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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
MFOS Ramp Core Layout Problem
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jbeuckm



Joined: Nov 30, 2008
Posts: 165
Location: Stockholm
Audio files: 9

PostPosted: Thu Sep 17, 2015 9:05 pm    Post subject: MFOS Ramp Core Layout Problem Reply with quote  Mark this post and the followings unread

I am working on building the core of Ray's VCO with just the ramp shaper.

Core: http://www.musicfromouterspace.com/analogsynth_new/VCO20120618REV0/pdf/vco20120615wsoic_page_1_assembly.pdf

My ramp output shows a little curly distortion around 0V and increases in amplitude with the frequency to the point of clipping at high frequencies.

At point "RAW" I see a clean ramp. BUT, it goes between about 1-2V at low frequencies and 0-4V at the highest frequencies. It is supposed to always go 0-2.2V

I am using an MPF102, unmatched 2N3904's and three TL082's. Do you see any problem with the schematic attached? I think the MPF102 has the same pinout as the 2N5457. I also use a 5pF for the 4.7pF in Ray's schematic. Is that a precision cap or something?

Gah - I just finished my second layout prototype and it has exactly the same problem! There must be something wrong with my schematic.


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Low frequency - 2V per division. RAW going between about 1-2V. Ramp output (larger) shows the distortion around 0V.
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High frequency - 2V per division. RAW going between about 0-4V. Ramp output way bigger / more distortion.
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jbeuckm



Joined: Nov 30, 2008
Posts: 165
Location: Stockholm
Audio files: 9

PostPosted: Tue Sep 22, 2015 11:43 am    Post subject: Reply with quote  Mark this post and the followings unread

Could this be a bad batch of cheap Chinese TL082? I now see the exact issue on two layouts of this core and a known-good layout of MFOS VCO Old Design VI.
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elmegil



Joined: Mar 20, 2012
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PostPosted: Tue Sep 22, 2015 1:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

That seems a reasonable theory, you'll need to get some other TL0 op amps to test though. If you have TL072, they'll sub directly, and there are several other types of dual op amps that do as well (but double check the data sheet).
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jbeuckm



Joined: Nov 30, 2008
Posts: 165
Location: Stockholm
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PostPosted: Wed Sep 23, 2015 7:19 am    Post subject:   Reply with quote  Mark this post and the followings unread

Welp, that was exactly the problem. I am DONE with cheap Chinese ebay parts. 10% resistors marked as 1%... capacitors that seem totally random... and who knows what those TL082 were Mad I found an old drawer of op amps from 20 years ago and went from junk to three new VCOs in about five minutes Smile
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