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 Forum index » DIY Hardware and Software » Thomas Henry designs
VCO-1 and matched transistors
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philpeery



Joined: Nov 08, 2006
Posts: 137
Location: new jersey, usa

PostPosted: Fri May 21, 2010 8:13 pm    Post subject: VCO-1 and matched transistors Reply with quote  Mark this post and the followings unread

Hi everyone,

It's time for me to get back to my synth project after over a year layoff! So I had breadboarded Thomas's VCO-1 when it first came out, and used garden variety transistors for the expo pair, and it worked rather well, even on my old and worn out breadboard!

I have been looking at the datasheet for the THAT340 which has both matched PNP and NPN transistors in it, and was thinking that I could use this chip for the PNP pair for the expo converter with no issues, and then thought I could use the remaining NPN pair for the schmitt trigger section in the triangle part of the circuit. Makes a bit of sense, right?

My question is, does anyone know (Thomas or Scott, maybe???) if there would be any issues with the schmitt trigger section because of the temp compensation needed for the expo? Since they are on the same die/chip/whatever, could there be interaction from the tempco compensation imposed onto the triangle wave? Or would it actually be of benefit? I know that Thomas used the schmitt trigger approach for speed reasons, so I thought that a well matched pair would be a good thing here. I was just wondering about the temperature issue. I did a google search on temperature effects on schmitt trigger performance, but didn't find anything.

It's good to be back working on analog circuits again!

Thanks and Regards,

Phil
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Scott Stites
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PostPosted: Fri May 21, 2010 10:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hey Phil,

Though I've never tried it, by sheer coincidence I did add a section on the subject on the page when I transferred it to my site last week:

http://www.birthofasynth.com/Thomas_Henry/Pages/VCO-1_resources.html

I hadn't thought of the Schmitt trigger section, but I did muse over using the NPN pair in the sine converter. Again, having never tried it, I have a pretty good idea it would work. I think if you used the NPN pair for either, it should be fine - the datasheet mentions grounding the substrate to minimize crosstalk, and I'd bet that would be enough. Perhaps the pair for the sine converter might be a better bet(?) because it's not going to have that sharp "yank" per cycle. It might require (perhaps) a little fiddling with transistor values - or not. I'm perfectly willing to use you as a guinea pig. Laughing

I'd ask TH himself, but I think he's out galavanting across the prairie in some flat section of the country, as seems to be his tradition when he's released from a semester of heavy duty mathematics instruction.

Take care,
Scott

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philpeery



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PostPosted: Sat May 22, 2010 9:14 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Scott,

Thanks for the reply, I will order myself a bunch of THAT 340's and start testing to see how it works with either the sine converter or the schmitt trigger in the same chip as the expo converter. I'll post my results. If I take this all the way to a single sided PCB design, do you think TH would have any issues with me posting that here?

Thanks!

Phil
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Scott Stites
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PostPosted: Sat May 22, 2010 5:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Phil,

That sounds like a plan! I don't think either TH or I would have a problem with you posting a PCB here.

Best Regards,
Scott

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Photon



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PostPosted: Tue May 25, 2010 5:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Phil,
This doesn't pertain directly to your question, but this seemed like a good place to post this.

I'm going to build a couple of VCO-1's in the not to distant future as part of a portable synth I've been planning for a while now. I decided to use an SSM2220 for the expo converter and came up with a cute way to adapt it to Thomas's layout without having to modify any traces. I bent up the pins of a wire wrap socket to fit the pinout on the board and snipped off the two that weren't needed. Checkout the pics, its pretty easy to see whats going on. ( Apologies for my ghetto macrophotography. I just stuck an eye loupe on the front of my camera. Rolling Eyes )


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philpeery



Joined: Nov 08, 2006
Posts: 137
Location: new jersey, usa

PostPosted: Sat Jun 05, 2010 2:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hey Photon,

That's a cool trick, bending the socket leads like that, I may use that!

Update, I got my THAT340's they other day. I've breadboarded page 1 of the schematic using the NPN pair in the schmitt trigger, and everything seesm to work just fine! I'll try it again using the NPN pair in the sine converter as well, maybe during the week next week. It should be fairly easy to make the change on my breadboard as it sits right now. I am assuming that it should also work without issue, but better to test, right?

Once I am done with testing, I will work out and alternate single sided PCB layout with the THAT340. I'll post my restults when I am done.

Regards,
Phil
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philpeery



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Location: new jersey, usa

PostPosted: Sat Jun 05, 2010 2:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

By the way Scott, I really dig your new web site layout! Keep up the great work! It's a resource I go back to over and over again. Very valuable!

Phil
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