BOSS RPH-10 uses Roland VCF chip. Other phaser-filters?

[ExpAnonColin]

http://modezero.com/bitmaps/rph10-Ex.jpg

That's the RPH-10's innards, it uses roland's IR3109 chip, which isn't a gigantic surprise because of the era, but I thought the IR 3109 was more of a multimode/transconductance amp sort of thing and didn't have a way to specifically work as an allpass (unless it is really just a simple transconductance amp), so there must be something this chip is doing in there... possibly with the phaser itself, unless they are just using it to filter out some frequencies because they had so many.  Are there any other phasers that use multimode, not allpass, filters specifically?

-Colin

[Tim Escobedo]

I believe the Roland chip was simply a multiple OTA chip. Perhaps a bit simplified regarding biasing. Which would make perfect sense, since it could do various functions, multimode filter, S&K filter, several allpass filters, VCAs, so on.

Some of the old CEM filter chips were similar, multiple OTAs on a chip, simplified so that biasing was easier, and easily configurable for a variety of VCA/VCF circuits.

[Mark Hammer]

The IR3109 is apparently the not quite so glamourous cousin of the more esteemed SSM2040.  Both are chips packing 4 independent (re: audio) but ganged (re: control current) OTAs that can be readily configured into a variety of filter topographies by moving a few parts around, and from what I gather, you can do pretty much anything with the IR3109 that you could do with the SSM-2040, though they have different pinouts.  It is a proprietary chip made for Roland (don't know by who) and used as a 4-pole filter on a number of their analog polyphonic synths, as well as the RPH-10.  There is a busted Juno-106 in my basement right now packing 6 of them.

I've seen pinout diagrams for them posted around but no datasheet as such (after all, mere mortals couldn't buy them).

[puretube]

the 3080/13600/13700 variety circuits can easily be modded for multifunction filtering...
don`t ask me how, though  :wink:
(once again, it`s all hidden in the app-notes: just combine...).

[Mark Hammer]

Ton's absolutely correct.  In fact it was years of staring at the SSM2040 appnotes and FINALLY staring at the Small Stone schem the right way that prompted the phasefilter mod.  John Blacet had done this decades earlier using the 2040, but no one had really applied it to a guitar pedal.  Score yourself the datasheets/appnotes for the SSM 20xx series (all of which are available here: http://www.synthdiy.com/view/filecat.asp ) and start staring at things that use OTAs.

[puretube]

Oberheim...

[Mark Hammer]

And let's not forget the ever-productive Jurgen Haible's all discrete clone of the SSM2040: http://www.oldcrows.net/~jhaible/tonline_stuff/jh2040.gif

[ExpAnonColin]

So it's just an OTA chip.  That sure clears things up!

-Colin

[Mark Hammer]

"Just" underplays the strengths to be had when 4 of them share a common control voltage source and have exponential conversion in common in a package the size of 2 of them individually.  But yeah, it's *just* an OTA.

[puretube]

actually, the following link was intended for a different thread, which I can`t find at the moment (about a custom B*ss VCR-IC for tremolo or compressing purposes,iirc...) :

http://www.pat2pdf.org/patents/pat3446987.pdf

and, as this is a phaseshift thread:

http://www.pat2pdf.org/patents/pat4052679.pdf

[SeanCostello]

Here's the classic David Rossum patent about using OTA+buffer stages for phase shift, high pass, or lowpass:

http://www.pat2pdf.org/patents/pat3969682.pdf

When the SSM 2040 chip came out, Rossum published an article in Electronotes that described the above modes, plus using the chip for state variable as well as Sallen & Key lowpass, highpass and bandpass. It was EN #78, for anyone who has the full set (and for those people interested in analog synthesizer DIY, I STRONGLY recommend purchasing the full set from Bernie Hutchins).

Barry Klein described the same thing in his book, IIRC, but my copy of his book is sitting 800 miles away, so I can't check it right now.

All of the OTA based generic filter chips that I can recall (CEM 3320, SSM 2040, IR 3109) used the OTA+buffer topology, so you should be able to convert any of the SSM filter tricks to the other filters without too much math. The later CEM and SSM filter chips seemed to be more single purpose, such as state-variable or 4-pole lowpass only. Still, you can obtain a lot of filter response curves from a 4-pole lowpass, assuming you have access to the output of each filter stage as well as the input. Electronotes discusses this in a later issue.

I would assume that the above tricks can be applied to modern circuits based around the NJM13600/13700 without too much difficulty, with the one caveat that the buffers in those chips may not be as precise as the buffers in some of the custom filter chips, or may have different imprecisions that result in different artifacts.

Sean Costello