The VCO-555

elmegil · Posted: Sat Jan 25, 2014 7:08 pm Post subject:

It is worth noting that one measure to compare VCOs by is how well they track--how many octaves they stay in tune.

It's certainly only one measure, but on that score, I find the 555-VCO to be the best tracking of any of the analog ones I've built. I've only built the MFOS soundlab VCOs, but also el Cerrito (another TH design), X4046 (another TH design), and the Teezer (a through zero VCO by Ian Fritz).

jukingeo · Posted: Sat Jan 25, 2014 10:05 pm Post subject:

Ok, guys, fair enough.

Judging by your answers and what I read so far, I think I would be more happy with the 555 VCO than those from MFOS. So I put myself down for 3 555 VCO circuit boards. BUT for the LFO I am going to stick with the MFOS Variable Skew.

Thanx,

Geo

jmcecil · Joined: May 11, 2013 Posts: 20 Location: Prattville, AL

I'm having a hard time translating the thermistor requirements to a datasheet. Will this one work?

Mouser: Vishay 2k Radial Thermistor

mcop · Joined: Apr 27, 2012 Posts: 46 Location: Brighton UK

Try these: http://www.thonk.co.uk/shop/tempco-resistor-krl-bantry-3500ppm-x2/
or these: http://synthcube.com/cart/forums.html?route=product/product&path=74_18&product_id=147

jmcecil · Joined: May 11, 2013 Posts: 20 Location: Prattville, AL

jmcecil · Joined: May 11, 2013 Posts: 20 Location: Prattville, AL

damn, I tried to find them on synthcube but it kept taking me to a lesser tempco. I could find that one.

mosheen · Joined: Jun 19, 2013 Posts: 21 Location: NC

Magic smoke carries them. http://www.magsmoke.com/tempco-resistors.asp

andreschouhy · Posted: Thu Mar 13, 2014 7:52 pm Post subject:

Hi, i'm building 2 of these vco 555 modules. But I'm having troubles to get some components like the tempco (which, reading above, i'll be replacing with a fixed resistor until i get the tempco), but also some resistor values. So, my question is: Is it ok to replace the following resistors?
R9, R10 from 2k to 2.2k
R13 from 3k to 3.3k
R21, R22 from 20k to 22k
R26* from 300k to 330k (* this is not really R26, but the page 2 on the schematic presents a couple of resistors named R26 with different values)

Thank you very much

aladan · Joined: Aug 13, 2011 Posts: 52 Location: Australia

John.R.Strohm · Joined: Feb 09, 2014 Posts: 8 Location: Huntsville, AL, USA

Thomas's designs call out a PT-146 2K ohm +3300 ppm/degree C tempco.

The part is manufactured by Precision Resistor Company.

It is not a stocked standard part. They make them to order. For a US customer, there's a $50 setup charge. Minimum order is 10 parts. They get about $5/each, for quantity 10-99. Unit price declines slowly as order quantity increases. To make it simple: 10 parts will cost you US$100, plus shipping: $50 setup and $5/each x qty 10 = $50. Plus shipping. That's IF you're in the US.

I emailed them recently. The above is taken from the quote they gave me.

Music From Outer Space offers them for US$9/each.

Magic Smoke offers them for either US$3.50/each or US$1.90/each, depending on whether you look at their Tempco page or their Price List page. I emailed them a few weeks ago to inquire as to which was the correct price, but have not heard back, and I am VERY nervous about ordering from someone who isn't answering emails asking about pricing. (I have my doubts about those prices, as they are significantly below the manufacturer's price at even the highest quantity break.)

andreschouhy · Posted: Fri Mar 14, 2014 7:13 am Post subject:

elmegil · Posted: Fri Mar 14, 2014 7:34 am Post subject:

Tim (Magic Smoke) has moved and had other chaos in his life right now. He's a solid guy--I've never had any parts NOT show up that I've ordered from him.

That said I understand your nervousness, and I would not be able to reassure you about how long it might take.

There are other sources of 2K tempcos out there though:

http://synthcube.com/cart/forums.html?route=product/product&path=74_18&product_id=205

wackelpeter · Posted: Sat Mar 15, 2014 1:20 am Post subject:

I'm thinking of an substitution for 2k tempco when i'm doing this in the future...

Rene Schmitz has an article written about this over here:
http://www.schmitzbits.de/
it's that article: whats this tempco stuff...

and as he wrote it works and is much cheaper with nearly the same results in the "normal" temperature range... only at very low or very high temperatures the difference in range between tempco and the substitutes will raise...

acidblue · Joined: Jun 26, 2009 Posts: 226 Location: The Darkside

Excuse me but what exactly is the triangle on sheet 2 with pins 9 and 10???
Its right between the two R26's

elmegil · Posted: Mon Mar 17, 2014 10:10 pm Post subject:

The LM13700 has a darlington buffer in it.

Here's the view on the datasheet...

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acidblue · Joined: Jun 26, 2009 Posts: 226 Location: The Darkside

I did not know that, thank you

fonik · Posted: Wed Mar 19, 2014 6:31 am Post subject:

wackelpeter · Posted: Wed Mar 19, 2014 9:27 am Post subject:

fonik · Posted: Thu Mar 20, 2014 2:30 am Post subject:

fonik · Posted: Thu Mar 20, 2014 4:53 am Post subject:

wackelpeter · Posted: Thu Mar 20, 2014 8:29 am Post subject:

better do not ask how i select them... to be moderate i would call it intuitive... Razz

surely have no idea which i should select so i took 2 different 1k and 2 different 2k sensors...
as René said they should be in series with a normal resistor reaching the value of the tempco... i'm not sure if it works, so i can only try it myself...
at least i have spent 2 Euros to experience that it doesn't work as supposed for me...
then i still have the NTC method to try...

hm... as you didn't have any pcb's left i see myself soldering and drilling stripboards my whole life.... Wink

but no worries about that...

jukingeo

Hello All,

Ok, so I bit the bullet and bought 3 PCB's from Fonik to build 3 of these 555-VCO's. For the most part getting the parts isn't too difficult, but it seems the hardest part to get are the matched transistors and the special Tempco resistor.

I don't have as much test gear as I used to and I don't think I am going to be building a bunch of voltage tracking VCOs, so it seems like the easy way (but not the cheapest) is to buy the transistor set and Tempco that Ray offers on his MFOS site. The trouble is that the transistors are 3904's and not 3906's. The Temco is also different being a 3300 and not a 3500. So I am curious if I can use these transistors and Tempco or is this something that has to be exact. If it has to be exact, then were can I source the Tempco and matched transistors necessary to build the VCOs.

Thank You,

Geo

mcop · Joined: Apr 27, 2012 Posts: 46 Location: Brighton UK

You'll need matched 2N3906's as they're PNP rather than 2N3904's which are NPN.
Ian Fritz's method for matching them is fairly easy and does the job perfectly. There's a thread about it over at Muff's : http://www.muffwiggler.com/forum/topic-81990-0.html&postdays=0&postorder=asc&highlight=

You can get the exact tempco's from either Synthcube in the US or Thonk in the UK although I think others have been successful with 3300ppm tempco's.

Thomas_Henry · Posted: Tue Apr 08, 2014 9:32 am Post subject:

If you check the exponential relationship, you'll see that 3300 is a better fit. I only used 3500 because that's what I had. Either will work acceptably, though.