PreenFM > preenfm2 and preenfm3

Subtle distortion on one channel

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SirPrimalform:
My PreenFM2 seems to have a subtle distortion on one side of the stereo image. It doesn't sound like clipping, but rather that kind of digital distortion you get when the steps in the DAC aren't quite the same size. I've ignored it for a long time as it's very subtle and I rarely use it in stereo but I'd like to diagnose it now if I can.

Can anyone suggest any steps? Swapping the DAC ICs would be an obvious one... see if the distortion follows the DAC or not.

lokki:
can you record a file and create a spectrogram of both channels to compare? are both channels set to exactly the same sound? i.e. a mono sound centered?

SirPrimalform:
Good idea, will try and do that this evening.

Xavier:
Each channel is the sum of 2 Dac outputs (12 bits + 6 bits).
The resistors pair 115K and 1.8K have a ratio of 63.88 (6 bits = 64) which is what is used in the sum.

You can verify with a multimetter that you mesure the same value on the 2 115K and the same on the 2 1.8K.
You'll get a lower value for the 115 because of the other components in the circuit.

(You can verify the same thing for all the paired resistors around the op amp, they are all part of the output audio.)

clément:
Hi there, I believe I was referring to the same issue here.
In my case I notice it mainly when I'm working on "soft" sounds, not with strong FM. Is it the same for you?

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