grrr_arrghh said:
ahh, ok - i will call them RCAs in the future - i shall explain: my all-in-one hi-fi has two 'RCA' sockets in the back for an AUX connection - that was all i meant by phono input.
I'm English, and always call them phono plugs and phono sockets, you caused confusion by using 'phono input', which is an input for a 'phonograph' (record player).
in the diag you pointed me at, are you saying I could replace ALL the op amps in the diagram with 741s?
Yes, 741's have often been used in these sorts of designs, but it is a very old chip, and far better ones are available - probably cheaper then 741's. 741's though were such a ground breaking chip, that even today most opamps are '741 compatible' - which means you could fit sockets for the chips, and simply plug better opamps in when you wish.
Many years ago I built a complete 16 channel PA mixer, using all 741's, by operating at sensible levels it worked beautifully - I ran the PA for a rock band with it, and did disco's in between sets. If I was doing it now, I'd use better opamps - but back then 741's were cheap, others were not, and they did the job!.
also, I am slightly confused (!), in figure 2 on that page, both diagrams are almost exactly the same, is one for left audio, and the other for right? again, if i was using stereo signals, would i have to make 2 of the input circuits (figure 1) for each channel (left or right)?
Figure 2 is for stereo, as you quite rightly say. Figure one is mono, you need two for each stereo input channel - but only if you want to add tone controls. As I mentioned previously, simply adding pots on the front of figure 2 would give a working line level mixer without tone controls.
if I had three inputs (six chanels in total) would i be able to run all from just 2 9v batterys?
If just using figure 2, you could have as many channels as you liked, it wouldn't increase the current consumption. But if adding figure 1 as well, each extra pair of opamps will take more current - I don't know how much it would be, but it should be pretty low - certainly 3 stereo channels wouldn't be a problem.
p.s. 741's are still used alot in English secondary schools..., but only as comparators
Schools tend to be rather old fashioned in that respect, unfortunately teachers who know much about electronics are few and far between.