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| Building a sound mixer within my guitar amp with line IN & mic input |
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| dazz:
--- Quote from: Yansi on June 04, 2019, 07:23:44 am ---Look for single opamp differential amplifier at google. Gain way to easy! In this circuitabout 316. And you are supposed to trim the bottom feedback path to exact resistor ratio as the upper one for best rejection. However wit the input impedance imbalance it is pointless anyway. --- End quote --- Oh, OK. I thought the trimpot was meant to balance the impedance of both inputs, even though that's not what the simulation showed. So the trimpot is there to make the ratios between R1/R4 and R2/R5 equal, to maximize common mode noise rejection, but of course, it's imposible to pick the resistors so that those ratios are equal and the input impedances are also the same. Well, at least I think I'm starting to get it (yeah, I'm a bit slow). I've order the components I needed to build the preamp you posted. I think I'll still try to build the differential one just because those components will take a couple of weeks to arrive. As for the gain control, I was considering adding a non-inverting opamp with variable gain after the differential stage, because adding a pot to the differential opamp would mess up the R1/R4 and R2/R5 ratios. I increased the values of the input resistors from 1k to 10k to lower the gain of the differential stage. I get 1k64 and 1k24 ohms of input impedance. A bit low and quite a significant imbalance, isn't it? |
| Zero999:
I wouldn't bother with the potentiometer. I dare say 1% tolerance resistors will be good enough. If you're using screened twisted pair cables for the microphones, it should minimise common mode noise anyway. Another possibility is matching the resistor values by measuring them with a decent multimeter. The problem with the TL082 is it's relatively high voltage noise, which is why the NE5534 was suggested. What parts in the design Yansi posted, are you having difficulty sourcing? There doesn't appear to be anything non-standard or difficult to find. |
| dazz:
I just need PNP transistors. Ordered 100 bc557 already. Actually now that I think of it I might have a couple of cannibalized PNP transistors lying around |
| Yansi:
NE5534 on by itself (no discrete diff amp before it) is still quite noisy (TL0xx is heck of noisy) for a general purpose MIC amp. Do not go this way. Noise levels will get unacceptable when you will go with gains like 40dB or more. You can make a rough calculation of output SNR: NE5534 has a rating of 3.5nV/sqrtHz. At an audio bandwidth, this gets you to 0.65uVrms of equivalent input noise voltage. At a gain of 40dB, this results in 65uVrms noise. Considering a nominal signal level of 0dBu, that is a SNR = 20log(775/.065) = 80dB. Okay, 80dB aint that bad, but considering there will be a lot of added noise from the surrounding passive components and other culprits here and there, 80dB is the best case. In reality? 65 to 70dB I'd say. Even if 75dB SNR. Now imagine that mic amps are often used with gains over 40dB, this will worsen the SNR even further. So really, it is best to design for lowest noise right away. The discrete PNP stage is nothing complicated, in fact very cheap well known and robust solution to the problem. Also it is (or at least was) widely used back in the days, where engineers were engineers and not people gluing together fancy blackboxes (unnecessarily expensive ICs). |
| Zero999:
That's a bit of a generalisation. Whether the NE5534 is too noisy or not, depends on the application. The microphone impedance, sensitivity and how loud the sound being recorded, will all be factors. If the microphone is used in a fairly loud situation, such to record singing or a pick-up on a musical instrument, I doubt it will be an issue, as the ambient sound will always be higher than the NE5534's noise floor. If it's being used to record quiet sounds, such as insects moving around, then it certainly won't be good enough. I'm currently designing an intercom system, using the NE5534 as a dynamic microphone pre-amplifier, simply because there's no point in using anything better. It's not a balanced system: the headsets all have single core screened cable and tests have found that noise pick-up from the cable, exceeds that in the NE5534, even with a noise gain of 60dB (a two channel mixer with a gain of 50dB). I would love to use a balanced system or headsets with a built-in pre-amplifier, but it has to be compatible with an existing system, which is unbalanced, so is a no-goer. |
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