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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
Single supply VCA?
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Hammer



Joined: Mar 13, 2018
Posts: 72
Location: Russia, Zhukovsky
Audio files: 23

PostPosted: Wed Apr 11, 2018 6:54 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm back with my version of TDA7052A VCA!

Volume pin have something like pullup resistors. If volume pin is not connected, there will be max volume level. If you will connect cap. between volume pin and ground, cap. will be chargered.

Transistor draws this pin to ground. But 0.4V is minimum voltage, when amp output level is already zero. That's why I added diode to minor shoulder of voltage divider.

Left part of this circuit have trigger for drum envelope. MOSFET is for fast cap. charging. This circuit is triggering even when you touch input cap. by finger.


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TDA7052A VCA release example

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mph



Joined: Aug 25, 2007
Posts: 87
Location: France

PostPosted: Sat May 23, 2020 6:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Hello
I've tried your circuit and it's working like a charm.
However I wanted to create some "snap" effect on the attack.
It reduces the overall amplitude but allows to have "plucked" very percussive attacks; I use it with 40106 and ICL8038 oscillators mainly.
You can try 1K lin/2K lin for the "Pluck" control.
You'll need a bit of Release if the Pluck is not at min.
Here's a simple mod :


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richardc64



Joined: Jun 01, 2006
Posts: 679
Location: NYC
Audio files: 26

PostPosted: Tue Jun 02, 2020 6:00 am    Post subject: Reply with quote  Mark this post and the followings unread

Here's one I thought I hadn't tried.

I had trouble understanding the Korg KR55 drum machine VCAs until I flipped the schematic vertically, changed the pnp transistors to npn and reversed the diode. Then I recognized it as similar to a VCA I had tried, and used successfully years ago: The VCA from]Ralph Burhans'Digivox, posted elsewhere in this very forum.
http://electro-music.com/forum/viewtopic.php?highlight=digivox&t=44343

Here's the relevant excerpt: http://www.sdiy.org/richardc64/rtlsynth/vcatxt_071209a.gif

In both Korg's and Burhans' circuits the bottom transistor gets the input signal and the top transistor is the control element.

The CA3053 Burhans used is obsolete, but its internals are easily replicated with two 2N3904 or similar, or three if one wants the dual outputs of the original design. I have labeled the resistors (in blue) with the closest 5% values. The "Bias" trimpot would be adjusted for minimal thump caused by CV feedthru.

Rs may need tinkering for voltages other than 3.6V RTL levels. Because the circuit is linear it will work on any waveshape as Input.


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dk



Joined: Feb 12, 2019
Posts: 115
Location: Europe

PostPosted: Tue Jun 02, 2020 1:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'm sure this leaves "simple vca" land, but the right side of what you posted looks awfully similar to this: https://www.aleph.co.jp/~takeda/radio/img/farmVCA-scheme.gif

Obviously, they only look similar... tying both of the outputs on yours to that opamp would minimize CV bleed through and make it mono. I was actually thinking about trying to get Takeda's VCA to work single supply, although I'm not sure I can get it there.
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richardc64



Joined: Jun 01, 2006
Posts: 679
Location: NYC
Audio files: 26

PostPosted: Wed Jun 03, 2020 11:57 am    Post subject: Reply with quote  Mark this post and the followings unread

dk wrote:
I'm sure this leaves "simple vca" land, but the right side of what you posted looks awfully similar to this: https://www.aleph.co.jp/~takeda/radio/img/farmVCA-scheme.gif

Obviously, they only look similar

Yep, only in appearance, the operation of the two designs is quite different. In Burhans' VCA the "top" transistors are the control elements; in Takeda's they form a differential amplifier handling the audio. Note that the base of Q2 is grounded. Someone more knowledgeable could explain the operation of Takeda's design (which I only vaguely understand) better than I possibly could.

Quote:
... tying both of the outputs on yours to that opamp would minimize CV bleed through and make it mono. I was actually thinking about trying to get Takeda's VCA to work single supply, although I'm not sure I can get it there.

That would be a stretch.

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Steveg



Joined: Apr 23, 2015
Posts: 182
Location: Perth, Australia

PostPosted: Thu Jun 04, 2020 9:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

Takeda's design looks like a simplified LTP VCA ... see: https://sound-au.com/articles/vca-techniques.html#s51
Q2 looks like it is connected to ground but this is a dual rail supply design. In a single rail design it would be at 1/2 VCC.'
Q3 modifies the gain of the LTP controlling the amplification. Q4 & Q5 are there to convert the control voltage to a base drive sink for the PNP transistor Q3. It looks like Q4 is intended to keep Q3 from saturating ... Q5 makes sure that only negative control voltages are applied.
The effective gain of the LTP is the ratio of the top resistors R5/R6 to the effective resistance of Q3.
You could use Q1 alone which is what the earlier two transistor VCAs are doing ( KR55 & Burhan's designs) but using Q1 & Q2 with U1 as a LTP differential amp keeps down power supply noise and CV bleedthrough.
For a lunetta CV bleedthrough and power supply noise probably aren't issues, especially if you are modulating rail to rail square waves. Smile
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dk



Joined: Feb 12, 2019
Posts: 115
Location: Europe

PostPosted: Sat Jun 06, 2020 4:52 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for that reply Steveg. I've gotten a somewhat similar transistor arrangement working already on single supply for a Steiner-Parker-esque filter, so I figured that part of Takeda's circuit would be doable (or at least fudge-able). What I'm entirely unsure about is how U1 will work on only single supply... since the idea is to null out the DC control signal, won't I end up with at least a static DC signal (whatever Vref ends up being?) on the output if the circuit is not centered around 0V?
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Steveg



Joined: Apr 23, 2015
Posts: 182
Location: Perth, Australia

PostPosted: Sat Jun 06, 2020 7:39 am    Post subject: Reply with quote  Mark this post and the followings unread

Now that I look at it longer Q3 and Q4 form a conventional current mirror ... Q3 passes exactly the same current as Q4 does ... It is just drawn in a different way.

U1 will work just fine on a single supply.
Wherever you have a resistor to "ground" you need to establish a 1/2Vcc supply level. This can be a capacitor and either an op amp or a transistor driving it to 1/2Vcc.
Or else you can put a divider at each location so for R8, a 33k resistor, replace it with a divider of twice the value to VCC and ground ... so for 33k two 68k resistors should work. Similarly R4 and R7 could be each replaced by a pair of 1k resistors forming a divider from VCC to ground.

You can probably leave Q5 out completely.

Just another edit ... all three of these designs are simplified voltage multipliers ... the signal input and control inputs are to some degree interchangeable. Look up "Gilbert Cell" if you want more info on the full deal.

re-read your question ... U1 will output the difference between the voltage drop across R5 and the drop across R6. It should null out the effects of the Q3 voltage changes and that won't change by moving from dual rail to single rail power. If you replace R8 as i indicated it should keep the output signal centred around 1/2 Vcc.
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dk



Joined: Feb 12, 2019
Posts: 115
Location: Europe

PostPosted: Sat Jun 06, 2020 3:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

Cool, thanks for the pointers... hopefully I'll get to breadboarding it in the next few days and report back!
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