Mono/Poly VCO heater (was: beating a dead DAC )

[jhaible]

Hi again,

I've been digging deeper into the schematics now, and this is how I see it:

The Mono/Poly has two temperature regulation loops.

For the record: the SSM2033 has an on chip temerature sensor,
an on-chip heater, and the necessary control amplifier, and it's
intended to be a self contained thermostat circuit. This means
drawing a rather high current, of course.

Now the Mono/Poly circuit does limit this current to a smaller
value by inserting a 150R resistor into the heater's power supply
connection. An external circuit watches the voltage drop across
this 150R resistor and controls an external heating circuit, including
the power resistor that is located next to the 2033.

Think of it as a cascaded servo loop. When the chip is too cold,
the internal heater is activated. But this internal heater is artificially
starved, and can only draw so much current. As soon as the internal
heater hits its (reduced) limits, an external "booster" is turned on.

This has the clear advantage that the chip suffers much less stress
from high currents. It still has the advantage of a fast internal
servo loop for small errors. Only fast or large deviations from the
reference temperature are handled by the external heater.
In both cases, the internal sensor controls the loop.

I cannot tell where they set the threshold between the two loops.
Maybe the external resistor handles only the rough stuff (like
heating up on power-on) - maybe the external resistor handles
all but the tiniest deviations.

Apart form the advantage of increasing the chip's life, there
might or might not be an advantage of better regulation, too.
Remember the thread about the disadvantage of the heater
method vs. tempco method ? Non-symmetric heat up and
cool down time constants ? This might be improoved here,
as part of the thermal capacity (the power resistor) is
outside the isolation of the chip package.

All right, part of this last one might be speculation. But
I have a feeling that I've learned a lot today. I think it's a brilliant
solution, and once more a Korg circuit has caught my highest
admiration !
And thanks to Osamu for pointing this out!

JH.

PS.: I just noticed that they also replaced the internal CV summing
opamp with an external JFET type. Both modifications must have
been designed after the main circuitry (external pcbs).









> I checked the article again.  The designer said:
>
> "In Mono/Poly, temparature is directry controlled by heaters
> (resisters).  Surface temparature is about 65 degrees.  Other makers
> were using thermisters but it is very difficult to stabilize.  Direct
> temparature control is much easier and more stable, even though its
> primitive.  Stabilization took 3 or 4 minutes."
>
> I've never looked at Mono/Poly schematics.  But, I also have a real
> Mono/Poly.  It has big resisters on/above VCO chips.  (Maybe the
> resisters are temparature sensors.)
>
> - -- ---- ----------- --- --- - ----- -- --- --- - - ------ - - - --
>  Osamu HOSHUYAMA          Digital Signal Processing Technology Group
>                     C&C Media Research Laboratories, NEC Corporation
>                Email: houshu at ccm.CL.nec.co.jp,  Fax: +81-44-856-2232
> --------------------------------------------------------------------
>
>