# Transistor Ladder Filter by Gerbrand Sterrenbur (Gerbster.nl)

**URL:** <https://lookmumnocomputer.discourse.group/t/transistor-ladder-filter-by-gerbrand-sterrenbur-gerbster-nl/6219>\
**Category:** DIY STUFF\
**Created:** [May 30, 2023, 5:56pm UTC](https://lookmumnocomputer.discourse.group/t/transistor-ladder-filter-by-gerbrand-sterrenbur-gerbster-nl/6219 "2023-05-30T17:56:57Z")\
**Posts on this page:** 1\
**Showing post:** 4

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**Author:** ![bowanderror](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@bowanderror](https://lookmumnocomputer.discourse.group/u/bowanderror)\
**Post date:** [July 13, 2023, 12:08am UTC](https://lookmumnocomputer.discourse.group/t/transistor-ladder-filter-by-gerbrand-sterrenbur-gerbster-nl/6219/4 "2023-07-13T00:08:54Z")

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I’ve built two of these as well and have run into similar issues. Maybe we can try and troubleshoot this one together.

First off, the module just sounds really thin for a 4-pole LPF. In addition, the Cutoff & Resonance are very much affected by the Input level attenuation control. As I increase the signal level into the module (AKA decreasing the amount of attenuation), the slope of the filter seems to decrease. Like at below 50% on the Input knob, I get that very obvious carving 4-pole filter sound (even if it’s just carving noise). As I increase the Input knob setting, the filter loses its sharp slope & sounds more like a 2-pole LPF. As a result, I’ve removed both filters from my rack as they were unpredictable to patch and I have 2-pole filters that sound more like a Moog VCF.

On my builds, I used polyester film caps and did match the capacitors to \<1% on a cheapo TC-1 Multifunction Tester (although I doubt the tester is really accurate down that far):  
 ![Screenshot 2023-07-12 at 7.53.18 PM](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/3X/8/d/8d69cda35f60a301c0ea9f32d7ba86c0f547be2e.png)

For transistors, I used BC547B instead of BC547C (it’s what I had on hand). Maybe this could be part of my issue, does anyone have experience with how hfe affects ladder filter performance? I also used Ian Fritz’s transistor matcher on ~75 transistors from the same batch, and found 3 pairs that were matched in VBE to 0.01mV (although I don’t trust my meter going down that far.)

I also tried a bunch of different CA3086s from several vendors (including a bunch of NOS ones sitting in storage at work). There may have been some difference between their performance, but not enough to make one set solve the issue.

I would love to get this thing figured out as I enjoy the sound at a narrow band of settings, but it’s just kinda useless as-is.

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