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WIGGLING 'LITE' IN GUEST MODE

New module: Discrete Ladder Filter
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author New module: Discrete Ladder Filter
Synthbuilder
Just a quick heads up on a new module that I'm developing.



http://www.oakleysound.com/dlf.htm

It's not terribly ground breaking but I'm trying out some ideas with printed circuit board design that I thought I would try in 5U before I start putting them into a completely new series of 5U, 19" and Eurorack designs that I'll be working on later in the year.

The Discrete Ladder Filter is pretty much a modern day clone of the 904A low pass filter and combines it with a clone of the CP3 mixer. It also has an interesting drive feature which can overdrive the filter section without changing the output volume significantly.

The boards are due in late next week and it'll take me a while to do the documentation and test the design. Heck, the PCB may not even work, so don't go holding your breath for this one to become available. It would nice if it could ship before the end of the month though.

It has been some while since I brought out a new 5U board. That should change with my new career direction. I closed my repair and service shop last month so I'm now spending all my time getting some of the many projects completed that have been on the back burner for far too long.

Tony
terjewinther
Being a Moog addict and a very positive 5U user, this is really good news. Best of luck with your new ideas, but whatever you do, keep up the 5U side of things.
Gizmo
Interested in 5U version ...
- Will Paul build some of these?
- Can the mixer output be available separately?
Synthbuilder
Gizmo wrote:
Will Paul build some of these?

Paul has expressed an interest. Once the PCB is out it may be worth letting Paul know you are interested.

Quote:
Can the mixer output be available separately?

Not on either of the current front panel designs. However, the output can be accessed on the PCB and taken out to a socket.

Now if Paul does build this module in MOTM format there is a possibility that it'll end up as the filter core panel - which is always my preferred option since I like to keep things as modular as possible. It's worth noting that when using multiple inputs the sound is not changed whether the inputs are mixed externally to this module or internal - both panel designs have the audio running through the CP3 mixer.



If an MU version is made this could be made more flexible but that'll be Paul's decision. I'll be chatting to Paul in the next few weeks about the possibility of making some dedicated MU designs in the future. Mostly to take in the benefit of the extra space that a double width MU panel design allows, but also to reduce the depth of the modules.

Tony
Synthbuilder
terjewinther wrote:
Best of luck with your new ideas, but whatever you do, keep up the 5U side of things.


Thanks Terje. I've no plans to ditch the MOTM format and, as stated above, we'll also look into the possibility of some dedicated double width MU modules too.

That said, I need to get of my butt and design some cool Euro modules. I've pretty much missed the boat on that one but hopefully I'll have something that people will want.

Tony
Noiseconformist
Synthbuilder wrote:
Gizmo wrote:
Will Paul build some of these?

Paul has expressed an interest.

But Paul isn't expanding to SMT, is he?
Just teasing! wink hihi
Michael.
Noiseconformist
Synthbuilder wrote:
I've no plans to ditch the MOTM format and, as stated above, we'll also look into the possibility of some dedicated double width MU modules too.

*breathes a sigh of relief*
5U is the format!
Quote:

That said, I need to get of my butt and design some cool Euro modules. I've pretty much missed the boat on that one but hopefully I'll have something that people will want.

Tony I'm sure you'll have to offer something which people are going to queue up for! thumbs up
Looking forward to talking to you in August!
Michael.
Blake Smith
Great to see new 5U designs.. Glad to hear the MOTM format will get continued support. My personal favourite, especially when it comes to building.
Awazleon92
Hello Tony,

I was wondering, which filter will be the next ? hihi
Fortunately, you did it...
However, one question, I already own the TSL filter, so, does it sound like it or is it different from the TSL ?
I know that CP3 mixer is full of character and can obviously modify the sound, but...

Have a nice day
Pascal
Synthbuilder
Awazleon92 wrote:
I already own the TSL filter, so, does it sound like it or is it different from the TSL ?


I got the new boards on Monday so I'm currently still testing and tweaking. It is sure to have a different sound - mostly because of the additional 'drive' control - but I don't have a TSL module here to compare it with, so more precise comparisons will be difficult. Both are Moog ladder filters though so there will be some similarities. But the big differences will show once you start to overdrive the inputs and use the drive control.

I'll get some sound samples up and maybe even a video to show the new guy in action. It won't be until later next week though.

Tony
terjewinther
Can you tell what kind of discrete components will be used for the filter core? Will we have to match induvidual transistors (or will that be possible)?
In comparison the TSL use individual BC549/BC550 transistors if I remember correctly, while the original moog modular filter uses 2N3392/2N4058 transistors.
Synthbuilder
terjewinther wrote:
Can you tell what kind of discrete components will be used for the filter core? Will we have to match individual transistors (or will that be possible)?


All transistors are BC550 (BC549) and BC560 (BC559). Nearly all of them have to be Vbe matched in pairs to +/- 1mV. Matching to this degree is pretty straight forward. All you need is a decent voltmeter, a 9V battery and a couple of 0.1% resistors. Actually, I tested a whole bunch of transistors that I bought from Farnell that all came off the same strip of cardboard (bandolier) and all were with within +/-1mV.

Tony
Synthbuilder
[s]https://soundcloud.com/takla-makan/904a-rubyone[/s]

Two Oakley VCOs, one saw and one square wave. Fourmix controls the input levels. The VCF-EG is a VRG and the output of the filter goes straight to the ADSR/VCA module.

I'm just tweaking the input levels, drive, frequency and resonance. A little delay for bounce and a bit of reverb for ambience.

Tony
Synthbuilder
Issue 1 PCBs are now available for sale.



Because it's issue 1 (that's the prototype version) I managed to let through two small errors pertaining to the way the pots are mounted to the board. These faults are easily corrected and if you're not mounting the pots to the board it won't make any difference at all. However, I'll put full details on how to do it in the Builder's Guide.

Because of these errors the issue 1 PCBs will be for sale at just 14GBP each.

I will make an issue 2 with these errors corrected at some point.

I'm nearly finished the documentation. This should go up tomorrow morning if all goes well.

Tony
Synthbuilder
Builder's Guide now up on site.

The User Manual will follow tomorrow morning - 5th July.

Tony
Synthbuilder
All documentation completed and now available on site.

Tony
Noiseconformist
nice'n tasty as always! applause
terjewinther
Very nice! Will order one right away.
I read the docs quickly, but didn´t found any recommendation as to what types of transistors and caps recommended for the filter core. Does it matter what kind of BC550 is used (A, B, C alternatives), and have you tried different caps? It might be "snake oil" asking for this, but I have seen discussions here and there regarding caps in filters. I especially noted that Jürgen Haible (RIP) used large, oversized caps in his moog ladder filter core, but then again: caps then (1960s) and now (2016) is a completely different matter when it comes to quality.
Synthbuilder
terjewinther wrote:
Does it matter what kind of BC550 is used (A, B, C alternatives), and have you tried different caps?


I used C transistors. I'll make a note of that in the Builder's Guide.

As for caps... I used polyester. I'm not a great believer in different caps sounding different so long as a good dielectric is used. And polyester film is a good dielectric. You can get better (eg. polypropylene & polystyrene) but whether it'll sound different I have no idea.

Tony
terjewinther
Synthbuilder wrote:
terjewinther wrote:
Does it matter what kind of BC550 is used (A, B, C alternatives), and have you tried different caps?


I used C transistors. I'll make a note of that in the Builder's Guide.

As for caps... I used polyester. I'm not a great believer in different caps sounding different so long as a good dielectric is used. And polyester film is a good dielectric. You can get better (eg. polypropylene & polystyrene) but whether it'll sound different I have no idea.

Tony


Very good. Thanks for the feedback regarding this.
Noiseconformist
Synthbuilder wrote:
All transistors are BC550 (BC549) and BC560 (BC559). Nearly all of them have to be Vbe matched in pairs to +/- 1mV. Matching to this degree is pretty straight forward. All you need is a decent voltmeter, a 9V battery and a couple of 0.1% resistors. Actually, I tested a whole bunch of transistors that I bought from Farnell that all came off the same strip of cardboard (bandolier) and all were with within +/-1mV.


I didn't scrutinise the original circuit, but I wonder how that was done in the 1960s?
I reckon that the match of the transistors significantly influences the sound (whether to get extra even harmonics or not).
I'm not sure how specifying 0.1% resistors for the selection process influences the audible result.
I'd like to speculate chances are more likely than with different types of caps. wink
I'm remembering a Multimoog which sounded more aggressively due to a bad ladder.
Michael.
Noiseconformist
this gets OT, but I just stumbled upon this:
"transistors are relatively new circuit elements, and working with them gives one a little of the pioneering feeling."
http://i0.wp.com/moogfoundation.org/assets/1956-2-Bob-Letter.jpg
Noiseconformist
... or maybe it's just utter nonsense and it's really just about the CV getting through. hmmm.....

Noiseconformist wrote:
Synthbuilder wrote:
All transistors are BC550 (BC549) and BC560 (BC559). Nearly all of them have to be Vbe matched in pairs to +/- 1mV. Matching to this degree is pretty straight forward. All you need is a decent voltmeter, a 9V battery and a couple of 0.1% resistors. Actually, I tested a whole bunch of transistors that I bought from Farnell that all came off the same strip of cardboard (bandolier) and all were with within +/-1mV.


I didn't scrutinise the original circuit, but I wonder how that was done in the 1960s?
I reckon that the match of the transistors significantly influences the sound (whether to get extra even harmonics or not).
I'm not sure how specifying 0.1% resistors for the selection process influences the audible result.
I'd like to speculate chances are more likely than with different types of caps. wink
I'm remembering a Multimoog which sounded more aggressively due to a bad ladder.
Michael.
Synthbuilder
Noiseconformist wrote:
... or maybe it's just utter nonsense and it's really just about the CV getting through. hmmm.....


Matching keeps CV breakthrough to a minimum. Since the audio signal running up the chain also slightly modulates the cut-off frequency there is a possibility that matching will affect the sound too. Just how much I can't say. I think some quality time with a circuit simulator might be useful here. I don't think matching all the transistors up the ladder is necessary - just the two bottom pairs and the top ones are probably all that is needed. But since this is a clone (of sorts) I matched them all in pairs.

Of course, the other key thing to remember here is that we are matching for just Vbe. Really we should probably be looking at matching Hfe too particularly for the transistors in the differential amplifier. Ultimately, I just matched for Vbe +/-0.5mV and was happy with the sound so left it at that.

Those 0.1% resistors are for the matching circuit. I use this circuit:



I don't use the 5K trimmer but use 0.1% 100K resistors instead. A perfect match will give you 0.0mV on your meter.

Tony
mooneh
It would be great if you could do more dry demo someday. Its easier for me to hear the character. Preferably with bass sounds like this one :


https://www.youtube.com/watch?v=_j5bbpVCuPc
Synthbuilder
mooneh wrote:
It would be great if you could do more dry demo someday. Its easier for me to hear the character.


Dry did you say... without delay? Dead Banana

How's about this then?

https://soundcloud.com/takla-makan/oakley-sound-discrete-ladder-filter -simple-bass-sequence

[Sorry for some weird reason it's not linking properly in the embedded player]

A very simple sequence of three notes repeating. The patch is the standard subtractive thing: Two VCOs, one discrete ladder filter, VRG for filter envelope, ADSR/VCA for final volume control.

As the sequence is playing I'm tweaking the cut-off, resonance, drive and input levels to the filter.

Tony
mooneh
Thanks sounds great. Must have w00t

I guess the sound around 1.36 is made with the drive knob to overdrive the filter? So nice.
Noiseconformist
Synthbuilder wrote:
It's not terribly ground breaking but I'm trying out some ideas with printed circuit board design
that I thought I would try in 5U before I start putting them into a completely
new series of 5U, 19" and Eurorack designs that I'll be working on later in the year.


In fact, that's what I'd consider a major teaser! cool
Synthbuilder
Issue 2 boards should be available later in the month and the price for the bare board will be 20GBP. Issue 2 PCBs will be nearly identical to issue 1 boards but will allow the pots to be mounted as normal. If you want an issue 1 board to save a bit of money and don't mind the extra hassle in mounting the pots now is the time to get a board at the current cheaper price.

Tony
emi2345
Hi there. I am planning my first modular and am pretty set on the Oakley system. I've built guitar amps and pedals before. I'm a full-time musician in a band. My favourite synths that I've played are (rather unoriginally) the vcs3, minimoog and arp 2600.

I'm thinking of a simple starter package around 12U, with a view to future expansion to around 20U, along the lines of:

mididac, vco controller, 2x vco, filter, adsr, multimix?

I'm trying to decide which filter and format I should go for. Would the discrete ladder filter be the most appropriate for minimoogy/early arp2600 sounds or the transistor ladder filter? Is there much difference, apart from obvious features? And the other question I had was, are there any advantages to the 2U format DLF? Could I use it as a distortion/mixer unit if the filter is fully 'up' or will it always sound to some extent 'filtered'?

Thanks for the help and apologies if I've posted this in a bad place, I can't make a new thread yet.
Synthbuilder
emi2345 wrote:
mididac, vco controller, 2x vco, filter, adsr, multimix?


I'd add another ADSR or, my favourite, a VRG.

Quote:
I'm trying to decide which filter and format I should go for. Would the discrete ladder filter be the most appropriate for minimoogy/early arp2600 sounds or the transistor ladder filter? Is there much difference, apart from obvious features?


They do sound different and the Superladder does have the rather useful one pole and bandpass modes. But for older synth sounds I'd take the DLF.

Quote:
And the other question I had was, are there any advantages to the 2U format DLF? Could I use it as a distortion/mixer unit if the filter is fully 'up' or will it always sound to some extent 'filtered'?


The 2U DLF behaves much like a 1U DLF with a simple passive mixer in front of it. Personally, I'd have the 1U DLF and an additional Fourmix module. Having the mixer separately allows for filter free (and therefore very clean) mixing and allows flexibility later on when you add more filters to your system. The Fourmix also has an interesting overdrive mode as well as a useful way to add constant voltages to your audio which can create asymmetrical distortion.

Tony
emi2345
Thanks for the reply Tony!

That slope/bandpass pot in the TSL is really cool, I'm assuming it wouldn't be easy to implement it in the DLF instead of one of the CV depth pots?

One thing I'm not quite clear about the DLF is the way the CVs work, I'm guessing they both control the frequency and not the drive or the resonance like the TSL? Couldn't the same effect be produced by mixing two CVs together using a multimix and then patching that into one CV input?

The VRG is definitely on my long-list, I just want to get a simple system up and running first and then I can add each new module one at a time and learn it really well instead of having a tonne of complex stuff to learn in one go.

One final thought, and this may not be the best place to ask, does anyone know of a simple 2U or 3U loudspeaker/amplifier/headphone module I can get in the 5U format? I've been searching and I can't find one. One of the things I love about the VCS3, 2600 and Gleeman Pentaphonic is you can sit in a corner of the studio away from everyone else working on a patch without hogging the main studio speakers and making everyone else get itchy feet. They are complete instruments you can take anywhere and play without any external stuff. I'd be happy to try to build one from a schematic.

Thanks for all the help.
Synthbuilder
emi2345 wrote:
That slope/bandpass pot in the TSL is really cool, I'm assuming it wouldn't be easy to implement it in the DLF instead of one of the CV depth pots?


It would not be a trivial modification since you'd need another differential amplifier and a crossfader circuit. There's not a huge sonic difference between the sound of the TSL and DLF though - they are both clearly Moog ladder filters.

Quote:
One thing I'm not quite clear about the DLF is the way the CVs work, I'm guessing they both control the frequency and not the drive or the resonance like the TSL? Couldn't the same effect be produced by mixing two CVs together using a multimix and then patching that into one CV input?


Yes. Neither drive nor resonance are voltage controlled on this one. If you have a Multimix summing your cut-off CVs then the extra CV input on the DLF is superfluous. However, with the simple need for just envelope and keyboard CVs to control the filter there is no need for the Multimix - just plug them direct into the DLF and save that mixer for other duties.

Quote:
One final thought, and this may not be the best place to ask, does anyone know of a simple 2U or 3U loudspeaker/amplifier/headphone module I can get in the 5U format?


Inbuilt speakers and me don't get along well. I'd always settle for a nice little monitor speaker sat on top of my modular rather than one internal one taking up module space and which would inevitably not give the sonic results worthy of the modular connected to it.

A headphone amp and output module has been on the cards for some time.

Tony
Blake Smith
Quote:
A headphone amp and output module has been on the cards for some time.


That would be very handy.
emi2345
Well it seems like the best solution would be to get the DLF and then add a diode ladder later, which I think is similar to what the VCS3 has.

A 1U headphone output module would be great!
trebmonster
I'm a newbie so I'm not allowed to start a new thread...!
I'm just starting to build a DLF for use with my Moog Voyager. I shalln't be building a separate modular system but may add a couple of VCOs later. So I need to insert the DLF between the Voyager's mixer and filter. Moog kindly provide a jack for this purpose but the signal lever from it seems too low. I've not hooked up the scope yet but a DVM shows about 0.5v (AC) with all 3 VCOs on. My understanding is that the Oakley (and other MU systems run with analogue signals at around the 10V peak level. Is this right? So do I need to add an op-amp amplifier with gain of say 10 at the front and an attenuator at the end?
Many thanks smile
Synthbuilder
trebmonster wrote:
I've not hooked up the scope yet but a DVM shows about 0.5v (AC) with all 3 VCOs on.


The DLF is designed to work with signals ten times that, ie. 10V peak to peak. The input 'mixer' of the DLF will start to overload at around 8V peak to peak. Ideally then you need to amplify the signal with something like your mixing desk or an EFG module. Bear in mind that the output of the DLF will also be ten times what the Voyager wants on the way back in. So you'll need to cut it down a bit; a -20dB passive attenuator would be best. Not entirely straightforward then...

Alternatively, you could make the following resistor value changes to the DLF:

R18 220K, R24 15K - this will increase the input sensitivity by a factor of ten or so. I think this should work without introducing too much instability.

R27 3K3 - this will reduce the output by a similar amount.

I've not tested this so do let us know whether you get the results you are looking for.

Tony
trebmonster
Thanks Tony. I'll change R27 as you suggest, but as I may add some VCOs in the future, I'll leave the input sensitivity as it is and boost the Voyager with an Op-amp instead. Cheers.
trebmonster
All perfect. op amp giving gain of 10 on the input and your 3k3 change works spot on. Levels with Voyager just right. DLF calibrated perfectly - all trims to 0.000 V smile Sounds great. Love how you get resonance AND bass end at the same time. Many thanks.

905 clone reverb being built now when the tank arrives. But what next...???
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