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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 22nd Jun 2022, 12:54 am   #21
SiriusHardware
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Default Re: Commodore PET Speaker

If you feel like trying to make one from scratch there are circuits for LM386 based amplifiers all over the net, here is the first example I found, which includes a volume control and also explains how to change the gain (number of times amplification) of the circuit.

https://www.electroschematics.com/lm...dio-amplifier/
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Old 22nd Jun 2022, 4:15 am   #22
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Default Re: Commodore PET Speaker

I would be tempted to salvage the piezo from a scrap smoke detector.
If you strip it down enough there should be a very efficient sounder in it.
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Old 22nd Jun 2022, 8:36 am   #23
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Default Re: Commodore PET Speaker

The sounders in smoke detectors are usually highly resonant at one frequency and they often include a third (feedback) wire so that the oscillator or other circuit driving the sounder can self-adjust or 'self-tune' to the peak resonant frequency of the sounder.

This application requires the sounder to sound equally loud across a range of digitally generated frequencies, although you'll never get much bass from a little Piezo sounder.
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Old 4th Jul 2022, 10:33 pm   #24
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Default Re: Commodore PET Speaker

Did you get anywhere with any of this, Colin?
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Old 5th Jul 2022, 9:32 am   #25
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Default Re: Commodore PET Speaker

I bought this kit which works. It's still quiet but it works fine for me. I guess something with more volts would be louder, but I'm happier with this one for now.

Thanks for your help.

https://cpc.farnell.com/kitronik/216...-ddd7-00001003

Colin.
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Old 5th Jul 2022, 10:57 am   #26
SiriusHardware
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Default Re: Commodore PET Speaker

The continuing lack of volume problem concerns me, I have asked a couple of times if you could measure the logic 0 / logic 1 levels coming out of the 'M' output but I don't know if you ever got around to it.

It just strikes me as suspicious that every single diverse kind of amplifer or sounder you try is quiet, the only common factor being the signal you are feeding them with.

There may be a way to change the gain (amplification) on that little Kitronik amplifier although I notice that they have gone away from the LM386 in the V3 version to another IC that I am not familiar with.

Also - you probably know, but if you take any speaker drive unit and put it in a sealed box with the front of the speaker facing outwards through a hole in the box, it will sound much louder. Even placing the speaker face down on a smooth surface will have a similar effect, but putting the speaker into a sealed enclosure is better.

Last edited by SiriusHardware; 5th Jul 2022 at 11:08 am.
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Old 5th Jul 2022, 11:20 am   #27
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Default Re: Commodore PET Speaker

The attached is lopped from the PDF instructions for the Kitronik Mono Amplifier V3. it shows that the feedback resistor R2 largely controls the gain (amplification) of the amplifier IC.

Try changing R2 to 220K or 470K, but first, please do measure the size of the signal coming out of the 'M' output and make sure that it is at the expected logic 0 / logic 1 levels - when the port is actually producing sound, of course.
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