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| Just wondering... how to: digitally programmable gain on MIC inputs? |
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| DaJMasta:
You can have a fixed gain stage in the preamp, though, and then an adjustable stage with much less dynamic range for adjustment and a much higher noise floor because the noise floor of the system will already be set by the noise floor of the input - 10x gain on the initial preamp stage means any variable gain stuff following it can have a noise floor 10dB higher and only contribute the same noise to the signal as the preamp, making the requirements for the fancier adjustable gain hardware much more generous and making the thermal noise for a given value resistor much less impactful. Once you get amplified up a bit, ten microvolts here or there count for little, where at that first input stage a few nanovolts difference can be a big deal. Analog muxes do a reasonable job switching as well, the THD can easily be below 0.001% for reasonable signal levels into high impedances, so switching in attenuation is a viable choice on an amplified signal. Can't really say as to the exact architecture used, but there seem to be some options for inexpensive architectures that could yield more than 100dB of dynamic range for a complete system. |
| dmills:
My point about the ADCs was really that the front end for such a thing wants to have a gain of about -20 - + 40dB instead of 0 ->60dB that would have been appropriate to a old school desk (Which makes the gain setting resistor and DC block cap much more reasonable). Further the fact that you are only going for 2V or so at the ADC means that you CAN afford 10dB or so of excess gain followed by a pad without smacking into the rail. Say 20dB pad followed by 0/+20/+40dB switched amp and ADC driver, AKM have an 8 channel ADC that IIRC is specified at 115dB DR for about $4 at 1,000 quantity even paying Digikey pricing (AK5558VN), now I never quite believe the headlines on these things but it seems likely to do 110+ in reality, which is a non issue in a budget mixer. Doing the lazy IC sort of way, the THAT 6266 does all the programmable gain and ADC driving for two channels for £3.75 per channel pair at 1k quantities, and 6dB steps is just fine for this kind of thing, so one option is 4 * £3.75 + £4 per 8 channels (Plus a bag full of passives, connectors and tin) = ~£20 per 8 channels of mic input (Basically Digikey pricing at 1k off), £100 for a 24 channel desk front end seems workable at modest volumes. Bet you could cut that down by 30% or so with a good buyer and some clever engineering instead of going the easy route. Connectors and metalwork will cost as much as the 'tronics. Regards, Dan. |
| dmills:
Note that that old school preamp Yansi posted has a few issues... Firstly the wonderful, and much missed '737 low noise PNP is unobtainium and there is nothing else quite like it, that thing was superb. Secondly if you have a mic cable short circuit pin 2 or 3 to ground with the P48 on, the coupling cap will dump **amps** of current into the base of said transistor, more modern designs add a little series resistance and some clamp diodes to make this less destructive. Thirdly, reverse log pots have another name when you try to source the things "Rocking horse shit". Regards, Dan. |
| Yansi:
Well, it probably could be done with just a few steps. I have only experience with building pure analog frontends, hence asking in the first place. If the dynamic range of the ADC is good enough, that just few 20dB steps would be enough and the rest interpolated digital gain, well probably why not? That AK5558 seems dirt cheap for its spec, compared to for example CS5368. Well, we call the pots exponential, but in English I see them commonly as "reverse log" or "reverse audio". And they are definitely pain in the ass to get. I know about the issues, it was the first schematic I had on hand, from my small Behringer mixing desk. Regarding the "amp dumping action". I'm not sure, but those BAV21 diodes might protect it a bit, if they were connected a bit earlier in the path. Not sure what they are doing there else anyway. |
| nctnico:
--- Quote from: DaJMasta on January 07, 2019, 12:25:50 am ---You can have a fixed gain stage in the preamp, though, and then an adjustable stage with much less dynamic range for adjustment and a much higher noise floor because the noise floor of the system will already be set by the noise floor of the input - 10x gain on the initial preamp stage means any variable gain stuff following it can have a noise floor 10dB higher and only contribute the same noise to the signal as the preamp, making the requirements for the fancier adjustable gain hardware much more generous and making the thermal noise for a given value resistor much less impactful. --- End quote --- Indeed. IIRC A microphone will need 20 to 30dB gain to start with so there is no use to having a very large amount of gain in the controlled gain stage. |
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