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Unity Gain Voltage Buffer Op Amp Circuit
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Unity Gain Voltage Buffer Op Amp Circuit
olbert
Hi,

I've used the same circuit for the buffer as you can see on Wikipedia ( http://en.wikipedia.org/wiki/Buffer_amplifier#Op-amps ).
My question is, do I really need no feedback resistor or any other resistor after the op amp? Because the op amp is getting hot.
But I guess, it gets hot because of the high supply voltages (+- 12 V), doesn't it?

Other question:
The VCA is self-oscillating when the control voltage e.g. a ADSR signal is falling very fast, like for very short attack or release times. So you hear always short pops in the attack or the release time in your audio signal. Without a audio in signal you can hear only the short 'pops' themselves, when fast VC signals are coming in. The pops are getting louder the faster the control voltage is falling.
Does somebody know, from what that can come from? I guess I need a capacitor and a resistor anywhere to damp the oscillations, but where and which? Maybe in the feedback loop of my voltage buffer?

Thanks,
o

PS:
I've used the THAT2181 for VCA job and the AD8599 for the buffer and inverter job.
I've attached the schematics, maybe there are also other beginner's mistakes... wink
urbanscallywag
you do not need a resistor in the feedback path of a non-inverting buffer. i'm not sure why its getting that hot.

sounds like you have DC going into the VCA which causes the pop. you'll need some AC coupling caps or to trim the DC out of the VCA circuit.


on your schematic why is THAT2181 pin 7 V+ attached to +12V through a capacitor? this should be a DC supply, right?
Sound
Hello,

About the ADA8599 why you are supplying it with +/-6V with the voltage divider ? Try with +/-12

You can also configure the CV in as a inverting amplifier -you will add protection and standard impedance- at its input, and you can calculate the ratio with R in and R feedback instead of R13 and R12.
The only thing, you should connect it in the opposite sense control port.
You achieve lower impedance output. In addition you can add a 2200pf capacitor in series with a 51Ω resistor, output to ground, in order to prevent oscillations. See the THAT application notes "improving VCA performance" I,II and III.

You can also replace the trim resistors(except the trim distort).
Lets see, -600mV in the EC+ is about 100dB of attenuation. You could achieve it with a voltage divider but maybe you should buffer it...why not ground this EC+ and use only the EC-?
If you configure a summing inverting amplifier with -12V, Rin=240V, R feedback=12K you will have +0.6V at EC- that its about 100dB attenuation.
Now we want gain unity at +5V ( standard envelope peak), so adding the input Rin=100K you will have the -0.6V to achieve the unity gain.
Doing this is not needed neither R2 as a trim resistor.
Sound
Other thing that can improve your VCA is buffer the Audio In with a inverting amplifier.
Check also if there is improvement blocking the DC in audio IN with a capacitor.
Check R3, if 5.1 K for 15V as shown in data-sheet. for 12V should be around 4K.
And see all the THAT applications notes regarding the 2180 2181 they are very interesting.

Hope it helps.
olbert
urbanscallywag wrote:

sounds like you have DC going into the VCA which causes the pop. you'll need some AC coupling caps or to trim the DC out of the VCA circuit.


Thanks! DC coupling shouldn't be a problem, since the signal comes from a filter which is AC coupled anyway. And I want also amplify DC voltages with this VCA.

I've found out that the pops came from to much swinging supply voltages caused by the bypass capacitors C3, C4 and the two below. I removed three of them, and the pops are gone. I've seen that arrangement in many other circuitries to keep the supply voltage more constant, but I guess the capacity has to be chosen right.
How do I calculate the capacitiy for this purpose?

urbanscallywag wrote:
on your schematic why is THAT2181 pin 7 V+ attached to +12V through a capacitor? this should be a DC supply, right?

You are right. I misunderstood the datasheet. This cap caused a really strange behavior. smile
Sound
Standard bypass capacitor in the supply input are 10uf, and 0.1uf close the supply IC legs.
olbert
@Sound:

Thanks! That sounds very good, I will do it like this and I will check the THAT design notes.
olbert
Sound wrote:
Standard bypass capacitor in the supply input are 10uf, and 0.1uf close the supply IC legs.


Also thank you for this very help full info! -- There is so much to learn!
Sound
You are welcome, actually you could add easily unity gain control over its offset. Just add a potentiometer +12V to ground, through another 240K resistor to the node of the summing inverting amplifier. You will have also an active attenuator, actually VC attenuator.

Regards.
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