cheap but effective external filter for the SID.

[t_xen]

Hi all.

Well, On my MBHP_Sid, I have to get this repaired. I've blown the 5v regulator, after attempting to diagnose a huge amount of hum I was getting from the sids. (likely a ground loop as they were working fine before I stuffed everything into a case).

Anyhow, I built a Midibox_CV using an AOUT_LC to drive linear VCOs, but then discovered that this will not work due to accuracy of the voltage output, so.........

I'm going to use the AOUT_LC I've constructed to drive a filter I've etched and built by hand. I have yet to test the filter, and figure that this would be a pretty good time to do it.

The filter is based off the LM13700 chip.

I have 20 of these ICs. seems like a lot, eh? Well, I only needed 5 of them for other "projekts", but could only order them in a bulk of 25, and have a bunch of spares. these things sound great in the minisynth (so I knicked the schematic of the filtersection)! I'll update this thread when I've actually finished it off. Here's an image of the filter for your enjoyment. After I have this built and working, I'll post an audio clip of it in action.

Before I forget, the filter will take a CV controlled cut-off, but the downside is it is not capable for CV controlled resonance. meaning you will need an extra knob on your SID frontpanel to adjust the resonance.

I'll give the details on the wiring later.

;)

[rutgerv]

Very cool t_xen! I'm curious how it sounds! You can probably make the resonance controllable over CV too with half a 13700. You can put the 13700 in a VCA-configuration and use the VCA to control the feedback path. I did the same thing with the Rick Jansen Moog schematic and it works great.

Greetings,

Rutger

[drsyncenstein]

Hi,

your filter capacitors look like ceramic types to me, conventional wisdom is to use styroflex for those.

[AndrewMartens]

That's a nice, easy-looking layout there.  Should even be pretty simple to put on a veroboard.  Bipolar power supply, I assume?

Regarding ceramic vs. styroflex caps, of course the conventional wisdom is to use better caps in filters, but... meh, you don't really *need* to.  Sometimes it's easier to just use whatever you have on hand.  ;)

[t_xen]

yea, the ceramic caps are pretty much what I have on hand.  ;D

anyhow, it's gonna be a bit longer before I post more info on it. just tried testing it and got no audio. only osillations from the resonance. I have a few resistors coming out to only half their rated value for some weird reason, sooooooo.......

Still working on it. I'm pretty confident that this will work when I get the bugs worked out since this is a prototype.  :P

and yes, it is Bipolar voltage required. Operates at +/-9v, but can handle a max of -/+12v.

[t_xen]

Well, after numerous attempts, this is currently non-operational as I have yet to get this working.  :-[

if anyone would like to take a shot, here's the circuit.

Here's the schematic I was working with for parts and value references.

[AndrewMartens]

Just taking a quick look at the circuit, I noticed one problem... in the middle of the schematic, R42 (20k) should be connected between pin 8 and -9VDC.  Pin 9 should also have a 20k resistor R43 connected to -9VDC.  On your PCB layout, I believe these correspond to E$14 and E$15, except that you have them connected to +9VDC instead.

This might cause it to be maxing the resonance value, hence why all it does is oscillate.  Fingers crossed!

[uPDATE]:

On a slightly longer glance, you seem to have V+ connected to pin 6, and V- connected to pin 11.  From the schematic, V+ should be on pin 11, and according to the LM13700 datasheet, V- should be on pin 6.

... aaaaaaaand all that really means is that on your PCB layout, the + and - labels are reversed.  But assuming that you hooked it up with red = V+, then you at least have connected it correctly.

[t_xen]

Argh!  :o

messed up the labels. it's the tiny things I usually end up overlooking.  ::)

Going to swap the +/- voltage lines and test again.

;)

thnx.

[sonofdirt]

hey cool i've been looking for the simplest filter design for my mbsid too and i like what you've got here. I looked up the datasheet on that IC and it sounds pretty cool, looks like it can do VCO's, VCA's, VCF's, S&H, all kinds of good synth stuff! The datasheet shows diagrams for a few filter designs: Hi-pass, Lo-pass, and 2pole Lo-pass "Butterworth" filter. It also appears that the mini-synth designer drew up a circuit for Bandpass as well (im guessing it's the HPF and LPF wired in series?). Anyways good luck w/ the circuit and keep us posted, ill order a couple of these IC's soon and start breadboarding. It would be cool if we could make it a multimode filter, somehow switchable by MIOS & store settings with patches... Anyways, I intend to use something like this to completely replace the filtersection of my 6581 cuz it sounds like crap, especially the lower cutoff settings which come out all distorted for some reason (is that normal 6581 behavior?) I think many people will have interest in this as an alternative to the CEM/SSM filters because its so small and simple. Keep up the good work!  ;D

update: k so i've done some research on how to make filters w/ op-amps like this IC, check out section 5-8 of this PDF for lots of filter recipes: http://www.analog.com/library/analogDialogue/archives/39-05/Web_Ch5_final_PtB_F.pdf