Strange oscillations

Headroom

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Oct 12, 2012
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From
The Hague, Netherlands
Website
sites.google.com
Gear owned
DM4800
Strange oscilllations are ruining the S/N ratio in my DM4800. I can get any fat channel to produce oscillations at a level of about -50 dBu. They start when I feed some signal into the channel with the equalizer on and continue after the signal is switched off. By tweaking the equalizer settings I can produce any tone. It happens on more than one channel simultaneously, so my DM has actually turned into a 48-tone polyphonic synth :) Oscillations stop when I switch off the equalizer and don't start again as long as there's no input signal. Now I know that there can't be oscillation unless there is some sort of feedback, but I can't imagine what can cause this.

I hope someone can help!

Frans
 
I discovered this behaviour when The Old Forum was alive and wrote some detailed posts about it. Conclusion was: oscilation only happens, when there's complete silence on the EQ after it has had some signal present.

Workaround is to feed some low-level noise into the EQ. Any analog signal (DM's A/D converters etc) has enough of noise to prevent this happening. If your input is a DAW, you can insert a noise-generating plug-in (like dithering plug-in) to stop the oscillation. So, while the oscillation is always below -80dBFS (usually unaudible) with sensitive material, you can always add some noise (below -140dBFS) to stop it.

This is definitely a bug in DM, which should be fixed, but definitely not a show stopper, just a nuisance.
 
Jarno I also have this oscillation problems, I believe it has something to do with tdif connections. I use a lot of digital connetcions (firewire card/adat and 24 tdif channels) Do you have a solution, or a way to feed een low noise signal into the digital path? Thanks very much for replying

erik
 
Strange Oscillations. Sounds like a great title for a song!
 
Lol now that's what I'm talking about!
 

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