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I put a o scope on the output of my mth gn S2 4-8-4 and the waveform distorts quite quicky and the volume is not all that loud when it happens.  If you are running the sound at wide open you are really getting a bad distortion of sound.  At low volume  a pure tone like a bell is quite good on the scope.  Lionel legacy sound can go much louder before distorting.  I tried using a 8 ohm speaker on mth and the waveform stayed better and on a 16 ohm speaker it did the best, but not real loud.  If you are not a volume freak try a much lower volume or change to a 8 ohm speaker.  8 ohms and reduced volume was quite good.    when the bell is ringing, engine chuffing and whistle blowing it is a mess.  The whistle takes allot of amplifier power.  Mth needs more clean power or trying different speakers. I now run my bell at volume 2, whistle at volume 3 and chuff at volume 4 and crew talk at volume 6.  Use a 8 ohm speaker and all is much better,

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I wouldn't worry too much about what a scope shows on the audio output as you're dealing with Class D amplifier and both + and - outputs get into a push pull mode  that will drive a scope crazy without the proper filtering. Then add a 250kHz switching frequency ontop of all that and you have what appears to be a real mess.  

 

Here's the PS2 audio amplifier datasheet which should be read before any analyzing is done.

http://www.ti.com/lit/ds/sgls137e/sgls137e.pdf

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