YM2149 mixer with pan pots

[Martin Giese]

I was a little bit unsatisfied with the way the stereo mixing of the three channels of a YM2149 is usually done. One shortcoming is that different chip tunes assume different assignments of the channels to the stereo image. The other is that pulling together the outputs with a handful of resistors doesn’t give the audiophile grade channel separation your chip tunes deserve ;-)

I spent a few hours this weekend implementing an output network with three separate pan (trim) pots for the three channels, feeding into an active mixer:

 

(The chip to the lower right is an SPLD for IO address decoding, the two above the YM are a 17.734MHz crystal oscillator and a /10 divider for the original Spectrum pitch.)

If anybody is interested, I could produce a document explaining the design.

Martin

[Joao Miguel Duraes]

So, that's an easy "yes, please" from me. Sounds like I'm not the only nutter out there about the joys of AY/YM sound variations. 

Also interested to know where you've managed to get the crystal from - they are like hens' teeth. 

[Martin Giese]

OK, will do!

Concerning the crystals, Google for 17.734475Mhz crystals.  Mouser has them stocked for instance.  I got mine from Ali but that particular seller doesn’t have them anymore and it’s a pain to search through their search results because most of them give quartzes with umpteen different frequencies, not including the one you were looking for.  You could get these and sell the spares https://www.aliexpress.com/item/1005005568392007.html

Divide by 10 and you get the famous 2/5 * PAL colour carrier of a Spectrum 128K

The ZX True Tone design uses a 35.4689 MHz crystal, so that would have to be divided by 20.  Can also be found on Mouser but I wasn’t lucky on Ali and I preferred using an old fashioned 74LS90 decade counter.

Martin

[Martin Giese]

Enjoy :-)

https://docs.google.com/document/d/1Dj0Znar9hl_cpoXNB9l8FJ7Z6Eyww2m4VPAxdHmTJzA/edit?usp=sharing

Please let me know if anything should be done differently, I still consider myself a beginner.

Martin

[Ed Brindley]

Very comprehensive write up Martin, thanks!

I was also never totally happy with the channel setup either, but too lazy to look into changing it.

Maybe the sound card is due a "hi-fi" revision with the speccy accurate oscillator and some pan pots. :-)

Cheers,

Ed

[dean.ne...@gmail.com]

Studying your write-up Martin -- cool stuff.  You call yourself a beginner?

Joao introduced me to the 'stereo' design for this chip - which led me to update my design with the basic resistor solution.  But then I had to figure out how to enable the stereo signal to be passed and mixed in with my YM2413 'music' module.  

The YM2413 only has 2 outputs (Rhythm and Melody).  I managed to figure out some op-amp magic and achieve something similiar.

Mixing the 2 signals of the YM2413 does not quite produce as cool an output as the stereo mix for the YM2149.  But hearing the combines outputs of the 2 chips in stereo is cool indeed. 

Dean.

https://www.dinoboards.com.au/

[Fernando Carolo]

Hi Martin,

That's a very cool setup there. As for getting a faithful ZX Spectrum 128k sound, I found that Digikey sells ECS-P85-B oscillators, which are programmable oscillators from ECS. You can specify a frequency when ordering and they come factory-programmed with it.

In my case, I ordered one asking for 7.0938MHz and it's working fine in an Ed Brindley's YM2149F sound card rev 6.1 with the default /4 divider.

Best,

Fernando

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[Martin Giese]

Glad y’all like it!

Fernando, I didn’t know about those one-time-programmable oscillators.  Not cheap, but a solution to remember for hard to obtain frequencies.

Dean, I can imagine that mixing the outputs of several of these old chips together can give amazing results!  You could have one pan pot per channel and move them around a bit :-)  With different types of chips I would also expect the output levels to differ somewhat, so mixing them also requires adjusting the level, right?

My favourite so far is the YM2151, really impressive for its time.  I’ve only been talking to it from an Arduino, but one day, I’ll have to make a Z80-based MIDI synth with it, complete with LCD display, a ROM full of cheesy presets, and inscrutable parameter menus.

Martin