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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
LC-tank bass drums
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elektrouwe



Joined: May 27, 2012
Posts: 45
Location: Germany

PostPosted: Sat Aug 30, 2014 4:32 pm    Post subject: LC-tank bass drums
Subject description: from theory to working circuit
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I recently got a donation of some 100 small power transformers. Because I don't need so many power supplies, I wondered if I could use the primary winding as a big L, add a small C. in parallel and have it resonate in the audio range.
Pic.1 shows an LC-tank simulation : L is 20H, copper windings have 1Ohm and C is 100nF. As you can see it's a really mighty base drum with a decay time in the minute range ! Unfortunately a 20H coil with 1Ohm resistance needs a lot of windings with fat wire. Although I did not mind the 100kg weight and the size of a washing machine, I decided to continue experimenting with my transformers...
Ok, reality check : Pic.2 shows the simulation with measured data from the primary winding of my 1VA, 230V:6V transformers.
L is still 20H, but a size of 2x2x3 cm3 cries for thin wire: 2kOhm is a lot, and this leads to a bass burst of only 0.1 secs Sad
Ok, we have to fight the 2k. I knew a circuit called "negative impedance converter" with the nickname NIC. It' made of a non-inverting amplifier with positive feedback.
Pic.3 shows the bad LC-tank from pic.2 with a NIC build with 1 opamp , 1 R for the positive feedback and a 100k potentiometer to adjust gain.
As you can see, the compensation works and let us control the damping between very short bass bursts (uncompensated) and oscillation (overcompensated) ! finding the narrow sweet spot with the pot. to get a nice 0.5 ..1 sec tone is not so easy, but I can accept it for such a simple circuit.
Last step was to realize it in Lunetta style : in pic.4 the opamp has been replaced by 2 inverting CMOS amps and a 3rd inverted is used for a 2nd distorted output than can be mixed with the clean sine of the LC-tank.
Have fun !


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