will a basic buffer be sufficient?

[AFex54]

So I need to buffer the gate and CV signals from my CV keyboard,
what kind of circuit would you recommend?
its a fairly lofi device but i still want to make it work as best as possible,
I suspect the diagram below is the ideal buffer?
would it need a few more components to tailor it for my signals (which are just varying DC voltages for CV and a straight 9V for gate)?

 

[Arouse1973]

Hello Afex54
What do you mean by buffer the gate? Yep this is an ideal buffer, but depending on the speed of the input you may need a few resistors. Tell us a bit more about what you are doing.
Thanks
Adam

 

[AFex54]

Hello Afex54
What do you mean by buffer the gate? Yep this is an ideal buffer, but depending on the speed of the input you may need a few resistors. Tell us a bit more about what you are doing.
Thanks
Adam

sorry by 'buffer the gate' i meant buffer 9V after it has ran through a series of wires and bus bars to remove impedance,
I call it a gate because it is gating an envelope generator.
this is a similar design to mine ,
everything after the buffer you can ignore:
:

 

[AFex54]

still need help with this,
http://www.muzique.com/images/buff7.gif
would that do it?
are the filter caps important?
I wouldnt need them since im only buffering a CV and not an audio signal right?

 

[Arouse1973]

Please be patient, we are all busy doing other things. I will look at this tomorrow if thats ok.
Adam

 

[AFex54]

Please be patient, we are all busy doing other things. I will look at this tomorrow if thats ok.
Adam

sorry, this is the very last thing I need to do before I can start wiring my panel so im a little impatient,
and unfortunately I cant really test this on a breadboard so i need external advice to continue

 

[CDRIVE]

The OpAmp circuit in your first post is a Voltage follower. It provides unity gain with very a high input Z. It should work well when used with the voltage divider you posted, as it will not load the divider network but will replicate the input voltage on the non inverting input.
Chris

 

[CDRIVE]

still need help with this,
http://www.muzique.com/images/buff7.gif
would that do it?
are the filter caps important?
I wouldnt need them since im only buffering a CV and not an audio signal right?

This one will not work for you because it's an AC coupled amplifier.

Chris

 

[AFex54]

The OpAmp circuit in your first post is a Voltage follower. It provides unity gain with very a high input Z. It should work well when used with the voltage divider you posted, as it will not load the divider network but will replicate the input voltage on the non inverting input.
Chris

ok thanks, also im using a virtual ground,any idea where I should connect it?
edit: to the non-inverting input via a resistor , the resistor determines the input impedance.
I will use a 1M resistor but im wondering if using an even higher value would have an effect other than further lowering the current draw

 

[CDRIVE]

:

[/QUOTE]

ok thanks, also im using a virtual ground,any idea where I should connect it?
edit: to the non-inverting input via a resistor , the resistor determines the input impedance.
I will use a 1M resistor but im wondering if using an even higher value would have an effect other than further lowering the current draw

Your resistive divider is made up of 5 100Ω resistors totaling only 500Ω. A 1MΩ between GND and the non inverting input should be sufficient.

I'm not sure how to answer your "virtual" GND question yet because I have some questions.

What is the -12V source to the Buffer + (Buss) through the 10MΩ for? Those two Diodes will drop ~ -1.2V and cause the Buffer output to go negative by the same amount when no keys are pressed. Is this what you want?

Chris

 

[Arouse1973]

sorry, this is the very last thing I need to do before I can start wiring my panel so im a little impatient,
and unfortunately I cant really test this on a breadboard so i need external advice to continue

That's ok, looks like Chris is on the case now to help.
Adam

 

[CDRIVE]

That's ok, looks like Chris is on the case now to help.
Adam

Adam, I suspect we may need more than one brain helping here. For instance; we don't know if he's intending to use a single ended (Vcc,Vdd) or bipolar OpAmp. If it's going to be a single ended model what would be his best choice, with good 0V output offset? Sadly, I've not kept current and new improved models seem to proliferate like skeeters in the Everglades!

Chris

 

[AFex54]

Your resistive divider is made up of 5 100Ω resistors totaling only 500Ω. A 1MΩ between GND and the non inverting input should be sufficient.

I'm not sure how to answer your "virtual" GND question yet because I have some questions.

What is the -12V source to the Buffer + (Buss) through the 10MΩ for? Those two Diodes will drop ~ -1.2V and cause the Buffer output to go negative by the same amount when no keys are pressed. Is this what you want?

sorry, my design is different to that,
that diagram just gives a nice visual explanation of what a CV keyboard is but
otherwise its confusing and over complicated,

basically the device im controlling with my keyboard is voltage controlled.
each key is connected to 9V and each has a potentimeter to 'tune' to the right voltage
the buffer will give all the keys an identical low current draw.
thats as complicated as my design gets.

this is the buffer im using:

im fairly confident it will work from what you told me

PS im using a TL084

 

[Arouse1973]

Ok so what is coming in on IN? Vr I assume is the voltage from the potential dividers. You will be supplying +V with 9 Volts and 0 Volts for the -V pin what is your lowest and highest input voltage?
Thanks
Adam

 

[AFex54]

Ok so what is coming in on IN? Vr I assume is the voltage from the potential dividers. You will be supplying +V with 9 Volts and 0 Volts for the -V pin what is your lowest and highest input voltage?
Thanks
Adam

9V through a 100k potentiometer

 

[Arouse1973]

And what about the other questions?
Thanks
Adam

 

[AFex54]

And what about the other questions?
Thanks
Adam

highest and lowest input voltage?
9V highest ofcourse but im not sure what the lowest would be.
I feel this is turning out to be much more complicated than it should be and its my fault
heres is the schematic of the device i hope to control: http://www.musicfromouterspace.com/analogsynth_new/NOISETOASTER/images/noisetoaster_page_1.gif
In the very top left is the normal voltage control pot that controls the frequency of the oscillator
the outside lugs are connected to 9V and 0V
i have basically duplicated that 20 times with keyboard keys as selector switches
only one key will be pressed at a time.
all the pot outputs would connect to a bus that I would buffer before I send into the device.
I hope this has made it clearer

 

[AFex54]

I tried out the unity gain buffer I posted above but Im getting strange readout from my voltameter,
0 resistance gives me +3.5V, 50k gives me +7.5V and 100k brings me back to +3.5V?
probably due to ground(BN) being tied to the other side of the pot.

 

[CDRIVE]

I tried out the unity gain buffer I posted above but Im getting strange readout from my voltameter,
0 resistance gives me +3.5V, 50k gives me +7.5V and 100k brings me back to +3.5V?
probably due to ground(BN) being tied to the other side of the pot.

As I stated earlier,... that is an AC coupled amplifier. It can't be used for DC applications. The first OpAmp circuit that you posted is the correct configuration.

Chris

 

[Arouse1973]

This is not complicated, it's getting confusing, as Chris said the first circuit works better. Why can't you draw your circuit and post it here. You only want us to choose the op-amp for you so I would have thought you had everything else sorted.

Or do you want us to design the rest of the circuit? Before I can recommend an Op-amp I need to see the circuit and you do need to tell me what the lowest voltage will be, it's actually very important. I don't want to spec a £500 Op-amp when a £1.00 one will do. We are not being awkward it's just we need the information to make a proper decision for you.

Cheers
Adam