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Old 3rd July 2008, 10:18 AM   #1
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Default Simple preamp / mixer circuit - need help!

Hello there,

I've done a lot of Googling for what I seek - I couldn't find anything that exactly fits my needs, so I've ended up taking a bit from here & a bit from there etc, etc.

(I learned about electronics about 35 years ago in the military, but haven't touched it much since, so excuse the naive line of questions!)

I'm after an analogue circuit that first splits a piezo pickup signal (sourced from one guitar string) into two, outputs 1 leg 'pure', with the other leg being 'summed' to a further five (guitar string) signals, giving a combined output of the six strings.

Therefore, what I think I need is....

A voltage follower (to split the signal) ->

then another voltage follower to buffer the split signal (ie the other 5 guitar strings must not have *any* presence with the 'pure' output 1A in the circuit below) ->

then a summing amplifier. (to combine all the 6 guitar strings into one)

Like I say, i've cobbled together some circuits taken from others!

Click the image to open in full size.

I have several questions.

1. When joining two voltage followers together...is there a need for decoupling capacitors (as you can see, I've not used any between the 1st & 2nd opamp stage)

2. Is a bias resistor needed in the second stage (the dotted line R? resistor) - if so, what would be an appropriate value.

Is C2 needed in the above circuit?

Are the capacitor C1 & C2 values appropriate.

Any other comments most welcome!
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Old 3rd July 2008, 10:50 AM   #2
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You can probably eliminate C1 and C2.
You may not need the second voltage follower as the output impedance of the opamp will be nearly zero into the 47k combining resistors. This impedance ratio is enough to prevent crosstalk.
You will want to keep the output coupling capacitor in the circuit to prevent any slight offset voltage from reaching the output.
10uf should be okay for C3.

Hope this helps.
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Old 3rd July 2008, 01:49 PM   #3
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Hi Frank,

Thanks for your feedback.....

I've simplified the circuit a little....

Click the image to open in full size.

I can't run the risk of any crosstalk whatsoever (the top output 1A feeds a unit that analyses the incoming signal for pitch information...so it needs to just be the 'pure' signal from that particular guitar string alone), therefore I've left the second voltage follower in.

I've also left input capacitor C1 in, as I'd already partly made up the circuit board with it installed & it's not taking any harm!

Is the circuit missing anything?

My main concern is any DC voltages that may/may not exist through summing this one signal with all the 5 guitar signals (ie the last stage)

Does output 1A also need a 10uf capacitor?
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Old 4th July 2008, 07:56 AM   #4
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A very slight offset voltage that may be present at the point of summing won't be a problem. The output capacitor will protect whatever equipment follows this circuit.
It would probably be a good idea to use a coupling cap on the 1A output as well. It certainly won't hurt.
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