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 Forum index » DIY Hardware and Software » Thomas Henry designs
8038-based LFO from "Synthesizer for the 21st Century"
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marcusriley



Joined: Dec 21, 2019
Posts: 4
Location: seattle

PostPosted: Sun Dec 22, 2019 11:18 pm    Post subject: 8038-based LFO from "Synthesizer for the 21st Century"
Subject description: Coupling and offsetting
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Hi all -

I recently purchased a copy of "Synthesizer for the 21st Century" looking for a design solution to get a decent LFO from an 8038 IC. (I posted about struggling a bit with the 8038 already here: http://electro-music.com/forum/topic-71465.html)

Essentially, Henry's schematic is a little more complicated than I need for more immediate purposes, but stripping away some of the delay/triggers/etc, I'm able to get nice low-frequency waveforms with a great range.

The only problem is that the synth is being driven on a a single supply -15v - and I'd like to get it to play nice with my +/- 15v system.

To deal with balancing the waveform, Henry uses a 3080 with an offset trim. (page 21).

I'm wondering about some other alternatives for doing this, since the 3080 is a bit pricey and I'm not using the other functionality of the chip.

My experience, so far, is that adding an offset voltage in parallel with the input waveform at the non-inverting input of a regular op amp doesn't quite work. As I raise the offset voltage, the waveform begins to dwindle and distort. Maybe there are better "sanctioned" ways of doing this or maybe there's some reason why this isn't possible with the 8038 output, etc.

I also know that potentially simply adding a capacitor at the output of the waveform would "couple" the signal, but I'm not sure what the trade-offs of doing things that way are.

Can anyone help?
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marcusriley



Joined: Dec 21, 2019
Posts: 4
Location: seattle

PostPosted: Mon Dec 23, 2019 8:39 am    Post subject: Reply with quote  Mark this post and the followings unread

I could potentially screenshot the circuit I'm looking to alter, if people think it would help and wouldn't violate legal issues.
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elmegil



Joined: Mar 20, 2012
Posts: 2117
Location: Chicago
Audio files: 16

PostPosted: Sun Dec 29, 2019 7:20 am    Post subject: Reply with quote  Mark this post and the followings unread

There are other OTAs that are available besides the 3080.

An LM13600 or more commonly LM13700 would probably drop into Thomas' circuit reasonably well without a lot of mucking about.

Any of the various 2164 clones (V2164 used to be the main one available, but now there are a couple other manufacturers) would also work, but you'd need to do more work to adapt the circuit.
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