The 386 trick

[mveitch]

Im trying to use an LM13700, however it doesnt seem to like normal pot divider biasing as it eats current for breakfast and lunch. Ive searched the forum and found reference to RG's 386 trick. Ive had a look around and cant find it anywhere.  Can someone point me in the right direction, or explain it?

[j0shua]

Hmmmm let me see if i undestand :

first : Im trying to use an LM13700, however it doesnt seem to like normal pot divider biasing ..... (  I DON'T KNOW THE LM13700 ) and i don't see any infor about LM13700  on NTE software ......



2- this is the linl of the information about LM386

http://www.nteinc.com/specs/800to899/NTE823.html

[B Tremblay]

http://diystompboxes.com/sboxforum/viewtopic.php?t=18596

[mveitch]

The LM13700N is a transconuductive dual op-amp - take a look at this article by RG:
http://www.geofex.com/Article_Folders/VCA%20Applications.pdf

cheers

[mveitch]

ps....thanks for the links!

[Paul Perry (Frostwave)]

That's probably no longer an LM13700 if you have been trying to wire it as a 'normal' op amp.
Have a look at the data sheets, plenty of application examples there. Don't be frightened by the +- V15 supplies in the examples, works fine down to +_4 at least.
www.national.com/pf/LM/LM13700.html

[mveitch]

I'm trying to use the LM13700 in a Tremolo application.  I guess I am using the chip as an op-amp, but with the gain set by a triangle wave LFO. The LFO produces 2V-6V oscillation, and i feed this into the LM13700N amp bias pin. However, measuring the voltage on that pin shows that it drags the voltage down to about 1.36V-1.38V oscillation.  This is a very nice tremolo sound. However, I wanted to build a 'depth' pot into this, which means i think i may need to put a buffer between the LFO and the LM13700N (like in RG Keen's article).  However when i started thinking around this, I started to wonder why the amp bias had dragged the voltage so far down. I wanted to be able to see that 4.5V was on the amp bias and therefore the amp was at unity - a 4V-5V would give a subtle effect - and 2V-6V would give a huge tremolo.  Therefore I started thinking of using the 386 as the buffer (just cause I reasoned that it was a need for current that had dragged the voltage down).  I guess im used to thinking in volts rather than amps!

Can anyone tell me if i'm on the wrong track? I've looked for examples of people using this for tremolos, but cant find any. Is this a reason?  

thanks

[puretube]

never use an OTA without a current limiting resistor (~10k @ Ub=9V),
(unless you know what you`re doing)!
This R will protect the current flowing into the IABC pin,
and will also take care of the voltage-to-current conversion of a CV...

[gez]

I've looked for examples of people using this for tremolos, but cant find any. Is this a reason?

They work fine in tremolos.  Connect the Iabc pin, via a resistor, to the output of a LFO.  Make sure you don't exceed the max Iabc rating (check the data sheet).  

The input signal needs to be divided down (you'll see this in schematics) as the OTA will give quite a lot of gain.  Doing this also helps to preserve linearity.

Paul's right though, if you've been biasing it as a normal amp then it's proabably up there in silicon heaven along with all the 3080s I sent there before I knew the above!  

[mveitch]

Hmmm....  

I may order another LM13700N just to check that mine hasnt been toasted.

I only used the normal bisaing on the second half of the chip, so i assume the  first half is still working because as far as I can remember i did have a current limiting resistor of 10k.  It does work just fine too. I guess im making too much out of actually seeing a swing of 2-6v on the amp bias pin.

My problem now is  creating the 'depth' pot stage that goes between the LFO and the lm13700n.

[gez]

I only used the normal bisaing on the second half of the chip, so i assume the  first half is still working because as far as I can remember i did have a current limiting resistor of 10k.  It does work just fine too. I guess im making too much out of actually seeing a swing of 2-6v on the amp bias pin.

My problem now is  creating the 'depth' pot stage that goes between the LFO and the lm13700n.

If you used a current limiting resistor then it should be fine.  It's ok to bias an OTA like a normal op-amp, it's just that you need a resistor (hooked up to a voltage source) or constant current source to set Iabc.  OTAs are usually (though not always) used without feedback, biasing is very similar to that used in op-amps though.

What type of LFO are you using, dual op-amp or sine-wave phase-shift oscillator?

If you measured the voltage at Iabc pin then you'd expect the readings you gave (couple of diode drops above ground).  If you have hooked a resistor from LFO output to this pin, then it's the other end of the resistor (the output of the LFO) that you'd have to measure (and with a scope too as the voltage at this output is varying).

[mveitch]

here goes....

I am using a dual op-amp LFO.  By the way, the tremolo circuit i am using is very much like the one in the 'Practical Electronic Musical Effects Units'  by RA Penfold (Babani).

I am using a 10k current limiting resistor.  The voltage on the output pin of LM13700N is 1.38V is +/- 0.01V (ish). The voltage on the output pin of the LFO is varying between 2 and 6V.  No real accurate readings i am afraid cause i am using a multimeter.  In this configuration (LFO->LM13700N) the circuit works.  I might be being dense with the meter, but the LFO apparently is drawing 35mA from the LM13700.

Is this normal readings for LFO and LM13700N? Am i being stupid?  I am beginning to see that its current thats is important here.

Now. Assuming that is normal, how do I reduce the peak to peak voltage of the oscillator in order to control the depth of the effect? My thinking is that I need to make the midpoint of the oscillation 0V (so it goes from 2V to -2V). Then apply this to an 'amplifier' which can increase or reduce this signal. Then add this signal to a 4.5V reference voltage - so that LFO swings about 4.5V. Then send the signal thro the limiting resistor to Iabc. (I assume that the limiting resistor will translate these voltages to amps on Iabc).

Am I making it too complicated???  Any ideas/help would be greatly appreciated. thanks for your time guys!

[mveitch]

Further to last post. Yes i was being dense with the meter - i forgot that ammeters go in series not in parallel. Schoolboy error  :oops: .  The current is more like 2mA and 4ma between the lfo and the lm13700, which sounds about normal going by the datasheet....

[gez]

You won't need to buffer if you've used a dual op-amp LFO, it would be inadvisable anyway as an emitter follower (with lower end of emitter resistor connected to Iabc pin instead of ground) would limit LFO swing by a further 0.7V.  That's one of the problems with the 13700, the extra diode drop limits LFO swing with a 9V supply.  Only slightly, but the tiny extra you get if you use a 3080 does make a difference in tremolo circuits (well, that's my experience).

10k current limiting resistor is fine, I doubt you've fried anything.  From your original post it read like you didn't include one at all.  :shock:

how do I reduce the peak to peak voltage of the oscillator in order to control the depth of the effect? )

Ton posted a little trick you can do to make amplitude variable with this type of LFO, but I don't have the link.  I'm pretty sure the amptitude mod (could have been a trimpot in his schematic) would affect the rate of the LFO as you turned the pot though.

One way to vary depth would be to stick a pot in series with the 10k current limiting resistor, but it's a bit of a compromise (it only 'subdues' the tremolo, you won't get a totally 'non-modulated' sound)

Another way would be to wire up an inverting op-amp with a pot in the feedback loop.  The pot is set up to totally short out at one end and its resistance increases as you turn it.  Make the pot equal to the input resistor.  Set your LFO to swing around a mid-supply axis (or a couple of diode drops higher if you use CMOS output amps).

[puretube]

http://cache.national.com/ds/LM/LM13700.pdf

your circuit might look similar to fig.2 & 5 ?
(probably modified for Ub=9V)

gez: I can`t remember what you`re referring to - maybe later...

[gez]

gez: I can`t remember what you`re referring to - maybe later...

Ton, it was a hand-drawn schematic you drew up for Brian.

[mveitch]

Apologies that this thread is a bit random. it no longer relates to 386's, more the amplitude of the dual op-amp lfo.  I think I was thinking out loud and have ended up going deeper into the operation of the circuit than i'd expected was required. However its been very helpful.

puretube: my circuit does look similar to Figure 5.  Im happy that the LM13700N works now, its more the output of the LFO that concerns me. That datasheet is pretty helpful though.

Gez: I used your suggestion of an inverting opamp biased at the half-voltage, and a pot in the feedback loop for the depth control.  The oscillator is set at the half voltage too.  Incidentally, I used an op-amp because I tried to do a depth control similar to that in the Zombie Chorus, with a pot tied to 4.5V(half supply) at one end.

http://www.geofex.com/PCB_layouts/Layouts/zombie.pdf

However, I found that the 4.5V supply was a bit unreliable if it is tied to the output of the LFO via this pot, and i lost valuable current at the Iabc. The opamp acts like a buffer and keeps eveything stable, and most importantly works.  Though the down side is that I now have three chips in a supposedly simple effect.

That trick for the amplitude would interesting to see, perhaps it'll get rid of the extra op-amp. Blimey, i feel like ive learned loads today! thanks again everyone.

[gez]

Gez: I used your suggestion of an inverting opamp biased at the half-voltage, and a pot in the feedback loop for the depth control.  The oscillator is set at the half voltage too.  Incidentally, I used an op-amp because I tried to do a depth control similar to that in the Zombie Chorus, with a pot tied to 4.5V(half supply) at one end.

http://www.geofex.com/PCB_layouts/Layouts/zombie.pdf

However, I found that the 4.5V supply was a bit unreliable if it is tied to the output of the LFO via this pot, and i lost valuable current at the Iabc.

I think we're at cross purposes here.  The inverting amp is set up so that it 'amplifies' the output of the LFO, gain being in the range 0 - 1.  The output of the inverting amp then supplies the Iabc pin via a suitable fixed resistor, there's no pot or anything coming off its output (the pot is in the feedback loop of the inverting amp to control its gain).

If you use a 3080 OTA then you can use a dual op-amp in the following way.  One half can be used for the depth control and the other for the OTA's buffer (no bias resistors needed - wire up as a follower and directly tie + input pin to the 3080s output, though you need a resistor from the 3080s output to Vref to bias it up at around half supply).  Just a thought...

[puretube]

gez: I can`t remember what you`re referring to - maybe later...

Ton, it was a hand-drawn schematic you drew up for Brian.

found it - not yet the thread, though...
IIRC, it was posted for other reasons, than amplitude-control,
(me thinx, Brian and Colin both couldn`t get constant amplitudes...),
and in fact, I don`t think the schem can control down to zero - the trimpot rather sets the thresholdvoltage for the schmitt-trigger - well, it`s worth a try...

[puretube]

AAAAAAAHHH - this is the stuff to drive ya insane:
after searching for almost 2 hours in the forum`s archives I gave up;
booted the PC which I use for up-loading
- went thru the hidden archives of my sites - finally found the schem:

it musta been in one of those few threads, that got deleted
(for flaming reasons... IP, or FF...)
:evil:  :twisted:  :evil:  :twisted:  :evil:  :twisted:


(carnaval do Brazil...)