I use a Tascam DR 05 to record my mates acoustic guitar and my bass in rehearsing some of his original material.I'm through an amp and hes playing and singing acoustically,My problem is that on playback,the playback sounds like it has an added 'tremolo' effect,emphasised at the bass end. We've tried different locations for the Tascam but that same effect is still there,I have discovered that the 'Tremolo' effect is only there when I use the 'headphone/line out' socket either into 'phones or straight into a stereo amp.If using the USB socket into my PC the recordings are 'clean' with no weird effects.What could be causing this?