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 Forum index » DIY Hardware and Software » Thomas Henry designs
3080 VCO current sources
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Conboy



Joined: May 01, 2009
Posts: 25
Location: London

PostPosted: Tue May 14, 2013 2:42 am    Post subject: 3080 VCO current sources
Subject description: a few issues with current sources from Making Music with 3080
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I have been having varying degrees of success building a simple VCO from the designs in Thomas's book Making Music with the 3080 OTA.

The VCO section is working nicely now on my veroboard design. But my problems are with getting the current source to work correctly.

I have tried building the "Easy Exponential Current Source" which ought to suit my needs. However the pots range is unusable.
All the range is within the 1/3rd of central position on the pot - at either end (upper 1/3rd and lower 1/3rd) the VCO bottoms or tops out with the frequency remaining the same for the rest of the range.
I can't work out how this is adjusted? Or why it should be so.

I actually get much better results just using the "simple manual arrangement" from the simple linear current source chapter! From which I am able to get the full range of the VCO from maybe 5Hz to 20kHz +

Seems wrong that the super simple solution works better! Smile

Also - with both designs I get a stepping effect. Almost digital in quality (ie not welcome!) - is this an inherent part of the design? Can it be smoothed out in any way

any advice much appreciated

Paul
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forbin



Joined: Jan 29, 2009
Posts: 120
Location: Fremantle, Australia

PostPosted: Tue May 14, 2013 5:30 am    Post subject: Reply with quote  Mark this post and the followings unread

It does sound a little bit like you may have one of the transistors the wrong way around... i had a similar problem with a vco where i had accidentally used the wrong polarity transistors NPN <=> PNP....
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widdly



Joined: Jun 25, 2007
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PostPosted: Wed May 15, 2013 2:51 am    Post subject: Reply with quote  Mark this post and the followings unread

If only the middle third of the pot is useful, add a 100k resistor between the 15v and the top of the pot and another 100k between the bottom of the pot and the -15v (assuming you are using a 100k pot).


I've built that vco using the precision expo source and there is no stepping.
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oculus



Joined: Oct 30, 2011
Posts: 35
Location: Iceland, Reykjavik

PostPosted: Thu Jul 11, 2013 9:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

i donĀ“t have this stepping issue but i have same problem with the range on the VCO-1 i just put a extra 100k on the middle pin of the pitch knob that seemed to fix things for me,
so basicly i just shortened the range of the pitch knob
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cslammy



Joined: Apr 27, 2018
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PostPosted: Mon Apr 30, 2018 4:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

I agree that you might have the transistors in wrong? I laid out my board (accidentally) for matched VBe BC560 type transistors vs. 2N3906 (the C/B/E pinouts are different for the different transistors--doh!).

Helps to pay better attention to Eagle parts placement Confused Just something to check I guess.

With BC559's the current source worked perfectly for about 8 octaves. Trimmed out the V/oct by ear. Works great and temp compensation is good as well.

I am having matched 2N3906 version fabbed up at EasyEDA now. I guess we'll see if that works....it should.

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cslammy



Joined: Apr 27, 2018
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PostPosted: Mon Jun 18, 2018 12:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

Update. Matched 3906 version of the TH 3080/13600/13700 expo current source works great. I have tested on both 3080 and 13700. Yep, the thing really works.

It's a really good design, simple and easy to build.

hats off the the mighty TH!

I'll post more on my website one of these days.

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cslammy



Joined: Apr 27, 2018
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PostPosted: Wed Oct 17, 2018 7:58 am    Post subject: Reply with quote  Mark this post and the followings unread

cslammy wrote:
Update. Matched 3906 version of the TH 3080/13600/13700 expo current source works great. I have tested on both 3080 and 13700. Yep, the thing really works.

It's a really good design, simple and easy to build.

hats off the the mighty TH!

I'll post more on my website one of these days.


UPDATE: I have notes on my blog about the builds:
http://audiodiwhy.blogspot.com/2018/10/thomas-henry-13700-based-vco-all-hail.html

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cslammy



Joined: Apr 27, 2018
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PostPosted: Wed Oct 17, 2018 7:59 am    Post subject: Re: 3080 VCO current sources
Subject description: a few issues with current sources from Making Music with 3080
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Conboy wrote:
I have been having varying degrees of success building a simple VCO from the designs in Thomas's book Making Music with the 3080 OTA.

The VCO section is working nicely now on my veroboard design. But my problems are with getting the current source to work correctly.

I have tried building the "Easy Exponential Current Source" which ought to suit my needs. However the pots range is unusable.
All the range is within the 1/3rd of central position on the pot - at either end (upper 1/3rd and lower 1/3rd) the VCO bottoms or tops out with the frequency remaining the same for the rest of the range.
I can't work out how this is adjusted? Or why it should be so.


Paul



Paul: one more thing I noticed when I was building this: make sure you have the right cap value in the VCO stage (tied to the OTA). I didn't when I was breadboarding and the thing only went to like 4K an never higher.

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