| Electronics > Projects, Designs, and Technical Stuff |
| Just wondering... how to: digitally programmable gain on MIC inputs? |
| << < (4/6) > >> |
| dmills:
There are also other ways of skinning this, you can for example build a mic amp out of a couple of transistors and a instrumentation style stage based on a few opamps and use an LED and LDR pair to swing the gain of the instrumentation amp, I see no reason to think it would not work.... If your input pad is say a relay switched 20dB (fairly typical) then you can do most of your gain changes by using a network of photomos switches in the instrumentation amp gain setting region, (CADAC did this in the excellent M16). Remember also that for a modern digital system you are only looking for a few volts peak at the ADC input pin pair (And you nominal level should be probably 30dB below that), so it is not like you are trying to swing 15V on a 17.5V rail like the old days. Then there is that trick Sony came up with, two preamp stages effectively about 30dB apart, two ADCs and some DSP you use the sensitive one until it approaches clipping then switch to a scaled version of the less sensitive one until the modulator in the sensitive stage recovers.... One nice trick if playing with this sort of thing is to run the ADCs at 96kHz, then deliberately inject a sniff of 24Khz sine into the inputs, that way you can trivially get the I/Q values for the 24Khz and use this to tweak the digital gains. It is worth noting that the best microphones have about a 130dB dynamic range or so, a good quality ADC about 120dB dynamic range so it should be entirely possible to cover the whole thing with just a pad and two or three switched gains, the rest can be done in the digital domain. For THAT parts, Profusion in the UK are good for modest quantities. Regards, Dan. |
| Yansi:
I do not think that it would be that easy with photomos. The gain setting resistor typically starts from couple of ohms and needs to have good linearity. Any switch in that path will have significant effect on the distortion. Increasing impedances will increase noise too. You are correct, that most ADCs want somewhere about 2Vrms. But excessive gain followed by attenuation still creates unnecessary noise and distortion. You are probably correct, that with a very high quality ADC (120dB of DR) it would be possible to have only few switched gains. However such ADCs are scarce in the cost optimized equipment mentioned above. Hence wondering how have they done it. But I think solution with PGA2505 comes cheap enough, it would cost very likely under $4 at quantity, which I think for a complete and a minimum external parts sounds quite reasonable. |
| Jr460:
--- Quote from: EEVblog on January 06, 2019, 10:16:16 pm --- --- End quote --- Except that mixer doesn't have digital controlled pre-amps. Not much of anything on that board is under digital control. Let's take a look at Yamaha 01V mixer. All digital right? Save all the setting, recall anything you want, right? Wrong, look close at the pre-amps for each channel. That control and also the 20dB pad are analog. Setup to LS9, MC7L, PM5D, and you get preamp gain controlled and saved digital. To take it further try a Rio3224D headend, the preamps are not even in the same box as the mixer. (Dave really needs to go do a tour of the Audinate/Dante company) For the Rio line, I know that they do not do the whole adjustment range with one amp/chip. There is a place in the range that you can hear a small relay click as it puts in or removes a pad. Of course when they do that they also have readjust the gain of the amp at the same time so things seem seamless. An early firmware update fixed an issue with just that issue, gain change not smooth just as the relay came in or out. |
| nctnico:
Back in my 'audio days' a simple digital potmeter in the feedback loop (to adjust the gain) of a low noise opamp (NE5532) did the trick. |
| Yansi:
But you simply can't cover a 60dB gain range with a simple digital pot in a differential low noise mic preamp. NE553x at 60dB gain in simple amplifier configuration would be noisy as hell. The THAT part datasheets shows quite nicely, how the preamp looks like and looked like even back in those analog days. The resistor they mark as "Rg" was/is a typically 5k reverse log potentiometer. (in the schematic below, the gain pot connects to the "X68" pins 1 and 2 in the right upper corner) |
| Navigation |
| Message Index |
| Next page |
| Previous page |