LM13700 tester circuits?

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Jackdamery
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LM13700 tester circuits?

Post by Jackdamery » Wed May 27, 2015 6:43 am

Anyone got a testing circuit for the LM13700. I have a whole bunch of them and some are probably half or totally dead. It would nice to be able to quickly test them.

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andrewF
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Post by andrewF » Wed May 27, 2015 6:51 am

whynot the VCA from the datasheet
Set it up on whiteboard with a socket and drop the chips in to test

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Post by Jack Gull » Wed May 27, 2015 9:56 am

remember that sometimes you might have a functioning OTA but with a broken buffer!
which is not a problem because many designs avoid to use it anyway

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marcusriley
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Re: LM13700 tester circuits?

Post by marcusriley » Mon Feb 10, 2020 3:34 pm

Hi all -
Sorry to bring up an old thread. Not sure what the etiquette is on this.
I'd like a simple LM13700 circuit to drive an LED. I was experimenting with the single-OTA oscillator in the application notes, but started to burn out all the buffers (I think). With the VCA circuit, it would require an input, a CV, etc... My hope is that there's a simple way to just get a blinking light.
Does anyone have anything they use?

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Re: LM13700 tester circuits?

Post by guest » Mon Feb 10, 2020 3:40 pm

you can make a simple blinker from a single opamp. search for "opamp relaxation oscillator".
openmusiclabs.com

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Re: LM13700 tester circuits?

Post by devinw1 » Mon Feb 10, 2020 4:26 pm

To just make an LED blink, yeah go with just an op amp, or the classic 555 timer. This isn't something where you need an OTA (as useful as they can be!)

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marcusriley
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Re: LM13700 tester circuits?

Post by marcusriley » Mon Feb 10, 2020 5:43 pm

Hi - I'm looking for a simple circuit that lets me test the lm13700. That's why I posted in this thread.
Thanks.

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Re: LM13700 tester circuits?

Post by guest » Mon Feb 10, 2020 6:44 pm

just connect the "gain control" to +12V or something, it will work just fine for testing your 13700.
openmusiclabs.com

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marcusriley
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Re: LM13700 tester circuits?

Post by marcusriley » Mon Feb 10, 2020 10:24 pm

i think some kind of blinking circuit would really help to debug some things, since i know power rails can get pinned.

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Re: LM13700 tester circuits?

Post by guest » Mon Feb 10, 2020 11:37 pm

would you like to debug the circuit you have built? do you have a link to the schematic?
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ege.gonul
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Re: LM13700 tester circuits?

Post by ege.gonul » Tue Feb 11, 2020 12:08 am

Anyone interested in more than LM13700 or LM13600 (same without four pins of independent dual buffers), I am taking Lanterman's Modular Synth class this term, you can find a slide chapter relating to OTA chips which can be used as tester circuit as well.

Lectures are great too, lot's interesting bits and pieces info, if you have time to watch whole series.




There is a homework question for a little bit of circuit analysis of a very cleverly designed PAIA module. Here is the intro of one of the HW questions:

In this problem, we will analyze the PAiA 9710 VCA. The schematic is available here:
http://www.paia.com/prodimages/9710scha.gif
It is in squintovision, so I would recommend opening it up in some sort of image viewing program
so you can zoom in a little. The 9700 series runs off a +/- 12V power supply, i.e. V- on the
schematic means -12V, and V+ means +12V.
The 9710 actually contains two VCAs, along with a “balanced modulator” (a four-quadrant
multiplier, which is good for special effects.) We will look at the “right” VCA. You will find the
core of it along the bottom part of the diagram. J2 is the audio input, and J11 is the output. The
circuit will we look at uses one-half of a dual OTA, the LM13600; the one of interest to us is labeled
IC5:B.
R45 and R43 create a slight voltage bias to compensate for non-ideal characteristics of the
LM13600. Real OTAs have an output bias current, which vexingly varies with the control current.
The 9710 uses a fixed resistor network to put a small bias voltage to try to correct for a “typical”
output bias current. Many other designers use a trim pot for this. In any case, we won’t worry
about such non-ideal effects here, so you may assume the OTA is an “ideal linear OTA” and that
the + terminal of IC5:B is at ground.
The current generating circuitry for the OTA may be found in the upper left corner of the
schematic. R14, R16 (at least I think it’s R16 - it’s the 220K resistor below R14), R12, R18 (at
least I think it’s R18 - it’s a 1500 ohm resistor to ground), IC2:C (an op amp), and Q2 form a
current source similar in spirit to the one from Chamberlin, p. 203 that I presented in lecture,
so if you have difficulties analyzing it, you will want to review your notes from that day. R44 is
a small current limiting resistor to help make sure the OTA doesn’t become an ex-OTA. D2 is
for protection; we will ignore it in our analysis. Similarly, we will ignore R48, the 18M resistor
(consider it to be infinite.)
To simplify our analysis, suppose the “pan” pot is turned all the way towards R, so the wiper
is at ground.
J5 is a “normalled” jack, meaning that if nothing is plugged into it, the “signal” part of the
jack (the top pin in the schematic, which appears to be labeled with a test point “H”) will default
to the signal going to the middle pin (which appears on the schematic to be labeled with a test
point “B”). We will assume that the user has inserted a control signal into J5, so point “H” (the
left side of R14) will be some user-created voltage we will call Vcon.
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Re: LM13700 tester circuits?

Post by devinw1 » Tue Feb 11, 2020 11:05 am

marcusriley wrote:
Mon Feb 10, 2020 5:43 pm
Hi - I'm looking for a simple circuit that lets me test the lm13700. That's why I posted in this thread.
Thanks.
In that case, I think the simple VCA posted is probably the best bet. That is if you are looking to test quickly if a LM13700 is good or not, unless we misunderstand you. This circuit will exercise the bias current, the linearizing diodes, and the buffers (if you choose to hook them up). Do the same for the other half and you'll have a good idea if the chip is OK or not.

All you need is some kind of signal (from a VCO or LFO or function generator or something) and a way to visualize it. :tu:

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Re: LM13700 tester circuits?

Post by mskala » Tue Feb 11, 2020 7:02 pm

Yep, ask for an LED blinker to test an LM13700, and you get LED blinker suggestions and LM13700 test suggestions, but not an LM13700-based LED blinker.

The "triangular-square wave VCO" from the data sheet should work. Swap in a bigger capacitor for C, and maybe a bigger resistor on Vc (both to bring the frequency down into the few-Hz LED blinking range), tie Vc to ground, and attach your LED with a suitable current limiting resistance to the output.
Screenshot_2020-02-11_18-59-53.png
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