Except they haven't, and their attempt was laughable
Assuming you actually wanted to make an AC/DC mixer (even though it's a completely bizarre idea), and didn't care about massive distortion dependent upon where you set the controls and the specific inputs) this wouldn't be how you would do it.
For a start throwaway the bogus AC coupled stage (the only stage that isn't distorted), use a proper virtual earth mixer (separate AC and DC gains can be easily set for it, if required), and throw away the no longer required second 'mixer' attempt, and use that opamp for an inverting buffer (so as to maintain the polarity through out). It's still going to be pretty naff, and dependent upon levels and settings, but at least it's actually a mixer.
I'm presuming the idea of this is that you can use it EITHER (not both at once) as a DC 'mixer' or an AC one?, in which case why not simply build two separate mixers?, twice as useful, both would work properly, and you'd use less parts than the existing effort.
For a start throwaway the bogus AC coupled stage (the only stage that isn't distorted), use a proper virtual earth mixer (separate AC and DC gains can be easily set for it, if required), and throw away the no longer required second 'mixer' attempt, and use that opamp for an inverting buffer (so as to maintain the polarity through out). It's still going to be pretty naff, and dependent upon levels and settings, but at least it's actually a mixer.
I think one thing I forgot to share is the fact that most of these modules have input and output buffers. It seems the first opamps in the mixer are buffers?
I'm presuming the idea of this is that you can use it EITHER (not both at once) as a DC 'mixer' or an AC one?, in which case why not simply build two separate mixers?, twice as useful, both would work properly, and you'd use less parts than the existing effort.
I assume the main purpose here is to keep it simple for the end user.
The bits are made for ages 8 and up and it mite be a bit confusing for a kid with no electronics background to pick the right one to use.
I assume the main purpose here is to keep it simple for the end user.
The bits are made for ages 8 and up and it mite be a bit confusing for a kid with no electronics background to pick the right one to use.
Here is the speaker or output module
It is certainly not high fi but its just a toy.
As expected its mono, but its interesting how they shift the audio thats above 0 volts back to above and below 0 volts.
Because there's no biasing for the opamps, and even assuming it's provided from the previous stages adjusting the volume settings will still alter the biasing on the later stages.
Here is the speaker or output module
It is certainly not high fi but its just a toy.
As expected its mono, but its interesting how they shift the audio thats above 0 volts back to above and below 0 volts.
Nothing particularly wrong with the output part, it's all you can expect when you've only got a low supply rail.
However, it's got yet another non-biased buffer at the front, for no reason whatsoever - that opamp isn't part of any DC system so could have been biased properly and capacitor coupled - for that matter the first buffer stage could have been left out entirely, and use the 100K input impedance of the second stage, or even increased it to 1Meg if there was any need (which I wouldn't have thought there was).
I'm a bit bemused by your use of 'interesting', for what is a standard and simple technique used in almost every circuit you come across - however it's just DC blocking, it's not referencing it to ground as that's not wanted (the usual place it is wanted doesn't apply here because it's a bridged output stage, a single ended stage though would reference the speaker back to ground).
I think I could take that as a hit, but I won't, admittedly I am still learning even at my age. At least I can still see things that don't seem quit right and find them interesting.
I thought it was interesting in this case how they feed the signal into the (IN-) of the output chip.
Because there's no biasing for the opamps, and even assuming it's provided from the previous stages adjusting the volume settings will still alter the biasing on the later stages.
And if you don't bias it then it rectifies an incoming AC signal, giving MASSIVE distortion.
I've been looking a little more at the littleBits website (confusing as it is), and the previous stages (at least the ones I've looked at) provide biasing for the following buffer stage - but this doesn't prevent the problem of the volume controls changing the bias of the following stages.
You should also bear in mind that you don't get 'rail to rail' opamps, only 'nearish to rail' ones
Unfortunately the entire littleBits system is crippled by it's lack of supply voltage, and too small a number of connections between modules
I dont think Nigel likes your machine. On the other hand, 12 and likes messing with music, maybe it will spark an interest.
Its a shame that he isnt into it enough to mess with 555 timers and piezo electric disc's for drums etc.
I have looked and could not find anything on biasing a unity gain buffer, as far as I can tell its a voltage follower.
Thats why I asked if there is anything specific you could point me to.