Trying to build an RC2014 soundblaster style card

[Alan Cox]

I'd been pondering building a soundcard like the early PC soundcards (covox, disney sound source) but was put off by the 20 odd resistors it needed and also the fact that for the Disney one it needed a small FIFO and a clock.

Today I found out Bourn make the entire R/2R ladder in a SIP so I've put together a design. The other thing I realised is that really I wanted to build a soundblaster type card so I've added a flip fop. The idea is that a Z80DMA card can use it as the ready signal and a CTC as a clock. The CTC keeps setting it and each DMA write in turn clears the ready until the next CTC edge thus controlling the sample rate using the CTC.

Does mean I have to stop using the DMA for disk I/O whilst playing sounds and go PIO but I think I can live with that.

I am not too sure about the audio side though - that's all a bit new to me!

Alan

[Greg Holdren]

Alan,

Looks like C7 is backwards if you put a polarized cap in.

Greg

[Richard Lewis]

As Greg mentioned C7 is the wrong-way around. TH ceramic caps of 10uF are relatively cheap and not hard to find. Reminds me of my first TRS-80 Model 1 where before developers figured out how to use the cassette output port for sound one would use a transistor radio tuned to a vacant channel, placed nearby (even across the room) and the sound would come via way of EMF. 

On Thursday, October 3, 2019 at 3:57:51 PM UTC-7, Alan Cox wrote:

[Alan Cox]

On Fri, 4 Oct 2019 at 20:51, Richard Lewis wrote:
>
> As Greg mentioned C7 is the wrong-way around. TH ceramic caps of 10uF are relatively cheap and not hard to find. Reminds me of my first TRS-80 Model 1 where before developers figured out how to use the cassette output port for sound one would use a transistor radio tuned to a vacant channel, placed nearby (even across the room) and the sound would come via way of EMF.

Thanks folks. I've fixed C7 and also got a bit carried away and added gain and volume trimmers.

Funny you should mention TRS80 because the first simple 8bit DAC card I know was the Orchestra80 card for the TRS80.

(http://www.trs-80.org/orchestra-80/)

Alan

[Mark T]

Also when mixing audio with digital circuits try to take as many opportunities as possible to filter digital noise as you can. So any op amps with gain control feedback can include a cap in parallel with the resistor. Not sure if that applies to lm386, maybe some application circuit would show how it should be added.

Mark