VCA Circuit

Posted: 3/16/2015 12:33:51 PM
carlmart

Joined: 3/16/2015

Has anyone designed or built an actual limiter using this VCA?

And tested, of course! To see how it sounds.

If possible compared to other limiter designs.

 

Carlos

Posted: 6/20/2015 3:01:12 PM
tinkeringdude

From: Germany

Joined: 8/30/2014

I built this VCA with parts I happened to have lying around, so I didn't have that fet, but rather two J113. And an NE5532P (at +/- 5V rails) as opamp, the second one in there used to invert a 0..+3V control voltage so I got the negative one for the gate.
I checked that indeed my input goes to 0..3V and the gate gets 0..-3V.

This circuit then only manages to attenuate the 1kHz @ 3V squarewave output of my scope to something like -30 dB, which is pretty poor I guess.

Now I'd like to remind you I'm a newb with only basic electronics knowledge and leraning something new here and there, but not the "sit down every day and work through a book XYZ chapter with excercises" way ;-)

What might be the reason for that? In spice it attenuated to I think -50dB which also doesn't seem great, although I don't know what the standards for simple analog VCAs are. But my real world assembly doesn't live up to that.
Is it simply differing characteristics of my J113's ?
What could I adapt to improve the performance?

(btw, I'm looking for a low parts count one for my synth stuff, not theremin. That's why I'm experimenting with this, it looks cheaper than OTA based ones I've seen, esp. when using some SMT JFETs I've seen for rather low price)

Posted: 6/21/2015 1:24:33 AM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

tinkeringdude, did you try replacing the two input resistors with a 1k pot?  Matching the FETs for VT and IDSS helps, as does somewhat higher supply voltage.  Square wave as audio test signal in this scenario (measuring attenuation) isn't recommended due to the strong higher harmonics.

For real use I would crazy glue the FETs together to keep them at the same temperature.

Posted: 9/26/2016 4:01:03 AM
dony

Joined: 9/26/2016

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Posted: 12/6/2018 12:34:09 PM
Dominique

From: Switzerland

Joined: 11/5/2018

This circuit seam to be among the best you can do with silicium. I like its simplicity. Thanks for sharing.

Posted: 5/29/2019 3:27:40 AM
Klaus He

Joined: 5/29/2019

Since the voltage control amplifier (VCA) can adjust its amplification factor by the external DC voltage, it can be used as a variable magnification component of the AGC amplifier and a low-frequency amplitude modulation circuit, in addition to a circuit with a variable amplification factor. The principle of multiplication is usually used. This circuit uses a variable GM type OP amplifier to change the mutual conductance by the external voltage to achieve VCA. Article from aluminum sheet price Circuit working principle CA3080 is an IC called OTA. It is a differential input / current output type OP amplifier. The mutual conductance GM is expressed by the formula GM = ΔI0 / ΔE1. While GM=19.2*IH, a GM proportional to the bias current IA can be obtained. Because it is the current output, its internal yin RO is very large, R ≈ 7500 / IA/10 negative 3 power. According to the above relationship, the output current I can be calculated by the following formula: I=GM.E1=19.2*IA.EIO The output voltage can be obtained after current-voltage conversion by A2. The article comes from Mingtai Aluminum. According to positive and negative EI=IO/19.2*IA, when IO=1MA, IA=1MA, the input voltage of CA3080 is positive and negative EI is 52MV; when IA=10UA, it is 5.2V. When IA is small, the conversion speed will decrease, so it should not be too small. The larger IA value should be used as much as possible, and the maximum input level should be adjusted at the input voltage dividing circuit composed of R1 and R2. The value of R1 is determined according to the application. If the output current IO is designed according to 1MA, when the maximum output is positive or negative 10V, the feedback resistance of the OP amplifier A: should be 10/10 minus 3 powers of kiloohms. The article comes from the aluminum sheet manufacturer R5=8.2 kohms. The VR2 is 5,000 ohms (adjustable). Input method of bias current IA: In order to enable a current proportional to the control voltage VC to flow, a base-grounded PNP transistor can be used. If the base current is negligible, the input current I1N=(VC-VBE)/R4, when the voltage VC is lower than VBE≈0.5V, the IA is almost equal to 0, and there is no amplification function. Since the entire circuit uses DC coupling, an offset adjustment circuit is added to the OP amplifier A2. If the input and output are DC-blocked, and only the AC current is passed, the offset adjustment circuit can be omitted. At the inverting input of A1, offset adjustment may be performed after series resistor R7. Note Other VCAs can also implement VCA using analog multipliers. Usually, the transmission characteristics of the multiplier can be expressed by EO=X.Y/10 (the input voltage range of X and Y is 0~ plus or minus 10V). The article comes from the 3004 aluminum sheet. If DC control voltage (0~ positive 10V) is applied to any of the input terminals X and Y, the amplification factor A can be changed between 0 and 1 times. There is also a monolithic IC made specifically for VCA, and Mitsubishi's M5241L belongs to this IC. When used for volume adjustment, it is also called an electronic potentiometer. Although called VCA, there is actually no gain and should be called a voltage controlled attenuator. Nippon Wireless' NJM4200 is an inexpensive analog multiplier. In its external OP amplifier and resistor, a high-precision VCA can be formed, which cannot be used in the video band because its upper limit frequency is several kilohertz.

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