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Hi,
Compared to other devices (such as sony ES SACD player) I'm hearing a faint noise in the OPUS DAC. This noise cannot be heard with normal listening volume, but only without music playing and with the pre all the way up (in this case 0 db). The noise sounds like a shhhhh (white noise) in the tweeter and like a hum in the bass. Disconnecting the toslink module did not cause any changes. Disconnecting the receiver board did not cause any changes. Disconnecting the Metronome, the bass noise disappeared. So it seems that the high noise comes from the DAC board and the low noise from the Metronome. Although this does not affect the music, I'm curious as to where is the noise generated. I don't think these are device intrinsic noises. Any hints would be appreciated.
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Mine is set up with a metronome and a balsie and it's dead silent.
PJN
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Hmmm, I was thinking about that. Perhaps the Balsie with the high CMRR would filter out that noise. Mine goes straight from the DAC to the preamp. Since I am planning of running it balanced, I think I'm going to try to use the TXD modules which can also eliminate the DC offset in balanced mode.
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Still interested in finding the source of the noise.
I am now using the Opus in balanced output mode into a balanced amp and noise is notably reduced. The current situation is the following:
When the DAC is just powered up, I hear a humming noise that goes away once music is sent to the toslink (in a previous experimentation, this startup humming noise seems to be associated with Metronome). Afterwards I hear a hissing sound (which most probably is coming from the DAC board):
At 0db gain in the preamp, the hissing is inaudible from normal listening position
At +10 db gain, the hissing is barely audible from normal listening position
At +35 db gain, the hissing is quite loud
The noise is not coming from the preamp (A PGA2320 based) because when I mute the inputs, there is not noise even at +35db
The loudest I listen to music is around -10 dB, son in normal listening conditions, I don't hear the hissing noise, but since other sources don't show this noise, I want to find out where is it coming from.
Any hints/info greatly appreciated
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I don't have enough info (would need to see pictures etc just to start) to diagnose your problem. But I have some thoughts to help you along. In my experience the PGA2320 and its siblings are pretty noisy EMI wise and even with careful layouts can hiss, unfortunately. I suspect your hiss is coming from EMI, either from inside the DAC, or from the PGA itself Its impossible for me to say which. You will not hear this hiss when you ground the input even if its coming from the PGA, as it effectively shunts the EMI to GND. That said, it may or may not be the preamp, I am just speaking from experience. This is specifically why I moved away from the PGA (I have used them all) and designed the Joshua Tree to take its place. Some SACD/CD players GND the outputs when music is not playing. This may be why you don't hear it from your other player when nothing is playing. The hum has to be a GND loop. The metronome on its own would not induce a hum. Cheers! Russ Edited by user Thursday, April 3, 2008 10:58:37 AM(UTC)
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Hi Russ, thanks for your support. The pictures are at http://picasaweb.google....sDac?authkey=YTOvsvrrU1Y and listed in the gallery section under glt Additional details - everything is connected to a Monster Cable power conditioning unit - The OPUS is connected to the PGA through a short cat-5 (1 foot)cable, one pair each for RV+, RV-, LV+, LV- - The so called "hum" only happens when after power up and goes away once there is music fed to the toslink (the hum is louder than the hiss). - I had replaced the bridge on the LCDPS with MUR diodes - Previously I was using a Classe integrated where the preamp section was based on the CS counterpart of the PGA and heard the same noise, but in single ended, the noise is noticeable louder Thanks again for your support.
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Ahhhh, ok that DAC. Very clever case work. :) I remember it now.
I don't see anything obvious wrong, but I can suggest this. twisting the digital signal wires to together with each other is probably not the best. If they were twisted with GND or something it would be ok, You want them to run short and straight, and not to interact with each other.
You may be getting a bit of noise from the trafo or the diodes. It very difficult to say.
I will mull it over. If I can think of anything else I will post it.
Cheers!
Russ
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Hi Russ,
Followed some of your suggestions so I unplugged the DAC from the preamp and hear almost the same amount of hiss. So the input of the preamp is now floating. Does this demonstrate that the hissing is coming from the PGA? The cables from the input socket to the PGA chip are fairly short (2-3 inches).
From your experience/memory can you say what kind of "tricks" can be used to lower the noise?
Thanks again for your help
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Russ, your suggestions were right on the money. Played a silent track both from the PC->DAC and from the SACD player. The level of hiss at +31 db setting in the preamp is comparable (although they sound somewhat different). Wanted to compare with a passive attenuator, but at 0 db, the PGA based preamp is also totally silent. I guess I should not worry about it
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Hi,
Unfortunately, I have never been able to fully get rid of the hiss from a PGA type preamp (at > 0db as you also observed). My theory is that this hiss come from the digital piece of the chip. I have tried all sorts of isolation tricks. Things like using small value resistors or ferrites or both. Also have split the GNDs and used ground planes. The quietest solution was using one single ground plane and using a ferrite and bypass cap at the analog power supplies, yet even this did not fully cure the hiss.
One solution is to change you firmware so that the gain never goes > 0db then you just depend on the gain you have down stream.
Cheers!
Russ
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Hi Russ,
Do you mean higher value bypass at the analog power input to the chip. I already have a 10uf oscon + 1 1uF film at each analog power input pin. I notice that your boards have 100 uF FMs
Also, as I am planning of getting an Ivy for balanced DC blocking, I suppose I can add gain at that stage.
Thanks again for your help.
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Hi,
Are you referring to the Kookaburra? That is just one of the PGA designs I have done. I have done several. 10uf + .1 is fine to decouple/bypass. What I meant was adding a ferrite or small value R(say 1ohm) in series with V+ and V- just before those caps.
What I would do if I were you is use the built in volume control of the Opus DAC. It is very easy to do. If you need some pointers or example code I can give it to you. Then you could use either IVY or Ballsie. Adding gain to either is pretty easy.
Cheers!
Russ
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