I know they used to exist as MANY old analog synths are based on them. These ones for example were used extensively http://en.wikipedia.org/wiki/Curtis_CEM

There are still a few of these chips out there, but at £50+ PER CHIP, they aren't exactly fit for experimenting / hacking, more for expensive antique synth repair :/

Anyway, I just wondered if anybody knows any similar chips - remember though, they must be analog.
And for the uninformed, just because something is in 'chip' form, it doesn't mean it must be digital. ICs are just a collection of components shrunk down and packaged in a more convenient form.

I did get a 24$ CEM3396 from this guy:
http://www.maad.net/ms6/

This guy makes new ones:
https://sites.google.com/site/hearnmorley/products
Aren't exactly cheap either but they're not 50 $.

Thanks for that - still a bit pricey and esoteric for hacking though.

it's actually not too difficult to build your own circuits using either dip-sized smd pcbs with pins in them such that they'll fit like a dip into a main board.

i prefer though using slotted pcbs where the side is etched into a pattern that sits sideway into a slot on the main board.

if you're only wanting to use one circuit though, why do you want them in IC form? you still need all the extra components such as resistors, capacitors and signal/control traces leading to/from an IC.

my favorite filter design actually only uses three transistors and a single opamp making it the same size as one of these IC filters.



the KORG35 filter IC is an example of this. the only difference is they coated it in black epoxy.



another method of mounting sub/daughter boards is to stack them:

Its not a space issue or anything - I've been thinking about building a synth for a while, and I'm weighing the options. I originally thought about building from discreet components, until I happened upon those CEM chips.
Then laziness said 'Hey, half the work has been done for you'.
Frugality replied 'Think of the cost!'.
Sense said 'The only point of building your own synth is so it is UNIQUE - build from scratch'
Then hunger said 'Eat cornflakes now'.

i have built synthesizers.

http://soundcloud.com/aciddose/metarandrome

you can hear an example of the "favorite filter" i refer to in the right channel. the left channel is a sh-09 with a filter built from four lm13700s - the typical four-pole with negative feedback.

the ideal solution is to build small modules which sit as in the first example - on their sides. those can go into SIL sockets, or just solder directly into the main board.

the main board can be built with the bussing on two sides, with the panel controls mounted on the side opposite to the modules.

that is both the cheapest and most flexible solution available.

there are a lot of alternatives but they all involve trade-offs. you can build your circuit directly onto the main board as in a ms-20 for example, but this adds extreme complexity to the layout and severely limits the flexibility for modifications.

you can use boards with other types of linkage, but by replacing bussing with free wires you'll get a wire nest as a result.

the solution of modules on their sides allows you to place modules directly below switches or pots which can be a huge advantage. the use of sockets also allows you to design a common bus architecture where your complete synthesizer can be made up of interchangeable parts. separating the interface and busses from the modules, and treating them as single components simplifies the design process.

you won't need to design a complete synthesizer, instead you can focus on each component one at a time.

the best way to prototype is to limit the functionality of modules to make them as small and simple as possible. plug these into sockets and wire the busses using free wires between modules. if something needs to be changed in a single module, or say you want to add another filter all you'd need to do is insert the new module and add additional busses where needed.

if you don't already have pcb fabrication tools, you can use an etchant made from HCL and peroxide, and you can print etch-resist onto plain copper pcb by using a hot iron and laser printer traces printed on glossy "photo" paper.

the next step up in expense is to use treated pcb and photo exposure, then development. HCL can still be used, but if you're going to be buying the developer chemical you might as well buy some persulphate or other etchant as well.

a far higher quality but more expensive prototyping solution is to use one of the many pcb fabricators - such as "express pcb".

the limitation of the iron/laser method is that it only works on very small pcbs, less than 3"^2 or so. that makes it ideal to use in the prototyping method i describe. for larger boards you pretty much must use photo exposure. (which requires any clean film sheet, and an ordinary laser printer, plus a lamp and development chemical.)

Do note that the 3396 chip is a fairly full featured chip with 2 waveshapers, filter and several vcas. All you need for a full synth voice is two squarewaves and modulators (envelopes, LFOs and such).

Maybe drifting OT but if it's simple circuits you're after for hacking around then this guys are pretty good:

http://www.electro-music.com/forum/topic-34550.html

All very simple and can be battery powered so no tricky PSUs to blow up while tinkering.

It's not normally the board mounted circuitry that takes the time to build it's the front panel and case.

Like Aciddose says a good way is to mount all the pots, switches and displays on one or two big circuit boards. That keeps the time consuming task of wiring the parts to the voicing boards to a minimum.

But having front panel leaves less flexibility if you want to change things around. It can also make repair and servicing harder. Many prototypes use hand wired front panel parts and wire up the modules in a way that you can change them later.

I do like the idea of using little daughter boards for the actual module parts like Aciddose says.

Whichever way you do it building synths can be a lot of fun and educational too.

Tony

Thanks for getting involved everybody - this is a great dialogue.


@aciddose

Could you post a schematic (if its not a 'trade secret' or anything)?

i don't have a tweaked version. it's a slim version of the korg-35 with a more standard feedback path. use single diodes, clip after scaling, not before in the signal path. the level is reduced to reduce frequency modulation by the input signal. feeds are provided for low/band/high. the current through the fet buffer is increased to reduce noise. each of the three circuits (cv, fet, opamp) are bypassed individually to reduce noise. second half of an opamp is used for the vca section - a simple pnp current sink feeding an npn is used. similar to the ms-10 implementation, but the pnp current sink provides much higher linearity.

also, you'll want to reverse the direction of the npns used for trans-conductance to eliminate feed-thru of the cv. the fet buffer can also be modified to work correctly with a wide variety of fets by adding the usual biasing resistor, and dc-blocking capacitor on the output feeds into the non-inverting high-impedance input of the opamp.

i've experimented a bit with improving cv linearity for the filter, but i found it doesn't really help much. not a filter to use for perfect key-tracking effects.

it's high noise, very sensitive to EMI, but it sounds great and has very low parts count. very adaptable by modifying the values in the fet buffer, the scaling pre/post diode saturator and the over-all level through the filter. addition of noise into the vca can be minimized by feeding directly with a very small resistor like 10k.