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PostPosted: Mon Jul 23, 2018 3:38 am 
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BYOC Envelope Filter modifications 23/07/18

The aim of the modifications -

A - Convert the Manual filter control into a control that sets the low frequency point from which the filter sweeps starts, let’s call it Sweep Bias.

B - Re-purpose the Auto/Manual switch into a selector for Low pass / Band pass filter characteristics.

1/. Cut all the tracks to the three switch pins.

2/. Cut the track between the 10K (that’s in series with a 100nF that goes to the pedal output via the bypass switch) and the three 22K’s connected to pin 8 of the LM13600. Rewire this 10K to the centre pin of the switch with a length of insulted wire.

3/. Re-wire the inner pin of the switch to the three 22K’s.

4/. Re-wire the outside pin of the switch to pin 9 of the LM13600.

At this point you will have the switch wired so that it switches the output of the pedal between high pass as it was originally and low pass. Low pass will be selected when the switch lever is flipped towards the centre of the pedal. The Manual pot will at this point be disconnected and will have no effect.

5/. If your kit shipped with a 100K Log (A) pot for the Manual control remove it and replace it with 100K Anti-log (C) pot. A Log pot has the incorrect curve and with it the Manual control is very abrupt. The Anti-log curve produces a much smoother frequency sweep.

6/. Cut the track on the outside edge of the top of the PCB that connects +9V to the outside end of the 10K that connects to the Manual pot.

7/. Re-wire that end of the 10K to the junction of the two 22K’s that produce Vref.

8/. Remove the 10K immediately next to the inside pin of the switch.

9/. The Manual pot should already have been disconnected from the switch during the switch mods, so re-wire that end of the pot (pin 3) to the pad of the 10K just removed that is nearest to the switch. This connects the pot to pin 16 of the LM13600 which is one of the control current inputs.

10/. On the back side of the PCB connect the two control pins, 1 & 16 of the LM13600 together with a short insulated wire jumper.

11/. Connect the emitter of the transistor to the end of the 10K that is connected to pin 1 of the LM13600. This end of the resistor previously connected to the switch.

At this point you should have the control current inputs of both halves of the LM13600 connected together and both the ‘Manual’ pot and the output of the envelope follower (emitter of the transistor) should be driving those joined inputs. That is the end of the basic modifications.

Other tweaks to consider -

1/. Increasing the value of the filter capacitors – the two 0.01uF connected to pins 5 & 7 and 12 & 13 of the LM13600. Doubling these up to 0.022uF will shift the filter down by one octave.

2/. Improving the efficiency of the rectifier in the envelope follower by using diodes with a lower forward voltage drop. You could use germanium diodes but I used Schottky diodes 1N5819s.
3/. Stiffen up the bass response and the stability of the Vref supply by replacing the 22K with 10K and increasing the 4.7uF to something more substantial such as 100uF.

4/. Increase the filter swing / sweep sensitivity by reducing the 10K from the transistor emitter to the control inputs of the LM13600 to 4.7K.

5/. Select the transistor for a high current gain and increase the 150K. I used a 330K.

I may post a modified circuit diagram when I have something presentable.


Attachments:
File comment: The front of the Env. Filter PCB showing the mods.Cuts in light blue, links in green, removal in magenta.
BYOC Envelope Filter PCB front mods 300.png
BYOC Envelope Filter PCB front mods 300.png [ 256.77 KiB | Viewed 2259 times ]
File comment: The back of the Env. Filter PCB showing the mods.Cuts in blue, links in green.
BYOC envelope filter PCB back mods 300.png
BYOC envelope filter PCB back mods 300.png [ 236.69 KiB | Viewed 2259 times ]


Last edited by Tark on Fri Jul 27, 2018 7:13 pm, edited 1 time in total.
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PostPosted: Mon Jul 23, 2018 8:14 am 
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Joined: Tue Jan 02, 2007 1:24 pm
Posts: 16196
Location: Albany, NY
Very interesting modifications. Diagrams or marked up photos to support the step-wise instructions would be very helpful, I think.

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