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Noise in 192k opto signal input

gamma3, gamma24 (plug-in for the γ3)

Noise in 192k opto signal input

Postby Caudio12 » May 21st, 2017, 5:21 am

Hi,
After finnished the gamma3, very close to reference built, I have made a more systematic testing.
The input tested are opto, phono and USB. No signal to test the coax or XLR.
All works fine, but in 192k sampling the opto gives a sound with distortion, high. frequency ticks. The other input the 192k works fine.
I also tested the opto signal out, and converted this signal with a gamma 2, and then the sound is fine.
Also tested the same signal direct to gamma2, no error, so no problem with the source.
This puzzled me, then this indicate the digital source reach the gamma24 unit without distortion. I thought it was a error in the optical receiver units.
I have used the test files supplied by Linn Audio, for test purpose.
http://www.linnrecords.com/recording-test-files.aspx
Looking forward to some advice how to locate the error in the DAC unit.
/
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Re: Noise in 192k opto signal input

Postby amb » May 21st, 2017, 11:02 am

Try a different (hopefully shorter, high quality) optical cable.
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Re: Noise in 192k opto signal input

Postby Caudio12 » June 3rd, 2017, 12:28 am

Hi,
Made some new test, still odd resultars.
1) new opto cable, digital output to gamma3, still same cracking sounds in 192k.
2) coax cable from same source, fine sound in 192k.
3) connect gamma3 , output optical (or coax, same result) to a gamma2, fine sound in 192k. Using the same two opto cables used in the test 1. Source-> opto cable#1 -> gamm3 -> opto cable#2 -> gamma2-> amplifier.
The last test shows at least to my understanding that there is no problem with the opto cables.
What can it be?
The signal goes to the SRC4392 circuit inside gamma3/24 and routed back to the opto/coax output.
Based on this, the signal is ok then reaching the SRC circuit.
The same signal source from the coax input give the digital output result, but the DAC Is now without the cracking sound.
The DA circuits are the same in gamma2 and gamma3, so both circuit would have the same sensitivity against errror in the signal.
Question is if the SRC circuits, not the same in both units, have diffent sensevity of error in the signal.
But there is no oversampling in this case due to the source us already 192k. But the circuit is also doing some corrections, possible only for the outgoing stream to the DAC circuits and not to the digital output. (I need to reading through the data sheet)
Any other test I can do to nail down the problem?
I can out shopping a really expensive opto cable and see if this will solve the problem, the current is 0.5 to 1.0 m long, and cost around 10€ each.
Looking forward to any good ideas.
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Re: Noise in 192k opto signal input

Postby amb » June 3rd, 2017, 3:01 am

What's your γ3 menu setting for "Up-sample"? 96KHz or 192KHz?
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