What is -12v used for?

[naujoks]

I've built a universal power supply for use with many vintage computers. It has +5v, +12v and +9vac available.
I'm looking to get an Amiga 600 connected to it as well, but my PSU doesn't provide -12v, so my question is what the -12v is actually used for. If it's just for power in the ports, I could live without those, as I just have the joystick/mouse port in use.
Does anyone know if the Amiga is runnning without -12v?

[Karlos]

Audio and serial both spring to mind.

[alexh]

Originally Posted by Karlos

Audio and serial both spring to mind.

Quite right.

http://ianstedman.co.uk/amiga-power-supply-guide/

Originally Posted by steady

What is the -12V supply on the Amiga used for?
It is used for the RS232 transceiver chips and to power the Op-Amps in the audio circuits.

If there is an issue with +/-12v you get audio distortion. It is well documented.

[naujoks]

I see.
I might put in a little something like this:
https://www.ebay.de/itm/384939237261...Cclp%3A2047675

[pandy71]

Originally Posted by naujoks

I see.
I might put in a little something like this:
https://www.ebay.de/itm/384939237261...Cclp%3A2047675

This will not work for you - you need inverting power regulator, something using inductance or capacitance to create from positive voltage negative one - there are some modules based on 7660 (using so called charge pump) or something made on MC34063 where inverting topology is used to invert incoming voltage or there are some new IC based modules or alternatively modify commonly available boost regulators. Nowadays there is plenty of such modules available on internet.
Search for inverting voltage regulator - 7660 solution may not provide sufficient power as they primary purpose was to create bias current (so usually only few mA capable).

I would search for something capable to deliver at least 50..100mA in -12V line.

For example https://www.ebay.de/itm/302708721231 should work but probably you can easily find similar modules.

this one https://www.ebay.de/itm/113236210368 may work too but probably you need connect +12V line trough diode to prevent reverse current flow so single voltage output is way easier to be used directly without introducing severe Amiga modifications.

[Karlos]

Originally Posted by alexh

Quite right.

http://ianstedman.co.uk/amiga-power-supply-guide/

If there is an issue with +/-12v you get audio distortion. It is well documented.

Indeed. The DAC aren't able to produce the negative waveform values without it. Serial may or may not work, depending on how fussy the device on the port is about correct signalling levels.

[Jope]

https://www.dimensionengineering.com/products/negatron

[pandy71]

Originally Posted by Karlos

Indeed. The DAC aren't able to produce the negative waveform values without it. Serial may or may not work, depending on how fussy the device on the port is about correct signalling levels.

Are you sure that DAC in Amiga produce negative voltage?
OPAMP after DAC require symmetrical (bipolar) supply (circuitry is not made to use unipolar supply).

Serial port level translators(1488/1489) may not be designed to be powered by unipolar power supply.
Nowadays serial level translators (MAX232 and similar) usually use single power supply and they are equipped with charge pump to create negative voltage on board.

[Daedalus]

It's only a very minor thing (the main point being that audio requires +/-12V), but indeed, my understanding of the audio circuitry is that the DAC doesn't produce a negative voltage, or actually a voltage waveform at all, but produces a varying *current* signal. This is then converted into a varying voltage audio output signal by the op-amps (which do need the dual rail supply).

[Karlos]

Originally Posted by Daedalus

It's only a very minor thing (the main point being that audio requires +/-12V), but indeed, my understanding of the audio circuitry is that the DAC doesn't produce a negative voltage, or actually a voltage waveform at all, but produces a varying *current* signal. This is then converted into a varying voltage audio output signal by the op-amps (which do need the dual rail supply).

Thanks for the detail. I was using DAC in a very broad context to mean the entire set of electronics responsible for generating the output analogue audio signal from the digital data.

[pandy71]

Originally Posted by Karlos

Thanks for the detail. I was using DAC in a very broad context to mean the entire set of electronics responsible for generating the output analogue audio signal from the digital data.

Usually DAC's are supplied by single voltage and as such usually they not output bipolar (positive/negative) voltage - usually 0 (zero) is shifted (offset) to half of the supply voltage - similar circuitry (resistor divider) generate 'Aref' voltage in Amiga (used to bias I/C converter OPAMP) - later you usually use capacitors to provide AC coupling so you remove DC bias...

btw this bias in Amiga may severely impair audio quality - better is to use active bias circuit and prevent noise (also power supply) injection to OPAMP inputs but this is beyond thread.