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VCA circuit options - advice needed
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Author VCA circuit options - advice needed
ucacjbs
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Joined: 02 Feb 2013
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PostPosted: Fri May 17, 2013 10:15 am    Post subject: VCA circuit options - advice needed Reply with quote Add User to Ignore List

Hello there,

Just getting my feet wet with Synth DIY, and am designing a circuit (for now in LTSpice, will move to breadboard and find out what really happens soon) to which I'm thinking of adding a VCA (never too many, etc).

In any case, my first instinct was to just implement the circuit in the LM13700 schematic, but while reading the ethics thread (no, I'm not linking to it twisted), I saw Dr Sketch-and-Etch cite the Mike Irwin FET-based VCA as having become something of a standard implementation

So I was wondering what would the advantages of Mike Irwin's circuit (my guess would be that it enables control over the expo/linear response of the VCA). Anyone care to enlighten me?

Cheers!

ben

p.s. I do have some analog electronics background, albeit a couple of undergrad courses in the early 90s, so don't be afraid to get technical - if I don't get it right away, I can bust out Horowitz and Hill and do some thinking.

[Edit: got the good Dr's name backwards. Corrected!]
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J3RK
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PostPosted: Fri May 17, 2013 10:35 am    Post subject: Reply with quote Add User to Ignore List

Here's my version of the Mike Sims low noise LM13700 VCA. It uses both OTAs from the LM13700. This VCA is very nice. I use it frequently. This will fit on a 1"x3" PCB.

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TOPLEL
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PostPosted: Fri May 17, 2013 2:11 pm    Post subject: Reply with quote Add User to Ignore List

J3RK wrote:
Here's my version of the Mike Sims low noise LM13700 VCA. It uses both OTAs from the LM13700. This VCA is very nice. I use it frequently. This will fit on a 1"x3" PCB.

VCA circ.jpg


That looks like something i can throw on a perfboard pretty easily.

Will it survive higher CV-s than 5V?
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J3RK
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PostPosted: Fri May 17, 2013 2:48 pm    Post subject: Reply with quote Add User to Ignore List

TOPLEL wrote:
J3RK wrote:
Here's my version of the Mike Sims low noise LM13700 VCA. It uses both OTAs from the LM13700. This VCA is very nice. I use it frequently. This will fit on a 1"x3" PCB.

VCA circ.jpg


That looks like something i can throw on a perfboard pretty easily.

Will it survive higher CV-s than 5V?


I regularly use it with CVs ranging from 0 to 15V. 0-10 is the typical range though.
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ucacjbs
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PostPosted: Fri May 17, 2013 4:04 pm    Post subject: Reply with quote Add User to Ignore List

Options, options.

Thanks for that circuit - I'll need to spend some time digesting what's going on with the CV input circuit there.
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Nordcore
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PostPosted: Fri May 17, 2013 4:13 pm    Post subject: Reply with quote Add User to Ignore List

CV here is not too complicated:
first guess, that the op amps works as regular amplifier. If it doesn't there would be no linear control.

If it operates linear, than + and - pins must be (nearly) at the same level. Plus is at ground level, so minus is also on ground level.

So any input CV will give a current solely defined by R5. with a maximum of +15V/39k=0,38mA.

The minus input (simplified) doesn't take any current, so all goes to the emitter of Q1. Ignoring the (tiny) base current, everything goes to the OTA control pin. And that pin might get up to 2mA.

(R9 does normally nothing, except for some voltage loss: 0.38mA*10k= 3,8V, pin 16 is AFAIR about two diode forward voltages (1.3V) from the negative supply, collector voltage of Q1 might go up to one saturation voltage below 0. So R9 kicks in at a current of about 1.3mA (15V supply), and protects the OTA even from input voltages above 39k*1.3mA=50V! )

From a more general view:
The LM13700 OTA is controlled by a current that goes in pin16 (@ -13.something volts) . But you want to control it by a *voltage* referenced to ground.
The voltage is converted to a current by R5. The op amp regulates the transistor (via its base voltage) in a way, that the level is referenced to ground. And the transistor supplies the final current through its collector.
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ucacjbs
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PostPosted: Fri May 17, 2013 4:49 pm    Post subject: Reply with quote Add User to Ignore List

Thanks for the explanation - I follow what you wrote, but I'm still hazy about the function of C3 and D1 in the circuit. Is C3 there for stability? D1 for to turn off Q1 for negative CVs?
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Nordcore
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PostPosted: Sat May 18, 2013 1:00 am    Post subject: Reply with quote Add User to Ignore List

Yes, C3 is for stability. (Especially around 0 Volts. The semiconductors in the feedback path gets very high impedance here, so the op amp turns, thanks to C3, to an integrator. Without it, it tends to oscillate in that region (=zero input voltage) with a few 10 mV and around one Mhz. )

D1 keeps the (possible) negative voltage on B-E low and provides the feedback path for negative input voltages.
Without D1 the output of the op amp would go very negative on negative input voltages. At around -7V the B-E junction will break trough and a current starts to flow. As the collector is still more negative no collector current, and the full input current flows here (base to op amp output). With more negative input voltage this will grow fast above the maximum allowable current (which is *very* low) and the transistor might get destroyed.
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ucacjbs
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PostPosted: Sat May 18, 2013 1:19 am    Post subject: Reply with quote Add User to Ignore List

Thanks so much for your responses - your explanations are very helpful! When I redrew that part of the circuit I saw it as an integrator with a mysterious diode in the feedback path and an even more mysterious transistor bridging the input and outputs (which I didn't trace properly through the OTA). Really illuminating to get the circuit broken down differently - thanks again!
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amdagan
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PostPosted: Sat May 18, 2013 1:32 am    Post subject: Reply with quote Add User to Ignore List

J3RK, thanks for the schematic - I have a few 13700's and TL072's lying around, so would like to give this a try once I have my PSU sorted out.

Question: the first OTA stage is just some kind of constant gain buffer / preamp stage, right? What is its gain? i'm thinking I'll replace it with 1/2 TL072 so I can make 2 VCAs like this with only one 13700 and 3 TL072's, seeing as TL072's are cheaper and smaller than 13700's.
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qfactor
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PostPosted: Sat May 18, 2013 2:27 am    Post subject: Reply with quote Add User to Ignore List

Does this circuit work with 12V instead of 15V, as is?
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Nordcore
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PostPosted: Sat May 18, 2013 3:08 am    Post subject: Reply with quote Add User to Ignore List

@amdagan: http://www.teaser.fr/~amajorel/sims/
(the trick is that the first OTA (at fixed gain) pre-distorts the signal complementary to what the input of the second ota (variable gain) will do to the signal. So in total distortion gets much lower! )

12V: To keep the operating point the same, change R2 from 61.1k to 47k.
For same trim range change R3 to 820k.
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Stinktier86
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PostPosted: Sat May 18, 2013 10:08 am    Post subject: Reply with quote Add User to Ignore List

Hey all,

i'm still learning to follow schematics from scratch as an excercise more than anything else and wouldn't dare building active circuitry until i feel i fully understand everything.

I see various pins (7,8,2,10,9,15) without apparent connections - but in other schematics i've seen, unused pins aren't even described. Am i misunderstanding something here?

Also, i can't see what number the most top-left pin of the lm13700 reads? Is it 8 or 6? :S Because i already see 8 and 6 in other places.

Thanks
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nigel
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PostPosted: Sat May 18, 2013 11:35 am    Post subject: Reply with quote Add User to Ignore List

The unused pins on the 13700 are probably shown because they connect to circuitry inside the chip, but those parts of the chip are not being used in this particular circuit. Pins which are not connected to anything inside the chip are often not drawn in the circuit diagram.

The 13700 contains two separate amplifiers, but the chip only has one set of power connections. This diagram shows the (same) power connections attached to both amplifiers inside each chip, so the power pins (6 and 11) have been drawn twice. The same is true of the 072 - pins 4 and 8 are drawn twice. Usually the power connections for a chip are only drawn once, connected to one of the internal components.
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J3RK
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PostPosted: Sat May 18, 2013 11:45 am    Post subject: Reply with quote Add User to Ignore List

Yeah that's not typical. I just draw all of the connections because it makes it clearer to me at a quick glance. Also in my software logic chips can be configured for various logic signal levels, so I jyst label and configure everything. Then I'm sure it's all set up correctly.
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