The 386 trick

[mveitch]

I think we're  talking about the same thing.

Like I said, I have a pot in the feedback loop of the amplifier.  The output of the op amp is tied to Iabc by a fixed resistor. I set the gain of the opamp so it goes from '0' through to about '2'. At 2 it causes the lm13700 to fully modulate the signal, and beyond - though i think the sound probably wont be to everyone/anyone's taste...

I probably caused confusion with the Zombie Chorus bit?  Instead of an  inverting amp to control the depth in that circuit, they have just used a pot. One leg of the pot is connected to 4.5V (half supply), one leg to the output of the LFO, and the wiper goes to the next section of the circuit. I found that this was no good because keeping the 4.5V at 4.5V was too difficult. I could have stuck with it a bit longer but I used the inverting op -amp instead.

Sorry about the confusion. Im pretty new to this forum stuff. Ill be clearer from now on.

Am I right in saying that the CA3080 is just a stripped down, single, version of the 13700?  Thats a good point about the dual op amp too - at the moment i have one side of dual op amp and one side of a lm13700 completetly free so if i could get rid of them it would be great.  

When you've used the Lm13700 have you found it to be at all noisy? I expect not because its used in a lot of synth stuff i think, id just like to be able to rule it out .
cheers.

[puretube]

noise: @ the end of the signal chain, there`s no problem.
(some will find it noisy, when putting a higain device after it...).
13700 is better than 3080 & 13600 conc. noise, and CV bleedthrough,IMHO

[R.G.]

Just some quick level setters:
- the forward gain of OTAs, 13700 and 3080 included, is linear with the current going into the Iabc pin. 10x current, 10x gain.
- more than 1ma of Iabc (3080) or 2ma (136/700) kills the OTA
- it is possible to use an OTA in the same manner as a normal opamp by setting the gain fixed and high, then using negative feedback. In this mode, the Iabc is fairly useless, as the gain is set by the feedback resistors unless you run the gain too low. In any case, it is NOT an amplitude modulator in this mode.
- Tremolos come in three versions: modulates up (all variation is above the base loudness level) modulates down ( all variation is down from the base level) and up/down, which is what you want. Up/down is quite hard to do without some additional circuitry to hold a base level and modulate around it. Best is the one you've come onto, an LFO with a base DC level and then pots to vary the up/down level around the LFO base level. See
the "improved LFO" schemo at GEO for some hints.
- The touchiness of the chips in dying with too much Iabc and the need to current feed says that for all simple applications, you use a voltage source and a resistor to do the Iabc into the chip. However, this is not the optimal for audio sound. Ideally, you would like an exponential amplitude response. One way to do this is to feed a linear LFO input to an exponential converter. The other, and simpler way is to note that the Iabc terminal is two silicon junctions in series and feed a linear *voltage* source to the summing junction, arranging that the voltage is limited so that the *current* it feeds is never more than 1 or 2ma. The resulting current in the Iabc node is then an exponential function of the applied voltage, and the resulting gain is exponential.
- the output node of an OTA is a current source. This is incredibly useful for making certain time-delay functions.

[puretube]

& then there is (a fourth) method, that probably will be cloned a couple of years from now...

[gez]

Am I right in saying that the CA3080 is just a stripped down, single, version of the 13700?  Thats a good point about the dual op amp too - at the moment i have one side of dual op amp and one side of a lm13700 completetly free so if i could get rid of them it would be great.  

More or less.  It's a single OTA and doesn't have an on-board buffer, but it's really easy to do one with an op-amp - no extra components needed and, if you use a CMOS output op-amp, you'll get more headroom than you do with an emitter follower (especially if it's a Darlington follower).  

Only down side to using a dual amp in the way I described is you'll have to be really careful with layout, so as to avoid LFO bleedthrough (one half of the chip will be wired up to the output of the LFO).

When you've used the Lm13700 have you found it to be at all noisy? I expect not because its used in a lot of synth stuff i think, id just like to be able to rule it out .
cheers

I agree with Ton, the 3080 is slightly noisier in my experience, but a simple cap (small value) from its ouput to ground often sorts out the problem.  The reason I like them in trem circuits is that the Iabc pin is only a diode drop above ground, rather than two as with the LM13700.  This allows you to swing the LFO's output a further .7V lower.  Might not sound like much, but the peak-to-peak voltage will be up to 1.4V greater, which does give you slightly more depth.  Don't swing the LFO output any lower than .7V above ground though (or too close to .7V), you'll get an audible thump if you do.

Due to the linearity RG mentioned, getting any real level of depth from a 9V supply is difficult, unless you go down the route he mentioned (or something similar).  Keeping things simple, if you use a CMOS amp for the 'depth' control you can bias it so that its axis is a diode drop above half supply, this will allow you to get max output swing to drive the Iabc pin (be aware of its max current rating though) of a 3080.  Little tricks like this all help to get more depth.