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digital attenuation

77.12.218.62
Posted on January 15, 2010 at 03:27:43
play-mate
Audiophile

Posts: 558
Joined: November 21, 2008
 dear inmates,

running my system with only digital attenuation, I was wondering if I would gain any higher resolution by inserting a resistor between the lynx TWO-B and my ICEpower amplifiers.

there are three digital attenuators in the chain (cPlay, allocator & lynx) which are all attenuated by apprx.10 dB. -further the lynx is switched at -10dB at it´s analog relais operated output.

my digital-sceptic highend dealer says there is nothing to fear, although I´ve always "heard" digital attenuation is an evil and is costing "bit-resolution"

the lynx output is 100 Ohms in balanced mode.
the acoustic reality eAR´s input is 10kOhms.

if a resistor would be beneficial, where to put it ?
1. inserting it close the output (lynx standard break-out cable)
2. at the XLR inputs of the amps.

-and finally, which values and since balanced, is a resistor required on all three pins ?

thanks a lot in advance !

RE: digital attenuation, posted on January 15, 2010 at 04:56:30
DBB
Audiophile

Posts: 575
Location: NYC, USA
Joined: November 11, 2004
 I realize this is not a technical answer to your question but more of a practical approach. You might first try to put an attenuator/passive preamp, if you have access to one, in your system. Then compare. I have done this using the Lynx mixer for volume control on the software side against my Gold Point passive. I was not at all sure I could hear a difference. Sometimes do not miss those alleged missing bits.

RE: digital attenuation, posted on January 15, 2010 at 07:58:34
soundchekk
Audiophile

Posts: 816
Location: DUS
Joined: July 11, 2007
 Hi there.

What you're describing is the typical problem of many patchwork systems.
I'd say the majority is configured like that. Very many amps just have much too much gain.

You need to bring down a 4V DAC output signal to estimated approx. 0.125V (30db=20*log(0.125V/4V) at the amp input

I think it is easy to understand that this won't work properly. This will heavily impact the signal and SNR. For sure you'll suffer.

We would have to do some engineering first to find a 100% matching solution. You really need to get the analog side straight:

What we need to know is

1. output voltage of your DAC
2. voltage gain of your amp
3. level db/W SPL of your speakers
4. DC resistance of your speakers
5. preferred maximum listening level

From here we can do some reverse engineering. If you come up with above numbers I can show you how to do a calculation as an example.

I can tell you already that you need to run your DAC-amp connection single-ended, which will save you 6db on the voltage side.
And the other 24db you need to gain on the amp input stage (for reducing the amp gain on class-d amps usually opamp-inputstage resistors need to be changed).
I'd guess that you have to set the gain of your amp input stage close to 0 , my guess is that your amp runs at least a voltage gain of 36.
(I had to do it also on my Hypex modules).
Perhaps you check with Acoustic Reality. They might change the resistors for you.

I doubt that you get the output voltage of your sound card further down. That I regard more complicated.

And forget to put a resistor on the lines! ;)

Cheers

CAUTION!, posted on January 15, 2010 at 08:07:33
Tony Lauck
Audiophile

Posts: 4582
Location: Vermont
Joined: November 12, 2007
Contributor
  Since: February 24, 2009
 Sound quality should be the least of your worries, and only after you have addressed the more important question: the continuing integrity of your amplifier and speakers.

If you have too much analog gain in your system then any sort of a computer glitch can easily blow things up. You should reduce the analog gain by some means (such as resistors at the amplifier input, passive attenuators, restrapping the sound card, etc...). Ideally, you will not be able to damage your gear no matter what miscreant software may do, while still having enough gain that you never feel limited by the 0 setting on your digital volume control when listening to a recording made at a low level.

Tony Lauck

"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar

RE: digital attenuation, posted on January 15, 2010 at 09:52:40
Dawnrazor
Audiophile

Posts: 8992
Location: N. California
Joined: April 9, 2004
 Hey Playmate,

I like this attitude, even if it es probably over kill :)

The absolute easiest thing is to get new amps or have AR modify them.

As an example, I recently replaced my beautiful Pass Labs x-150 and its 30db of gain with 2 Parasound Halo A21s. The gain on that amp is listed as 28 db @ 1v. Anyhow, with the Parasounds I am about 10-15 db higher in the cPlay volume control ( though I am using them at consumer levels and had the pass at Pro Levels) My back up was that they had an analog volume contol on the input so if necessary I could use that.

My challenge though seems to be the opposite of yours. FOr some reason when I have allocator going, I lose about 10 db of volume. I checked everywhere I could think but all the sliders I see are maxed. Are there any hidden vcs there? I have to check with Parasound as I still can resort to pro levels if for some reason the allocator just happens to suck 10db of gain.

And I was looking at Gilmore. They have a switch that changes the amps gain according to their website:

Gain

* 27 dB in high gain position nominal
* 20 dB in low gain position nominal


But ultimately finances won out and I went with the Halos.



No one here remembers the bending of our minds
RE: digital attenuation, posted on January 15, 2010 at 15:00:14
mäç
 When using digital attenuation, you are effectively throwing away resolution. Attenuating the signal off your soundcard in the analog domain is a much better idea. Since your signal is balanced, you will need to attenuate both positive (pin 2) and negative (pin 3) pins to ground. You might see if NHT is still selling their Pro attenuator at a discount as they will work perfectly for your application (and use the fabilious Jensen transforms, which will provide you with better isolation between your PC and amp).
RE: digital attenuation, posted on January 15, 2010 at 16:18:25
aljordan
Audiophile

Posts: 1064
Location: Southern Maine
Joined: November 4, 2003
 Hi,

Why is attenuating the signal in the analog domain better than the digital domain?

I realize you are throwing away resolution in the digital domain, but in the analog domain, with a standard preamp, you are raising the noise floor by excessively amplifying the signal, then attenuating that signal which doesn't attenuate the amplified noise. You are not necessarily raising the noise floor with a passive attenuator, but you can very negatively impact dynamics with a passive attenuator if it is driving a low impedance amplification load.

If digital attenuation was implemented in with 32 or 64 but math, is it necessarily worse than analog attenuation? The reason I ask is because my ears tell me that one problem is just being replaced with another when comparing digital to analog attenuation.

Any comments or theory would be welcome.

Thanks,
Alan

pinging soundchekk, posted on January 15, 2010 at 17:46:43
play-mate
Audiophile

Posts: 558
Joined: November 21, 2008
 thank you all for your replies,

....I´m not entirely sure if I´m not even more confused....:-)
-but I guess it may be worth a try to insert some resistors and listen to the result.

soundchekk : cannot find the output voltage of the lynx card, but here is what the spec sheet says :


Type
Electronically balanced or unbalanced, XLR connectors on L2Audio Cable

Level
+4 dBu nominal /+20dBu max. or -10dBV nominal / +6dBV max., software selectable in channel pairs

Bandwidth
10Hz - 92 kHz @ 200 kHz sample rate
10Hz - 46 kHz @ 96 kHz sample rate
10Hz - 23 kHz @ 48 kHz sample rate
(analog input to analog output)

Input Impedance
Balanced mode: 24 k , Unbalanced mode: 12 k

Output Impedance
Balanced mode: 100, Unbalanced mode: 50

Output Drive Capability
600 impedance, 0.2 µF capacitance

8, 16, 24 or 32 bit file types
On-board Buffer Size

(Analog performance specs measured at 44.1 kHz sample rate, 24-bit, card installed in computer)
Frequency Response
20 - 20 kHz, ± 0.05 dB at 44.1 kHz sample rate

Dynamic Range
117 dB, A-wtd.

Signal-to-Noise
116 dB, A-wtd.

Channel Crosstalk

-120 dB, 1 kHz signal @ -1dBFS
THD+N

-108 dB (0.0004%) @ -1 dBFS
-104 dB (0.0006%) @ -8 dBFS
1 kHz signal, 22Hz - 22kHz BW

Analog Out Performance
(Analog performance specs measured at 44.1 kHz sample rate, 24-bit, card installed in computer)
Frequency Response
20 - 20 kHz, ± 0.05 dB at 44.1 kHz sample rate

Dynamic Range
117 dB, A-wtd.

Signal-to-Noise
117 dB, A-wtd.

Channel Crosstalk
-120 dB, 1 kHz signal @ -1dBFS

THD+N
-97 dB (0.0014%) @ -1 dBFS
-104 dB (0.0006%) @ -8 dBFS
1kHz signal, 22Hz - 22kHz BW


.....and the acoustic reality amps have following specs:

voltage gain : 30dB
input impedance : 60kOhm
input voltage : 30dB 2V RMS


I´m not sure either of what the sensitivity of the speakers are, but 3way magnepans do need a lot of "juice" - I believe that taking 24dB off the the volume (from the top so to speak), will do.
this is a 3 way active system with 1x 600watts on the bass & 2x 200watts on mid-panel and ribbon.
-do you wonder I need a bit of attenuation ? :-)

so how to put together a formula ?


thanx a bunch !
RE: digital attenuation, posted on January 15, 2010 at 18:23:45
Tony Lauck
Audiophile

Posts: 4582
Location: Vermont
Joined: November 12, 2007
Contributor
  Since: February 24, 2009
 Actually, with some DACs you might get better sound by doing some digital attenuation. This can happen for several reasons. The DAC might upsample and there might be an "intersample" peak that goes over and which would be clipped, something that would be avoided with digital attenuation. The DAC output circuitry might have non-linearity or slew rate limits on the output, and giving it less work to do might reduce the distortion. Of course on the low end, the reduced signal would be more subject to noise, low level non-linearities, cross over distortion, etc. It's all going to depend on the specifics, assuming you are working with 24 or 32 bit DACs. With 16 bit DACs digital attenuation is never a good idea, as there is only the bare margin of dynamic range available (if that).

Tony Lauck

"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar

RE: digital attenuation, posted on January 15, 2010 at 19:08:36
Dawnrazor
Audiophile

Posts: 8992
Location: N. California
Joined: April 9, 2004
 Mac,

WHile you are quoting the textbook, I dont think that it is necessarily right. Todays digital volume controls can be every bit as good as analog ones. It all depends on the implementation. What digital vc did you experience?

I re-visited the whole analog digital debate about a year ago. I put in my old dac that had an analog vc and was looking forward to all that resolution especially at low volumes. You know, it wasnt there, and I couldnt notice much if any difference between the 2 volumes. Granted the comparison used 2 different dacs so it wasnt exactly comparing just the vc implementation, but I sure was disappointed with the analog. I have to go with Alan on this one.

Also playmate is tri-amping. How is he going to get the volume precise using 3 of the analog devices you mention?? And for him to get 1st class preamplification for 6 channels that is going to cost a ton.

Which jensens are you recommending?


No one here remembers the bending of our minds
RE: digital attenuation, posted on January 15, 2010 at 22:32:04
mäç
 Although he mentions that he's using the Thuneau Allocator, he didn't state that he's multi-amping.

If that's the case then the equation changes in that he'd need three dual-stepped attenuators rather than one. He also mentions that he's interested in reducing gain (thus the use of fixed-value resistors). A stepped attenuator may be overkill for the purpose of setting-it-and-forgetting-it, especially if he needs to attenuate six channels.

The OP would probably be best slapping a dual-ganged buck twenty-five attenuator into his system to determine if reducing the gain brings about the sonic improvement he's seeking. If it does, then it's a very simple task to replace the pots with fixed high-quality resistors as I previously described.

As an aside, I also triamp and use Hypex modules (same input Z as his B&O amplifier modules). In the past I have used a DCX-2496 and I can state that it greatly suffers by attenuating the volume either before the inputs to it, or digitally by using the gain settings from within the unit. I also use a SB that has a digital volume control and it suffers sonic degradation when using its volume control. This is why I recommended analog attenuation between the sound card and amplifiers.

IME he should not have S/N issues using a resistive dividing network as described. Changing the gain of either his sound card or amplifier is another option but it's an option that would be much more difficult to implement.

The NHT passive volume control that I linked to in my prior post includes a set of internally mounted Jensen JT-11P-1HPC transformers along with a dual-ganged Alps Blue Velvet pot.

RE: pinging soundchekk, posted on January 16, 2010 at 00:58:43
soundchekk
Audiophile

Posts: 816
Location: DUS
Joined: July 11, 2007
 Your list is not complete yet!! Checkout my requirement spec again.

>>I believe that ..... -- that we do in church. ;)

We want do do some serious engineering. ;)

It is funny how difficult it is to find out the very basics, isn't it.

So - what do we have?

1. Voltage gain of your amp.

1 out of 5.

All the other stuff is irrelevant.

You need to squeeze out Google a bit more.
I'am sure well get it done.
RE: digital attenuation, posted on January 16, 2010 at 08:58:29
fmak
Audiophile

Posts: 5241
Joined: June 1, 2002
 There is no noise and no loss of resolution with a top notch constant impedance passive attenuators. Otherwise one has to have a preamp with the right gain structure. Even early preamps have tackled this issue.

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