Electronics > Beginners
Building a sound mixer within my guitar amp with line IN & mic input
dazz:
Not positioning myself on the issue, but I did some tests yesterday with the gain maxed out and an attenuation pot and I'm probably leaving it that way. Eliminates the need for a reverse log pot and I have full control of the volume down to zero. Doesn't seem to be noisier either
Zero999:
--- Quote from: Yansi on June 12, 2019, 11:45:16 am ---you don't see, so tell us why yet all manufacturers of mixing desks make the input preamp gain variable, including those digital ones?
Mic preamps are not all about CMRR only. There are other important factors, such as noise, PSRR, linearity (distortion THD, IMD, TIM), headroom, etc.
However that is nothing new, that circuit design is a mixture of compromises.
--- End quote ---
I don't know, you tell me?
--- Quote from: dazz on June 13, 2019, 05:41:47 am ---Not positioning myself on the issue, but I did some tests yesterday with the gain maxed out and an attenuation pot and I'm probably leaving it that way. Eliminates the need for a reverse log pot and I have full control of the volume down to zero. Doesn't seem to be noisier either
--- End quote ---
Yes, that's a common design. Lots of microphone pre-amplifier circuits also have a fixed gain, with a an attenuator afterwards. It makes it easer to design for a fixed gain. You still need a log potentiometer though.
dazz:
--- Quote from: Zero999 on June 15, 2019, 02:36:15 pm ---Yes, that's a common design. Lots of microphone pre-amplifier circuits also have a fixed gain, with a an attenuator afterwards. It makes it easer to design for a fixed gain. You still need a log potentiometer though.
--- End quote ---
Interestingly, a linear pot seems to work best in this case. I initially went with a log pot, but there was very little volume up until 3/4. Not sure why considering it's a volume pot.
Zero999:
--- Quote from: dazz on June 15, 2019, 02:59:35 pm ---
--- Quote from: Zero999 on June 15, 2019, 02:36:15 pm ---Yes, that's a common design. Lots of microphone pre-amplifier circuits also have a fixed gain, with a an attenuator afterwards. It makes it easer to design for a fixed gain. You still need a log potentiometer though.
--- End quote ---
Interestingly, a linear pot seems to work best in this case. I initially went with a log pot, but there was very little volume up until 3/4. Not sure why considering it's a volume pot.
--- End quote ---
What's connected to the wiper of the potentiometer? If there's a significant load, then it will have a more logarithmic response.
Here's a simulation, comparing a truly logarithmic potentiometer with a linear 100k pot, with an 8k load: note the similarities.
By the way, if it's not obvious how my potentiometer simulation works, here's a link to a tutorial I did awhile ago.
https://www.eevblog.com/forum/beginners/simulating-potentiometers-using-ltspice/
dazz:
Yeah, that's it. The mic preamp's output goes to the mixer along with the guitar preamp output and the line in (where I connect my phone to play backtracks). According to LTSpice the input impedance is fairly low, some 5K Ohms. Rin is 22K, I probably should have picked larger resistors to increase the input impedance
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