Moog Ladder Filter – gain loss correction

Here is a detail of my Moog Ladder Filter implementation. The Moog Ladder Filter suffers from gain loss when the emphasis / feedback is turned up, as many other filters do. Here is my take on correcting this. With this method the gain loss can be compensated to 100% (or more) if wanted. I made the feedback voltage controllable. So I have already a control voltage proportional to the feedback. This voltage can be used to control a second OTA in the output stage of the filter. The output of the OTA is then added to the regular filter output. No feedback CV means no additional output. Increasing the feedback CV causes the OTA to add to the regular output. The amount is determined by the load resistor (R18 on the schematic) of the OTA. You can adjust it to your needs. It is possible to use a potentiometer here as well.

Moog Ladder Filter. Gain loss correction. Schematic detail.

Moog Ladder Filter. Gain loss correction. Schematic detail.

 

Here are some screenshots from the output with different amounts of gain correction starting with 0% up to 100%

Moog Ladder Filter, output with no gain loss correction. As you can see, the output (blue line) drops significantly.

Moog Ladder Filter, no gain loss correction

Moog Ladder Filter, no gain loss correction

 

Moog Ladder Filter, output with gain loss correction. R18 30k:

From here on the peek of the output signal is above the input signal. Watch the headroom of your system!

Moog Ladder Filter with gain loss correction R18=30k

Moog Ladder Filter with gain loss correction R18=30k

 

Moog Ladder Filter, output with gain loss correction. R18 50k:

Moog Ladder Filter with gain loss correction R18=50k

Moog Ladder Filter with gain loss correction R18=50k

 

Moog Ladder Filter, output with gain loss correction. R18 100k:

Moog Ladder Filter with gain loss correction R18=100k

Moog Ladder Filter with gain loss correction R18=100k

 

Moog Ladder Filter, output with gain loss correction. R18 200k:

As you can see the gain loss is completely corrected here. If you look at the last picture you can see that the peeks of the signals are about 10V. If you use 100% gain loss correction you should have a look at the headroom of your system.

Moog Ladder Filter with gain loss correction R18=200k, Scale 2V

Moog Ladder Filter with gain loss correction R18=200k, Scale 2V

Moog Ladder Filter with gain loss correction R18=200k, Scale 5V

Moog Ladder Filter with gain loss correction R18=200k, Scale 5V

Moog Ladder Filter with emphasis correction and multiple outputs

Why another Moog Ladder Filter? There are many decent implementations out there in the SDIY domain. Any reasons beside the joy of making something yourself? Well, I wanted to have some features that are not found in this combination elsewhere. First of all I wanted to get rid of loosing gain when the emphasis / feedback is turned up. And I wanted to bring all filter poles out. This means 6dB, 12dB, 18dB and 24dB outputs (Discussed in my next post). As a side effect off the gain corrections I got the emphasis voltage controlled as well. The feedback loop can be opened so you can bring in some filtering or what ever you want in the feedback loop. The exponential circuit can be stuffed with discrete transistors or LM394 as well. And this circuit is temperature compensated with KTY81-110.

Moog Ladder Filter with LM394

Moog Ladder Filter with LM394

The PCB is made for use with ordinary (matched) transistors and with matched pairs like LM394.

Moog Ladder Filter with discrete Transistors

Moog Ladder Filter with discrete Transistors

The schematic:

Moog Ladder Filter schematic

Moog Ladder Filter schematic

Some closeups:

Moog Ladder Filter LM394 closeup

Moog Ladder Filter LM394 closeup

Moog Ladder Filter with discrete transistors closeup

Moog Ladder Filter with discrete transistors closeup

Moog Ladder Filter temperature compensation with KTY81-110 and discrete transistors

Moog Ladder Filter temperature compensation with KTY81-110 and discrete transistors

Moog Ladder Filter temperature compensation with KTY81-110 and LM39

Moog Ladder Filter temperature compensation with KTY81-110 and LM39

Tool: Transistor Matcher

Matched transistor-pairs are often needed while building modules for synthesizers. Especially for exponentiators and differential amplifiers. Not to forget the Moog style ladder filters. I build an transistor matcher for me many years ago and still use it. I know that there are easier matching circuits out there today but mine works great for me so there is no need to change. Mine is based on the original transistor matcher found in the “Technical Service Manual for Moog Modular Systems” on page 53. I added some circuitry for stabilizing the used voltages. That was done to be able to repeat the measurement without bothering about the power supply adjustment. We are dealing with 1mV – 2mV here!

Transistor Matcher schematic

Transistor Matcher schematic

 

There was no need for a PCB. I just put it on perfboard and used IC sockets for the DUT.

Transistor Matcher

Transistor Matcher

 

The usage is easy but you have to consider some precautions. The measurement is very temperature sensitive. You need to keep your environment stable. Use pliers to mount the DUT. Place the transistors into the socket. Measure base to emitter voltage. Don’t touch the transistors with fingers. The finger heat will cause the readings vary. Mark down the Vbe and find two transistors that the Vbe matches within 2mV or better.

Transistor Matcher usage

Transistor Matcher usage

 

With today standards of fabrication transistors it is not unusual to find nearly every transistor within the 2mV Vbe range in a batch. You can easily match your pairs to better standards. Depending on your equipment 0.5mV Vbe match should be easily to reach. But always remember: The measurement is very sensitive. Be careful with your setup!

NGF VCF 36dB: First steps

While waiting for the PCB’s to connect my Kimber Allen contacts to the electronics of the keyboard scanner I worked on my long planned 36dB VCF. I wanted a replacement and additional VCF for the original 24dB Elektor Formant filter. I kept the original structure and just added two more 6 dB filter cells. The emphasis (Q) is now voltage controlled. I added a linear timbre modulation and a “gender changer” for the ENV pot for better usage as well. And I replaced the mechanical LP/HP switch with electronic switch DG419. Maybe I am looking in 48dB next time not knowing if this makes sense musicaly.

To start I tested one 6dB cell with one half or a LM13700 and the DG419 switch on breadboard.

VCF 6dB cell on breadboard

VCF 6dB cell on breadboard

Drawing the schematic:

VCF 2x6dB schematic

VCF 2x6dB schematic

Next was to make some PCB with two 6dB cells each.

VCF 2x6dB cell PCB

VCF 2x6dB cell PCB

The red PCB on the picture is a prototype of a logarithmic voltage controlled current source with multiple outputs (Will come later in this blog). The green PCB is the filter. Those two 6dB cells makes quite a good 12dB VCF like the original 12dB Elektor Formant filter.

Adding two more for 24dB

VCF 24dB

VCF 24dB

Those four 6dB cells makes quite a good 24dB VCF like the original 24dB Elektor Formant filter.

Adding two more for 36dB

VCF 36dB

VCF 36dB

Putting it together and adding the voltage controlled feedback path(Q). The feedback path was added with means of the partly stuffed blue PCB. This one contains the prototype of my implementation of the Moog ladder filter, where I will use the same feedback technique (More in a later blogpost).

VCF 36dB with feedback

VCF 36dB with feedback

Elektor Formant 24dB VCF rebuild (part one)

While waiting for the PCB’s of the Elektor Formant VCO rebuild, NGF VCA DC uno and the Vocoder project, I started the rebuild of the Elektor Formant 24dB VCF. I have already an modified version of the original. Now I want to rebuild the original with easily available parts with better specs and better overall performance. I will add the features of my modified version as well. The features are: Voltage controlled Q, voltage controlled linear slope controll, better ENV controll for the HP with “gender changer”, simple level control. To start with I just put up one 6dB Filter cell on breadboard to verify some part values.

Elektor Formant VCF 6dB filter cell

Elektor Formant VCF 6dB filter cell on breadboard

Four of this cells makes the core of the 24dB Elektor Formant VCF rebuild. You can add two more to go for 36dB as well. Let’s see.

Elektor Formant 24dB VCF 6dB filter cell

Elektor Formant 24dB VCF – 6dB filter cell