Been designing a simple MIC/Line mixer for a mobile disco.
It has 2 inputs, a line level input and mic input.
The mic input has a extra gain stage to match the line input.
Output stage just mixes line and mic stages together.
Using a 12ax7.
Power comes from 12vac wall wart, into a reversed transformer to get up to around 200vDC.
Heaters run off 12vac input.
I have elevated heaters to DC 20v ish.
B+ goes into 100uf then I have split mic stage and mixer stages power supply into two parts with 4k7 and 100uf.
It has 2 inputs, a line level input and mic input.
The mic input has a extra gain stage to match the line input.
Output stage just mixes line and mic stages together.
Using a 12ax7.
Power comes from 12vac wall wart, into a reversed transformer to get up to around 200vDC.
Heaters run off 12vac input.
I have elevated heaters to DC 20v ish.
B+ goes into 100uf then I have split mic stage and mixer stages power supply into two parts with 4k7 and 100uf.
Interesting project.
You don't need grid stoppers on a 12AX7 because its gm is too low to be a serious cause of oscillation. And anyway, all it does is add unnecessary noise to the mic input.
The output impedance is quite high so HF loss could be an issue with long cables and LF loss could be an issue with lowish impedance loads. What is it going to be connected to?
Exactly what type/make./model of mic do you plan to use with this?
Cheers
Ian
You don't need grid stoppers on a 12AX7 because its gm is too low to be a serious cause of oscillation. And anyway, all it does is add unnecessary noise to the mic input.
The output impedance is quite high so HF loss could be an issue with long cables and LF loss could be an issue with lowish impedance loads. What is it going to be connected to?
Exactly what type/make./model of mic do you plan to use with this?
Cheers
Ian
The amplifier is 47K input impedance.
Its just a mic without active circuit in it. It was a cheapie I picked up at Maplin's.
Its just a mic without active circuit in it. It was a cheapie I picked up at Maplin's.
OK, so the 100nF output cap and a 47K load turns over at 33Hz. Are you sure you want your LF -3dB point that high.
Is it a high or low impedance mic? If it is high impedance then you are OK. If it is low impedance then you will need a step up transformer.
Cheers
Ian
This is a mobile disco. I've run lo-Z dynamic mikes to a grid, and it is OK in loud ambient and strong talker. If he needs more, the XLR-1/4" adapter with transformer is the quick 14dB fix.
Dunno who needs a tube disco today. Done that, done with that.
However: Square Dance sound system. (Read to the end: the marvelous speakers turned-up!)
Tube disco…. One tube here - not a whole bunch of 6550’s. But as one who has this on his bucket list…. It is being considered.
The main amp is solid state. I just wanted a valve mixer to give it a little warmth.
Got it in a box today. Good fun punching out holes for 3 jack sockets and the power 12VAC plug.
Just waiting for knobs for the pots now.
