First Shruthy: a Voltage Controlled Synth?
  • Hallo Shruthers,
    I finisced my first board whit SMR-4 filter.
    It was a very teaching experience, vary satisfactory, I enjoed myselt a lot.
    Only trouble I found was at first the tuning of oscillator. The sine tone was fading quickly and, as is it told in the forum, I presume it was a problem of over heating. Before digging it, I lowered voltage in and I found that at 7,5V the Shruthy sounded well. I tuned it perfectly whit the tubeohm utility and checked later with a scope.
    Responsible of over heating was 7805; with 9-12V in, it was untouchable.
    Changed it presuming a defective unit. Nothing changed.

    Checking the power section I mesure:

    Volt in 7,45

    LT1054
    pin 5: -6,18V
    pin 8: +6,69V same tension than after diode

    7805 out: +4,89
    7905 out: -4,94

    Same voltages on + – rails.
    Feeding 9V change nothing, heathing a part.
    With note C4 sine wave is around 3V pp.

    My explanation is: if 1054 must output -5V and +5V the 7805 works too much and heats. But why 7905 not?
    Electromistery to me.

    As soon as possible I’ll replace 1054. May be I wave it half fried with a power supply too “hot”.

    Anyway, at 7,5V the instrument sounds sweet. Waves dance on scope graciously and now I can play the oscillator over 9 octaves.

    I believe that every machine has is own voice. Maybe the regulator is gone, but mine has found is own regulation.

    Is it a Voltage Controlled Synth? VCS

    All the best to all

    Gengisthecan, 56, Roma, Italy

  • It is normal for the 7805 to get hot. It gets 9V or 12V at its input, and has only 5V at its output, sourcing 120mA of current or more. There’s no miracle of physics, the excess energy must go somewhere – it gets dissipated as heat – close to 1W when the power supply approaches 12V; but only 300mV when the input is 7.5V voltage. This is not due to a defective part so don’t bother replacing it at the risk of damaging your board.

    “But why 7905 not?” -> Because the 7805 gives power to the LCD display, the LEDs, the digital circuitry, while the 7905 only has to give a few mA to a bunch of op-amps!

    Is it a Voltage Controlled Synth? VCS -> Voltage control is used internally for the filter and amplifier; but the Shruthi has no voltage-control inputs; and voltage-control is not used for the oscillators (that’s why they are stable). The oscillators are digital and don’t need tuning – what you’re trying to tune is the filter!!!

  • Merci for your feedback.
    I used the expression VCS quite ironically because it seem to me that the voice of the machine is changing when voltage changes. Anyway, at 12V in, 7805 is untouchable and the filter fades. (12V measured with a tester) Have I investigate for some defect around the resonance section (due by the heat?) as I read in the forum? Or go on whit the gem tuned for 7,5V in?

    Great experience, Olivier.
    Thanks again.

    Gengisthecan

  • Don’t use 12V, not only this makes the 7805 overheat, but a slightly voltage surge above that will zap the LT1054.

  • Rule for hardware Synth lover. First, have a perfectly controlled power supply!
    Now I have to learn to play well the Shruthy…
    Gengisthecan