Eventually the transistor ladder filter was the very first synth module I built, initially in a cardboard box and later transplanted to fit behind a eurorack-format panel. My implementation was a kludge on prototyping board, with later modifications bodged in and not-quite-quality components. While the circuit still sounds fine, it now has intermittent issues which are simply not worth tracking down and fixing anymore. Instead, I designed a proper PCB for the circuit and built a few new ones. The PCB fits behind an 8HP eurorack panel with all through-hole components and no wiring required. I sometimes have some of these PCBs available for sale, and the build documentation can be found at the bottom of this page.
My version of the filter is not intended to be an exact replica of the Moog filter. While it uses the traditional transistor ladder core and exponential converter, I used an opamp instrumentation amplifier (inspired by Rick Jansen) as output stage replacing the original discrete outputs. I added a CV input with attenuverter and a Drive control, allowing to adjust the input saturation that is a significant part of the filters characteristic sound.
A warning to all: if you intend to use 2N3904/2N3906 for the transistors, the pinouts are NOT THE SAME as the BC547 and BC557. I repeat: pinouts NOT THE SAME.
ReplyDeleteNo problem, you must it rotate at 180°
ReplyDeletehi there...do you still have a pcb available, please? how much does it cost along with postage to Malta, EU? thanks. Mike
ReplyDeleteHi! I have PCBs available. You can see the prices on the page "PCBs for sale" on top!
DeleteThe sound of this Ladder Filter is phenomenal! What an outstanding value-priced PCB + an easy build. At this price I should have bought some more ;^)
ReplyDeleteI'm scratching my head about a few points in the schematics.
ReplyDelete1. Where's C5? I can't see it in the pics of the board.
2. I want to use SMD pairs. Does that mean that BIAS1 is not connected to transistors?
Hi!
Delete1. C5 is on the bottom side of the board
2. BIAS1 connects to R6 and R7, and then the netlabels IN_L and IN_R connect to the transistors.
Best,
Caspar
I scratch built this filter from your schematic I must say it is a very nice sounding filter! I built the YuSynth ladder filter after this one and to me it doesn't sound as nice. I also had noise problems with the YuSynth transistor differential amplifier. I think I'm going to scrap it and make another one of yours - its easier to build and it works a lot better!
ReplyDeleteHello, i'm having some problems understanding the schematic. Should I connect the OUT_R's together? like "OUT_R" to "OUT_R"; "IN_R" to "IN_R" etc... or should i connect them in pair like "OUT_R" to "IN_R" and "OUT_L" in "IN_L" and so on... and also the Resistors (R1,R2,R3,...R5) are they connected between the BIASES? Thank you for your help!
ReplyDeleteHi! You should connect OUT_R to OUT_R, etc. Also BIAS1 through BIAS5 are connected to the corresponding label at the transistors. Good luck!
DeleteJust to be 100% sure. The “long line” for the BCM847BS is located on the left side of the board.
ReplyDeleteYes, pin 1 is the bottom left corner for all of them.
DeleteThanks!
DeleteGot them all in place now. Not as difficult as I feared they might be. Plenty of flux did the trick.
Hi,
ReplyDeleteThanks for this!
is there alternative to BCM847BS ?
Thanks
Hi, nice build (as usual!). Do you think it is possible to run this circuit on -/+ 9V ? I expect that at least a few adjustments on parts values would be required, but if that's all I guess I could still use one your PCBs and find the adjustements myself. My attempts to test it out on a breadboard were unsuccesful, but up to now I have been unable to tell if there's an issue in my rat nest of wires or if there is a fundamental reason for it not to work below -/+ 12V (I'm not very knowledgeable about transistors, these three legged weirdos have always puzzled me).
ReplyDeleteIn any case, thanks !
Hi! As far as I can see it should work just fine on +/-9V. You can change R21 to a smaller value accordingly (75k, but for example 68k is also fine) to keep a similar frequency range with the knob.
DeleteThank you for your quick answer ! I got it to work on breaboard, and it sounds great !
ReplyDeleteThe PCB definitely have a place in my wishlist, I need to go through the rest of your stuff too to get a few shipped at the same time, I could use a few more of these. Anyway, keep up the good work !
Is there an error in the v1.1E schematics around RV5? I believe the wiper of RV5 should be connected to the wiper of RV6 and non-inverting input of U1A, rather than to the CV input. As configured in v1.1E, RV5 is just acting as a variable resistor between the CV input and ground.
ReplyDeleteI could not source the trimpots with teh side screw - but you can put other trimpots on the backside without problems (there's a big cap, anyways)
ReplyDeleteI've built a few of your modules but this is the first one I'm having some problems with. No matter what I can't seem to get the output to go to zero when tuning with RV1.
ReplyDeleteShould I set any of the pots eg: drive, resonance, cv to a particular position when I'm tuning this? I initially zeroed them all but in that case the output is 0V, the problem only arises when turning up the pots, so I get a continuous tone in the output, without any input signal.
Hi! The resonance should be set to zero, otherwise the filter can self-oscillate indeed. The other knobs can be at any position. If the offset voltage changes a bit when changing frequency it doesn't matter, this adjustment is only approximate. Good luck!
DeleteLove the sound of this filter, sounds amazing when driven hard!
ReplyDeleteI am getting a lot of low-end loss & gain reduction with the Resonance set to anything but fully CCW. Is there any way to add gain compensation to the Resonance circuit?
Hi! The loss of low-end is a well-known effect of the original Moog filter, and many others. There are indeed ways to compensate it, some more information can be found for example in this topic: https://modwiggler.com/forum/viewtopic.php?t=212760
DeleteOn 24.6.20, Chris asked about RV5 in the attenuverter section. Looking at your Attenuverter module schematic, it does look like the wiper of RV5 should go to the wiper of RV6. I have a center detent pot for RV6 and want to use the RV5 trimmer. Is there a new version of the schematic/PCB? If not, can I modify the PCB by cutting and jumpering? Many thanks, Donald.
ReplyDeleteThe current version of the PCB is still 1.1, so unfortunately this hasn't been fixed yet.
DeleteTo use and the RV5 trimmer in your case, the easiest way is to solder de trimmer directly in parallel to the RV6 pot on the backside of the PCB, so all three legs of the RV5 trimmer connected to the corresponding pin of RV6.
I hope this helps, apologies for the inconvenience!
The PCB and panel arrived and after an easy assembly, the VCF works wonderfully. I chose to cut the PCB traces and made the new connections using the RV5 trimmer leads. I also found a sufficiently short 220uF capacitor, so the only part on the back of the PCB is the power connector. And the wood panel is excellent -- beautiful! Your design and documentation are excellent -- I think I'll add some of your other modules. --Donald.
DeleteThanks, and great that it worked out!
DeleteAny ideas why am I getting >10V on the output with no input? Transistor ladder bias voltages are in the right ballpark...
ReplyDeleteI'm stuck at -5v on my out put. Checked everything I can think of :(
ReplyDelete