Monday, 9 December 2019

Synthesizer Build part-7: THE MOOG LADDER FILTER.

Okay, here we have a filter who's sound is iconic and instantly recognizable if you know the music by Jean-Michel Jarre, Giorgio Moroder or Vangelis and (thousands of others really). The Moog Ladder Filter produces that sharp, snidy, ripping the fabric of the universe synthesizer sound and also that wet, watery sound you sometimes hear. That's why I had to include it in my synthesizer build project of course.

I used the schematic from Yusynth's website which is a great resource for synth DIY.

Before we start. Most people always want to know if it works on 12V. I tested the filter on dual 12V and it works just fine.
In this schematic the top and bottom transistors are applied in the form of a transistor array chip, the CA3046, but I couldn't get hold of that quickly enough so I decided to use all transistors and that works just as well. It makes the layout easier too. It is always mentioned that you must use matched pairs of transistors for this filter but really, that's a throw-back to the early seventies when transistors were not as consistent and reliable as they are now so if you have transistors from the same batch they will probably be matched well enough but put them through the transistor tester on your multimeter and match them on hfe value. The only place where the transistors must be matched well is on the place in the schematic where they use the CA3046; the top and bottom of the filter and the output on the side. I personally matched all my transistors by using the Transistor Curve Tracer I described in an earlier article on this website.
I built a second ladder filter as a test for the layout below and I used all unmatched transistors. The layout works fine but using unmatched transistors did not turn out well. I could not get the resonance trimmed correctly and there were enormous differences in volume when using the resonance potmeter. I used a squarewave for testing and the top of the squarewave had an angle to it instead of being horizontal. So you must used matched transistors!
This filter has a few quircks that you need to know about but which are normal for this design.
- The Resonance potmeter has only a small area of influence. For most of the throw of the potmeter you will hardly notice anything. This is normal for this design.
- When the Resonance is fully open, the output volume drops. This too is normal for this filter and even the original Moog ones have this. Yusynth also talks about this on his website.

The build is quite straight forward but you need to be very accurate. The 50K anti-logarithmic potmeter for the Emphasis or Resonance control was an other thing I couldn't get a hold of so I made my own by using a linear potmeter with a 5K resistor between pins 1 and 2. This works very well, In the layout I used a reversed logarithmic potmeter and I show the alternative that I myself used, next to it.

I made a layout for stripboard including the wiring. I used this layout to build a second filter and it worked straight away so this layout is verified. (All potmeters viewed from the front):

(Last revised: 24-June-2020: Corrected polarity of C3. 15-July-2020 added alternative for reversed potmeter.)

Stripboard only:

Here are a few pictures of the finished circuitboard:

As you can see in the pictures, I added two potmeters which are not on the stripboard layout above. These are two 200K trim pots and they go over pins 1 and 2 of each opamp, to make the gain adjustable. It says in the schematic to 'adjust the value of the feedback resistors according to audio level'. These pots make that possible without having to use the soldering iron. It's a bit awkward with the wires but I had to put the potmeters on the print where there was room enough to accommodate them and the wires. Plus I added them as an after thought, so after I made the layout. At least they are all neatly in a row. :-)
For clarity I made a second layout which includes these alterations. If you decide to replicate this then don't forget to remove (or not solder in) the original resistors over pins 1 and 2; the 56K on IC-1 and the 120K on IC-2, as this layout shows. Lay-out is verified not only by me but at least two other people who built this filter successfully.:

I lowered the Control Voltage input resistors from 100K to 5K6. In the schematic they are 100K but after installing the filter in my synth set-up I noticed that the CV hardly had any effect at all when I connected a LFO to it, so I lowered these to 5K6.

After finishing the build I tested the filter on the oscilloscope first and set all the trimmers to the right positions. It's easy enough to do, you just watch the scope for the best response. Then, after installing the filter you can adjust the trimmers to get the best sound out of it. This filter is self-oscillating, meaning that if you have nothing connected to the inputs and you turn Frequency Cut-off and Resonance all the way up the filter will oscillate of its own.

Here's a video showing the test results on the oscilloscope so you have an idea of what the waveforms should look like:


A demonstration of the sound of the filter:

Here's a look at my PCB for the filter. This is what is actually in my synthesizer and I build it from a different layout that I made earlier especially for the Eurocard format of stripboard:

Here's the panel I made for it. You can see the Resonance potmeter works reversed:

As you can see in the picture, I've installed a bypass switch for this filter. It's great to have this filter in series with the Korg MS-20 or the ARP2600 filter but sometimes you want to be able to easily switch to one filter. With a bypass switch there's no need to constantly connect and disconnect patch cables so this is really helpful. The switch only works with audio input 1 and it sends the signal straight to the output jack and disconnects the in- and output from the in- and output jacks at the same time. Here's the wiring diagram for the bypass switch:

Okay that's it for this episode. Stay tuned for more synthesizer build articles and while you're here leave a comment please!


  1. Hey Eddy, Roland from Synth DIY (snd some other group) here. I am a tad confused by the vactrol circuit you added to freq cutoff. There is already CV for that in the schematic. There is none for the emphasis/resonance though. Isn't it the resonance you have put the vactrol on?

    1. Oh yes, you are right. I mixed that up. The vactrol is connected to Resonance and not cut-off. The one parameter that isn't already controlled by a CV input in the schematic. I'll correct that straight away. Thank you!

    2. I have corrected it in the article now. Thank you for pointing it out Roland!

  2. Hello Eddy, thanks for publishing this. However, I have a remark about the polarity of C3. In my opinion, the schematic is correct.
    The DC-level at the opamp U3 pin 7 is at 0 Volts, the DC level of the base of U1b pin 7 is determined by the resistor ladder from +15V to ground, i.e. 220/(220+4*150+330)*15 = +2.86 Volts. So the plus-side of C3 shall indeed be at the base of U1b pin 7.

    1. Ah thank you for noting that. I had wondered about it. This is one of the first filters I built and I'm no expert by any means. Thanks, I will alter this in the article.