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Thread: PET 2001 Sound

  1. #31
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    Quote Originally Posted by tanuki View Post
    That makes sense. Would a standard PC speaker set (which are amplified) work for this purpose or is the line level wrong?
    I think so. From a PC, the line out voltage is maybe around +/- 2 V into 600 Ohms (which would be 3 milli-Amp max). On the PET, the CB2 output is 0 to +5V square wave but with little drive current capability limited to 800 micro-Amps at 2 V. I assume a PC amplified speaker input would be AC coupled which should block any DC voltage to the amplifier.

  2. #32
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    Yes, if you are not careful you will overdrive the amplifier input.

    My suggestion would be to use an external potentiometer to drop the voltage from 5V down to something more suitable for inputting to an amplifier.

    My suggestion would be to use a 100 kOhm log potentiometer. Wire the CB2 output from the PET to one end of the potentiometer and GND/0V to the other end of the potentiometer. You would then wire the GND/0V to the screen of the amplifier input and the wiper of the potentiometer to the signal input of the amplifier.

    The potentiometer will act as a potential divider to match the signal level from the PET to the amplifier input.

    You could adjust the potentiometer to get an acceptable input to the amplifier (no clipping etc. - although the signal from the PET will be a binary signal level anyhow!) and then replace the potentiometer with two fixed resistors.

    Dave

  3. #33

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    Hi Dave,

    Please excuse my sloppy freehand circuit drawing. Are you suggesting something like this?

    PET_sound_proto_small.jpg

  4. #34
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    Almost.

    The lower end of your potentiometer (as drawn with the black dot) needs to be wired to ground.

    It is debatable whether you actually need the 74LS00 and could wire the ‘top’ of the potentiometer directly to the PETs CB2.

    As Dave_m mentioned previously, this signal (plus ground) is brought out to J2 (the user port), therefore you may not need to modify the PET at all. The CB2 signal is brought out to J2/M. I think GND/0V is on J2/N?

    Dave
    Last edited by daver2; December 3rd, 2019 at 10:52 PM.

  5. #35

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    He still might add a capacitor. Something like a ceramic disk of about 1uF should be fine.
    Dwight

  6. #36

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    Something this simple I think. (?) This will let you adjust the output voltage from the full 5v to 0v. Without grounding one side of the pot, the voltage won't change, only the current.
    Should a cap go between CB2 and the pot? My thought is to prevent dc coupling from CB2. The smaller the cap the sharper the square wave but a very small cap there might distort low frequencies.
    Capture.JPG

  7. #37

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    Which side the cap goes on is usually not important. There is a slight difference in the low pass as the wipper of the pot is moved from one end to the other, if the capacitor is on the output side of the pot. There is an even slighter change if it is on the input side as the pot is moved. In either case, this forms a high pass circuit. It will attenuate the lower frequences more than the higher. Most tones that you'll be creating will still hand many higher frequency harmonics that will easily pass.
    You'd need different circuits to get rid of these higher harmonics. If a circuit was toned for one frequency, it wouldn't work well for another. Since your circuit is producing a single square wave, you wil not be able to create a filter that works well for all frequencies. It is possible to use pulse width sound that will be cleaner but this requires processing power that a 1MHz 6502 is unlikely to be able to create.
    An alternate way to do filtering is to use multiple square waves of harmonics and use them to cancel the higher frequencies. This is the way many old electronic organs worked. New ones use fully sampled sounds.
    Dwight

    Dwight

  8. #38

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    Thanks for all the input. If I understand everything correctly, then the updated circuit is as follows:
    PET_sound_proto_3_small.jpg
    Does this look right?

  9. #39
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    No, the NAND gate is not needed. Do not use the diagnostic signal in this speaker circuit. It may be used in the 8032 PET to disable the speaker if the Diagnostic pin is grounded, but the 2001-8 PET does not have the machine language monitor in ROM so there is no quick access to the ML monitor.

  10. #40
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    +1.

    As per the drawing in post #36 but without the capacitor marked as '?'.

    Dave

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