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 Forum index » DIY Hardware and Software
XR-2206 Questions
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mikeb



Joined: Nov 20, 2006
Posts: 53
Location: The Automotive Capital of Canada

PostPosted: Thu Dec 14, 2006 11:15 pm    Post subject: XR-2206 Questions
Subject description: A few Q's about TH's VCO and other uses for the chip itself..
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After having introduced myself to VCO design and becoming very familiar with TH's XR-2206 VCO circuit while laying out a board for my own variation of an XR-2206 VCO.. I have some questions about the XR-2206 chip.

If I want to ditch the SKEW portion of TH's circuit, can I use the AM pin as a basic VCA?

Figure 6 on the XR-2206 datasheet shows that 0V on the AM pin should give an amplitude of 0, while 4V or higher should give an amplitude of 1 (max). It's also worth nothing that -4V does the same, but reversing the phase. This isn't a feature I'd need. Just going from 0-1 while applying 0-4V will be great for me. Is this doable and as simple as I think? THe reason I'm doubting it is that the example circuits all show this AM pin already grounded. Well, if it's grounded.. shouldn't the aplitude be at 0?? But it's not..

Another reason I'm slightly confused about the XR-2206 is that it has the "analog multiplier input" pin. Where it states that the output level is approximately the same as the DC bias at this pin. Again it seems like this is something I could use for controlling the amplitude by a control voltage..

The reason I ask is that I'm trying to do a basic synth on a very small single board. Having the ability to have VCA-type control over the output right on the XR-2206 would be awesome.

I know TH's goal was to make this thing into the best VCO it could be... which he's obviously done. But I wouldn't mind a few alternate features.. like a VCA or something.

Also, between pins 13 and 14 is the switch to go between sin/tri. A pot can be placed here as well to shape the waveform to minimize harmonic distortion... and I'm wondering if a large value pot (instead of the one recommended) was placed here, if it could let all waveforms between sine and tri be generated, intead of just a switch? I can try this myself soon though...

I think that's all the questions I have right now.
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Scott Stites
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PostPosted: Sat Dec 16, 2006 4:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Figure 6 on the XR-2206 datasheet shows that 0V on the AM pin should give an amplitude of 0, while 4V or higher should give an amplitude of 1 (max).


Actually figure six on my datasheet shows that at Vcc/2 on that pin, the amplitude approaches 0. That's why the examples you see have pin 1 grounded - that's actually max amplitude. Vcc would also be max amplitude, but with reversed phase.

Quote:
Another reason I'm slightly confused about the XR-2206 is that it has the "analog multiplier input" pin. Where it states that the output level is approximately the same as the DC bias at this pin. Again it seems like this is something I could use for controlling the amplitude by a control voltage..


I'm thinking this is pin 1 again? The multiplier is the VCA (it's actually a four quadrant multiplier, or a 'ring modulator' actually).

Quote:
Having the ability to have VCA-type control over the output right on the XR-2206 would be awesome.


Yes it would. Bear in mind it wouldn't be terribly ideal; 55 dB is a pretty small amount of range for a VCA.

Quote:
Also, between pins 13 and 14 is the switch to go between sin/tri. A pot can be placed here as well to shape the waveform to minimize harmonic distortion... and I'm wondering if a large value pot (instead of the one recommended) was placed here, if it could let all waveforms between sine and tri be generated, intead of just a switch? I can try this myself soon though...


I suppose it's possible. If you listen to a sine wave, then a triangle wave, you'll notice, though there is obviously some difference, it's not terribly dramatic, so this would be a sort of 'fine' adjustment kind of thing for an unfiltered timbre. If the adjustment allows the signal to be clipped at some point in the pot's travel, it certainly would give you much more harmonic variation.

Cheers,
Scott
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mikeb



Joined: Nov 20, 2006
Posts: 53
Location: The Automotive Capital of Canada

PostPosted: Sat Dec 16, 2006 4:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hey Scott, thanks for replying.. I very much appreciate it.

You are totally right about Figure 6. My mistake.. gnd at pin 1 would be max aplitude.. Vcc/2 would be 0, and Vcc max. Exactly like you said.. Oops Smile

As for the Multiplier, For that I am talking about pin 3. "maximum output amplitude is inversely proportional to the external resistor, R3, connected to Pin 3. (see Figure 3). In the TH circuit you have up, this pin is used to switch the resistor while switching between sin/tri to keep the amplitudes about the same. I'm wondering if getting rid of that convenience would let this pin be used for VCA type function.

Mainly for now, I just want to have the ability to also input a Gate signal and use that to switch the XR-2206 from outputting audible sound, to nothing at all when the gate signal is absent. I wonder if I could use that FSK input, sort of like how TH's Sync input works...

Thanks for responding to my question.. The XR-2206 VCO has been a great introduction to me, and has really had me reading anything I can get my hands on regarding VCO design.

Mike
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