[sdiy] LC delay lines and scanner chorus/vibrato taps

Tom Wiltshire tom at electricdruid.net
Sun Sep 8 17:39:33 CEST 2019


Hi all,

I’ve been playing with a modified version of Jürgen Haible’s scanner chorus/vibrato, described here:

	http://jhaible.com/legacy/interpolating_scanner_and_scanv/jh_interpolating_scanner_and_scanvib.html

Since JH has conveniently done the calculations for the delay line part and the 33mH inductors he used are still cheap and available, I’ve used that part without modification:

	http://jhaible.com/legacy/interpolating_scanner_and_scanv/scanner_vibrato_2008_page2_sch.pdf

What I’ve changed is the control and crossfade circuit. My version has a pair of DG406 16-to-1 switches for selecting taps from the delay line, and then an AS3360 dual VCA for crossfading between those taps. Control signals for these are generated in firmware on a PIC 16F1778. This chip usefully has  10-bit DAC channels that can be used to drive the VCAs, and by setting the DAC reference to the correct level, the output is perfectly scaled for the VCA. I’ve done a variable speed “LFO” in firmware that can scan the delay line at three depths (as per the Hammond organ, more or less), but you can also feed in a 0-5CV and use that to scan the delay line.

However, while I’m comfortable with all that part, I know very little about transmission lines, beyond what I’ve read here:

	http://www.rhombus-ind.com/dlcat/app1_pas.pdf

My question is “Are JH’s buffers on each tap necessary?”. The paper noted above suggests that a load on each tap should be at least 10x the impedance of the delay line (calculated by JH as 840 ohms). That’d mean resistors to take signal from each tap should be roughly 10K or greater. That would work perfectly into an inverting mixer or virtual ground node (such as I have on the VCAs) and I could simply use weighted resistors values to get all the taps at the same level. However, I can’t get this part to work, and looking at the signals in the transmission line doesn’t tell me much since I don’t know what I’m looking at or looking for.

So, do I need the buffers? Would a 10K to virtual ground be too much load for each tap? At this rate, I’m going to finish up putting simple op-amp buffers on each tap as an experiment.

Thanks,
Tom

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Synth & Stompbox DIY
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