Electronics > Beginners
Building a sound mixer within my guitar amp with line IN & mic input
Yansi:
--- Quote from: schmitt trigger on June 02, 2019, 10:17:15 pm ---Yansi:
Would general purpose PNP transistors work here?
--- End quote ---
Why wouldn't they? :)
BC557C, BC560C, or even some higher current ones provide for less noise, like BC327-25.
In the old days, the golden transistor for the preamp was a 2SB737.
Other trickery may be to use two parallel transistors to get the noise another 2dB down*.
Here is an example of a mixer input stage:
*Noise relations for parallel connected transistors: http://leachlegacy.ece.gatech.edu/papers/Parallel.pdf
If I remember right, such trickery was used in some Allen&Heath mixing desks.
Zero999:
--- Quote from: Yansi on June 02, 2019, 10:49:11 pm ---
--- Quote from: Zero999 on June 02, 2019, 10:11:36 pm ---
--- Quote from: Yansi on June 02, 2019, 09:21:59 pm ---Jeez. Why 12GBP part for something that can be solved with a couple of discretes?
--- End quote ---
--- Quote ---Your differential mic amp is useless. Instead of simulating stuff, where it tells you nothing more that the simulation is worth, try breadboard it.
--- End quote ---
LTspice can simulate noise, but I've not played with it much.
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I thought more of a practicality standpoint. Noise-wise it will be horrible no doubt, but the input impedance imbalance is a real issue in practical application. With such imbalance, you can't even design a sensible EMI protection around it that will not screw the CMRR even more.
H.
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Which circuit/IC are you referring to?
Let's also not forget that there's little point in designing an amplifier with an input noise voltage much lower than the noise figure of microphone. A 200R dynamic microphone will have thermal resistance noise voltage of 1.8nV/√Hz at room temperature, plus some thermal acoustic noise, roughly matching its frequency response. The air molecules and the actual transducer itself will randomly vibrate, at temperatures above absolute zero.
dazz:
I'm reading about 3 op amp instrumentation amplifiers now and I'm a bit confused. Apparently it's a differential op amp with both inputs buffered through 2 low noise op amps. But that would have a very high input impedance, didn't I need low impedance at the inputs?
--- Quote from: Zero999 on June 03, 2019, 08:20:42 am ---Which circuit/IC are you referring to?
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Pretty sure he was talking about my circuit
Zero999:
--- Quote from: dazz on June 03, 2019, 12:09:15 pm ---I'm reading about 3 op amp instrumentation amplifiers now and I'm a bit confused. Apparently it's a differential op amp with both inputs buffered through 2 low noise op amps. But that would have a very high input impedance, didn't I need low impedance at the inputs
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Yes, that's correct, it will have a high impedance input. The good thing about it is it doesn't require adding any resistors in series with the microphone, which increase noise. The impedance can easilly be reduced by adding resistors in parallel with the microphone.
Bassman59:
--- Quote from: dazz on June 02, 2019, 08:56:24 pm ---Those INA163 go for 12€ on ebay
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They're easily less than half that through a proper distributor. eBay? Why bother.
Also consider THAT 1510 or 1512.
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