tirsdag 24. oktober 2017

BA662 clone

While studying the Juno 106 filter I came across Analogue Renaissance's BA662 clone. It is dirt simple, but when I tried simulating it, I got massive distortion on the output compared to my Xonik linear VCA (based on the research by bergfotron etc, see separate post on the VCA)

When running a 2V p-p signal through it, using a 0.110mA (15V/136k) control current I got an output signal with the same amplitude, but the sine wave had been visibly distorted. Suspecting that this was only a simulation problem, I tried breadboarding the circuit. The distortion was even more visible on the oscilloscope.

2V p-p input (green), distorted output (blue)

I quickly realised that the amplification was not linear close to the maximum signal amplitude, much like a compressor/limiter circuit. Reducing the input to 1V p-p showed that the gain was in fact not 1 but more like 1.5.

Adjusted input, 1V p-p
After readjusting the control current to get unity gain again, I could still see some distortion:

Adjusted amplitude. Output wave is still distorted, though not as much as earlier

After reducing the input further, to 0.5V p-p, and readjusting the control current again, I got this, which seems much better. The control current is now 61.9uA


Then I took a closer look at a document I printed a few years back, that shows what voltages and currents to expect in the Juno:

An annotated Juno 106 service manual showing the various voltages. I can't quite remember where I got it, but I THINK Analogue renaissance made it?

To see how the BA662 is actually used in the circuit, I added the resistors between VCF out and VCA in (though not the VCA offset pot) and set the pot to 25k. Without changing the control current I got this:

Clone as used in the Juno 106 - with the 50k pot centered. Ampitude is MUCH lower.

Notice the significantly lower amplitude. I then adjusted the control current to the maximum according to the document above and got this:

Same configuration but with the control current at 0.3mA, which according to the picture above is the maximum. Notice that the amplitude is alost a quarter of the input. Even if we increase the control current, we are unable to reach unity gain.
So, there is no way to get unity gain out of this. I guess it is compensated elsewhere in the circuit. What troubles me is that with that much attenuation, wouldn't the VCA be unnecessarily noisy?

I will have to recheck the Xonik Linear VCA to see it it suffers from the same distortion problems. From my notes I see that I run it using a control current up to 1.5mA.

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