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]

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.

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]

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]

Will Paul build some of these?

Paul has expressed an interest.

But Paul isn't expanding to SMT, is he?
Just teasing!
Michael.

[Noiseconformist]

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!

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!
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 ?
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]

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]

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]

https://soundcloud.com/takla-makan/904a-rubyone

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!

[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]

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]

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]

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.
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/ass ... Letter.jpg

[Noiseconformist]

... or maybe it's just utter nonsense and it's really just about the CV getting through.

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.
I'm remembering a Multimoog which sounded more aggressively due to a bad ladder.
Michael.

[Synthbuilder]

... or maybe it's just utter nonsense and it's really just about the CV getting through.

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 :