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Precision heater chips (fixing temperature instability)?

Discussion in 'Electronic Design' started by Richard Rasker, Jul 8, 2009.

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  1. 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
     
  2. Jim Thompson

    Jim Thompson Guest

    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 | |
    | Voice:(480)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
     
  3. 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
     
  4. 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
     
  5. Jim Thompson

    Jim Thompson Guest

    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 | |
    | Voice:(480)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
     
  6. Jamie

    Jamie Guest

    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.
     
  7. 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
     
  8. 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
     
  9. Jim Thompson

    Jim Thompson Guest

    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 | |
    | Voice:(480)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
     
  10. 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
     
  11. Jim Thompson

    Jim Thompson Guest

    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 | |
    | Voice:(480)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
     
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