Electronics > Projects, Designs, and Technical Stuff
Just wondering... how to: digitally programmable gain on MIC inputs?
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Yansi:
Hello,

I have randomly came across this very interesting component: http://www.thatcorp.com/datashts/THAT_5171_Datasheet.pdf
http://www.aes-media.org/sections/pnw/ppt/other/low_cost_programmable_microphone_preamp_gain_control.pdf

I have always wondered, how do they implement programmable gains on the MIC inputs in all those digital audio mixing desks and racks. So now I know... or don't I?

Out of curiosity, I have looked how much does the part cost. Wow. I've almost fallen of my chair.  Insane pricing strikes again! :o :o :o
https://mouser.com/ProductDetail/THAT/5171N32-U?qs=sGAEpiMZZMvMUzLd0qnXYkT0A1%252b9vFe5

Okay, there is a cheaper part no. THAT 5173 with only 3dB gain step, costing just 2/3 of that.

Okay, there is even a cheaper part, THAT 5263, a dual channel within single package with even less gain range.

One could also state, that at quantity, the cost is acceptable, about $4-5 a pop. But that's just the gain control IC, you still need to add cost of the pre-amp itself. The cost of that per channel will very likely be at least the same amount, likely much more.

You could also probably state, that those digital mixing desks are very expensive. Yes and no. There are several smaller models available on the market, that I think just could not justify the cost of such ICs.

As far as I have looked, I have not found any similar ICs from any major IC vendor. Availability of these THAT parts is also piss-poor, I have found only Mouser (at least in EU) has them, and I mean not much of them is really in stock (if any, for example no THAT 5263 in stock - which is the cheapest one of them all) 

So how do they achieve the programmable gain on the MIC inputs in those entry level* mixing desks? Do they really use these THAT parts, or others I haven't found yet? Or are there some interesting topologies, that might be more cost effective for these? Does anyone know? Unfortunately, I have never had any chance to see any of these modern mixing desks inside (but plenty of those analog ones).

By entry level, I mean like Allen & Heath Qu16, which is a 16 MIC input desk that must cost like a 1000 USD or less to manufacture (retail price is like 1500 USD)
Or the Mackie DL32S, which is a freaking 32 channel input mixrack with a retail price of 1100 USD.

Would be nice if there is anyone on the forum, who got hands on one of these, so we could look inside.  8)
mrpackethead:
 the pricing on mouser/digikey in no way reflects the pricing that you will pay when you are buying many many reels of them.   


Yansi:
Still the pricing will reflect what the manufacturer quotes on their web page, won't it?

http://www.thatcorp.com/5171_Digital_Preamplifier_Controller_ICs.shtml
 
Quotes $6.7 for a 1000pcs. I do not think that digital mixing desks are made in such quantities, the pricing could drop significantly lower.
Yansi:
I have found these cute images of what's inside that Qu16 desk: https://imgur.com/a/yHnJI Unfortunately, no photos of the preampy stuff.
DaJMasta:
Quantity goes a long way, having a relationship with a manufacturer goes a long way, and THAT corp is sort of a boutique that does good work, but is by no means the only player in the game.  Sure, you may only use a dozen of the chips on a console you're only going to make a few thousand of, but if you can use the same chips across your product line, you could easily need tens of thousands, so the volume is fairly high.  That said, large mixing racks are actually in demand, in addition to venues and recording studios, you need them for TV and film work, they use them in large churches, and they're starting to come in for streaming and other uses, so selling a thousand high end mixers is probably easy for a large company.


You could also try a different approach, a fixed gain preamp with a programmable attenuator.  Since audio is low bandwidth, getting 100x or more gain out of an input stage is pretty easy, and line level is probably the internal analog bus/what works for the DACs/ADCs used in a digital console, so you don't need a particularly high signal level internally.  That means if your split rail is like +-5V or more, you could easily overamplify your signal by a fair margin and still stay away from the rails, then just switch in digital attenuation as needed.  Of course there's a lot of finesse involved with audio stuff, but as long as you can keep the noise floor low enough for 100dB or so of dynamic range and you can keep the distortion down, it will probably sound fine even on the high end.  Sure, it's no simple feat, but it's also well within reach of many modern ICs, so the constraint usually comes down to layout and analog concerns more than IC performance, so you don't often need the very top spec'd parts for your audio path.
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