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transconductance amplifier

darren adcock

Sep 26, 2016
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Mostly I've read that the LM13700 IC is used to substitute for the lm3080 or CA3080. I can find CA3080's for sale but they are far too expensive for me. I know the lm13700 has a different pin out so my question is, is the lm13700 the best alternative to the ##3080? It's for use within the attached VCA circuit.
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Are you building a modular synthesizer?

My approach would be to ask Nathan (NLC). I've built a couple of dozen of his modules (mostly for the Zen of soldering) and he would probably be able to comment.

From memory, at least some of his designs use an optocoupler based on a LED and a CdS cell. However I'm sure I've seen transconductance amplifiers used (I can probably check)

This has been an issue for over 10 years now. I certainly wouldn't be using RCA designs for new projects :-D

The 3080 is a relatively low spec device, and the 13700 send to be an adequate replacement, although it isn't a direct replacement. See here for some valuable information, including information on the difference between the two (in part 2)
 

darren adcock

Sep 26, 2016
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HI Steve. hanks for your reply.

Yeah I'm building a modular synthesizer. Mostly I've been building all MFOS modules but keep running out of money. So was just nosing around for a VCA circuit that i could knock up on stripboard, i liked it's design, low part count and it's duality in that i could run it as a mixer as well.

Don Tillman's blog is fascinating, thanks

Yeah I spontaneously bought a bunch of Thomas Henry books only to then find out about the 3080 problem.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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The modules we build cost around £30.

Maybe you should call in and build some :)

Are you using euro rack modules?

Here is my synth:

IMG_20171119_125031.jpg
 

Johnyradio

Oct 30, 2012
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The modules we build cost around £30.
Maybe you should call in and build some :)
Are you using euro rack modules?
Here is my synth:
hardware analog synths! great! love those module names.

who's we?
call in and build some?

i'm developing an open-source hardware Lunetta synth, played with gestures.
75s6YTA.jpg

You know about Lunettas?
i've run classes teaching people how to build them.

how did you know i'm into synths!
 
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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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hardware analog synths! great! love those module names.

:)

who's we? call in and build some?

Assuming you're somewhere near the place you mention to in your sig then the place is about 9150 miles from you.

i'm developing an open-source hardware Lunetta synth, played with gestures. You know about Lunettas?

No, I haven't heard anything about them.

how did you know i'm into synths!

As a moderator I have unique powers to look into the private lives of all participants. That, OR I posted in the wrong thread by accident, having no idea that my post was strangely relevant to both.
 
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Johnyradio

Oct 30, 2012
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@(*steve*) your synth is a beauty. Yes, i'm on the west coast.

ok i get what happened. guess my post here is out of place too. carry on.

Mod edit: tidy up
 
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darren adcock

Sep 26, 2016
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WIN_20171013_17_01_40_Pro.jpg
The modules we build cost around £30.

Maybe you should call in and build some :)

Are you using euro rack modules?

Here is my synth:

View attachment 37547

Hi Steve, i'm not sure what defines eurorack?

flipping heck that synth looks good! envy :)

You live n Manchester?

my synth looks a bit like this....there's two more vco's to be added, two more VCLFO's, a Noise generator, a mixer, a signal splitter, built and tested, just need to be painted.
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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I'm probably not in the neighborhood. I'm 14,617 km away as the crow flies.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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, i'm not sure what defines eurorack?

It's the size of the modules, and their power supply requirements. See here for more information.

Nice looking synth. What size jacks are you using, 6 mm?

One of the problems with eurorack modules is that the controls are often very closely spaced.
 
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darren adcock

Sep 26, 2016
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It's the size of the modules, and their power supply requirements. See here for more information.

Nice looking synth. What size jacks are you using, 6 mm?

One of the problems with eurorack modules is that the controls are often very closely spaced.

Hi Steve yes 6 (.3) mm jacks.

The modules i'm making started out at 13cm by 25cm.... their size is difined a bit by what scrap wood I find and how much i can be bothered cutting this to size, my next modules will all be 13cm by 21 cm as I found a chest of draws i'm currently cutting up.

I like the idea of 3.5mm jacks, nice to have less bulky wires around. Problem i'd find is because i use found wood i'd probably struggle with the length of jack socket and thickness of wood. I've minly been gettin g around this by threading the panels with a tap and die set.

I'm very early on in build and am deffo a newbee, i'm already thinking this synth will end up huge, but then i've oggled those 60's floor to ceiling synths for a long long time.

Sacking off the above VCA circuit after discussions here and your links. Will build the standard VCA's from MFOS instead on stripboard, fairly straight forward circuit, most of the parts I have to hand anyhow.

Some of your module names are hilarious!!!!!!!!!
 

darren adcock

Sep 26, 2016
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At the minute i'm working on this Foolish (won't be the last foolish tape head) cassette module. 3 personal cassette player mechanisms nicked form cheap walkmans. They all have internal speed control, zero to about double speed (really need to check what expected regular play back speed should be)... each cassette player can be individually modulated from external CV's. The CV input jacks simply go through a variable voltage divider then to an LED. There is then a photoresistor TIP120 circuit which tracks the LED and modulates the cassette motor. I have it about adjusted well enough that I can have it light or dark fast. However at times the TIP120's get rather hot and can stall if i'm not super careful. There's also a record in which i've not had chance to test yet, I wanted a module that i could sample the synth with and then modulate, this about does that however it is rather crude and not fully tested, but the results are better than i expected so far. If i get it working I doubt i'll ever try to make another as it;s been a nightmare, defo will just buy a tape echo at some point rather than think i can make one. This is a week or so old now as a photograph, i'll upload a working video later this week i hope.
 

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