MPX Filters

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I wonder if anyone can help me with the following conundrum, which is driving me CRAZY:
I play a Yamaha PSR-9000 keyboard, purely for my own entertainment. When I record any of my tunes to my Technics cassette deck, on playback the sections containing certain instrument sounds (notably sounds with lots of attack, and rich in harmonics, such as trumpet and saxophone etc.) have a peculiar "gargling" effect. I can record to the cassette deck from any other source (CD’s, vinyl LP’s, FM radio etc.) and the results are perfect. I can record to my PC from the PSR-9000 and then burn a CD from the captured recordings, and this plays perfectly. However, if I then record this CD to cassette tape, the "gargling" is still present.
Having bought a copy of the service manual for the cassette deck, and spent a fruitless 2 weeks tweaking adjustments, and even building a complete new power supply, I still haven’t nailed this problem.
I’m now inclined to believe that the root cause of the problem is probably detritis left on the analogue waveform by the digital method of sound generation used by the PSR-9000. I’ve noticed that the more recent cassette decks have a facility called "MPX Filter", which, I imagine, addresses this particular issue.
The question is : Does anyone out there have a circuit diagram, or know where I can obtain a circuit diagram, for such a filter ??

Fingers crossed ……….
 
MPX filter

na thats not what it is for. It is for recording off air FM. In order to get a stereo signal, it uses a pilot carrier at around 19kz (there are more added for SCA)..(multiplexing, thus the MPX) This is usually not heard, and is filtered out in the reciever anyhow. BUT <p>

when recording to tape, it is possible that this noise will rise to the level that it could be recorded, thus adding to the overall signal level, and causing distortion (especially intermod). Turning on the MPX filter just slams everything above some freq so that the 19kz is notched filtered out. So while it might take of your immediate problem, the reality is that a notch filter period might help since it sounds like this is an intermodulation or beat freq problem. <p>

You can build one of these using a simple op-amp.
 
Many thanks for your thoughts on this one, Sawzall.
I am familiar with op-amp-based active filters.
I was hoping that someone else had experienced similar problems and had developed a "killer" fix, complete with tried and tested component values.
Looks like it's going to be down to some happy hours of experimentation.

Alec L
 
take a look at TI and AD sites

Both I think give sample applications with values for using op-amps as filters. It sounds like your keyboard's analog outputs are pretty trashy. If it has MIDI or other digital outs, I might think about building/buying a better digital to analog interface to solve the problem as opposed to just filtering the problem.
 
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