My initial hypothesis was that this part of the circuit was used to decrease the amplitude of the audio signal when the resonance was increased. My thinking was that, because the audio signal usually gets very large once the filter starts to self-resonate, this circuit was used to lower the volume and keep that self-resonating signal under control. It turns out, though, that this hypothesis was totally wrong. It turns out that this circuit increases the volume of the signal when the resonance is increased. Let's look at the data to see why...
|Measuring the voltage across R152 to see how the LM13600 is being controlled.|
|Excerpt from the Polysix Schematic for the Resonance-Controlled VCA on KLM-368 Prior to the Effects Section.|
To dig a little deeper, I went back and recorded the audio signal going into this circuit and coming out of this circuit. The easiest place to grab the input signal is from TP1 on KLM-366. The easiest place to grab the output is TP4 on KLM-368. My procedure was to set the resonance knob at the desired setting, to play a note on the keyboard (C1), and to record the audio with my M-Audio recorder. I repeated the recording at several resonance settings. I then brought the recordings to the PC and analyzed them in Cool Edit Pro V1.2, where you could immediately see what was happening...
|Raw Audio Waveform Recorded With Difference Resonance Settings.|
As you can see, at TP1 (the input to this circuit) we see that the amplitude of the signal decreases as the resonance is increased. This means that, if we were listening to the audio at TP1, it would get quite a bit quieter when we turned up the resonance. Compare this to TP4. At TP4, the amplitude barely changes in response to the resonance knob. That's what this circuit must be doing...it must be compensating for this drop in volume. With this resonance-controlled VCA, the user can twiddle with the resonance knob without experiencing a big drop in volume. Cool!
I'm not satisfied, yet, though. Let's further quantify things. With these signals in Cool Edit Pro, I can use the "Analyze" feature to assess the RMS value of each of these blocks of audio. This is one way to estimate how loud we would perceive each block of audio. By comparing RMS values, you can guess how much louder or quieter each bit of audio would be. These values are summarized in the middle columns of the table. As could can see, at TP1, the volume would drop by about 10 dB when you turn up the resonance. That's a lot! By contrast, at TP4, the volume only drops by a couple of dB. That's pretty good compensation!
Ideally, I'd be able to relate the amount of compensation (gain) back to the performance parameters of the LM13600 IC that is doing all of the work. To try to understand how the LM13600 is working, I already computed the control currents, which should control the amount of gain of the LM13600. Using my measured RMS values, can can compute the (relative) amount of gain actually produced by the LM13600 in response to the resonance knob. As you can see in the last column of the table, the effect of the resonance knob is to increase the signal gain by 6 dB once you get to a knob setting of "3". Is this what we should be getting?
|Measured Control Current to IC20 at Difference Settings of the Resonance Knob. Also shown is the Measured Gain from IC20 Relative to the Gain at a Resonance Setting of Zero.|
Sadly, the analysis of the LM13600 proved too challenging for me. The datasheet has a couple different equations for the gain of the LM13600, but these equations assume a certain configuration (or values) of the circuit elements around the LM13600. The Polysix circuit doesn't quite match any of these examples. I tried to use a PSpice model of the LM13600 (via WinSpice) without any luck...the results were garbage. So, sadly, I cannot confirm if this is the way that it is supposed to work. Given that we've already seen that the circuit seems to nicely compensate for the resonance-induced change in volume, I'd say that the circuit is working well. If it's working well, it's probably working as intended.
So, what's the take-away from this? First, that the designers of the Polysix appear to have added this circuit so that you can play with the resonance knob without also getting changes in volume. That's great. Second, in my own notions of by-passing the whole KLM-368 board (to eliminate the added noise of the effects unit and to avoid the changes in frequency response due all the other circuit elements), I will lose this nice functionality. That gives me pause.