raph36 wrote:... the UA726 needed a heater because of its transistors instability over temperature change, so the idea was to heat them at the same temp so they would stay at equal spec, thus providing a better matched pair....
but if we have no trouble with the transistors staying matched without a heater, i guess that's it ...
It is nothing to do with instability. All transistors are subject to the Ebers Moll equation giving the relationship between collector current and Vbe. This contains a term in the exponent of k.Vbe/q.T, where q and k are constants, but T is the absolute temperature in degrees Kelvin. This means that ALL transistors can be used to make exponential converters and ALL are temperature sensitive. Differences in the transistor die structure can introduce errors, and a low bulk emitter resistance is desirable for a wide range as this causes a significant voltage drop at higher currents. Most matched transistor pairs and quads have been designed with this in mind, the 3046 was not. HF compensation is sometimes used to correct this aspect.
A better converter can be made by having two identical transistors at the same temperature for
temperature balancing and this is the most common circuit. The transistors have to be at the same temperature and that means the silicon itself, not the outside of the package, which means a monolithic pair. The temperature of the transistors will change with self heating as the current changes so two transistors stuck together do not give good results.
If that circuit remains at a constant temperature it will function as required, but if it is subject to temperature changes of a few degrees the output will be in error proportional to the change in absolute temperature. It will still be exponential and will track another, but the scale will have changed and an octave CV will not be exactly an octave change. At 25 deg C a 1 degree change is 1/(25 + 273.15) = 0.003354 so if you can multiply the Vbe by something that changes by the same factor it will be
temperature compensated. Alternatively the temperature can be prevented from changing. Neither technique is as easy as it sounds. The problem with temperature compensating resistors is that the coefficients have a +/-10% tolerance whereas the transistors don't, so you can't just drop one in and expect it to work. If compensation was really working you would be able to change the temperature with a hot air blower or a freezer spray and hear very little change.
The problem with temperature ovens is that it has to run at a temperature higher than any that will be encountered in use, say above 60 degC, and the servo loop will have a time lag so it will be cycling between a maximum and minimum temperature.
In practise several VCOs of the same design in the same synthesizer at the same temperature will change pitch a similar amount and appear to track each other, but have an absolute error. Trying to get different VCOs in different places in a modular synthesizer to track is far more difficult and is only going to work if the temperature compensation is exact or if the temperature can be held above any ambient changes within a few degrees.
Also two identical synthesizers may not track together if built at different times using a different batch of tempco resistors
question is will this replace the UA726 in any given situation, meaning like in a minimoog
Only the last Minimoog models used µA726s, most of them used 3046s and Q81s.
It's seems to be working for the 208 though, i guess Don Buchla wouldn't sell a synth that doesn't track well ?
Try playing a Buchla with a five octave keyboard...
I would put more trust in Dave Smith.
Hasn't anybody noticed that the BEMI pcb pictured above does not match the µA726 pinout?
Buchla seems to use a quite simple solution.... does anyone know if they use this as a direct replacement solution? No added circuitry? no tempco ? seems almost too simple...like everyone has been scratching their head way too much....
