SGCD Preamplifer to increase Signal to Noise Ratio?


#1

Hi there,
I’m pretty new to the stereo separates side of audio, so I apologize if this is an exceedingly easy question to answer.

I currently run my DirectStream Junior with single ended RCA interconnects and the 20db attenuator “in/on” to a pair of M700s. My speakers are extremely sensitive, (101db/watt), so even with this setup I can still hear white noise from the speakers at my 13ft away listening position. The noise is not overly loud or distracting but it is noticeable. If I swap the RCA’s for a pair of XLR’s the 6db of extra gain makes the background noise unacceptable, even with the attenuator in-line.

If I understand how all this works correctly I think I can lower my overall signal to noise ratio by inserting a preamplifier like the SGCD into the chain is that correct? If I run the DSJ DAC at 100 volume with the attenuator in-line then I’m maximizing the S/N of that component. In my current setup the DSJ@100 (attenuator in) is louder than I’ll likely ever want to listen, 70 is jamming out pretty loud.

Since the SGCD is line level at volume 76 I would be hearing equal volumes with these two scenarios correct? DSJ@100 = DSJ@100 + SGCD@76

So therefore if I insert the SGCD into the signal path and run it at any volume between 0 and 76 I should be decreasing the amount of white noise that ends up in the music since it will be scaling down a maximized S/N signal correct? Or does it not work as cleanly as that because the SGCD inserts its own noise into the system?


#2

Yep, you’ve got it (except possibly for the literal numbers on the volumes controls - I don’t know that both are 1/2 dB per unit.)


#3

Thanks Ted!

Listening to the white noise with my ear about 1ft away from the compression driver of my speakers there is only a very slight increase in noise when the DSJ is added to the signal chain so a majority of the background is coming from the amps. I’m extremely happy with the sound from my system and I’m just wanting to make sure I’ve got everything setup as optimally as possible. The DSJ does some wonderful things to digital recordings! Can you say piano and electric guitar!? I’ve never heard those two instruments they way they sound on this DAC, incredible air and “analogness”.

Next question:
It sounds like adding XLR cables increases the entire signal by 6db over RCAs, including the background noise. Adding the XLRs to my current preamplifier-less setup turns the DSJ into a major contributor of background noise.

Would there be any advantage to running the XLRs from the DSJ, with their added background gain, and then reduce it via a SGCD over running the 6db quieter RCAs directly from the DSJ to the amps? Is there a difference in the S/N ratio between the RCA and XLR outputs of the DSJ? With a preamplier would there be any background noise advantage of the RCA’s over the XLR’s?


#4

Minor technical correction: XLRs don’t have gain and people don’t usually talk about a change in background noise if the signal itself changes exactly the same way.

Anyway unless your preamp gets noisy when turning things down 6dB (extremely unlikely) you’re better off using the XLR out of the DS and cutting the extra level in your preamp. Some general theoretical reasons: keep a signal as high of a level as possible to lessen the relative effects of interference, balanced signals allow the receiver to cancel out common mode noise better (e.g. ground loops) and it also allows the DAC to cancel any incoming noise (e.g. ground loops). In the case of the DS and DS Jr using 1/2 of a transformer or 1/2 of an opamp is incrementally worse in terms of distortion, noise, etc. - these differences are tiny, but I’m sure some will have a preference one way or the other whether that difference is good or bad.


#5

Thank you again Ted, that is most helpful! I was noticing a quiet hum from my woofers as well so based on your last post I re-hooked up the XLR’s and the hum is completely gone. The advantages of the balanced connections that you just listed are definitely evident in my system.

I’m going to keep the XLR’s in the chain for a week or two then decide if I want to take the next step and add a dedicated preamplifier…which I probably will!

Thank you again for your help, I feel as though I have a much better understanding of how it all fits together now.


#6

I’m fairly confident I’m now in the market for a preamplifier to aid my extremely sensitive speakers.

Are there any dissadvantages of running the Directstream 20db attenuated signal (@volume 100) vs. the non-attenuated signal to the BHK preamplifier if the final listening volumes are below the line level volume of the preamp?

I understand and take advantage of the signal to noise ratio benefit of running the DS 20db attenuator “in” since I have plenty of volume headroom remaining in the DAC. I am curious though with the addition of a preamp if there are any advantages of a regular signal fed into the preamp vs. the attenuated signal that would be worth considering? IE does the BHK preamp function better with a higher level signal vs. lower and will it function as desired with the 20db attenuated signal or will it be underdriven?


#7

The BHK pre will be perfectly happy with the lower level signal.

Of course, you can try it both ways. Things sometimes work out differently than we expect and you may prefer the higher level signal. My expectation however is you will be happier with the lower level signal.


#8

Without a preamplifier in my system the sound is surprisingly loud with the DSJr’s volume set to 1. Volume 1 is loud enough for low level critical listening and too loud for background music when trying to have a conversation in the same room. The signal cuts off at 0 so 1 is as quiet as I can go.

Am I correct in thinking that the addition of a analog volume control preamplifier will allow for additional low level volume resolution between my current lowest digital volume and signal cutoff?


#9

Yes a preamp would definitely help. If you haven’t tried using the “DAC Level” button (the 20dB attenuator) you should at least see what that does for you.

I am surprised tho, a volume of 1 is 49.5dB quieter than most people listen, there isn’t that big of a difference between typical speaker efficiencies combined with typical amp gains. Are you sure you don’t also have a preamp in there already? Perhaps a phono preamp?


#10

Hi Ted. I don’t have any other audio components in the system other than the DSJr and the M700’s. I only utilize two sources in the DSJr: optical, which is fed by the audio return channel of my TV, and the Bridge II. I do run the 20db attenuator “in” at all times and my observations in this discussion are made with the attenuator “in”. I didn’t take the time to break out my UMK-1 microphone and REW but my phone app is showing a 10db difference between volume 0 and volume 1 on the DSJr when streaming over the Bridge II. I could take some more accurate measurements if you were interested but the volume level @ 1 isn’t too big an issue for me, more of an interesting observation in this path of discovery and optimization.

My listening position is 12ft away from the speakers. The quiescent idling noise of the M700’s becomes audible within 1ft or so of my typical listening position with normal levels of background noise in the living room; I can’t really pick it out sitting back in the couch but leaning forward to the front plane of the couch it becomes consistently observable. Turning the DSJr on adds enough background noise that it is easily noticeable sitting in the listening position, even with low level music playing, and something I’d like to reduce as much as possible with the addition of the BHK Preamp.

I know the BHK pre can attenuate the background noise of the DSJr down to nothing so I’m not worried about that part of it at all. My only real remaining concern is if and or how much noise the BHK itself will add on top of the M700’s quiescent idling noise. If it is nearly silent or only adds a small amount of noise to the amps then I think I’ll be set for life or at least until the next upgrade bug hits. However if the BHK adds in it’s own noise to a level similar to that of the DSJr then I don’t think I’ll be gaining enough to justify the extra expense of adding a preamp.


#11

The BHK preamp won’t be adding noise, in general even if it did it would mostly be relative to the output level (the opposite of the DS which is more or less constant regardless of output level.)


#12

Thanks again as always! I think this will seal the deal on the BHK later this month. I’m extremely happy with my setup already but getting this last few percent out of it will be very satisfying. I do a lot of business process systems design at work so I find this optimization process an enjoyable part of the audio hobby.

I realize that I represent a statistical anomaly in the customer base for system setup considerations and I really appreciate your time in helping me learn more about this topic.