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AMDEK-MXK-600 (DIY ROLAND) MIXER MODIFICATIONS

I bought this lovely little mixer from a friend. Thanks Joep! I'd been looking for a small and simple mixer for a while. The MXK-600 looks awesome, and is just the size I was after:

It had a US 110v transformer, which I needed to replace to run the mixer in the EU.


Here is the article concerning the kit when it was released:

http://www.muzines.co.uk/articles/amdek-stereo-mixer-kit/5905


After looking around, replacements for this type and size especially (3x4cm!) were not available. I decided to remove AC operation and go for a standard DC input.


After researching the IC specs, I realised it was possible to run this thing really hard, up to + and -16v, so 32v across the opamps - Nice! It ran on +8/-8 previously!


I ordered a 31vDC supply, from an HP printer. I have used one like this before in an audio project and it was just fine, very stable and clean. +24v would have worked as well, though these are harder to come by in a convenient package like the printer supply. Also I wanted the max voltage.


After the virtual ground does its work, the 31vDC is changed to +/-15.5v, almost the highest possible with these opamps.

I had to replace some caps inside to deal with the higher voltage rating. The mixer was very time consuming to disassemble and reassemble. All parts are Roland parts, the mixer is well constructed (hats off to the DIYer who assembled it) There are THREE parts to the metal body, and all pots and switches are wired to the board (no pcb mount). They don't build like this now! It's not that old, early 80s.


Now the mixer is clean, with new caps, and a new DC inlet. It's a round peg in a square hole, though it fits fine and stays put! Also, double the voltage input has increased the headroom.


If you have a device which runs on +9v, chances are it could also run on +12v. Consider the datasheets of the opamps/ics/and capacitors in the design and see what the voltage ratings are. I have a BOSS BX4 which runs without any modification on +12v.


Here we see the new DC inlet. The panel on the left was the mounting point for the AC transformer:

I could not do any comparison tests, (the previous psu was 110v) so there is no before and after observation. If it can run on a higher voltage, it's generally better to do so, as it does increase the headroom.


I think I may have inserted the switches backwards in the case, though they are all the same orientation, so it does not really matter too much. It's a big, long task to open it to move them around. I will leave them as they are. When -50 is selected, this is the most gain, and I think this should be the least, which is why I surmise they are just in the wrong way around!


Still to do is to add a label on the case with the new voltage information. I left space for this by placing my logo sticker higher up:

www.artworkfromewaste.eu

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