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Rene Schmitz VCO3 updated schematic anyone?
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NuttyMonk



Joined: Jun 30, 2020
Posts: 62
Location: UK

PostPosted: Thu Jul 08, 2021 10:17 am    Post subject: Rene Schmitz VCO3 updated schematic anyone? Reply with quote  Mark this post and the followings unread

Hi all,

been looking at the Rene Schmitz VCO3 schematic recently and wondered if anyone had an up to date version with replacement components in it.

A lot of the components are old and out of date or really expensive. Just looking for good replacements. I know he has suggestions for replacements on his webpage but a lot of them are also out of date or expensive too.

The biggest problem i have is with the 2N4391 N-Channel JFET's. What are the important stats i need to match for a replacement of the 2N4391's?

Any help would be appreciated.

Cheers

NM
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analogmonster



Joined: May 30, 2011
Posts: 58
Location: Germany

PostPosted: Fri Jul 09, 2021 6:13 am    Post subject: Reply with quote  Mark this post and the followings unread

They are obtainable at least on ebay or at kessler electronic, germany, probably elsewhere also:

https://www.ebay.de/itm/310620986331?hash=item4852741bdb:g:0tMAAOSwnXde37oU

If I don't want to use a special component any more or can't find it, I do an LTSpice analysis of the circuit with a corresponding replacement component. Then I can see whether this works or not.

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Sven



Joined: Mar 10, 2017
Posts: 48
Location: Norway

PostPosted: Fri Jul 09, 2021 8:08 am    Post subject: Re: Rene Schmitz VCO3 updated schematic anyone? Reply with quote  Mark this post and the followings unread

NuttyMonk wrote:
. What are the important stats i need to match for a replacement of the 2N4391's?
NM


Hi,

my guess is the reason why he chose this JFET is it's low leakage current when turned off. In the datasheet it is the drain cutoff current. This is not specified in a lot of other FETs datasheet.

Everything is described here https://www.schmitzbits.de/vco3.html even optional compobnents.
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NuttyMonk



Joined: Jun 30, 2020
Posts: 62
Location: UK

PostPosted: Fri Jul 09, 2021 9:41 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for your advice guys.

I have checked the schmitzbits page already. Even with the help on there i couldn't get the circuit to work in Falstad. I went over it a lot of times making small changes to the values of some items but couldn't get it working. Using a standard N-channel JFET in Falstad lets over 3mA of current through when it should be off which is probably the problem with it not working right.

I decided to redesign the reset part of the oscillator core myself and managed to get it working with fewer components. The only problem i have is with the waveform not looking too good and there is a big difference between the peak-to-peak voltages between different frequencies which i don't understand.

I can get the 2N4391 components from some places here in the UK but they aren't currently in stock. I might be able to get them elsewhere though as analogmonster suggested.

The reason i like this design so much is the fact it can get such a wide range of frequencies making it useful as a LFO, Clock or Oscillator. And it has a Sync function which could work with syncing clocks up with each other when doing per-step CV control of the clock speed.

I might just have to bite the bullet and hope it all works when i put it together in the real world using the components i can get my hands on.

I've included the two Falstad files in case anyone wants to have a look at them.

Thanks for your help

Cheers

NM


VCO3 Original.txt
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 Filename:  VCO3 Original.txt
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VCO3 Redesign.txt
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 Filename:  VCO3 Redesign.txt
 Filesize:  3.23 KB
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Sven



Joined: Mar 10, 2017
Posts: 48
Location: Norway

PostPosted: Fri Jul 09, 2021 10:18 am    Post subject: Reply with quote  Mark this post and the followings unread

I would build it instead of simulating it. Simulations are always different than the real circuit. On Schmitz's page he wrote he tried BC245C that should be BF245C.

I found the MMBF4391/2/3 series these are SMD versions of the 2N439x. They are much cheaper. Maybe the current "chip crisis" also is a "transistor crisis"?
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NuttyMonk



Joined: Jun 30, 2020
Posts: 62
Location: UK

PostPosted: Fri Jul 09, 2021 10:30 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm planning to use the ICM7555, 2N4391, TL07x op-amps, 2N390x (instead of the BC5xx), AS394CH Tin-Can (instead of the 2SC1583), AS3080 (instead of the CA3080) and CA3140.

Hopefully they will all work together ok.

It's quite a weird parts list compared to most VCO's. Must have taken him a lot of time to put that together on a breadboard and get it working well.

Cheers

NM
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NuttyMonk



Joined: Jun 30, 2020
Posts: 62
Location: UK

PostPosted: Tue Jul 27, 2021 11:16 am    Post subject: Reply with quote  Mark this post and the followings unread

In the VCO3 it uses a differential transistor pair for the exponential voltage to current converter. When i simulate the input of that expo converter i get a voltage of -200mV to +200mV going into the expo converter. As far as i know, the exponential region of a NPN transistor doesn't start until you get to approximately 350mV and stays in that exponential region until about 550mV.

Is there something i am missing here about how the differential (long-tail) pair of transistors works? With the way the schematic is drawn, it seems that the transistors will never get into the exponential region and therefore never be a true exponential converter.

Any help would be appreciated.

Cheers

NM
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elektrouwe



Joined: May 27, 2012
Posts: 143
Location: Germany

PostPosted: Wed Jul 28, 2021 3:48 am    Post subject: Reply with quote  Mark this post and the followings unread

Rene himself explains it very well here:
https://www.schmitzbits.de/expo_tutorial/index.html
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