[sdiy] Potted expo converters

[John L Marshall]

I have a couple of  "Philbrick/Nexus" Logarithmic Operator modules, nice
potted plastic block. Operating temperature range is -25C to +85C + or -
0.04%/C. These potted blocks are probably older than many lurkers on this
list.




----- Original Message -----
From: "Magnus Danielson" <cfmd at bredband.net>
To: <peter at buzzclick-music.com>
Cc: <synth-diy at dropmix.xs4all.nl>
Sent: Tuesday, December 30, 2003 12:08 PM
Subject: Re: [sdiy] Potted expo converters


| From: Peter Grenader <peter at buzzclick-music.com>
| Subject: [sdiy] Potted expo converters
| Date: Tue, 30 Dec 2003 06:24:16 -0800
| Message-ID: <BC16CA89.FD35%peter at buzzclick-music.com>
|
| > Ladies Tangents,
|
| Peter Buzzington,
|
| > I remember one day I got all excited at this brainstorm I got that you
could
| > (sort of) duplicate the functionality of the ua726 by using the
remaining
| > trannies in a 3046 as a self heater, only to find it had been done a
billion
| > times before when I posted the idea on this list.
| >
| > d'OHI!
| >
| > So, here's another lightbulb that appeared over my head that I'd thought
I'd
| > bounce off you all  that I'm also willing to bet ain't original,
either...
| >
| > I remember when I was an engineering tech two billion years ago, we used
to
| > glue thermocouples to the windings of 1/2 horsepower electric motors
using
| > thermally conductive epoxy for UL lock-rotor tests. the material seemed
to
| > be was astonishingly heat conductive. But...we were dealing with
| > thermocouples which could detect a match being lit in the next room (not
| > really of course, but you know what I mean).
| >
| > Along the same lines, has anyone tried potting the tempco/expo converter
| > pair in conductive epoxy to A) improve heat transfer from one another
and B)
| > aid in isolation from ambient conditions?
| >
| > Just curious.
|
| Well... yes!
|
| If you use separate transistors, it have been done for quite some time and
in
| many different fields. For instance, I've seen many cases where the TO-72s
have
| been put "face-to-face" and with a copper-clamp around them for both heat-
| conduction and just plain mechanical clamp. dBX used a small aluminium
case on
| top of the pair for their famous VCAs.
|
| When you use MAT-02s, LM394s or what have you, the transistors sit on the
same
| silicon-chip, which as such binds the transistors together. However, when
you
| do that you make 4 transistors in a square and bind them together in a
diagonal
| fashion, so that the (1,1) and (2,2) transistors forms one transistor and
the
| (1,2) and (2,1) transistors forms the other transistors. This makes the
| compound transistors form in a diffrential pair situation a better
thermical
| stability from thermical gradients, i.e. a temperature difference between
the
| sides of the transistor setup. The compensation is not perfect, but nocks
off
| the first-grade difference.
|
| Now, if we now succseeded having the transistors of the diff-pair at the
same
| temperature, where' all set? No!
| The temperature difference between the transistors helps to null-out the
"Is"
| differences, but doesn't null-out the temperature differences of the
exponents
| q/(kT) term. There is two traditional approaches to this offset, either
| external compensation of the gain difference, usually done with the
Tellabs Q81
| temperature-dependent resistance which *almost* nulls out the temperature
| dependence in the scale. The other approach is to lock the temperature of
the
| transistors through a oven-design and then make a static compensation of
the
| gain constant (trimmable ofcourse, but not under electrical/temperature
| control).
|
| Those being alert have seen the discussions and experiments (ending up
very
| succsessful) of a tempco-resistorless compensation scheme, by the use of
the
| same temperature-dependence, namely that of a semiconductor. This is just
a new
| approach to the external compensation, but on a theoretical point of view
| reaching for a perfect compensation.
|
| The external compensation tricks however all depend on the sensing of the
| temperature of the NP-junctions of the transistors in the diff-pair, and
the
| missmatch there will cause missmatching. Therefore it is important to have
the
| tempco-resistor (or whatever sense-method in use) thermically ties to the
| NP-junctions of the diff-pair transistors.
|
| For instance, in one version of the MiniMoog, the tempco-resistors where
| sitting fairly far away from the expo-transistors, causing the
compensation not
| to track the correct temperature. Moving the actual resistors over to the
| diff-pair, tieing it thermically to the diff-pair and then wires over to
the
| original solderingpoint made the same electrical design much more stable.
I.e.
| the lesson is that it's not all in the schematics.
|
| There is one additional lesson to learn. The expo-pair transistors and
whatever
| else you have thermically tied to them is not "dead" components, they are
quite
| active (or they would be useless) and this means they themselfs dissapate
heat.
| The outgoing transistor dissapates a very large range of heat energy,
since it
| depends on the current it currently is transmitting. So, in order for
correct
| compensation, it is hard to keep the same temperature of the components
| individually, but tight thermical coupling between them is necessary to
keep
| them close in temperature.
|
| > I know a few manufacturers, Steiner-Parker included,  used to place all
| > three parts in a little metal box filled with thermal compound.  The
| > alternative I'm mentioning here seems much less messy.
| >
| > Considering we're dealing with comparatively low voltages, I assume
there
| > wouldn't be a lot of difference, but if it meant an improvement in
stability
| > even a little may be worth it.
|
| I don't think you have to go to such extremes. Tying things together
| thermically is what you want. However, I would spend a few moments caring
about
| the output collector off the diff-pair. You don't want to create a too
low-
| impedance path to anything else. So you might creating a new problem when
| trying to solve another. Don't do that, it could spoil your low-freq
tracking
| if you care about huge ranges of frequency-tracking. It's more about
watching
| out and avoid a few selections than anything else.
|
| > I tell you what - it's a lot easier than
| > scanning the globe for 726s all the damn time.
|
| Indeed.
|
| > anyone?
| >
| > anyone??
|
| How's bad?
|
| Cheers,
| Magnus