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

David G Dixon dixon at mail.ubc.ca
Tue May 9 06:22:31 CEST 2017

```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
>>
>> That's the one...
>>
>> MC
>>
>>> Anyway, I think Michael E C was referring to this posting of his:
>>> http://search.retrosynth.com/
<http://search.retrosynth.com/ah/search/lookit.cgi?-v9901.2166>
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/
<http://search.retrosynth.com/ah/search/lookit.cgi?-v9901.2166>
ah/search/lookit.cgi?-v9901.2166
>>>
>>> sorry, Michael E C, Michael.
>>>
>>>
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