucho812]

Keef are you refering to this one :?


http://i6.photobucket.com/albums/y236/pucho812/LineInputAmplifier9000.jpg

 

[SSLtech]

Actually picturing a different layout, but essentially they're doing a similar trick with a slightly-different layout in the one you show. -The two inputs are fed-back using R26 and R27 to present a similar impedance on each leg.

The two halves of IC19 are the two input op-amps, the third op-amp is IC8. The two other op-amps shown (IC18) are DC servos, replacing coupling caps.

It is overall an exceptionally complicated input stage, doing a usually unity-gain unbalancing job. (how often are the input controls left at 0dB/unity compared to set at any particular gain stage) and the component requirements are pretty onerous... the potentiometer for example has to be a VERY accurately-tracking (over an overall 40dB range no less) item, otherwise you can kiss any CMRR advantages of this design goodbye.

As a DIY project, and since I do not usually consider essentially unity gain input stages to be in ANY significant way contributive to the "sound" of a console, I say DO NOT attempt this as a DIY; it's simply not cost effective. The line gain stage on a soundcraft is probably simpler and cleaner. If supreme CMRR rejection and a moderate gain adjustment range is what you desire, Get a THAT balanced input stage chip and follow it with a ±10dB single-op-amp gain trim stage... I't what I'd do. -There's no way on earth that I'd devote this many components and that much board real-estate to such an insignificant part of a console...

Keith

 

[pucho812]

[quote author="SSLtech"]Actually picturing a different layout, but essentially they're doing a similar trick with a slightly-different layout in the one you show. -The two inputs are fed-back using R26 and R27 to present a similar impedance on each leg.

Keith[/quote]

hmmmmmm I pulled that right out off of our 9000XL Schematics.... :wink:

 

[Samuel Groner]

Pucho, can you please link such large pictures... :roll: I've got a 23" monitor but no everybody here.

The potentiometer for example has to be a very accurately-tracking (over an overall 40 dB range no less) item, otherwise you can kiss any CMRR advantages of this design goodbye.

I don't think so--it looks to me as if CMR happend in the first stage of this design. Transfer to the second stage is balanced but without CM voltage, probably to maintain headroom at lowest gain settings.

Samuel