Fade in/out idea (little help needed)

[~arph]

Hi all,

After reading about attenuation and how to properly implement this with opamps I decided to give a fade in/out circuit a shot. I came up with the schematic below and while it works really great for fading in I do have issues on the fade out, which does not gradually fades out, but it happens in a short period of time after some delay. So there definatly are issues with the circuit and I was hoping someone here could give me some hints. It's possible that it's related to the logarithmic perception of sound level differences, so maybe it's a question of using different ramps for fading in and out.
How the circuit below (supposedly) works:
The leftmost opamp is an integrator circuit that generates a ramping voltage between Vref and 0V decided on the position of the switch at the negative input. The 100KL pot determines the speed of the ramp.
The output ramping voltage is fed between the two 10K resistors on the input of the inverting amp (slightly above unity gain) when the ramp is at Vref the signal is fully attenuated, when it's at zero volts there is no (0dB ) attenuation.

I have seen VCA's using the LM13700, but I wanted to do this with regular opamps. I used 4558's and TL072's



Any help greatly appreciated,

Arnoud

[R.G.]

The circuit as shown has some issues.

It works by changing the resistance of the diode between the ramp opamp and the 10k's. When the diode is reverse biased, there is no attenuation; when the diode is forward biased, it has a low (few ohms) resistance, and you get a lot of attenuation. The diode actually does this over quite a small voltage range, from about 0.2V to about 0.5V. Your large ramp is doing a lot of moving the DC output of the signal opamp around, I think.

The asymmetry in response is from the difficulty in changing just the diode's bias.

Try this: Move your input DC-blocking capacitor to just before the opamp input. Replace the 1N4001 with a 220K resistor and add a 2N5088 with its capacitor to ground and its collector to the junction of the 10k's. The 220K goes to the 5088's base. What this does is modulate the collector-emitter resistance of the transistor to give you a fade.

Diode modulators work, but they are very, very picky about the DC levels you feed them for changing the diodes' resistances. The transistor modulator I described is the one used in the Pulsar; it's a lot less finicky about the gain change because (a) it's fed with a current through that base resistor and (b) the base-emitter feed is at least modestly isolated from the collector-emitter which does the modulation.

[~arph]

Thank you for you reply R.G.

I see what you mean, but I was trying something different here.
Originally the attenuation circuit had a variable resistor between its positive input (which is at Vref) and the 10k-+-10k juntion.. so the amount of Vref fed to the junction determines the amount of attenuation, instead of leaking of the input signal with a variable resistance to ground. I What I tried to to is replace that variable resistor with a ramp voltage that ramps up and down towards Vref (which, btw, I discovered it doesn't in my setup.. it goes all the way to V+ (9v), why?) . I put in the diode to experiment and found out it actually  faded in much smoother, so I kept it.
f I can't get it to work this way I will try your suggested route.

Regards,

Arnoud

[R.G.]

Originally the attenuation circuit had a variable resistor between its positive input (which is at Vref) and the 10k-+-10k juntion.. so the amount of Vref fed to the junction determines the amount of attenuation, instead of leaking of the input signal with a variable resistance to ground.

Actually, Vref **is** ground to AC signals. That variable resistor was the same as a variable resistor to ground, not a variable amount of Vref.

I What I tried to to is replace that variable resistor with a ramp voltage that ramps up and down towards Vref (which, btw, I discovered it doesn't in my setup.. it goes all the way to V+ (9v), why?) . I put in the diode to experiment and found out it actually  faded in much smoother, so I kept it.

It fades in because of the variable AC resistance of the diode, not because of the varying amount of Vref you're supplying.