Electronics > Beginners
Building a sound mixer within my guitar amp with line IN & mic input
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dazz:
And the simulation works great with the differential amplifier! Looks like I might have a winner here.
I can use the other half of the dual opamp to buffer the non inverting input of the mic preamp, but if I pick large resistors for the differential opamp, the input impedance will be large, right?
Yansi:

--- Quote from: Audioguru on June 01, 2019, 11:30:31 pm ---Years ago I was told that a TL081 is "general purpose" and that a TL071 is a TL081 selected for "audio" low noise and low distortion.
But todays datasheets have identical spec's.

--- End quote ---

Thanks for mentioning that, I always wondered what the difference between 07x and 08x series was. I have also used always the 07x series for audio - or so was I thought these could be used with decent result.
magic:
I think OPA1642 is a good replacement for OPA2134 these days, with less noise, lower power and otherwise similar specs.
Sadly no DIP8.
Zero999:

--- Quote from: Yansi on June 01, 2019, 11:19:43 pm ---TL082 will actually be better for battery life, than a NE553x or similar audio opamp.

I think that a lot of older DI-boxes used 062 opamp also fed from a 9V battery.

Also I do recommend you to get a book and read more on this topic:  Douglas Self, Small signal audio design.

--- End quote ---
The TL062 has a lower operating current. The problem is the NE5532, TL072 and TL082 aren't properly specified at 9V, so there are no guarantees they'll work well off a 9V battery, especially as it discharges. Anyway, the original poster is using a mains powered supply, so this is a non-issue.


--- Quote from: dazz on June 01, 2019, 11:31:50 pm ---And the simulation works great with the differential amplifier! Looks like I might have a winner here.
I can use the other half of the dual opamp to buffer the non inverting input of the mic preamp, but if I pick large resistors for the differential opamp, the input impedance will be large, right?

--- End quote ---
A high input impedance is good. The only downside to using high resistor values is increased noise, but with a J-FET op-amp, such as the TL082, it's less of an issue, as the input noise current density is a fraction of a bipolar op-amp, such as the NE5532 or LM358. With the TL082 thermal noise will dominate, with input impedances above around 40k.
dazz:
Thanks guys. I'm buffering the non inverting input, just because I can. If that allows me to use smaller resistors that's probably good (I'm googling that stuff about input noise/thermal noise)
I'm also looking up that book Yansi recommended

What I have simulated right now is attached bellow. But I probably rely too much on simulations and need to learn a lot more about real life issues like noise rejection and stuff. 
I found this mic preamp design and the author aimed for a low CMR, which I believe is not simulated in LTSpice (not sure though). Actually I thought CMRR was just a function of the noise frequency while the actual configuration of the opamp circuit didn't matter. Off to google again I guess  :-DD
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