Simple discrete synth

Tenine · Joined: Sep 13, 2006 Posts: 44 Location: Coventry

I'm planning to build a monosynth to use in my music (Think 80's 4AD ethereal stuff and MBV) and for live performances.
I have a pretty good knowledge of synthesis and have owned a Clavia NL2X for a about 4 years or so.

I want the synth to be pretty compact (maybe 13" square), so I'll be using a 32-note mini keyboard from an old Casio.
I'd be happy with the architecture being as simple as
one Oscillator with waveshaping,
one AR Envelope Generator
and a Lowpass Gate (maybe two of these).

I know I will be basing my oscillator on the UJT osc shown here - http://www.aaroncake.net/Circuits/torgan.asp

I don't plan to allow any external control interfacing, as it will be very much a performance synth (No, I don't mind the mini keyboard haha!).
Because of that, there's no real need for it to follow any industry standards as such.

Just preparing and writing down thoughts a the minute.
I will need -
Info on resistor-chain keyboards.
Keyboard CV S&H circuit.
UJT Oscillator circuit.
Waveshaping circuit.
AR Generator circuit.
Lowpass Gate circuits.

Am I overlooking anything?

Oh yeah, and this is the good part, this is kind of setting a challenge for myself, because I want to keep it as simple as possible and avoid using ICs!

Alternatively, I could make a fully polyphonic synth (I know I said simple ha!) using said circuits (less the Keyboard CV S&H), effectivly one synth per key. I could add global control over different parameters via lots of homemade vactrols (it may be frowned upon by some, I'm not sure, but I quite like this idea!).

Does anyone have any thoughts or suggestions please?

There's not many people I know who are into synth DIY so I just had to type out my thoughts and see what other people thought and if anyone gets my vibe on this thing!

Cheers!

snufkin · Joined: Apr 06, 2007 Posts: 57 Location: the world

hi just wanted to say

i am building a synth in a similar vain

i went for the sn voice circuit board it uses ICs but its easy as pie (i had only built guitar pedals before making it)

the sn voice is almost a whole mono synth in a single board !

a good power supply is also a must!

Dave Kendall · Posted: Sat Nov 22, 2008 7:26 am Post subject:

Hi.

Juergen Haible has some excellent info here which may give you some ideas.
The single transistor VCA has had favourable comments on the sdiy list IIRC.

good luck Smile

cheers,
Dave

Tenine · Joined: Sep 13, 2006 Posts: 44 Location: Coventry

Hey, thanks!
The single transistor VCA looks ideal - It'd go well with the Lowpass Gate, I'm sure.
I think some other Korgs use a similar circuit.

Tim Servo · Posted: Sat Nov 22, 2008 7:51 pm Post subject: Simple discrete synth

Hey Tenine,

Another source for some simple schematics (many use only transistors, or just a few op amps), would be the PAiA 2700 series. Maybe check the PAiA web site, or just Google away. You should also check the slightly improved PAiA 4700 series. You might also look up Korg MS-10 and MS-20 schemos. I was wondering why you want to avoid ICs? There are cases where an IC (like an op amp) can actually make a circuit easier to construct. For example, look at the PAiA Gnome, which uses a bunch of transistors and one LM3900 IC to make a pretty neat compact synth.

Have fun with it!

Tim (not a Gnome, but I play one on TV) Servo

stolenfat · Posted: Sun Nov 23, 2008 1:19 pm Post subject:

what about adding some glide/portamento to give it some thing special.
I've seen schematics out there for some really simply lag units.

Pehr

nicolas3141 · Posted: Tue Nov 25, 2008 3:04 am Post subject:

I am currently building op-amp based stuff, but tending towards simplicity rather than sophistication in my designs. So I am very interested in the all-discrete challenge. One thing that would make a discrete synth easier to achieve is not attempting to go for accurate 1V/oct tuning. If you go for the V/Hz standard as per MS20 it makes it a bit easier. And if you don't worry about perfect linearity it makes it even easier. The difficulty is then that your controller (keyboard, etc) has to be correspondingly non-linear. Not such a problem if you plan to use a ribbon controller. Slightly harder if you are building a keyboard. More of a headache if you want midi control. The guys doing tube stuff are my inspiration for this approach. See http://www.cgs.synth.net/tube/index.html

Keep us posted on your thoughts and progress.

richardc64 · Posted: Tue Nov 25, 2008 6:20 am Post subject:

The R portion of an AR generator won't be of much use with that oscillator: it stops oscillating when a button or key is released.

Richard (wet blanket) C64

nicolas3141 · Posted: Tue Nov 25, 2008 12:09 pm Post subject:

The release sound only is a problem with a conventional organ/piano type controller where the key press is both a pitch selector and a gate/trigger/volume all in one (although volume is generally a pedal thing with organs).

With a more guitar-like controller you might have one hand selecting the pitch and the other hand doing the gate/trigger/volume control.

Or a sax-like controller with both hands controlling pitch, while the gate/trigger/volume stuff is under breath control.

That's the beauty of DIY instrument making. Outside the square is a place we are free to go to.

Tenine · Joined: Sep 13, 2006 Posts: 44 Location: Coventry

Tenine · Joined: Sep 13, 2006 Posts: 44 Location: Coventry

richardc64 · Posted: Wed Nov 26, 2008 4:15 am Post subject:

Here is a reprint of the orginal Radio-Electronics article for building the PAIA 2720 modular synth. Uses very few ICs. The VCO uses a UJT.

http://www.paia.com/manuals/docs/2720-buildamodular-article.pdf
(2.6MB)

I also found this
http://web.media.mit.edu/~joep/papers/EarlySynthesizerReport.pdf
which is largely based on the 2720, but more complex. Not the clearest of documents. (10MB)

nicolas3141 · Posted: Sat Nov 29, 2008 1:40 am Post subject:

Have a look at this stuff - lots of the modules are discrete transistor based :

http://home.earthlink.net/~synthfred/h_toctop.htm

Tenine · Joined: Sep 13, 2006 Posts: 44 Location: Coventry

Thanks, Nicolas. I looked at the link briefly on Saturday (I was at work) and it looked very promising, but unfortunately it's dead now.
Do you have any of the schematics saved please?

How's your synth going?

phdinfunk · Joined: Jun 04, 2008 Posts: 119 Location: Taiwan

I briefly played with these. I actually really liked them as they're easy to make and mine worked fine (I didn't have a keyboard to test tracking or anything at the time). Also, the sound was cool. The UJTs I had sounded really grungy and badazz.

nicolas3141 · Posted: Mon Dec 01, 2008 2:41 pm Post subject:

A UJT used this way is a bit like a schmitt trigger in a single package. But if you want to build the equivalent circuit from more common components you can build a simple discrete schmitt trigger with an NPN a PNP and a few resistors. You give yourself more options for voltage switching points, etc. and you are more likely to have the bits onhand than a UJT. Add a cap and you have an oscillator.