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moog ladder filter mods?
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Author moog ladder filter mods?
lilakmonoke
hi all ... i have a behringer model d which i love except for the insane bass volume drop at resonances above ~50%. after that it turns into kind of a bandpassfilter :-)

there are a few ladder filter modules out now that dont have this problem at all so my guess is it can be easily fixed. also 1/2 pole outputs and maybe bandpass out would be sweet.

does anybody here have a deep understanding of the minimoog transistor ladder filter and can you explain? my understanding of circuits is extremely limited but i can solder.

my model d is an exact copy of the minimoog circuit afaik. im in the process of modding it with a breakout box so i could add filter functions no problem.

schematics:



boog filter:



ACL ladder filter with less bass drop and 1/2 pole outputs:

lilakmonoke
so im reading a GS thread about this and i understand that passband drop is true in all feedback based filters. they suggest 3 solutions:

- insert the unfilterd signal somewhere "strategically" into the filter circuit. where?

- "sending the cv from resonance position to vca, or pre filter mixer. so as you raise the resonance you de facto raise the total output level." this of course can create an overdriven vca.

- mix a sine wave of the fundamental with the filtered signal. the mini doesnt have sine but tri which is almost the same. thats a simple solution for sure but i have to deal with 3 oscillators..

https://www.gearslutz.com/board/electronic-music-instruments-and-elect ronic-music-production/1134740-reminder-most-analog-filters-lose-bass- high-resonance.html

a bandpass would be subtracting the LP from the HP signal?

now, where would i find 1/2/3 pole signals?
Huba-Swift
I'm not extremely knowledgable with filter design but thought this might be helpful: http://yusynth.net/Modular/EN/ARPVCF/index.html
It's the Arp 4072 filter, not the moog ladder filter, but I'm sure the principles behind both are similar. After all, they both exhibit the same problem. The dual gang pot in the upper right is there to compensate for the low-frequency gain loss. There's an explanation below the schematic on the page regarding how it works.
guest
the 303 used a similar trick, but with a single gang pot. it took a signal from the wiper of the resonance pot and fed it to the output. as resonance goes up, so does this signal (which is identical to the filter output), and therefore total output increases. its not a completely flat response, but is simple.
lilakmonoke
yes, all feedback based resonance filters have that "problem", even 2 pole state variable filters. the venerable sh-101 also but it still makes awesome basslines. its easy to understand, if you increase the amplitude of a certain frequency the other frequencies have to come down or the signal ceiling has to go up.

most designers include some compensation but bob never did. right now i just adjust the output volume as i increase the resonance - or i add bass with the analog mixer eq which is even better.

the arp filter is one of my favourite filters and sounds great even at high resonances. im not quite sure i understand the schematics. so there are two ganged pots, and as the resonance goes up something is added to what? just increase the volume of the signal, what im doing now manually? whats the blue signal in the right schematic?

without compensation vs with:



Quote:
it took a signal from the wiper of the resonance pot and fed it to the output.


ah, you mean add whatever comes out of the resonance pot and add it before the vca? thats simple indeed! maybe thats why the 303 sounds so overdriven if you crank the resonance. so whats the function of this dual gang pot?

.
.
Huba-Swift
guest is a lot more knowledgeable by me, I'm mostly here out of curiosity. Here are some of my thoughts though.

In my head, I've come up with a couple of ideas on how this circuit might be working, but I just want to be clear that I'm not too knowledgeable and could likely be wrong. This is more of an educated guess than anything.
If you take a look at the pb4 dual gang pot in the 4072 schematics (the upper wiper). That is the one that is responsible for the gain compensation. When the wiper is all the way to the left, or in other words the pot is turned fully CCW, the wiper is connected to gnd, and then through R43 to the inverting input on U2B pin 6. I'd imagine that since that op-amp is referencing gnd on the non-inverting input, having the R43 connected to gnd in this scenario would reduce the gain since gnd through R43 is already helping both inputs on the op-amp remain at gnd. I'm not 100% sure if this is the case though.

Another way I could see this circuit working is that as the potentiometer is turned fully CW, the resistance in the series branch of C9 and R43 is decreased, allowing more signal to pass through to U2B pin 6, increasing the gain. Actually, now that I look at it, this seems much more likely than the other speculation I made.
guest
i think the dual gang pot is doing the exact same thing, but with more components. if the two pots had different tapers you might be able to get a better volume control, maybe?
Grumble
HERE is my mod of the transistor ladder filter.


I have this ^^ replaced by an instrumentation amplifier.
guest
this is a good mod, as it eliminates the gain loss at low CV. this is different from the OPs issue - here as you sweep the filter frequency down, the gain drops a bit, which is particularly noticable at higher resonance, as the resonance feedback drops. you can replace R31,38 with 1M to get most of the way there.

the output impedance of the ladder filter is around 250k at the low end, so you just need to be much bigger than that.

and, if you want a 2 pole filter, you can place another instrumentation amp at a lower point on the filter.
Silesius
There’s this version as well... with gain compensation, separate pole outputs and even voltage controlled resonance. I want to give it a try someday.
Modified Moog filter
Grumble
In the schematics is an error: R32,31 are 100k, not 220E and I have shorted C16,17
Haralds:Werk
Maybe you can use my solution for inspiration. I have added an adjustable gain loss compensation for high Q to my take on the Moog Ladder . And brought out the 6dB, 12dB and 18dB poles on an additional board as well.
lilakmonoke
those are all great suggestions. thanks! ... the model d is all smd parts so i can only do something extremely simple. i also dont have the skills to debug circuits so this has to be trial and error ;-)

so i think i will go for the 303 gain compensation, do you have a schematic for that?

then if i feel adventurous ill get an instrumentation amp and pipe out a 2 pole output following somehow dr. grumbles schematics- although i have no clue how right now. i could to this on a breadboard in the breakout box and run a multicore cable from the synth .... ill leave the 4-pole output like it is, i want to avoid cutting traces. its behringer after all ;-)
Haralds:Werk
lilakmonoke wrote:

then if i feel adventurous ill get an instrumentation amp and pipe out a 2 pole output following somehow dr. grumbles schematics- although i have no clue how right now. i could to this on a breadboard in the breakout box and run a multicore cable from the synth .... ill leave the 4-pole output like it is, i want to avoid cutting traces. its behringer after all ;-)


The signal from the ladder is weak. So, you need to keep the wires as short as possible or you run in all sorts of trouble. This means your additional circuitry must fit in the the Behringer case. And don't forget you need to power your additions. Easiest done from the internal PSU.
Grumble
Haralds:Werk wrote:
lilakmonoke wrote:

then if i feel adventurous ill get an instrumentation amp and pipe out a 2 pole output following somehow dr. grumbles schematics- although i have no clue how right now. i could to this on a breadboard in the breakout box and run a multicore cable from the synth .... ill leave the 4-pole output like it is, i want to avoid cutting traces. its behringer after all ;-)


The signal from the ladder is weak. So, you need to keep the wires as short as possible or you run in all sorts of trouble. This means your additional circuitry must fit in the the Behringer case. And don't forget you need to power your additions. Easiest done from the internal PSU.

There was no "real" need for a single chip instrumentation amplifier, I could also have made an instrumentation amplifier with a quad opamp like the LM324 although with less good results in terms of noise, DC behavior etc.

Haralds:Werk
Grumble wrote:
Haralds:Werk wrote:
lilakmonoke wrote:

then if i feel adventurous ill get an instrumentation amp and pipe out a 2 pole output following somehow dr. grumbles schematics- although i have no clue how right now. i could to this on a breadboard in the breakout box and run a multicore cable from the synth .... ill leave the 4-pole output like it is, i want to avoid cutting traces. its behringer after all ;-)


The signal from the ladder is weak. So, you need to keep the wires as short as possible or you run in all sorts of trouble. This means your additional circuitry must fit in the the Behringer case. And don't forget you need to power your additions. Easiest done from the internal PSU.

There was no "real" need for a single chip instrumentation amplifier, I could also have made an instrumentation amplifier with a quad opamp like the LM324 although with less good results in terms of noise, DC behavior etc.



That's what i used for my additional board with good results.
lemberski
Found this yesterday:

"... but I added a “Q Compensation” control. The negative feedback resonance path in the 1100, like the Moog 904A, caused the amplitude of signals in the passband to be attenuated as the Q increased, which some users found undesirable. If instead the signal is inserted into the Q VCA, this effect is eliminated. In EVOLUTION, the Q Compensation control allows insertion of the signal with an arbitrary mix into either of these inputs, allowing the ratio of direct to resonant amplitude to be arbitrarily selected."

from http://www.rossum-electro.com/about-2/from-daves-lab/

Cheers
lilakmonoke
Quote:
the 303 used a similar trick, but with a single gang pot. it took a signal from the wiper of the resonance pot and fed it to the output


im going to try this. by "output" you mean the output signal of the filter before it goes into the vca? to be sure, where in the schematic would you insert the resonance signal?

Quote:
The signal from the ladder is weak. So, you need to keep the wires as short as possible


good point! maybe there is a way to keep the addon in the synth. now, dr. grumble, could you please point out what parts of your schematics would i have to graft onto my model d to get a 1/2/3 pole filter out. again sorry for being a total circuit fool :-)

lets keep this as simple as possible, all of this or which parts?



Quote:
If instead the signal is inserted into the Q VCA, this effect is eliminated. In EVOLUTION, the Q Compensation control


dave rossums evolution ladder filter has amazing functionality and part of the inspiration for all this. alas, the minimoog doesnt have a resonance vca so i cant do this i think. here you can hear how great this filter sounds compared to the moog way of doing it. i like especially the 1/2 pole sounds and feel master moog has robbed us of half of the goodness of the ladder filter.

you listen to this and all you really need for awesome synth sounds is that filter with all its possibilities and ONE vco with a suboscillator - just as good as a sh-101, to me.



here is the whole post because its such an amazing analysis of the ladder filter. i only understand the general parts but its still a good read:

http://www.rossum-electro.com/history/the-evolution-of-evolution/

Quote:
The fundamental core of the Rossum Electro-Music EVOLUTION Variable Character Filter is Bob Moog’s famous “ladder” filter, which was described in US Patent 3,475,623. This circuit uses the variation of the Bipolar Junction Transistor’s emitter resistance with current as the voltage variable element in an RC filter. In the Moog implementation, four identical stages each implemented a single real lowpass pole. The Moog 904A module included a “Regeneration” control that created a negative feedback path around the four poles. Since each pole provided 45 degrees of phase shift at its -3dB point, increasing this feedback produced a resonant peak at cutoff. The musical utility of the Moog filter is, of course, famous.

In 1973, E-mu Systems introduced their 1100 submodule, which was the heart of their 2100 lowpass filter module. The 1100 used a Moog ladder as its core element, but I wanted to isolate the innate audio characteristics of the filter ladder from those colorations resulting from the input level-shifters and output amplifier used in the Moog 904A. I also envisioned a DC-coupled design with a cutoff frequency range well beyond 10 octaves, as well as eliminating variations of the height of the resonant peak or oscillation amplitude with frequency control voltage.

I level-shifted the exponential generator to allow the filter signal input to be directly applied to the ladder base. I then designed a completely new output stage for the ladder; this circuit has never (to my knowledge) been used outside my designs. The entire signal path was DC coupled and the resonant feedback path phase compensated. The 1100 was my favorite sounding filter (I liked it more than the SSM2040 I later invented, and kept the 1100 as E-mu’s modular lowpass in preference to a cheaper 2040 design). The operational range of the cutoff frequency was from about 0.1Hz to 25kHz, with stable Q’s throughout.

In launching Rossum Electro-Music, I chose a new implementation of the 1100 filter as the first all-analog module for our Eurorack offering, based on its unique and outstanding audio characteristics. I re-engineered the basic 1100 core using modern available surface-mount components and then added a number of features to the original 1100 design.

Ladder filters self-oscillate, and can be used as VCOs. I was able to design the Rossum Electro EVOLUTION’s frequency control exponential generator to be extremely accurate and temperature stable, rivaling the specifications of the best analog VCOs. I also added a novel temperature compensation circuit for the ladder emitter resistance. The actual measured specs surprised and delighted me.

The resonance (“Q”) of the original 1100 was not voltage controlled. I implemented voltage controlled Q using one cell of an SSM2164 VCA. Since the SSM2164 is based on my 1979 design of the SSM2010, this is an apt choice. (Sadly, the original 2164 is no longer produced, so a replica source was needed.) The phase compensation has been maintained, but I added a “Q Compensation” control. The negative feedback resonance path in the 1100, like the Moog 904A, caused the amplitude of signals in the passband to be attenuated as the Q increased, which some users found undesirable. If instead the signal is inserted into the Q VCA, this effect is eliminated. In EVOLUTION, the Q Compensation control allows insertion of the signal with an arbitrary mix into either of these inputs, allowing the ratio of direct to resonant amplitude to be arbitrarily selected.

There is no inherently desirable taper for Q control. In highly resonant, but oscillation-proof filters such as state variable designs, it makes sense to exponentially control Q. But in ladder filters, oscillation is expected, and high Q’s without oscillation are not practically achievable. The Rossum Electro EVOLUTION implements approximately linear control of the Q VCA.

The ladder design, as I implemented it, has no inherent distortion for signals far below the cutoff frequency. The characteristic timbre of the filter comes primarily from distortions of frequencies near and above the cutoff frequency. The degree of distortion depends on the signal amplitude. Consequently, it is sonically interesting to modulate the signal amplitude going into the ladder, and modulate the output signal with the precise inverse gain. This is the function of the Species control. A high voltage into the Species input will go well beyond the linear region of the ladder and cause the filter to distort much more audibly.

Because ladder filters produce their resonance by feedback, the relationship of that feedback to the drive VCAs is critical. The Rossum Electro EVOLUTION places the Q feedback within the drive VCAs. This means that the filter’s resonance is unaffected by the Species setting, but that the amplitude of any self-oscillation will be inversely proportional to the Species level. When using the filter as an oscillator, the Species control can be used to amplitude modulate the output. If oscillation is combined with an input signal, the results become even more interesting. Like the Q circuit, SSM 2164 cells are used for the drive VCAs.

With these additions, the preliminary design of EVOLUTION looked pretty complete. Then Marco asked if it would be possible to add voltage controlled slope (those marketing guys are never satisfied). My first take was that this would not be practical, because varying the slope usually involves controlling complex pole pairs, and the ladder comprises only real poles.

Then I realized that I could steer the current around individual ladder stages in an analog manner, controlling the number of poles rather than the slope. A prototype proved this was both practical and audibly pleasing. Since the resonant frequency of a ladder filter is determined by the 180 degree phase shift point, it changes with the number of poles: 60 degrees for three poles, 45 for four, 36 for five, and 30 degrees for six poles. Modulating the number of poles produces a unique “bubbly” sound.

Two more tweaks were needed to complete the circuit. Because the number of poles not only affects the phase shift for resonance, it also changes the amount of feedback required for oscillation, the pole control circuit needs to control the Q VCA in a manner such that the same Q control voltage produces the onset of oscillation for each pole setting. And since it’s useful but difficult to tune the initial pole setting to be in the center of the range (exactly steering the current to the desired ladder poles), I added analog controlled LEDs to indicate the activation of the poles.

We then sent development versions of Evolution to a number of collaborating musicians who responded with some excellent suggestions. These resulted in a few more circuit tweaks to finalize the product.
Grumble

carefull! R32,31 = 100k and C16,17 are shorted!
also: this part is just an amplifier, it does not filter in any way!
lilakmonoke
yes i know ... thats what i would graft onto the minimoog filter circuit, in addition to whats already there ... see schematic in my first post.
guest
you will want to make a small buffer amplifier, and connect it to the wiper of the resonance pot. a gain of 10 or so should be fine on this amp. then connect the output of that amp with a resistor and capacitor to the VCA input (Q16 on the original). this will run in parallel to the resistor and capacitor that the filter output use.
loki
For those who missed the conversation a couple of years back about the Emu low pass filter.
lilakmonoke
Quote:
you will want to make a small buffer amplifier


thanks! ill solve that riddle somehow ...

Quote:
For those who missed the conversation a couple of years back about the Emu low pass filter.


that schematic is the old emu filter from the 70s. i now took a closer look at the new evolution filter and it mysteriously has 3/4/5/6 poles but no 1/2. who on earth needs a 36 db filter but no 12 db? and a 6 db filter is amazing for pad movements while a 36 db filter is pretty much useless, thats why nobody is using these in synths.

anyways all the more reason for figuring out a 12 db ladder output ...

Maffez
hey guys - inspired by your posts here i went a tad crazy with filter drive mods in my behringer model d. bit of a cack handed mod but imo brings some nice colours to the existing palette

https://soundcloud.com/uibkmedan/sets/boogdrive

https://www.gearslutz.com/board/showpost.php?p=13798544&postcount=145
Maffez
managed to tap the individual poles in the behringer model d & thought i post some recordings for completion's sake (or ocd if you will smile

https://soundcloud.com/uibkmedan/sets/behringer-model-d-filter-pole

was using an opamp circuit as descibed above first but got cleaner results by just breaking the signal path in the boog and feeding the tapped signal back in

kinda scary how much the level drops at 6db and 12db - so a booster circuit as decribed by haraldswerk might be a greta boon here!
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