Topic: Circuit help needed for battery line mixer

[Church-Audio]

Isn't 100k a little high for the input pot? and 100k for R3 

Is this passive input section going to be OK? It seems like a brute force way to combine the inputs.

I wish there was just an easy to follow (already thought out) schematic for this. It can't be that difficult right?

I guess I just don't understand the part of the circuit that makes the op-amp single supply rather than bi-polar supply. I wonder if it compromises the circuit in some way?

This depends on the input impedance of the circuit in question 100k seems high I would say 10k to 20k would be fine. But again you have to take a close look at the impedance of the input stage of the opamp.

[Church-Audio]

Chris, is there a brand and model number that you prefer? Also, wat are the best caps for bypassing at the + and - side of the op-amps? I also know that people bypass around those DC blocking caps too.

To be very honest I get most of my caps from eBay, the surplus market is great as long as you test them before you use them they are on average 50% cheaper then retail. For coupling caps I use anything from Tantalum to Polystyrene and even monolithic. Take a look at bc components they make some sweet caps at a very small size go to e-sonic.ca also check out digikey.com for bc components.
Check out the Cornell Dubilier type DME, or surface mount tantalum if you want really small cheap high capacitance values. Kermit type t197a106k050as.

[poorlyconditioned]

Chris, is there a brand and model number that you prefer? Also, wat are the best caps for bypassing at the + and - side of the op-amps? I also know that people bypass around those DC blocking caps too.

To be very honest I get most of my caps from eBay, the surplus market is great as long as you test them before you use them they are on average 50% cheaper then retail. For coupling caps I use anything from Tantalum to Polystyrene and even monolithic. Take a look at bc components they make some sweet caps at a very small size go to e-sonic.ca also check out digikey.com for bc components.
Check out the Cornell Dubilier type DME, or surface mount tantalum if you want really small cheap high capacitance values. Kermit type t197a106k050as.

If you're ever in Toronto, visit "Supremetronic" on College Street.  This is a small "hole in the wall" store, but they have tons of components, particularly caps and resistors, all values/types.  I can get 2.2uF film caps for something like 60 cents each!  Much cheaper than buying for Digikey or other sources.

  Richard

[kuuan]

notes on the circuit, mostly to high for me, possibly nothing new to you, taken from answers in a german electronic forum

not all apply to Chuck's version of the circuit

The resistors in the loop of the level pots are relatively small. Consequentely,with the level pots being log., level will first rise very slowly, but very steep at the end. Remedy: choose lin. level pots. or: level pot only 1/10 ( 10k ) and in exchange following ( pre) -resistores and pan pot 10x larger ( 150k and 100k )
log. pan pot doesn't work, medium position will not mean equal distribution to both channels.
in mid position of a pan pot, if lin., level diminishes 5/5+15 0 1/4. The amplification of the opamps is 390/100 = 3.9, just equalling.

how to optimize amplification I was told: one can make the resistores on the opams bigger than 390k. E.g. 1MOhm results in (1M/100k)/4 = 2.5 x amplification. Higher ohm however means higher sensitivity to interferences.

btw: I had drawn the caps in the input the wrong way around, if using 10uF instead of the 330 ploy caps.
he says: the coppeling condenser together with the following 100k res. build a 6db bass cut. With the 330uF the 'corner' frequence is 4,8 Hz. with 10uF it makes a sensational 0,159Hz. The improovment with the condenser's value therefore is academic only.

[Chuck]

I've given up on the original schematic too. I am in the process of changing my two channel recording rig, so my attention is not on this project for now. I may eventually build a mixer with two microphone inputs and 2-4 line inputs. I have been slowly buying the hardware (XLR & RCA panel mounts, teflon coated wire, Hammond box etc...) so when I find a circuit I like I will be all set.

Watch out for those higher value resistors. Resistors can add noise and some of those values you mentioned will add lots of gain. The original circuit is for a line mixer, so lots of gain was not the goal.

Richard (poorlyconditioned) made a remark in a thread that is slowly sinking in with me. He said, essentially...  you can go out and buy good sounding, well designed pre-amps. Designing and building them may not be worth the time. Well, when I finally do build another one, I want it to be good, so I'm going to proceed slowly.

[kuuan]

I have not given up on this schematic.
I enjoyed building the small preamp, I'd enjoy building this mixer.
However I am afraid that my mics don't make for mixing:
 
I have 2 stereo mics, an AT822 which is an X/Y stereo mic in a single housing, and a pair of CMC4s, two speparate mics but joined by one wire.
I ask myself: Does panning of any of these four channels make any sense at all? Would I not better leave clean left and right channels? The mixer also would not have level pots as I'd adjust their levels at their respective preamps.
If there are no pan pots and no level pots all that's left is simply an 'adder', but I was told that 'adding' 2 stereo mics is not done.

My question:
Do you think that 'adding' my two stereo mics, their left and right channels respectively, makes any sense?
Or could even panning indiividual channels of my mics make sense?

As I see it now the only possible config. to actually 'mix' would be if I had a mono mic, a lav on a person ( I have two Giant Squid omni lavs ) and mix it with ambient sound of a stereo mic.
For this again all I'd need is an 'adder' as I'd better make an adapter to feed the mono signal fully to left and right channles instead of panning it.

So instead of the mixer all I may be able to use is an 'adder', adding the left and right channels of two stereo mics, which also can be used to add a mono signal to a stereo mic fully to both channels.

well, I hope my questions are not too stupid.
I enjoy this hobby, finding out, solering asf.,  my experience recording equals zero though. I wanted to make better audio for my videos, found this site and see how a vast and fascinating field this is. I will never be at a paar with you guys, and I am very grateful that you kind folks bear with me, even teach me.

In case I learn that as adder can be useful for me I'll have to enquire further.

thanks,
a.