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Hifimediy CS4398 dac loud hissing noise

Hifimediy CS4398 dac loud hissing noise

Setup is: minidsp (toslink or rca) --> CS4398 Dac --> Hypex amp. No mods done on DAC!
Digital volume control is on the minidsp. Also tried the same setup with squeezebox (toslink out) and PC.

I connected the CS4398 dac to a Hypex UCD180, using a balanced connection. This produced a loud hissing noise (produced by tweeter). Also when volume is down (not related to volume). I tested this with the unbalanced connection and got the same result. The hiss with balanced is louder.

Then I checked the Hypex without dac, it is absolutely quiet. The hypex with a different dac: no hiss. So the CS4398 is the suspect.
I then connected the CS4398 dac to a T4 amp the hissing noise is there, but less then with the Hypex. I lifted the modules from the chassis, lifted ground, no improvement.

Any ideas?

Another issue: I tried the SRC module and it gave a loud clicking noise. The yellow led kept on blinking.
The SRC module is working now. When I checked it with a Magnifing class, I saw some dirt (very small wires) on the chip legs. I cleaned it and it seems to work ok.

When I use the CS4398 with the T4 amp, the hiss is audible up to 1 mtr from the tweeter. I could live with that. But the noise is too loud with a high gain amplifier like the Hypex.
Found it:

"The crucial parameter is dynamic range, which is the relationship between the peak amplitude of the audio signal and the underlying noise floor of the circuit. When a DAC with a fixed output is followed by a passive attenuator -- e.g., a potentiometer -- the peak signal and noise are reduced by the same amount, so that the dynamic range remains constant. With a digital volume control, the peak signal level is reduced, but the underlying noise remains the same -- consequently, there is a reduction in dynamic range. The degree to which this loss of dynamic range is a problem depends on the inherent noise level of the DAC."

My conclusion is that the noise level of the CS4398 DAC is not low enough for my active setup. I guess I will have to attenuate the signal after the dac.
Hi, you could also try to listen directly to the cs4398 dac to check the noise level. Attach headphones directly to the cs4398 output. The cs4398 doesn't like low impedance loads, so only do this for a short period of time, and set the digital volume level (from computer/source) a bit below max.
How sensitive are your speakers?
I have also noticed the cs4398 dac is not the most quiet, however it's not noticeable on normal sensitive to sensitive speakers.
Thanks for your reply! I will try the headphone. I hope I don't burn the dac chip.
My test speakers are average sensitivity (Vifa drivers). I think under 90db.
I tested the dac with a T1 amp. There is no noise. I have to put my ear against the tweeter to hear the noise.
I think a voltage divider circuit between the dac and the Hypex amp will solve the noise. 10 to 20db attenuation will do the trick.
Hi, if there is no noise with the T1 amp, then it's no need to try the headphone test.
Hi, if I place a voltage divider U-Pad (basically a volume control) between the dac output and the amp on the balanced out, would that short DC to ground? The balanced out has no DC blocking caps in its path.
a volume potmeter would short DC to ground. It might survive a 50K for some time, but might fail later. Not recommended! Add some coupling caps.
I mixed up an L-pad and a U-pad. With an L-pad the shunt resistor is in parallel (signal and Ground) and would short dc to ground. Not so with U-pad (for differential/balanced). The shunt resistor does not short to ground. I tried it and works fine (R1 = 570, R2 = 1k). It gives me about 20db attenuation and the hiss is gone.
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