Output from DS with no music playing


#1

I have 100 wpc Class A mono bloc amps which have VU meters. I’ve noticed that when I connect via the balanced outputs from the DS the reading on the meters is quite high with the output setting at HI on the DS. It is lower with the DS set at LO.

It is not so pronounced when I connect via the unbalanced route.

My question: is this normal or is there something wrong?


#2

There was a similar question asked and answered recently - I think it was the McIntosh one?

The DS output stage is always switching even when no music is playing. It has to, because it has only two states: fully positive or fully negative. The low-pass filter captures most but not all of the high frequency noise that comes out as a result. Your amp is probably doing work in the ultrasonics even when nothing is playing.

Possibly the Zobel filter idea that Joe Rasmussen posted about recently would help with that, but it might also have a detrimental effect on the audible part.

Now I’m wondering whether this partly explains my higher electricity bill from the past quarter…


#3

Normal.

The ultrasonic noise from the DS (and other single bit DACs) will confuse some VU meters. That noise isn’t audible but it is there whether the output of the DS is at full volume or a very low volume.

On the other hand the 20dB attenuator is analog and lowers the output overall (including ultrasonic noise) by 20dB.


#4

Ted, what frequency range is the ultrasonic? To clarify the impact of the attenuator, is the ultrasonic a fixed level, not impacted by the volume? As such the ultrasonic would be always 20dB lower with the attenuator engaged, correct?


#5

The noise level doesn’t depend on the volume setting of the DAC (or the level of the input digital signal), BUT the 20dB attenuator is an analog attenuator and does a 20dB attenuation of everything including the noise. Yes the 20dB attenuator lowers output noise by 20dB.

The noise spectrum is a little harder to describe. But it’s reasonably flat thru the audio band to about 60kHz then it rises to about 100k then falls again. I change the shape somewhat from release to release but that’s the general flavor.


#6

Noise attenuation may also be part of the reason some people prefer the sound of DS going through a preamp. The preamp offers analog attenuation of the high frequency signal which may compromise some amplifier’s performance.


#7

Hmmm. Also explains (to me) why I’ve preferred the attenuated DS sound – I run it this way full time (I use a preamp as well).


#8

Ted - this noise is probably well above our regular hearing - but assuming my speakers can reproduce any of the noise, would it be loud enough to bother pets? (I’m being totally serious)


  1. Cats have excellent hearing and can detect an extremely broad range of frequencies. They can hear higher-pitched sounds than either dogs or humans, detecting frequencies from 55 Hz up to 79 kHz (a range of 10.5 octaves) and dogs hear from 67 Hz to 44 kHz, which are both ranges of about 9 octaves.

#9

It’s still down at a very low level. For example the SACD spec limits the high frequencies to -40dBFS which might look bad on a VU meter with music playing, but is still not loud in an absolute scale.

Also as I mentioned the ultrasonic noise doesn’t grow above the system noise till around 60kHz, but that doesn’t mean it takes a big jump there, it grows slowly to approx. 100k.

My amps are flat to 200k, my speakers to 80k and the cats here never seemed to care. In fact it’s fun watching them follow things around in the sound stage: it doesn’t seem to bother them that they can’t see them :)


#10

Cool, thanks for verification, Ted. With all the high-frequency talks with PP - it made me wonder.

I’m more aware of my pets environment after reading a story about a guy who’s dog hangs out with him in his workshop. The dog has a bed in his workshop, and it didn’t occur to the owner that his dog watches everything he does while his in the shop, including welding. Well, you can guess what happened there.


#11

Yipes!


#12
Lonely Raven said
Cats have excellent hearing and can detect an extremely broad range of frequencies. They can hear higher-pitched sounds than either dogs or humans, detecting frequencies from 55 Hz up to 79 kHz (a range of 10.5 octaves) and dogs hear from 67 Hz to 44 kHz . . .
Once again cats rule, dogs drool.
Lonely Raven said I'm more aware of my pets environment after reading a story about a guy who's dog hangs out with him in his workshop.
I find disturbing that people let their dogs hang their heads out of a moving car. One small stone or hard-shelled beetle and there goes an eye, let alone the lesser damage done by fast moving grit, etc.
On topic, given the low level of ultrasonic noise and its broad spectrum I doubt critters even notice. It certainly does not contain sufficient energy to cause damage.

#13

Follow up, I also had this issue of noise with no output. I had to reduce the input gain on my amps to eliminate it. Worked perfectly. And, I am very happy with the direct stream. Not sure what the noise was caused by and I notified ps audio of the issue the week I upgraded.


#14

@Ted Smith

I’ve done some more experimenting and if I go from DAC to preamp (tube) to power amp the VU meters are at 0 when no music is playing as opposed to a significant reading when going direct to power amp.

Why would that be? Is the preamp filtering out the high frequency noise?

I prefer the SQ via the preamp. I’m wondering if this is because the power amp is not able to filter out the noise.


#15

The VU meters aren’t necessarily hooked directly up to the signal - it’s reporting an averaged value and hence is a low pass filtered version of the signal - it’s intended to report an approximation of the level of a signal that’s close to what we might report when hearing the signal.

Some digital implementations don’t accurately represent what a “standard VU” meter would report. You could argue that the VU meter definition is dated and we can do better now, others argue that you shouldn’t call it a VU meter unless it matches the readings that the electromechanical device from the 30’s or 40’s would give.

The net is that a standard VU meter will be filtering out some of the ultrasonics, but especially with a digital VU meter you don’t really know what that filter is. In particular it doesn’t really tell you if a preamp or amp is filtering the signal, it’s designed to indicate how loud you’d report the signal to be (and you have a filter that doesn’t respond nearly as much to ultrasonic energy.)

What I can tell you is that from release to release of the DS software all of you like the results filtering the signal less and less and letting more ultrasonic energy thru, i.e. that you’all like to hear more harmonics of the signal, go figure :slight_smile: - I have to balance that against the number of user’s who’s systems don’t react well with too much ultrasonic energy. Ironically some systems definitely sound better with more ultrasonic noise…


#16
alcarp said Is the preamp filtering out the high frequency noise?
Very possible.

#17
Ted Smith said

What I can tell you is that from release to release of the DS software all of you like the results filtering the signal less and less and letting more ultrasonic energy thru, i.e. that you’all like to hear more harmonics of the signal, go figure :slight_smile: - I have to balance that against the number of user’s who’s systems don’t react well with too much ultrasonic energy. Ironically some systems definitely sound better with more ultrasonic noise…


interested why that may be…