Precision heater chips (fixing temperature instability)?

Hi all,

I was asked by a musician if I could do something about the temperature
instability of his old Moog 342 synth. At the core of its oscillators is
the CA3046 transistor array: http://www.linetec.nl/electronics/moogosc.jpg

This design turns out to be quite sensitive to temperature changes -- as in:
at least a half tone for every 6 degrees centigrade. This isn't much of a
problem in a studio setting, with a constant temperature of some 20 degrees
centigrade, although even then, it's necessary to let the instrument heat
up internally for half an hour before playing. However outdoors, the synth
is totally useless.

I tried replacing the dual transistors with an SSM2210 matched transistor
pair (with a far better matching that the 3046), but to my annoyance, this
didn't result in the improved stability I hoped for. Perhaps this is
because the whole design isn't really symmetrical and balanced -- I haven't
yet taken the time to investigate & calculate exactly what is happening.

Anyway, it seems that it's not just the transistor matching that's
important, but also the absolute Vbe value. This latter can't be stabilized
in any way except by providing a constant temperature -- so I'm thinking
about another classic solution for cases like this: heat up the '3046's to
a precise temperature, e.g. 50 degrees centigrade.

Now I already drew up a simple design based on a TC1047 temperature sensor,
an opamp and a power transistor, to be stuck onto the transistor arrays,
but I wondered if there's a simpler or more integrated solution available,
such as a precision PTC or a special "heater chip". Or even a transistor
with a built-in temperature sensor would be nice already.

As always, thanks in advance for any suggestions.

Richard Rasker

 

Hard to tell, from the JPG, but it looks like a VCO with a triangle
output? Is that so?

Transfer function desired?

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine Sometimes I even put it in the food

 

Just two oscillators, each with its own CA3046, some three inches apart on
the PCB -- so I'd need two controlled heater circuits. No problem at all to
build (I estimate it at an hour or so), but I just wondered if there were
ready-made solutions available.

Richard Rasker

 

Yup, it produces a triangle or sawtooth.

Not really necessary, I just wondered a) what caused the temperature
instability, and b) why the built-in temperature compensation (one of
the '3046 transistor used as a heater) doesn't work properly -- but I'm
slowly beginning to understand both things. Especially the failing
compensation is something to take yet another look at, because fixing that
would mean I no longer need to build my own heater circuit.
Anyway, it's funny how you can get the "feel" of a circuit simply by looking
at it and thinking about it long enough ...

Thanks for the response anyway

Richard Rasker

 

Indeed! I've always maintained I "think" and "speak" circuits ;-)

That current diversion AGC doesn't do away with kT/q temperature
terms.

The reason I was asking is that there are trivial ways to amplitude
control without adding temperature sensitivity.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine Sometimes I even put it in the food

 

Have you thought about replacing the caps in the OSC section, with more
stable types ?

Also, you can place a diode in series with the circuit to bias it when
the diode heats up, place the diode near the hear source and have it in
the circuit to help balance it.

 

Did that at my very first attempt, with less than stellar results. But as
soon as I fix the instability with the transistor array, I'm sure that the
new caps (2% tolerance, as opposed to the 5% types previously used) will
pay off after all. And oh, I also replaced all 270-degree single-turn
trimmer potentiometers with multiturn precision types.

Hmm, I think I'll try precision temperature control of the transistor array
first, before introducing yet more components. But thanks for the
suggestion anyway.

Richard Rasker

 

Erm, this exact procedure is what led me to the CA3046 being the prime
suspect (and convicted and jailed already) in this case. But thanks for
your contribution anyway.

Richard Rasker

 

Until the 3046 configuration is changed, the OP will always have a TC
to contend with.

The OP should "do the math" to understand the issue, rather than
hand-waving.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine Sometimes I even put it in the food

 

Well, in theory you're right. But this isn't my synth, and the guy who owns
it doesn't want me to put dozens of hours into it, as that would be a bit
too costly.
So I resorted to "hand waving" after all -- or more precisely: waving a can
of freeze spray to cool down individual components, while monitoring each
oscillator's frequency. It turns out that the transistor arrays are by far
the most heat-sensitive components. So now I have those heated to a toasty
50 degrees centigrade and packed 'em in foam to prevent any air currents
(e.g. from aircos) from cooling them down, and everything is quite stable
now. A 20 degrees centigrade change in temperature of the rest of the
circuit only results in a half percent deviation in frequency -- and that's
barely audible.

Richard Rasker

 

Yep. That solves it because... that 3046 current diversion scheme is
fundamentally flawed... for amusement, analyze that transfer function
using the transistor non-linear model.

Also the triangle wave oscillator is about as hare-brained as they
come ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine Sometimes I even put it in the food