This is a voltage controlled audio oscillator using two LM13700s.

 

At 1 KHz ARTA software (http://www.artalabs.hr/) reports THD+Noise of about 0.3%.

 

I started this about 25 years ago as the backend of a simple Theremin. The front end used two 74HC14 RC oscillators. One was fixed, the other used the capacitance of your hand to change its frequency. The difference in frequencies produced a DC voltage to control the frequency of a VCO using LM13600s (not LM13700s).

 

Then the LM13600 was discontinued. National Semiconductor said the LM13700 was a suitable replacement. It wasn’t. The LM13600 would work at 9V (a 9V battery) with a resistor divider to produce a bias voltage of 4.5V. The LM13700 does not work at that low a voltage.

 

This version runs on +12VDC that I regulate down to +9VDC and use an ICL7660 to produce -9V. The reason I took +12V down to +9V is because I want to be able to use the very cheap +12V switching power supplies from China via eBay. Some of these power supplies have 0.5V of hash on the output. Most of them are sold as LED power supplies (LEDs shouldn’t mind the hash) but some are sold as CCTV supplies. I bought two identical supplies (12V 1A) labeled for CCTV. With a load of 120 mA one had 50mV of hash (excellent) but the other had 200mV of hash (crap).

 

Because the control voltage now starts at -9V I made a level shifter so the input control voltage goes from 0 - 5V.

 

There is a switch to select the control voltage between an external source and an internal pot. The internal pot can be a trim pot or a real pot. To use a real pot don’t stuff R25 and connect the Real Pot to the R25 pads using wires.

 

There are three outputs:

 

1. Line level controlled by R33.

 

2. Microphone level (fixed).

 

3. Headphone output controlled by R26. It can also drive an 8 Ohm speaker but not very loudly.

 

Trim pot R23 controls the bias to the LM13700s. You can adjust it for minimum distortion. But if you go too far the LM13700s will have trouble starting or won’t start at all.

 

The level shifter trim pot R40 controls the offset. When using the internal control trim pot R31 controls the voltage for the VCO. When using an external control voltage the external voltage source can have a range of 0 - 5V or so.

 

I used the LM1875 as the headphone driver because it has better specs than the LM386. It is not intended to produce very loud sound in a speaker. If you do use a speaker it should be 8 Ohms or higher. Since many speakers are 4 Ohms you can put two in series. Make sure they are in phase. Otherwise their sound will want to cancel. That assumes the speakers are pointed in the same direction. You could put them in a box with one in the front and one in the back (both pointing outwards). In that case wire them out of phase so the sound will be in phase. I wonder what that would sound like. You could make a speaker enclosure with a speaker on each side. Wire Speaker 1 and Speaker 2 in phase to form Pair 1. Wire Speaker 3 and Speaker 4 in phase to form Pair 2. Wire Pair 1 and Pair 2 out of phase. Orient the speaker enclosure so you are looking at the corner of pair 1. It should produce a nice diffuse sound field. I wonder if anyone has ever tried this. I wonder what it would sound like.

 

 If you are not already a Maker (and you want to be one) see my tutorial on Making Things at www.jmargolin.com/making/jm_making.htm


JM

May 24,2019
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