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Ian Fritz Sawtooth VCO
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fonik



Joined: Jun 07, 2006
Posts: 3723
Location: Germany
Audio files: 23

PostPosted: Mon Sep 01, 2008 4:30 am    Post subject: Ian Fritz Sawtooth VCO
Subject description: just a few short questions...
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hello ian, hello all,

looking at your schematic of the sawtooth VCO i see a 237K resistor (R6) at the summing node of the control voltages going to ground.
what is this resistor for? when i use a 1k termistor, will i have to lower this value as well?

you use a LM329 as voltage reference. if i wanted to use a say 5V reference, what should be changed? the 33K resistor? or said things different: why 6.9V? how has this value been calculated?

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matthias
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Last edited by fonik on Thu Sep 04, 2008 12:49 am; edited 1 time in total
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frijitz



Joined: May 04, 2007
Posts: 1709
Location: NM USA
Audio files: 54

PostPosted: Mon Sep 01, 2008 12:36 pm    Post subject: Re: Ian Fritz Sawtooth VCO
Subject description: just two short questions...
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fonik wrote:
looking at your schematic of the sawtooth VCO i see a 237K resistor (R6) at the summing node of the control voltages going to ground.

Ummm ... actually it's going to the -6.9 V supply. It sets the overall range.

Quote:
when i use a 1k termistor, will i have to lower this value as well?

Could you clarify that? You are using 1 k for the feedback resistor and 50 k for the 1 V/Oct input? If so, then you also need to halve all the other input resistors (R2, 4, 6, 7, 9).

Quote:
you use a LM329 as voltage reference. if i wanted to use a say 5V reference, what should be changed?

You need to scale all the resistors that are supplying current by a factor of 5/6.9. These are R2, 6, 15. You need to make R21,2 provide 4 V, e.g., R21 = 22k, R22 = 100k.

Quote:
or said things different: why 6.9V?

The LM329 is inexpensive and used to be available in a premium grade (but no longer). I now use 5 V subregulators.

I'm actually right now working on upgrading this design again. Several people have indicated interest in having a barebones saw vco (no waveshapers) for stacking, for polysynths, or for using with external waveshapers. So boards may be made available if there is enough interest.

Very Happy

Ian
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fonik



Joined: Jun 07, 2006
Posts: 3723
Location: Germany
Audio files: 23

PostPosted: Mon Sep 01, 2008 1:27 pm    Post subject: Re: Ian Fritz Sawtooth VCO
Subject description: just two short questions...
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frijitz wrote:
fonik wrote:
looking at your schematic of the sawtooth VCO i see a 237K resistor (R6) at the summing node of the control voltages going to ground.

Ummm ... actually it's going to the -6.9 V supply. It sets the overall range.?

ahh, i didn't se that. mixed the symbols.

Quote:
Quote:
when i use a 1k termistor, will i have to lower this value as well?

Could you clarify that? You are using 1 k for the feedback resistor and 50 k for the 1 V/Oct input? If so, then you also need to halve all the other input resistors (R2, 4, 6, 7, 9).?

yep, 1k in the feedback path and okeydokey

Quote:
Quote:
you use a LM329 as voltage reference. if i wanted to use a say 5V reference, what should be changed?

You need to scale all the resistors that are supplying current by a factor of 5/6.9. These are R2, 6, 15. You need to make R21,2 provide 4 V, e.g., R21 = 22k, R22 = 100k.?

okay, i will take a look at the schematic again.

Quote:
Quote:
or said things different: why 6.9V?

The LM329 is inexpensive and used to be available in a premium grade (but no longer). I now use 5 V subregulators.?

i had a hard time sourcing the LM329/129 but eventually found it (minimum order of 10 pieces, so i could build some VCOs)

Quote:
I'm actually right now working on upgrading this design again. Several people have indicated interest in having a barebones saw vco (no waveshapers) for stacking, for polysynths, or for using with external waveshapers. So boards may be made available if there is enough interest.

Very Happy

Ian

huh, i am still planning my keyboard synth. i still have this EFM wildcat PCB but finally decided not to use it Wink
so i started planning a synth built it from various circuits. i decided to use you VCO. furthermore i had the feeling that i am in need of some reliable, top notch VCOs for my modular, VCOs i can rely on. nevertheless i will etch my own PCBs, though. i want to use PCB mounted controls as much as possible.

however thank you very much for your patience and the clear answers, ian. very appreciated.

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cheers,
matthias
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fonik



Joined: Jun 07, 2006
Posts: 3723
Location: Germany
Audio files: 23

PostPosted: Thu Sep 04, 2008 3:25 am    Post subject: Reply with quote  Mark this post and the followings unread

one more question.

if i wanted to add a sync input, where should i apply the signal? at the negative input of the comparator (U1 pin 5) as seen on the ENS-76?

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cheers,
matthias
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