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Synth design help, please
Author: Tenine on October 24 2006 Viewed 4251 times. 3 people liked this blog. You can rate it below if you haven't already.
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So, as many of you know, I've planned to build a synth for a criminally long amount of time now.
After getting over my initial disliking of IC's, I've found that a synth can be made almost exclusively with LM13600 or LM13700 chips, using the circuits taken from the IC's own datasheet. Lovely simple circuits.
I think I've settled on an architecture now...
*Either 2-octave C-C keyboard or 2 and a bit octave E-C.
*One Oscillator [it has Saw and Pulse waveforms, but I never use Pulse, so it'd just be a Swatooth oscillator]
*Filter [12dB LP. Frequency, Resonance and Envelope Mod]
*Envelope [Simple Decay control]
*VCA [Maybe switchable between Gate and Envelope]
It has unwittingly turned out rather like a 303.
That doesn't really bother me though, I want to use this for bass sounds and maybe mild blippy stuff.
Probably more like the 303 was intended to sound rather than how people use it 
But there are a few things that bother me a bit, or I'm unsure of.
Which is where I'm sure you guys can help and suggest things
I'm not after a synth with a huge palette of sound but I fear this will be really limited.
I'm not really sure what to do about that.
I realise how stupid that might sound.
'Add another waveform!' you say, but I don't like Pulse waves much and to add another waveform [other than Pulse / Saw] then means adding another Oscillator.
There is one osc which produces Triangle and Pulse, but again, without the Pulse, I'm left with only one usable waveform.
Adding another oscillator pushes the amount of work involved to the limits of my patience.
I don't want to make a really phat synth. I think one osc should be enough.
It's not supposed to be a bass monster.
Also, I was wondering [Psyingo, TomBugs, I'm looking at you] if an LM13600/13700 could operate on 9V [+/-4.5V] It says nothing about minimum operating voltage in the datasheet that I can see.
Any help really would be appreciated.
I've been thinking about this all day and it's really got to me, it's literally made my head hurt 
Thanks,
Alex x | |
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10/24/06
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Squeal
I just checked the datasheet of the LM13600 and it looks like you can set it up to do triangle and square wave as an alternate to pulse and sawtooth. Did you notice that?
10/24/06
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Psyingo
use the squarewave output for the input of an integrator circuit to turn the sqr into tri?
also im pretty sure the ic should handle 9v, lemme check though....
in the datasheet it says the common is 12 -13.5 v... but it should handle under that, right?
10/24/06
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xik
LM13600 as in the OTAs? I can understand using OTAs in the control stage, but not so much for the wave shaping where OPAMPS would be more usefull.
Somehow I'm thinking the chip your using isn't an OTA. Do you have a link to the datasheet?
10/24/06
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Tenine
Thanks, Squeal, I had seen that, but my problem is with the Square/Pulse wave [I just don't like them much].
It'd be nice to have Triangle and Saw, but both the Oscillators that produce those waveforms also produce Pulse too .
Which makes them a bit of a dead weight in terms of building:functionality ratio.
Psy, that sounds pretty good.
Would that be done using something like a voltage follower with forward and reversed biased diodes on the input like an envelope generator? [breathes].
When you say the common, do you mean the Voltage between the power rails?
ie - 12V = +/- 6V
I guess I could build a Pulsewave osc using half of the chip [see Figure 18] and put a cap on the end to obtain a Saw, right?
Of course, the Saw falling time would be the same at any frequency as opposed to being relative to the frequency. There would also be an upper limit on frequency.
Does that sound like it could work?
Or would it be best to just make the Figure 18 osc and use the other half of the IC to make a wave shaper?
10/24/06
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Psyingo
naw, an integrator would be an op amp design...
and as far as common... i thaught youd know!
10/24/06
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Otterfan
xik said: "Somehow I'm thinking the chip your using isn't an OTA. Do you have a link to the datasheet?"
lm13600 datasheet
Definitely a OTA. You see them in the audio portion of a lot of DIY filter and vca designs. The Music from Outer Space Soundlab takes its entire filter and vca section straight from those app notes. PAIA uses lm13x00s in their filters too.
Apparently they're good for more than making voltage-controlled resistors.
The lm13600s have been obsoleted by the lm13700 b/c of poorer audio performance.
Aaron Lanterman gets positively gushy about OTAs in some of his lectures.
10/24/06
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Otterfan
Tenine said: "+/-4.5V"
No first hand experience with OTAs, but why such a low supply voltage? In a regular opamp that lead to a lot of noise problems...
10/24/06
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Tenine
Revaluating then, I could use an op-amp [TL072] as a waveshaper and envelope generator.
I could then make an oscillator and VCA from an single LM13x00 and use another to make a VCF.
Not bad, really.
10/25/06
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race4prize
wow you people are clever.
10/25/06
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Tenine
Otterfan said: " Tenine said: "+/-4.5V"
No first hand experience with OTAs, but why such a low supply voltage? In a regular opamp that lead to a lot of noise problems..."
I'd just like to be able to run the whole thing off an easily available PSU [or maybe a 9V battery, but a battery certainly wouldn't be my first choice].
Noise problems shouldn't be too serious at that voltage though, I imagine.
12V would be better though.
Would you advise against using a DC adaptor similar to a Boss PSU?
I've had some ideas...
Adding an attack control would be simple and add possibilities.
Maybe a Sub-oscillator, I'm not sure though.
edited: Oct 25 2006
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Tenine
race4prize said: "wow you people are clever."
Haha
Not really, if I were that clever it would be finished and I'd be playing it right now...
Psy, what did you mean about the 'common'?
10/25/06
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Squeal
12V dc supply shouldn't be hard to find.
10/25/06
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Tenine
Aye, true, but would anyone advise against using a DC wallwart PSU?
edited: Oct 25 2006
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Tenine
Thinking less electronically for a minute, though, and more as a synthesist,
how would go about adding a little flexibilty to the following synth architecture?
[Do remember, it's mostly for bass sounds, but it's not supposed to be some sort of Moog]
1 Saw VCO
1 12dB Lowpass Filter
1 AR Envelope Gen [hardwired to VCA, variable modulation depth of VCF Frequency]
1VCA
Any suggestions, please?
10/25/06
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Psyingo
SUB OSC.
and use your old friend the 4017 (most sub osc's are sqr on synths anyway.) you could even have switches for devide by 2, 4, 8 for differnt octaves if you wanted.
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