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Old 23rd June 2004, 12:47 PM   #11
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Harmonics.
I realize the proper way to do is to look at graphic readouts from precision measuring tools etc.
But does/has any one here actually listened to frequency sweeps/spots to ascertain the audible effect with 'pure' tones?

I still cannot come to terms with what I consider pretty dodgy sounding test tones and the fact that the dac sounds pretty OK with music


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Old 23rd June 2004, 01:10 PM   #12
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At what frequency is that "pretty dodgy sounding test tone"?
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Old 23rd June 2004, 03:48 PM   #13
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Quote:
Originally posted by setmenu
But does/has any one here actually listened to frequency sweeps/spots to ascertain the audible effect with 'pure' tones?
Very good idea.

I have CD players with PCMXX chips where distortion can be optimized with a pot.

In non optimal adjustment distortion of 1kHz is clearly audible.

I will unudjust the pot and measure at the point where distortion comes to my ears.
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Old 23rd June 2004, 03:49 PM   #14
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Quote:
Originally posted by Peter Daniel
At what frequency is that "pretty dodgy sounding test tone"?

Hi Peter
My 'scientific' test involved comparing my Arcam CD23 and the Nos dac by switching between them using a Stax 4040 rig
to monitor the sound.
Using a sweep the effect is of lower frequency warbling birdies at the upper reaches of the sweep.
Using the spot frequencies, up to 14k seems ok, 16K sounds like there is another lower tone involved and once I hit 18k there does not appear to be an 18k tone anymore or it is swamped by lower frequencies tone[s].
Same for 20K.
Obviously my sensitivity to the higher frequencies is less than the lower so their sounds are probably being masked.

The Arcam on the other hand is 'clean' until above my hearing ability.

If I remember correctly aren't the highest frequencies[nos] modulated by the sampling frequency or some such?


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Old 23rd June 2004, 03:51 PM   #15
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Quote:
Originally posted by Bernhard


Very good idea.

I have CD players with PCMXX chips where distortion can be optimized with a pot.

In non optimal adjustment distortion of 1kHz is clearly audible.

I will unudjust the pot and measure at the point where distortion comes to my ears.

Hi Bernhard
When you say 1k, do mean listening to a 1k test tone?


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Old 23rd June 2004, 03:55 PM   #16
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Quote:
Originally posted by setmenu
Harmonics.
I realize the proper way to do is to look at graphic readouts from precision measuring tools etc.
The SpectraLab software is pretty accurate and can be used for precision measurements of sound equipment. The problem lies in the sound equipment, therefore, it is recommended that professional computer soundcards are used with it. Or at least, the soundcard needs to have lower distortions and better characteristics than the equipment being tested.

I measured my soundcard(Audigy1, it's more stable than later Audigy generations), and with the 1kHz test tone the THD for it was 0.003%, well below the DAC specs, so it is rather suited for this application.

About the spikes, I'm not completely sure about them 100%, they appeared more audible on the first runs of testing, when the i/v load was higher (3.3k). The THD was much higher then as well. With the load of 1.8k they are much less visible on the spectrum, therefore, the sound must be purer. And it does sound cleaner with the less load. The disadvantage of the passive i/v with less load, however, is weaker output - now it's giving 1V instead of the standard 2V of lineout from the CD. In other words, it looks more attractive to switch to the opamp i/v stage using precision fast low-noise opamps, like LM6172 or THS4082.

Any suggestions how to deal with those dodgy spikes? If they are caused by noise from the power regulators or digital signal, proper filtering can be applied, I think, to eliminate them. They are not random, in fact, and appear on 2, 3, 5, 7, 9, 11kHz and so on for 1kHz test tone... odd frequency spike ringing, except for 2kHz, of course. Are they caused by improper grounding, since I put everything on a test pcb?
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Old 23rd June 2004, 04:06 PM   #17
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Do shoot me down in flames but with this passive IV lark,
the main consideration seems to be the need for volts from our end, and the need for low impedance form the dacs point of view?

I wondered would using a low low value Iv resister followed by a Op set at a sufficiently high gain [bandwidth] be a valid approach?
No doubt this has been already covered and resulted in sound eh?


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Old 23rd June 2004, 04:17 PM   #18
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Quote:
Originally posted by setmenu

Using a sweep the effect is of lower frequency warbling birdies at the upper reaches of the sweep.
Using the spot frequencies, up to 14k seems ok, 16K sounds like there is another lower tone involved and once I hit 18k there does not appear to be an 18k tone anymore or it is swamped by lower frequencies tone[s].
Same for 20K.
Setmenu, it may sound disappointing to you, sorry, but i ran the 15 to 20kHz test tones thru my DAC and it sounds very good to me. I didn't hear any audible distortions like additional lower frequency tones at all, really. At 20kHz you can hardly hear any tone, it's like flying on a plane when your ears feel some pressure or something, just some high-pitch feeling instead of the sound.

There must be something else with your DAC or your amp(why not?) since it gives you what you described in your post. Try different amps, connect the phones directly to your DAC without amp, etc. etc...

if the distortions don't go away, i would revise the schematics and try different configurations, parts, and so on, until the high frequency garbage is gone. My cheap Sanyo mp3 player makes the same noises in the high frequency area, but who cares about it? I only use it for testing purposes, not for reference or listening pleasure.
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Old 23rd June 2004, 04:29 PM   #19
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Quote:
Originally posted by IpsilonSound


Setmenu, it may sound disappointing to you, sorry, but i ran the 15 to 20kHz test tones thru my DAC and it sounds very good to me. I didn't hear any audible distortions like additional lower frequency tones at all, really. At 20kHz you can hardly hear any tone, it's like flying on a plane when your ears feel some pressure or something, just some high-pitch feeling instead of the sound.

There must be something else with your DAC or your amp(why not?) since it gives you what you described in your post. Try different amps, connect the phones directly to your DAC without amp, etc. etc...

if the distortions don't go away, i would revise the schematics and try different configurations, parts, and so on, until the high frequency garbage is gone. My cheap Sanyo mp3 player makes the same noises in the high frequency area, but who cares about it? I only use it for testing purposes, not for reference or listening pleasure.

Hmm all 3 of them do it, and with all the amps tested.
And I can't really connect phones directly to the dac,won't drive em!
As for layout and parts choice, yup could be [most likely] that is
to blame then.
But this thing needs to be small so choices are limited...

Still, based on the observed shortcomings it does seem to sound
nice

But I am not content, I want a better matchbox sized dac


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Old 23rd June 2004, 04:42 PM   #20
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Quote:
Originally posted by setmenu
No doubt this has been already covered and resulted in sound eh?
Setmenu, with all due respect, this is gross overgeneralization. Why being so dependent on others' mishaps with active i/v stage experiments? You can find dozens of different implementations of the i/v active stages, ranging from simple to complex. For some it did result with inferior sound, but that shouldn't preclude one from trying it out for himself and rushing to decisions only after all the possibilities have been explored.
I never said the passive i/v was that bad, to me it sounds great, but I cannot implement it with the same power supply feeding both my PPA and DAC. That's Reason #1. Reason #2 - attempt to reduce the distortions and spikes visible on the spectrum, and they are clearly dependent on the load/impedeance put on the TDA1543. Therefore, properly implemented 3 channel, virtual ground, class-a, precision, extremly low noise/distortion wideband active i/v stage may be the only remedy for this? Did you check the datasheet specs for LM6171 amp? I also read what others think about i/v active opamp conversion, after conducted some interesting opamp experiments. They are not all with negative results, indeed. Unfortunately, it's in another language forum, otherwise, i could send the link to prove it. Well worth reading...
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