[sdiy] MS-20 tempco (was Model D service)

[Quincas Moreira]

Oh boy, I'm gonna breadboard this! Thanks Dave :)

On Mon, May 8, 2017 at 11:22 PM, David G Dixon <dixon at mail.ubc.ca> wrote:

> Yes, that's right.  Linear CV control has no temperature effect.
>
> Here's an interesting thing.  A simple potentiometer is a linear voltage
> selecting device.  However, the current ratios of the two sides of a pot
> are exponential.  Hence, one can generate the proper voltages for Hz/V
> control by putting a potentiometer on an opamp, with the wiper going to the
> inverting input, one tap going to the output, and the other tap going to
> the input.  If a constant voltage is supplied to the input (say 0.8V), then
> linear movement of the pot will give exponential voltages at the output of
> the opamp.
>
> If the potentiometer is replaced with a bunch of equivalent resistors
> connected in a chain, and a keyboard is used to connect the various
> junction points to the inverting input of the opamp, then provided that
> there is a trimmer on the input end of the resistor chain, this would give
> the proper voltages for chromatic tuning (to be converted to currents to
> the integrator directly) at the output of the opamp.  Is that more or less
> the standard approach?
>
> ------------------------------
> *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org] *On Behalf Of *Adam
> Inglis
> *Sent:* Monday, May 08, 2017 7:30 PM
> *To:* synth-diy at synth-diy.org DIY
> *Subject:* Re: [sdiy] MS-20 tempco (was Model D service)
>
> Hz - per - volt synths don’t need tempcos, do they?
>
> I have no experience with the MS-20 but I do have a Korg 700 from a few
> years earlier. It never needs re-tuning.
>
>
> On 9 May 2017, at 11:53 AM, matt holland <matt at mattholland.org> wrote:
>
> ebers moll equation: Ic = Is*e^(Vbe/Vt -1)
>
> a single transistor itself is an exponential converter. the second
> transistor in the common two transistor expo converter compensates for the
> complicated temperature dependance hidden in the Is term (Is goes away to
> the extent that the two transistors are identical). the tempco resistor
> compensates for the less severe effect of the temperature dependance hidden
> in the Vt term. gory details of the equation terms can be found in your
> nearest device physics textbook.
>
>
>
> On Mon, May 8, 2017 at 5:36 PM, Tony K <weplar at gmail.com> wrote:
>
>> Hello, those so-called special Rx's are 0.1% or precision resistors , the
>> ones with the circle around the resistor symbol.
>>
>> When we recorded I always had to tweak the Master Tune as time went on. I
>> never bothered adding one either. In mono synths it's not critical , the
>> exponential pair handles the biggest part of the temp drift - forget the
>> equation offhand.
>>
>>  Just referring to my notes , it's been a while.
>>
>> TK
>>
>> > On May 8, 2017, at 7:48 PM, Declare Update <declareupdate at gmail.com>
>> wrote:
>> >
>> > wait a second... no tempcos in the MS-20? what's with all the special
>> resistor symbols in the schematic? and how does that super common and
>> known-to-be-temp-dependent expo converter not need compensation?
>> >
>> > Chris
>> >
>> >> On May 7, 2017, at 4:14 PM, Michael E Caloroso <
>> mec.forumreader at gmail.com> wrote:
>> >>
>> >> That's the one...
>> >>
>> >> MC
>> >>
>> >>> Anyway, I think Michael E C was referring to this posting of his:
>> >>> http://search.retrosynth.com/ah/search/lookit.cgi?-v9901.2166
>> >>
>> >>
>> >>>> On 5/7/17, Michael Zacherl <sdiy-mz01 at blauwurf.info> wrote:
>> >>>>
>> >>>>> On 7.May 2017, at 19:48 , Michael Zacherl <sdiy-mz01 at blauwurf.info>
>> wrote:
>> >>>>>
>> >>>>> On 7.May 2017, at 19:04 , Quincas Moreira <quincas at gmail.com>
>> wrote:
>> >>>>>
>> >>>>> Michael, thanks for saving me frustration, I will refer to your
>> >>>>> instructions.
>> >>>>
>> >>>> uhm .. I didn’t do anything -
>> >>>
>> >>>
>> >>> whoops  *blushes* … the other Michael.
>> >>>
>> >>> Anyway, I think Michael E C was referring to this posting of his:
>> >>> http://search.retrosynth.com/ah/search/lookit.cgi?-v9901.2166
>> >>>
>> >>> sorry, Michael E C, Michael.
>> >>>
>> >>>
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-- 
Quincas Moreira
Test Pilot at VBrazil Modular
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