I was digging into some old document to create stereo sound output for one of my ZX Spectrums.
I found the document, made the modification and I'm very happy with the stereo quality.

So I thought it would be nice to create a very small pcb that would fit on the ZX Spectrum's AY-chip, so I did the pcb-design:



And posted it on WoS: New stereo audio board for fitting on AY chip - interested?

Unfortunately I got a lot of bad response about limitations, such as that eastern-european Spectrum users use another 3-channel mapping than in the other parts of the world, and a lot of music comes from that area.
Also there were some remarks about component values etc.

So I decided to stop development, and just post the results here, so you can decide if you want to build this yourself.

Here's my design:



And here is Ingo Truppel's design, which I think is slightly better (designed) than mine:



Here's Ingo's mail to me with more info about his design:
"Are you using ACB stereo?
As I understand it ACB stereo is more common than ABC stereo. That's certainly the set up I'd want if it wasn't switchable.

The 33pF have no effekt because they are much too small. About 1.5 nF ... 2.2 nF would be better to avoid high frequencies from the audio output.

The 10µF are much too big because the load resistor on the sound output should not be lower than 10 kOhm. A capacitor of 0.22 µF gives a 3dB cut off frequency of 72 Hz. That is low enough in face of the fact that table top loudspeakers will have a lowest frequency of about 100 Hz. Higher capacitors will give a big "plop" signal while switching the computer off or on and raise the risk of a destroyed loudspeaker (if loudness is set very high).

To distribute the center channel (B in your case) with 50% to each of the stereo channels is not really correct. For the listener the power of the two channels adds not the voltage. And therefore each of the two stereo channels should get about 70% (1/sqrt(2)) to get the same power if only the center channel is driven as if one of the other channels is driven.

You added the beeper signal to the output but not the incomming loadig signal. Therefore you will not hear the signal while loading from tape.

I have additional two bistable relais to switch between ABC and ACB sound."

So, if you wish to make one yourself, go ahead.
I only tested my design.
But I probobly use Ingo's design if I am going to build another one.

I had the chance to present my laserharp finally, after months of work, on a Dutch computerfair yesterday 3 october 2010 in Apeldoorn.
Here are two video's from the harp itself at the fair:

Visitors playing the harp
An interview with the fair organiser (I'm not very good in interviews...)

I'm so happy ;)

I've created a document to enhance your ZX Spectrum 128K, +2, +2A, +2B or +3 with a real MIDI output.

Download it here!

I'm now really making progress on the laserharp project.
That's because it's almost time to present the laserharp at some retrocomputing shows, so I'm anxious to finish it in time!!

Here a small video showing the modifications I made to a ZX Spectrum +2B .. poor speccy!

http://www.youtube.com/watch?v=3KJxLAzopgg

I'm building 50 more, and 10 are ready.

So get one at eBay:
DivIDE interface with 512MB CF card and CF adapter
DivIDE interface without 512MB CF card (but with CF adapter)